US20070168033A1 - Prosthetic intervertebral discs having substantially rigid end plates and fibers between those end plates - Google Patents

Abstract

Prosthetic intervertebral discs and methods for using the same are described. The subject prosthetic discs include upper and lower endplates separated by a compressible core member. The prosthetic discs described herein include one-piece, two-piece, three-piece, and four-piece structures. The subject prosthetic discs exhibit stiffness in the vertical direction, torsional stiffness, bending stiffness in the saggital plane, and bending stiffness in the front plane, where the degree of these features can be controlled independently by adjusting the components of the discs. The interface mechanism between the endplates and the core members of several embodiments of the described prosthetic discs enables a very easy surgical operation for implantation.

Description

RELATED APPLICATIONS
• This application is a continuation of application Ser. No. 10/903,276, filed Jul. 30, 2004, which in turn, is a continuation-in-part of co-pending application Ser. No. 10/632,538, filed Aug. 1, 2003, which prior applications are incorporated by reference.
BACKGROUND OF THE INVENTION
• The intervertebral disc is an anatomically and functionally complex joint. The intervertebral disc is composed of three component structures: (1) the nucleus pulposus; (2) the annulus fibrosus; and (3) the vertebral endplates. The biomedical composition and anatomical arrangements within these component structures are related to the biomechanical function of the disc.
• The spinal disc may be displaced or damaged due to trauma or a disease process. If displacement or damage occurs, the nucleus pulposus may herniate and protrude into the vertebral canal or intervertebral foramen. Such deformation is known as herniated or slipped disc. A herniated or slipped disc may press upon the spinal nerve that exits the vertebral canal through the partially obstructed foramen, causing pain or paralysis in the area of its distribution.
• To alleviate this condition, it may be necessary to remove the involved disc surgically and fuse the two adjacent vertebra. In this procedure, a spacer is inserted in the place originally occupied by the disc and it is secured between the neighboring vertebrae by the screws and plates/rods attached to the vertebra. Despite the excellent short-term results of such a “spinal fusion” for traumatic and degenerative spinal disorders, long-term studies have shown that alteration of the biomechanical environment leads to degenerative changes at adjacent mobile segments. The adjacent discs have increased motion and stress due to the increased stiffness of the fused segment. In the long term, this change in the mechanics of the motion of the spine causes these adjacent discs to degenerate.
• To circumvent this problem, an artificial intervertebral disc replacement has been proposed as an alternative approach to spinal fusion. Although various types of artificial intervertebral discs have been developed to restore the normal kinematics and load-sharing properties of the natural intervertebral disc, they can be grouped into two categories, i.e., ball and socket joint type discs and elastic rubber type discs.
• Artificial discs of ball and socket type are usually composed of metal plates, one to be attached to the upper vertebra and the other to be attached to the lower vertebra, and a polyethylene core working as a ball. The metal plates have concave areas to house the polyethylene core. The ball and socket type allows free rotation between the vertebrae between which the disc is installed and thus has no load sharing capability against the bending. Artificial discs of this type have a very high stiffness in the vertical direction, they cannot replicate the normal compressive stiffness of the natural disc. Also, the lack of load bearing capability in these types of discs causes adjacent discs to take up the extra loads resulting in the eventual degeneration of the adjacent discs.
• In elastic rubber type artificial discs, an elastomeric polymer is embedded between metal plates and these metal plates are fixed to the upper and the lower vertebrae. The elastomeric polymer is bonded to the metal plates by having the interface surface of the metal plates be rough and porous. This type of disc can absorb a shock in the vertical direction and has a load bearing capability. However, this structure has a problem in the interface between the elastomeric polymer and the metal plates. Even though the interface surfaces of the metal plates are treated for better bonding, polymeric debris may nonetheless be generated after long term usage. Furthermore, the elastomer tends to rupture after a long usage because of its insufficient shear-fatigue strength.
• Because of the above described disadvantages associated with either the ball/socket or elastic rubber type discs, there is a continued need for the development of new prosthetic devices.
RELEVANT LITERATURE
• U.S. Pat. Nos. 3,867,728; 4,911,718; 5,039,549; 5,171,281; 5,221,431; 5,221,432; 5,370,697; 5,545,229; 5,674,296; 6,162,252; 6,264,695; 6,533,818; 6,582,466; 6,582,468; 6,626,943; 6,645,248. Also of interest are published U.S. Patent Application Nos. 2002/0107575, 2003/0040800, 2003/0045939, and 2003/0045940. See also Masahikio Takahata, Uasuo Shikinami, Akio Minami, “Bone Ingrowth Fixation of Artificial Intervertebral Disc Consisting of Bioceramic-Coated Three-dimensional Fabric,” SPINE, Vol. 28, No. 7, pp. 637-44 (2003).
SUMMARY OF THE INVENTION
• Prosthetic intervertebral discs and methods for using such discs are provided. The subject prosthetic discs include an upper endplate, a lower endplate, and a compressible core member disposed between the two endplates.
• In one embodiment, the subject prosthetic discs are characterized by including top and bottom endplates separated by a fibrous compressible element that includes an annular region and a nuclear region. The two plates are held together by at least one fiber wound around at least one region of the top endplate and at least one region of the bottom endplate. The subject discs may be employed with separate vertebral body fixation elements, or they may include integrated vertebral body fixation elements. Also provided are kits and systems that include the subject prosthetic discs.
• In other embodiments, the prosthetic disc comprises an integrated, single-piece structure. In another embodiment, the prosthetic disc comprises a two-piece structure including a lower endplate and a separable upper endplate assembly that incorporates the core member. The two-piece structure may be a constrained structure, wherein the upper endplate assembly is attached to the lower endplate in a manner that prevents relative rotation, or a partially or semi-constrained structure or an unconstrained structure, wherein the upper endplate assembly is attached to the lower endplate in a manner that allows relative rotation. In yet another, embodiment, the prosthetic disc comprises a three-piece structure including upper and lower endplates and a separable core member that is captured between the upper and lower endplates by a retaining mechanism. Finally, in yet another embodiment, the prosthetic disc comprises a four-piece structure including upper and lower endplates and two separable core assemblies which, together, form a core member.
• Several optional core materials and structures may be incorporated in each of the prosthetic disc embodiments described herein. For example, the core member may be formed of a relatively compliant material, such as polyurethane or silicone, and is typically fabricated by injection molding. In other examples, the core member may be formed by layers of fabric woven from fibers. In still further examples, the core member may comprise a combination of these materials, such as a fiber-reinforced polyurethane or silicone. As an additional option, one or more spring members may be placed between the upper and lower endplates in combination with the core member, such as in a coaxial relationship in which the core member has a generally cylindrical or toroidal shape and a spring is located at its center.
• In the various embodiments, the disc structures are held together by at least one fiber wound around at least one region of the upper endplate and at least one region of the lower endplate. The fibers are generally high tenacity fibers with a high modulus of elasticity. The elastic properties of the fibers, as well as factors such as the number of fibers used, the thickness of the fibers, the number of layers of fiber windings, the tension applied to each layer, and the crossing pattern of the fiber windings enable the prosthetic disc structure to mimic the functional characteristics and biomechanics of a normal-functioning, natural disc.
• Apparatus and methods for implanting prosthetic intervertebral discs are also provided. In a first embodiment, the apparatus includes three implantation tools used to prepare the two adjacent vertebral bodies for implantation and then to implant the prosthetic disc. A first tool, a spacer, is adapted to be inserted between and to separate the two adjacent vertebral bodies to create sufficient space for implanting the prosthetic disc. A second tool, a chisel, includes one or more wedge-shaped cutting blades located on its upper and/or lower surfaces that are adapted to create grooves in the inward facing surfaces of the two adjacent vertebral bodies. A third tool, a holder, includes an engagement mechanism adapted to hold the prosthetic disc in place while it is being implanted, and to release the disc once it has been implanted.
• In another embodiment, the implantation apparatus includes a guide member that engages the lower endplate and that remains in place during a portion of the disc implantation process. A lower pusher member slidably engages the guide member and is used to advance the lower endplate into place between two adjacent vertebrae of a patient's spine. An upper pusher member is preferably coupled to the lower pusher member and is used to advance a first chisel into place opposed to the lower endplate between the two adjacent vertebrae. Once in place, an upward force is applied to the upper pusher member to cause the first chisel to engage the upper vertebral body and to create one or more grooves on its lower surface. A downward force is also applied to the lower pusher member to cause the lower endplate to engage the lower vertebral body and to become implanted. The upper pusher member and first chisel are then removed, as is the lower pusher member. Preferably, a second chisel is then advanced along the guide member and is used to provide additional preparation of the upper vertebral body. After the completion of the preparation by the first chisel and, preferably, the second chisel, the upper endplate and core members of the prosthetic disc are implanted using an upper endplate holder that is advanced along the guide member. After implantation, the upper endplate holder and guide member are removed.
• Apparatus and methods for implanting prosthetic intervertebral discs using minimally invasive surgical procedures are also provided. In one embodiment, the apparatus includes a pair of cannulas that are inserted posteriorly, side-by-side, to gain access to the spinal column at the disc space. A pair of prosthetic discs are implanted by way of the cannulas to be located between two vertebral bodies in the spinal column. In another embodiment, a single, selectively expandable disc is employed. In an unexpanded state, the disc has a relatively small profile to facilitate delivery of it to the disc space. Once Operatively positioned, it can then be selectively expanded to an appropriate size to adequately occupy the disc space. Implantation of the single disc involves use of a single cannula and an articulating chisel or a chisel otherwise configured to establish a curved or right angle disc delivery path so that the disc is substantially centrally positioned in the disc space. Preferably, the prosthetic discs have sizes and structures particularly adapted for implantation by the minimally invasive procedure.
• Other and additional devices, apparatus, structures, and methods are described by reference to the drawings and detailed descriptions below.
DESCRIPTION OF THE DRAWINGS
• The Figures contained herein are not necessarily drawn to scale, with some components and features being exaggerated for clarity.
• FIGS. 1A and 1B provide a three dimensional view of two different prosthetic discs according to the subject invention.
• FIG. 2 provides a three-dimensional view of a fibrous compressible element that includes a polymeric nucleus and a fibrous annulus according to one embodiment of the subject invention.
• FIGS. 3A to3C provide different views of a fibrous component of the fibrous compressible elements according to an embodiment of the subject invention.FIG. 3C illustrates the manner in which the 2D fabrics inFIG. 3B are stitched together.
• FIG. 4A provides a three-dimensional top view of a prosthetic disc according to an embodiment of the present invention in which the fixation elements are integral to the disc, whileFIG. 4B shows the disc ofFIG. 4A implanted with the use of bone screws.
• FIGS. 5A and 5B show the mating interface between disc top endplate with an upper vertebral body fixation element according to an embodiment of the subject invention.
• FIGS. 6A and 6B show the mating interface between disc top endplate with an upper vertebral body fixation element according to an alternative embodiment of the subject invention. The top endplate is clamped by a clamping element connected to the upper vertebral body fixation element through a spring.
• FIG. 7 provides an exploded view of a disc system that includes both an intervertebral disc and vertebral body fixation elements, according to an embodiment of the present invention.
• FIGS. 8 and 9 provide views of vertebral body fixation elements being held in an implantation device according to an embodiment of the subject invention.
• FIG. 10 provides a view of disc implantation device and disc according to an embodiment of the subject invention.
• FIG. 11 provides sequential views of a disc being replaced with a prosthetic disc according to a method of the subject invention.
• FIG. 12 provides a cross-sectional view of a prosthetic disc having a one-piece structure.
• FIG. 13A provides a three-dimensional view of a prosthetic disc having a one-piece structure including a single anchoring fin on each of the upper and lower endplates.
• FIG. 13B provides a three-dimensional view of a prosthetic disc having a one-piece structure including three anchoring fins on each of the upper and lower endplates.
• FIG. 13C provides a three-dimensional view of a prosthetic disc having a one-piece structure including a serrated surface on each of the upper and lower endplates.
• FIG. 13D provides a three-dimensional view of a prosthetic disc having a one-piece structure including a superior dome.
• FIG. 13E provides a three-dimensional view of the prosthetic disc having a one-piece structure ofFIG. 13D, having no superior dome.
• FIG. 13F provides a three-dimensional cross-sectional view of the prosthetic disc having a one-piece structure shown inFIG. 13D.
• FIG. 13G provides a three-dimensional view of a prosthetic disc having a prosthetic structure design without a gasket retaining ring.
• FIG. 13H provides a three-dimensional cross-sectional view of the prosthetic disc having a one-piece structure shown inFIG. 2G.
• FIG. 13I provides a cross-sectional view of an upper endplate of a prosthetic disc having a one-piece structure design without a gasket retaining ring.
• FIG. 13J provides an inset view of a portion of the upper endplate shown inFIG. 2I.
• FIG. 13K provides a cross-sectional illustration of a prosthetic disc having a one-piece structure design with a center spring.
• FIG. 13L provides a three-dimensional cross-sectional illustration of the prosthetic disc having a one-piece structure shown inFIG. 13K.
• FIGS. 14A and B provide illustrations of uni-directional and bi-directional fiber winding patterns.
• FIGS.15A-C provide illustrations of an annular capsule.
• FIG. 16 provides a three-dimensional view of a prosthetic disc having a two-piece structure.
• FIG. 17 provides a three-dimensional view of an outer lower endplate of the prosthetic disc shown inFIG. 16.
• FIG. 18 provides a cross-sectional view of a prosthetic disc having a two-piece constrained structure.
• FIG. 19 provides a three-dimensional view of a prosthetic disc having a two-piece unconstrained structure.
• FIG. 20 provides a cross-sectional view of a prosthetic disc having a two-piece constrained structure.
• FIG. 21 provides a three-dimensional view of a prosthetic disc having a three-piece structure.
• FIG. 22 provides a three-dimensional view of a lower endplate of the prosthetic disc shown inFIG. 21.
• FIG. 23 provides a cross-sectional view of a prosthetic disc having a three-piece structure.
• FIG. 24A provides a three-dimensional view of a core assembly for a prosthetic disc having a three-piece structure.
• FIG. 24B provides a three-dimensional view of another core assembly for a prosthetic disc having a three-piece structure.
• FIG. 24C provides a three-dimensional view of another core assembly for a prosthetic disc having a three-piece structure.
• FIG. 25A provides a cross-section view of a fiber reinforced core assembly.
• FIG. 25B provides a cross-section view of another fiber reinforced core assembly.
• FIG. 25C provides a cross-section view of another fiber reinforced core assembly.
• FIG. 26 provides a three-dimensional view of a stacked fabric core assembly.
• FIG. 27 provides a cross-sectional view of a stacked fabric core assembly.
• FIG. 28A provides a three-dimensional view of a stacked fabric core assembly.
• FIG. 28B provides a three-dimensional view of another stacked fabric core assemble.
• FIG. 28C provides a three-dimensional view of another stacked fabric core assemble.
• FIG. 29 provides a three-dimensional view of a prosthetic disc having a four-piece structure.
• FIG. 30 provides a cross-sectional view of a prosthetic disc having a four-piece structure.
• FIG. 31 provides an expanded view of a core assembly for a prosthetic disc having a four-piece structure.
• FIG. 32 provides a three-dimensional view of a prosthetic disc having a four-piece structure.
• FIG. 33 provides a cross-sectional view of a prosthetic disc having a four-piece structure.
• FIG. 34 provides a three-dimensional view of a prosthetic disc having a four-piece structure.
• FIG. 35 provides an expanded view of a core assembly for a prosthetic disc having a four-piece structure.
• FIG. 36A provides a perspective view of a spacer.
• FIG. 36B provides a perspective view of the head portion of the spacer shown inFIG. 36A.
• FIG. 37A provides a perspective view of a double-sided chisel.
• FIG. 37B provides a top view of the head portion of the double-sided chisel shown inFIG. 37A.
• FIG. 38A provides a perspective view of a holder.
• FIG. 38B provides a perspective view of the head portion of the holder shown inFIG. 38A.
• FIG. 39 provides a perspective view of a guide member.
• FIG. 40 provides a perspective view of a first chisel and lower endplate insert apparatus.
• FIG. 41 provides a perspective view of an upper endplate holder.
• FIG. 42 provides a perspective view of a second chisel.
• FIG. 43A provides an illustration of a method step of advancing a first chisel and outer lower endplate.
• FIG. 43B provides an illustration showing a pair of adjacent vertebrae during an implantation procedure.
• FIG. 44A provides an illustration of a method step of providing a force separating a first chisel and an outer lower endplate.
• FIG. 44B provides an illustration of a pair of adjacent vertebrae during the method step shown inFIG. 44A.
• FIG. 45A provides an illustration of a guide member and outer lower endplate.
• FIG. 45B provides an illustration of a pair of vertebrae with an outer lower endplate implanted onto the lower vertebra.
• FIG. 46A provides an illustration of a method step of advancing a second chisel.
• FIG. 46B provides an illustration of a pair of adjacent vertebrae during the method step shown inFIG. 46A.
• FIG. 47A provides an illustration of a guide member and outer lower endplate.
• FIG. 47B provides an illustration of a pair of vertebrae with an outer lower endplate implanted onto the lower vertebra.
• FIG. 48A provides an illustration of a method step of advancing a prosthetic disc upper subassembly.
• FIG. 48B provides an illustration of a pair of adjacent vertebrae during the method step shown inFIG. 48A.
• FIG. 49A provides an illustration of a method step of withdrawing an upper endplate holder and guide member.
• FIG. 49B provides an illustration of a pair of vertebrae with a prosthetic disc having been implanted therebetween.
• FIG. 50A provides a three-dimensional view of a preferred prosthetic disc for use with a minimally invasive surgical procedure.
• FIG. 50B provides a three-dimensional view of another preferred prosthetic disc for use with a minimally invasive surgical procedure.
• FIG. 51 provides an illustration of a minimally invasive surgical procedure for implanting a pair of prosthetic discs.
• FIG. 52A provides an illustration of an alternative minimally invasive surgical procedure for implanting a prosthetic disc.
• FIG. 52B provides a schematic illustration of a dual prosthetic disc having a mechanism for separating the discs after implantation.
• FIG. 53 provides a cross-sectional schematic illustration of an anti-creep compression member.
• FIG. 54 provides a cross-sectional illustration of a mechanism for deploying and retracting fins and/or spikes located on prosthetic disc endplate.
DESCRIPTIONS OF THE PREFERRED EMBODIMENTS
• Before the present invention is described, it is to be understood that this invention is not limited to particular embodiments described, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims.
• Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range is encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges is also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the invention.
• Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present invention, the preferred methods and materials are now described. All publications mentioned herein are incorporated herein by reference to disclose and describe the methods and/or materials in connection with which the publications are cited.
• It must be noted that as used herein and in the appended claims, the singulars forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise.
• The publications discussed herein are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the present invention is not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided may be different from the actual publication dates which may need to be independently confirmed.
• As will be apparent to those of skill in the art upon reading this disclosure, each of the individual embodiments described and illustrated herein has discrete components and features which may be readily separated from or combined with the features of any of the other several embodiments without departing from the scope or spirit of the present inventions.
• Prosthetic intervertebral discs, methods of using such discs, apparatus for implanting such discs, and methods for implanting such discs are described herein. It is to be understood that the prosthetic intervertebral discs, implantation apparatus, and methods are not limited to the particular embodiments described, as these may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present inventions will be limited only by the appended claims.
• The following description includes three Parts. Part A contains a description of a first set of embodiments of the subject prosthetic intervertebral discs, a review of representative methods for using the prosthetic discs, and a review of systems and kits that include the subject prosthetic discs. The embodiments described in Part A are those illustrated inFIGS. 1-11. Part B contains a description of a second set of embodiments of the subject prosthetic intervertebral discs, methods for using the discs, and apparatus and methods for implanting the discs. The embodiments described in Part B are those illustrated inFIGS. 12-54. Each of the descriptions contained in Parts A and B will be understood to be complete and comprehensive in its own right, as well as describing structures, features, and methods that are suitable for use with those described in the other Part. Part C includes additional information about the descriptions contained herein.
• Part A
• I. Prosthetic Intervertebral Disc
• As summarized above, the subject invention is directed to a prosthetic intervertebral disc. By prosthetic intervertebral disc is meant an artificial or manmade device that is configured or shaped so that it can be employed as a replacement for an intervertebral disc in the spine of a vertebrate organism, e.g., a mammal, such as a human. The subject prosthetic intervertebral disc has dimensions that permit it to substantially occupy the space between two adjacent vertebral bodies that is present when the naturally occurring disc between the two adjacent bodies is removed, i.e., a void disc space. By substantially occupy is meant that it occupies at least about 75% by volume, such as at least about 80% by volume or more. The subject discs may have a roughly bean shaped structure analogous to naturally occurring intervertebral body discs which they are designed to replace. In many embodiments the length of the disc ranges from about 15 mm to about 50 mm, such as from about 18 mm to about 46 mm, the width of the disc ranges from about 12 mm to about 30 mm, such as from about 14 mm to about 25 mm and the height of the disc ranges from about 3 mm to about 13 mm, such as from about 5 mm to about 12 mm.
• The subject discs are characterized in that they include both an upper (or top) and lower (or bottom) endplate, where the upper and lower endplates are separated from each other by a fibrous compressible element, where the combination structure of the endplates and fibrous compressible element provides a prosthetic disc that functionally closely mimics real disc. A feature of the subject prosthetic discs is that the top and bottom endplates are held together by at least one fiber, e.g., of the fibrous compressible element, wound around at least one portion of each of the top and bottom endplates. As such, the two endplates (or planar substrates) are held to each other by one or more fibers that are wrapped around at least one domain/portion/area of the upper endplate and lower endplate such that the plates are joined to each other.
• Two different representative intervertebral discs are shown inFIGS. 1A and 1B. As can be seen inFIGS. 1A and 1B,prosthetic discs10 each include atop endplate11 and alower endplate12. Top andbottom endplates11 and12 are planar substrates, where these plates typically have a length from about 12 mm to about 45 mm, such as from about 13 mm to about 44 mm, a width of from about 11 mm to about 28 mm, such as from about 12 mm to about 25 mm and a thickness of from about 0.5 mm to about 4 mm, such as from about 1 mm to about 3 mm. The top and bottom endplates are fabricated from a physiologically acceptable material that provides for the requisite mechanical properties, where representative materials from which the endplates may be fabricated are known to those of skill in the art and include, but are not limited to: titanium, titanium alloys, stainless steel, cobalt/chromium, etc.; plastics such as polyethylene with ultra high molar mass (molecular weight) (UHMW-PE), polyether ether ketone (PEEK), etc.; ceramics; graphite; etc. As shown inFIGS. 1A and 1B, separating the top and bottom endplates is a fibrouscompressible element17. The thickness of the fibrous compressible element may vary, but ranges in many embodiments from about 2 mm to about 10 mm, including from about 3 mm to about 8 mm.
• The disc is further characterized in that it includes an annular region13 (i.e., annulus), which is the region, domain or area that extends around the periphery of the disc, and a nuclear region (i.e., nucleus)14, which is the region, domain or area in the center of the disc and surrounded by the annulus.
• While in the broadest sense the plates may include a single region around which a fiber is wound in order to hold the plates together, in many embodiments the plates have a plurality of such regions. As shown inFIGS. 1A and 1B,endplates111 and12 include a plurality ofslots15 through which fibers, e.g., of the fibrous compressible element, may be passed through or wound, as shown. In many embodiments, the number of different slots present in the periphery of the device ranges from about 4 to about 36, such as from about 5 to about 25. As shown inFIGS. 1A and 1B, at least onefiber16 of the fibrous compressible element is wrapped around a region of the top and bottom plates, e.g., by being passed through slots in the top and bottom plates, in order to hold the plates together.
• The fibrous compressible elements,17, are typically made up of one or more fibers, where the fibers are generally high tenacity fibers with a high modulus of elasticity. By high tenacity fibers is meant fibers that can withstand a longitudinal stress without tearing asunder of at least about 50 MPa, such as at least about 250 MPa. As the fibers have a high modulus of elasticity, their modulus of elasticity is typically at least about 100 MPa, usually at least about 500 MPa. The fibers are generally elongate fibers having a diameter that ranges from about 3 mm to about 8 mm, such as about 4 mm to about 7 mm, where the length of each individual fiber making up the fibrous component may range from about 1 m to about 20 m, such as from about 2 m to about 15 m.
• The fibers making up the fibrous compressible elements may be fabricated from any suitable material, where representative materials of interest include, but are not limited to: polyester (e.g., Dacron), polyethylene, polyaramid, carbon or glass fibers, polyethylene terephthalate, acrylic polymers, methacrylic polymers, polyurethane, polyurea, polyolefin, halogenated polyolefin, polysaccharide, vinylic polymer, polyphosphazene, polysiloxane, and the like.
• The fibrous compressible elements made up of one or more fibers wound around one or more regions of the top or bottom plates may make up a variety of different configurations. For example, the fibers may be wound in a pattern that has an oblique orientation to simulate the annulus of intact disc, where a representative oblique fiber configuration or orientation is shown inFIG. 1A. The number of layers of fiber winding may be varied to achieve similar mechanical properties to an intact disk. Where desired, compliancy of the structure may be reduced by including a horizontal winding configuration, as shown inFIG. 1B.
• In certain embodiments, the fibrouscompressible element20 has afibrous component21 limited to the annular region of thedisc22, e.g., to the region along the periphery of the disc.FIG. 2 provides a representation of this embodiment, where the fibrous component is limited solely to the annular region of the disc and includes both oblique and horizontal windings. Also shown is aseparate polymeric component23 present in the nucleus. The fiber windings of the various layers of fiber may be at varying angles from each other where the particular angle for each layer may be selected to provide a configuration that best mimics the natural disc. Additionally, the tension placed on the fibers of each layer may be the same or varied.
• In yet other embodiments the fibrous component of the fibrous compressible element may extend beyond the annular region of the disc into at least about a portion, if not all, of the nucleus.FIG. 3A provides a view of afibrous component30 that occupies both the annular and nuclear regions of the disc, where the annular region of the disc is made up of fiber windings that are both oblique and horizontal, as described above, while the nucleus of the disc is occupied by fibers woven into a three-dimensional network that occupies the nuclear space. Instead of a three-dimensional network structure, one may have multiple two dimensional layers' of interwoven fibers stacked on top of each other, as shown inFIG. 3B, where the multiple stacked layers may be stitched to each other, as shown inFIG. 3C. By adjusting one or more parameters of the fibrous component, such as the density of the fibers, number of layers, frequency of stitching, the wrapping angle of each fiber layer, and the like, the mechanical properties of the fibrous component can be tailored as desired, e.g., to mimic the mechanical properties of a natural intervertebral disc. Also shown inFIGS. 3B and 3C is the outline of apolymeric component32 in which thefibrous component30 is embedded.
• In certain embodiments, the fibrous compressible element further includes one or more polymeric components. The polymeric component(s), when present, may be fabricated from a variety of different physiologically acceptable materials. Representative materials of interest include, but are not limited to: elastomeric materials, such as polysiloxane, polyurethane, poly(ethylene propylene) copolymer, polyvinylchloride, poly(tetrafluoro ethylene) and copolymers, hydrogels, and the like.
• The polymeric component may be limited to particular domains, e.g., the annular and/or nucleus domains, or extend throughout the entire region of the fibrous compressible elements positioned between the two endplates. As such, in certain embodiments the polymeric component is one that is limited to the nuclear region of the disc, as shown inFIG. 2. InFIG. 2, fibrouscompressible element20 includes a distinctfibrous component21 that is located in the annular region of thedisc22, whilepolymeric component23 is located in the nuclear region of the disc. In other embodiments, the polymeric component is located in both the annular and nuclear regions. In yet other embodiments, the polymeric component may be located solely in the annular region.
• Depending on the desired configuration and mechanical properties, the polymeric component may be integrated with the fibrous component, such that at least a portion of the fibers of the fibrous component is embedded in, e.g., complexed with, at least a portion of the polymeric component. In other words, at least a portion of the fibrous component is impregnated with at least a portion of the polymeric component. For example, as shown inFIG. 3B, stacked two-dimensional layers of thefibrous component30 are present inside thepolymeric component32, such that the fibrous component is impregnated with the polymeric component.
• In those configurations where the fibrous and polymeric components are present in a combined format, e.g., as shown inFIG. 3B, the fibers of the fibrous component may be treated to provide for improved bonding with the polymeric component. Representative fiber treatments of interest include, but are not limited to: corona discharge, O₂plasma treatment, oxidation by strong acid (HNO₃, H₂SO₄). In addition, surface coupling agents may be employed, and/or a monomer mixture of the polymer may be polymerized in presence of the surface-modified fiber to produce the composite fiber/polymeric structure.
• As indicated above, the devices may include one or more different polymeric components. In those embodiments where two or more different polymeric components are present, any two given polymeric components are considered different if they differ from each other in terms of at least one aspect, e.g., composition, cross-linking density, and the like. As such, the two or more different polymeric components may be fabricated from the same polymeric molecules, but differ from each other in terms of one or more of: cross-linking density; fillers; etc. For example, the same polymeric material may be present in both the annulus and nucleus of the disc, but the crosslink density of the annulus polymeric component may be higher than that of the nuclear region. In yet other embodiments, polymeric materials that differ from each other with respect to the polymeric molecules from which they are made may be employed.
• By selecting particular fibrous component and polymeric component materials and configurations, e.g., from the different representative formats described above, a disc with desired functional characteristics, e.g., that mimics the functional characteristics of the naturally occurring disc, may be produced.
• Representative particular combinations of interest include, but are not limited to, the following:
• • 1. Biocompatible polyurethane, such as Ethicon Biomer, reinforced with Dacron poly(ethylene terephthalate) fiber, or Spectra polyethylene fiber, or Kevlar polyaramide fiber, or carbon fiber.
  • 2. Biocompatible polysiloxane modified styrene-ethylene butylene block copolymer sold under C-Flex tradename reinforced with Dacron poly(ethylene terephthalate) fiber, or Spectra polyethylene fiber, or Kevlar polyaramide fiber, or carbon fiber.
  • 3. Biocompatible Silastic silicone rubber, reinforced with Dacron poly(ethylene terephthalate) fiber, or Spectra polyethylene fiber, or Kevlar polyaramide fiber, or carbon fiber.
• In using the subject discs, the prosthetic disc is fixed to the vertebral bodies between which it is placed. More specifically, the upper and lower plates of the subject discs are fixed to the vertebral body to which they are adjacent. As such, the subject discs are employed with vertebral body fixation elements during use. In certain embodiments, the vertebral body fixation elements are integral to the disc structure, while in other embodiments the vertebral body fixation elements are separate from the disc structure.
• A representative embodiment of those devices where the vertebral body fixation elements are integral with the disc structure is depicted inFIGS. 4A and 4B.FIG. 4A showsdevice40 made up of top andbottom endplates41 and42. Integrated with top andbottom endplates41 and42 are vertebralbody fixation elements43 and44. The vertebral body fixation elements include holes through which bone screws may be passed for fixation of the disc to upper and lowervertebral bodies47 and48 upon implantation, as represented inFIG. 4B.
• In an alternative embodiment, the disc does not include integrated vertebral body fixation elements, but is designed to mate with separate vertebral body fixation elements, e.g., as depicted inFIG. 7. In other words, the disc is structured to interface with separate vertebral body fixation elements during use. Any convenient separate vertebral body fixation element may be employed in such embodiments, so long as it stably positions the prosthetic disc between two adjacent vertebral bodies.
• One representative non-integrated vertebral body fixation element according to this embodiment is shown inFIGS. 5A and 5B.FIG. 5A provides a representation of theupper plate50 of a prosthetic disc mated with a vertebralbody fixation element51, as the structures would appear upon implantation. Vertebralbody fixation element51 is a horseshoe shapedstructure having spikes55 at locations corresponding to the cortical bone of vertebrae and porous coating to enhance bone fixation. Thefixation element51 also hasgear teeth52 such thatcorresponding gear teeth53 of thedisc upperplate50 can slide through the gear contact resulting in the right location of prosthetic disc with respect to the fixation element. The gear teeth have a shape such that only inward movement of the upper plate upon implantation is possible. Also present areslots56 in the spiked fixation elements next to the gear teeth that provide for the elastic deformation of the whole teeth area upon implantation and desirable clearance between mating gear teeth of the disc and fixation element so that incoming gear teeth of the disc can easily slide into the fixation element.
• In the embodiment shown inFIG. 5A, as the disc is pushed into the fixation element, the protrudedrail57 on the disc slides along the corresponding concave rail-way58 on the fixation element until the protruded rail on the most front side is pushed into the corresponding concave rail-way on the fixation element, as shown inFIG. 5B. This rail interface is devised to prevent the upward/downward movement of the top disc endplate and the bottom disc endplate with respect to the corresponding fixation element. This interface between the fixation elements and the top and bottom endplates of the disc enables an easy surgical operation. Specifically, the fixation elements are transferred together to the disc replacement area (disc void space) with an instrument and pushed in the opposite directions toward the vertebrae until they are fixed to the vertebrae, and then the prosthetic disc is transferred by the instrument between the fixation elements and simply pushed inward until the stoppers mate the corresponding stoppers. The prosthetic disc can also be easily removed after long-term use. For its removal, the gear teeth on the fixation element are pushed to reduce the gap of the slot so that the gear engagement between the disc endplate and the fixation element is released.
• An alternative embodiment is depicted inFIGS. 6A and 6B. In the embodiment shown inFIGS. 6A and 6B, thefixation element61 and theendplate62 have a different mating interface from that depicted inFIGS. 5A and 5B. As shown inFIGS. 6A and 6B, the gear teeth in the endplate are brought in contact with the corresponding gear teeth of the clampingelement63 that is attached to thefixation element61 through aspring64. In this mechanism, the slots next to the gear teeth shown in the embodiment depicted inFIGS. 5A and 5B are replaced by a spring attached to the fixation element and this spring deformation provides the necessary recess of the clamping element as the disc endplate is pushed in upon implantation. The gear teeth contact between the endplate and the clamping element allows one way sliding. The disc endplates and the fixation elements have the rail interface as inFIGS. 5A and 5B to prevent the vertical movement.
• II. Systems
• Also provided are systems that include at least one component of the subject prosthetic discs, as described above. The systems of the subject invention typically include all of the elements that may be necessary and/or desired in order to replace an intervertebral disc with a prosthetic disc as described above. As such, at a minimum the subject systems include a prosthetic disc according to the present invention, as described above. In addition, the systems in certain embodiments include a vertebral body fixation element, or components thereof, e.g., the fixation elements shown in FIGS.5A to6B, bone screws for securing integrated fixation elements as shown inFIGS. 4A and 4B, and the like. The subject systems may also include special delivery devices, e.g., as described in greater detail below.
• One specific representative system of particular interest is depicted inFIG. 7. Thesystem70 ofFIG. 7 is depicted as an exploded view, and includes upper andlower fixation elements71A and71B, anddisc74 made up of top andbottom endplates72A and72B, as well as the fibrouscompressible element75, made up of both afibrous component73 andpolymeric component76 of the prosthetic disc.
• III. Methods of Use
• Also provided are methods of using the subject prosthetic intervertebral discs and systems thereof. The subject prosthetic intervertebral discs and systems thereof find use in the replacement of damaged or dysfunctional interverterbral discs in vertebrate organisms. Generally the vertebrate organisms are “mammals” or “mammalian,” where these terms are used broadly to describe organisms which are within the class mammalia, including the orders carnivore (e.g., dogs and cats), rodentia (e.g., mice, guinea pigs, and rats), lagomorpha (e.g. rabbits) and primates (e.g., humans, chimpanzees, and monkeys). In many embodiments, the subjects will be humans.
• In general, the devices are employed by first removing the to be replaced disc from the subject or patient according to standard protocols to produce a disc void space. Next, the subject prosthetic disc is implanted or positioned in the disc void space, resulting in replacement of the removed disc with the prosthetic disc. This implantation step may include a vertebral body fixation element implantation substep, a post implantation vertebral body securing step, or other variations, depending on the particular configuration of the prosthetic device being employed. In addition, the implantation step described above may include use of one or more implantation devices (or disc delivery devices) for implanting the system components to the site of implantation.
• A representative implantation protocol for implanting the device depicted inFIG. 7 is now provided. First, the spine of a subject is exposed via a retroperitoneal approach after sterile preparation. The intervertebral disc in trauma condition is removed, and the cartilage endplates above and below the disc are also removed to the bony end plates to obtain the bleeding surface for the bone growth into porous cavities in the spikedfixation elements71A and72A. The gap resulting from these removals is measured and the proper artificial disk assembly is chosen according to the measurement.
• The spiked fixation element plates are loaded onto adelivery instrument80 as shown inFIGS. 8 and 9 such that relative location and orientation between the upper spiked fixation element plate and the lower spiked fixation element plate are kept at a desired configuration. This configuration can be realized by providing appropriate mating features on the instrument and the corresponding mating features on the spiked plates. One of the possible mating features would be the pocket of the instrument and the corresponding external faces of the spiked plates as shown inFIG. 8. The pocket has the same internal face as the external face of spiked plates but with a slightly smaller size such that the spiked plate fits tightly into the pocket of the instrument. The instrument together with the spiked fixation plates is delivered to the area where the disc was removed and the spiked plates are pushed against the vertebra using the distracting motion of the instrument as shown inFIG. 9.
• Once the spiked fixation plates are firmly fixed to the vertebra, theprosthetic disc75 is held by a different tool and inserted into the implanted spiked fixation plates such that its gear teeth go through the matching gear teeth on the spiked fixation plates.FIG. 10 shows the tool holding the disc. The grippers inFIG. 10 hold the fiber area of the disc when it is in grasp position. The disc accommodating the grippers has the circular concave area in contact with the disc and is pushed into the spiked fixation plates through this contact. When the disc is inserted all the way into the spiked plates, the protruded rails on the disc at its most front side are in contact with the female railway of the spiked fixation plates and the disc is secured between the spiked fixation plates and therefore the vertebra.
• The above-described protocol is depicted inFIG. 11.
• The above specifically reviewed protocol is merely representative of the protocols that may be employed for implanting devices according to the subject invention.
• IV. Kits
• Also provided are kits for use in practicing the subject methods, where the kits typically include one or more of the above prosthetic intervertebral disc devices (e.g., a plurality of such devices in different sizes), and/or components of the subject systems, e.g., fixation elements or components thereof, delivery devices, etc. as described above. The kit may further include other components, e.g., site preparation components, etc., which may find use in practicing the subject methods.
• In addition to above-mentioned components, the subject kits typically further include instructions for using the components of the kit to practice the subject methods. The instructions for practicing the subject methods are generally recorded on a suitable recording medium. For example, the instructions may be printed on a substrate, such as paper or plastic, etc. As such, the instructions may be present in the kits as a package insert, in the labeling of the container of the kit or components thereof (i.e., associated with the packaging or sub-packaging) etc. In other embodiments, the instructions are present as an electronic storage data file present on a suitable computer readable storage medium, e.g. CD-ROM, diskette, etc. In yet other embodiments, the actual instructions are not present in the kit, but means for obtaining the instructions from a remote source, e.g. via the internet, are provided. An example of this embodiment is a kit that includes a web address where the instructions can be viewed and/or from which the instructions can be downloaded. As with the instructions, this means for obtaining the instructions is recorded on a suitable substrate.
• It is evident from the above discussion and results that the subject invention provides a significantly improved prosthetic intervertebral disc. Significantly, the subject discs closely imitate the mechanical properties of the fully functional natural discs that they are designed to replace. The subject discs exhibit stiffness in the vertical direction, torsional stiffness, bending stiffness in saggital plane, and bending stiffness in front plane, where the degree of these features can be controlled independently by adjusting the components of the discs, e.g., number of layers of fiber winding, pattern of fiber winding, distribution of impregnated polymer, and the types of impregnated polymers, etc. The fiber reinforced structure of the subject discs prevents the fatigue failure on the inside polymer and the surface treatment on the fiber of certain embodiments eliminates the debris problem, both of which are major disadvantages experienced with certain “rubber-type” artificial disks. The interface mechanism between the fixation plates and the disc plates of certain embodiments of the subject invention, e.g., as shown inFIG. 7, enables a very easy surgical operation. The surgeon simply needs to push the disc inward after fixing the spiked fixation plates onto the vertebrae. Such embodiments also enable easy removal of the disc in case the surgery brings about an ill effect. The gear teeth on the fixation elements are easily pushed from outside such that the gear engagement between the disc endplates and the fixation elements is released and the disc endplates are pulled out from the spiked plates. In view of the above and other benefits and features provided by the subject invention, it is clear that the subject invention represents a significant contribution to the art.
• Part B
• With reference to the embodiments illustrated inFIGS. 12-54, the subject prosthetic discs include upper and lower endplates separated by a core member. In one embodiment, the prosthetic disc comprises an integrated, single-piece structure. In another embodiment, the prosthetic disc comprises a two-piece structure including a lower endplate, and an upper endplate and the core member. The core may be assembled or integrated with either or the two endplates. The two-piece structure may be a constrained structure, wherein the upper endplate assembly is attached to the lower endplate in a manner that prevents relative rotation. Alternatively, the structure may be a semi-constrained or an unconstrained structure, wherein the upper endplate assembly is attached to the lower endplate in a manner that allows relative rotation. In yet another embodiment, the prosthetic disc comprises a three-piece structure including upper and lower endplates and a separable core member that is captured between the upper and lower endplates by a retaining mechanism. Finally, in yet another embodiment, the prosthetic disc comprises a four-piece structure including upper and lower endplates and two separable core assemblies which, together, form a core member. Those of ordinary skill in the art will recognize that five-piece, six-piece, or other multi-piece structures may be constructed by further division of the core member and/or the upper and lower endplates, or by the provision of additional components to the structure.
• The implantation apparatus and methods are adapted to implant the prosthetic discs between two adjacent vertebral bodies of a patient. In a first embodiment, the apparatus includes three implantation tools used to prepare the two adjacent vertebral bodies for implantation and then to implant the prosthetic disc. A first tool, a spacer, is adapted to be inserted between and to separate the two adjacent vertebral bodies to create sufficient space for implanting the prosthetic disc. A second tool, a chisel, includes one or more wedge-shaped cutting blades located on its upper and/or lower surfaces that are adapted to create grooves in the inward facing surfaces of the two adjacent vertebral bodies. A third tool, a holder, includes an engagement mechanism adapted to hold the prosthetic disc in place while it is being implanted, and to release the disc once it has been implanted.
• In another embodiment, the implantation apparatus includes a guide member that engages the lower endplate and that remains in place during a portion of the disc implantation process. A lower pusher member slidably engages the guide member and is used to advance the lower endplate into place between two adjacent vertebral bodies of a patient's spine. An upper pusher member is preferably coupled to the lower pusher member and is used to advance a first chisel into place opposed to the lower endplate between the two adjacent vertebral bodies. Once in place, an upward force is applied to the upper pusher member to cause the first chisel to engage the upper vertebral body and to chisel one or more grooves into its lower surface. A downward force is also applied to the lower pusher member to cause the lower endplate to engage the lower vertebral body and to become implanted. The upper pusher member and first chisel are then removed, as is the lower pusher member. Preferably, a second chisel is then advanced along the guide member and is used to provide additional preparation of the upper vertebral body. After the completion of the preparation by the first chisel and, preferably, the second chisel, the upper endplate and core members of the prosthetic disc are implanted using an upper endplate holder that is advanced along the guide member. After implantation, the upper endplate holder and guide member are removed.
• As will be apparent to those of skill in the art upon reading this disclosure, each of the individual embodiments described and illustrated herein has discrete components and features which may be readily separated from or combined with the features of any of the other several embodiments without departing from the scope or spirit of the present inventions.
• I. Prosthetic Intervertebral Discs
• The prosthetic intervertebral discs are preferably artificial or manmade devices that are configured or shaped so that they can be employed as replacements for an intervertebral disc in the spine of a vertebrate organism, e.g., a mammal, such as a human. The subject prosthetic intervertebral discs have dimensions that permit them to substantially occupy the space between two adjacent vertebral bodies that is present when the naturally occurring disc between the two adjacent bodies is removed, i.e., a disc void space. By substantially occupy is meant that the prosthetic disc occupies a sufficient volume in the space between two adjacent vertebral bodies that the disc is able to perform some or all of the functions performed by the natural disc for which it serves as a replacement. In certain embodiments, subject prosthetic discs may have a roughly bean shaped structure analogous to naturally occurring intervertebral body discs. In many embodiments, the length of the prosthetic discs range from about 15 mm to about 50 mm, preferably from about 18 mm to about 46 mm, the width of the prosthetic discs range from about 12 mm to about 30 mm, preferably from about 14 mm to about 25 mm, and the height of the prosthetic discs range from about 3 mm to about 15 mm, preferably from about 5 mm to about 14 mm.
• The prosthetic discs include upper and lower endplates separated by a core member. The resulting structure provides a prosthetic disc that functionally closely mimics a natural disc.
• A. One-Piece Structure
• Representative prostheticintervertebral discs100 having one-piece structures are shown inFIGS. 12 through 15. The prosthetic disc includes anupper endplate110, alower endplate120, and acore member130 retained between theupper endplate110 and thelower endplate120. One ormore fibers140 are wound around the upper and lower endplates to attach the endplates to one another. (For clarity, thefibers140 are not shown in all of the Figures. Nevertheless,fibers140, as shown, for example, inFIG. 12, are present in and perform similar functions in each of the embodiments described herein.) Thefibers140 preferably are not tightly wound, thereby allowing a degree of axial rotation, bending, flexion, and extension by and between the endplates. Thecore member130 may be provided in an uncompressed or a pre-compressed state. Anannular capsule150 is optionally provided in the space between the upper and lower endplates, surrounding thecore member130 and thefibers140. Theupper endplate110 andlower endplate120 are generally flat, planar members, and are fabricated from a physiologically acceptable material that provides substantial rigidity. Examples of materials suitable for use in fabricating theupper endplate110 andlower endplate120 include titanium, titanium alloys, stainless steel, cobalt/chromium, etc., which are manufactured by machining or metal injection molding; plastics such as polyethylene with ultra high molar mass (molecular weight) (UHMWPE), polyether ether ketone (PEEK), etc., which are manufactured by injection molding or compression molding; ceramics; graphite; and others. Optionally, the endplates may be coated with hydroxyapatite, titanium plasma spray, or other coatings to enhance bony ingrowth.
• As noted above, the upper and lower endplates typically have a length of from about 12 mm to about 45 mm, preferably from about 13 mm to about 44 mm, a width of from about 11 mm to about 28 mm, preferably from about 12 mm to about 25 mm, and a thickness of from about 0.5 mm to about 4 mm, preferably from about 1 mm to about 3 mm. The sizes of the upper and lower endplates are selected primarily based upon the size of the void between adjacent vertebral bodies to be occupied by the prosthetic disc. Accordingly, while endplate lengths and widths outside of the ranges listed above are possible, they are not typical.
• The upper surface of theupper endplate110 and the lower surface of thelower endplate120 are preferably each provided with a mechanism for securing the endplate to the respective opposed surfaces of the upper and lower vertebral bodies between which the prosthetic disc is to be installed. For example, inFIG. 12, theupper endplate110 includes a plurality of anchoringfins111a-b. The anchoringfins111a-bare intended to engage mating grooves that are formed on the surfaces of the upper and lower vertebral bodies to thereby secure the endplate to its respective vertebral body. The anchoringfins111a-bextend generally perpendicularly from the generally planar external surface of theupper endplate110, i.e., upward from the upper side of the endplate as shown inFIG. 12. In theFIG. 12 embodiment, theupper endplate110 includes three anchoringfins111a-c, although only two are shown in the cross-sectional view. A first of the anchoring fins,111a, is disposed near an external edge of the external surface of the upper endplate and has a length that approximates the width of theupper endplate110. A second of the anchoring fins,111b, is disposed at the center of external surface of the upper endplate and has a relatively shorter length, substantially less than the width of theupper endplate110. Each of the anchoringfins111a-bhas a plurality ofserrations112 located on the top edge of the anchoring fin. Theserrations112 are intended to enhance the ability of the anchoring fin to engage the vertebral body and to thereby secure theupper endplate110 to the spine.
• Similarly, the lower surface of thelower endplate120 includes a plurality of anchoringfins121a-b. The anchoringfins121a-bon the lower surface of thelower endplate120 are identical in structure and function to the anchoringfins111a-bon the upper surface of theupper endplate110, with the exception of their location on the prosthetic disc. The anchoringfins121a-bon thelower endplate120 are intended to engage mating grooves formed on the lower vertebral body, whereas the anchoringfins111a-bon theupper endplate110 are intended to engage mating grooves on the upper vertebral body. Thus, theprosthetic disc100 is held in place between the adjacent vertebral bodies.
• The anchoringfins111,121 may optionally be provided with one or more holes orslots115,125. The holes or slots help to promote bony ingrowths that bond theprosthetic disc100 to the vertebral bodies.
• Turning to FIGS.13A-C, there are shown several alternative mechanisms for securing the endplates to the respective opposed surfaces of the upper and lower vertebral bodies between which the prosthetic disc is to be installed. InFIG. 13A, each of theupper endplate110 andlower endplate120 is provided with asingle anchoring fin111,121. The anchoringfins111,121 are located along a center line of the respective endplates, and each is provided with a plurality ofserrations112,122 on its upper edge. Thesingle anchoring fins111,121 are intended to engage grooves formed on the opposed surface of the upper and lower vertebral bodies, as described above. InFIG. 13B, each of theupper endplate110 andlower endplate120 is provided with three anchoringfins111a-c,121a-c. TheFIG. 13B prosthetic disc is the same as the prosthetic disc shown inFIG. 1, but it is shown in perspective rather than cross-section. Thus, the structure and function of the anchoringfins111a-cand121a-care as described above in relation toFIG. 12. Finally, inFIG. 13C, each of theupper endplate110 andlower endplate120 is provided with a plurality ofserrations113,123 over a portion of the exposed external surface of the respective endplate. Theserrations113,123 are intended to engage the opposed surfaces of the adjacent vertebral bodies to thereby secure the endplates in place between the vertebral bodies. Theserrations113,123 may be provided over the entire external surface of each of the upper and lower endplates, or they may be provided over only a portion of those surfaces. For example, inFIG. 13C, theserrations113 on the upper surface of theupper endplate110 are provided over three major areas, a first area113anear a first edge of theupper endplate110, asecond area113bnear the center of theupper endplate110, and a third area near a second edge of theendplate113c.
• Turning toFIG. 54, in an optional embodiment, the anchoringfins111 are selectively retractable and extendable by providing adeployment mechanism160 that is associated with theupper endplate110. A similar mechanism may be used on thelower endplate120. The deployment mechanism includes aslider161 that slides within achannel162 formed in theupper endplate110. Thechannel162 includes a threadedregion163, and theslider161 includes matchingthreads164, thereby providing a mechanism for advancing theslider161 within thechannel162. As theslider161 is advanced within thechannel162, a taperedregion165 engages the bottom surface of adeployable fin166. Further advancement of theslider161 causes thedeployable fin166 to be raised upward within aslot167 on the upper surface of theupper endplate110. Reversing thedeployment mechanism160 causes thefin166 to retract. Thedeployment mechanism160 may also be used in conjunction with spikes, serrations, or other anchoring devices. In an alternative embodiment, the threadedslider161 of the deployment mechanism may be replaced with a dowel pin that is advanced to deploy thefin166. Other advancement mechanisms are also possible.
• Returning toFIG. 12, theupper endplate110 contains a plurality ofslots114 through which thefibers140 may be passed through or wound, as shown. The actual number ofslots114 contained on the endplate is variable. Increasing the number of slots will result in an increase in the circumferential density of the fibers holding the endplates together. In addition, the shape of the slots may be selected so as to provide a variable width along the length of the slot. For example, the width of the slots may taper from a wider inner end to a narrow outer end, or visa versa. Additionally, the fibers may be wound multiple times within the same slot, thereby increasing the radial density of the fibers. In each case, this improves the wear resistance and increases the torsional and flexural stiffness of the prosthetic disc, thereby further approximating natural disc stiffness. In addition, thefibers140 may be passed through or wound on each slot, or only on selected slots, as needed. Two exemplary winding patterns are shown inFIGS. 14A and 14B. InFIG. 14A, thefibers140 are wound in a uni-directional manner, which closely mimics natural annular fibers found in a natural disc. InFIG. 14B, thefibers140 are wound bi-directionally. Other winding patterns, either single or multi-directional, are also possible.
• As described above, the purpose of thefibers140 is to hold theupper endplate110 andlower endplate120 together and to limit the range-of-motion to mimic the range-of-motion of a natural disc. Accordingly, the fibers preferably comprise high tenacity fibers with a high modulus of elasticity, for example, at least about 100 MPa, and preferably at least about 500 MPa. By high tenacity fibers is meant fibers that can withstand a longitudinal stress of at least 50 MPa, and preferably at least 250 MPa, without tearing. Thefibers140 are generally elongate fibers having a diameter that ranges from about 100 μm to about 500 μm, and preferably about 200 μm to about 400 μm. Optionally, the fibers may be injection molded with an elastomer to encapsulate the fibers, thereby providing protection from tissue ingrowth and improving torsional and flexural stiffness, or the fibers may be coated with one or more other materials to improve fiber stiffness and wear. Additionally, the core may be injected with a wetting agent such as saline to wet the fibers and facilitate the mimicking of the viscoelastic properties of a natural disc.
• Thefibers140 may be fabricated from any suitable material. Examples of suitable materials include polyester (e.g., Dacron®), polyethylene, polyaramid, poly-paraphenylene terephthalamide (e.g., Kevlar®), carbon or glass fibers, polyethylene terephthalate, acrylic polymers, methacrylic polymers, polyurethane, polyurea, polyolefin, halogenated polyolefin, polysaccharide, vinylic polymer, polyphosphazene, polysiloxane, and the like.
• Thefibers140 may be terminated on an endplate by tying a knot in the fiber on the superior surface of an endplate. Alternatively, thefibers140 may be terminated on an endplate by slipping the terminal end of the fiber into a slot on an edge of an endplate, similar to the manner in which thread is retained on a thread spool. The slot may hold the fiber with a crimp of the slot structure itself, or by an additional retainer such as a ferrule crimp. As a further alternative, tab-like crimps may be machined into or welded onto the endplate structure to secure the terminal end of the fiber. The fiber may then be closed within the crimp to secure it. As a still further alternative, a polymer may be used to secure the fiber to the endplate by welding. The polymer would preferably be of the same material as the fiber (e.g., PE, PET, or the other materials listed above). Still further, the fiber may be retained on the endplates by crimping a cross-member to the fiber creating a T-joint, or by crimping a ball to the fiber to create a ball joint.
• Thecore member130 is intended to provide support to and to maintain the relative spacing between theupper endplate110 andlower endplate120. Thecore member130 is made of a relatively compliant material, for example, polyurethane or silicone, and is typically fabricated by injection molding. A preferred construction for the core member includes a nucleus formed of a hydrogel and an elastomer reinforced fiber annulus. For example, the nucleus, the central portion of thecore member130, may comprise a hydrogel material such as a water absorbing polyurethane, polyvinyl alcohol (PVA), polyethylene oxide (PEO), polyvinylpyrrolidone (PVP), polyacrylamide, silicone, or PEO based polyurethane. The annulus may comprise an elastomer, such as silicone, polyurethane or polyester (e.g., Hytrel®), reinforced with a fiber, such as polyethylene (e.g., ultra high molecular weight polyethylene, UHMWPE), polyethylene terephthalate, or poly-paraphenylene terephthalamide (e.g., Kevlar®).
• The shape of thecore member130 is typically generally cylindrical or bean-shaped, although the shape (as well as the materials making up the core member and the core member size) may be varied to obtain desired physical or performance properties. For example, thecore member130 shape, size, and materials will directly affect the degree of flexion, extension, lateral bending, and axial rotation of the prosthetic disc.
• Theannular capsule150 is preferably made of polyurethane or silicone and may be fabricated by injection molding, two-part component mixing, or dipping the endplate-core-fiber assembly into a polymer solution. A preferredannular capsule150 is shown in FIGS.15A-C. As shown, the annular capsule is generally cylindrical, having an uppercircular edge153, a lowercircular edge154, and a generallycylindrical body155. In the embodiment shown in the Figures, thebody155 has two bellows156a-bformed therein. Alternative embodiments have no bellows, one bellow, or three or more bellows. A function of the annular capsule is to act as a barrier that keeps the disc materials (e.g., fiber strands) within the body of the disc, and that keeps natural in-growth outside the disc.
• Additional examples of the one-piece structure embodiment of the prosthetic disc are illustrated in FIGS.13D-F. Each of these embodiments includes anupper endplate110,lower endplate120, and acore member130, as described above. Theupper endplate110 includes an outer portion110aand aninner portion110b, and the lower endplate also includes an outer portion120aand aninner portion120b. The inner and outer portions of each of the endplates are bonded to each other by methods known to those of skill in the art. Each of theendplates110,120 also includes anchoringfins111a-c,121a-con the upper surface of theupper endplate110 and the lower surface of thelower endplate120, as also described above. Additionally, with reference toFIG. 13D, asuperior dome116 is provided on the upper surface of theupper endplate110. Thesuperior dome116 is a generally convex portion that extends upward from the upper surface of theupper endplate110. Thesuperior dome116 is optional, and functions by filling space between theupper endplate110 and the vertebral body upon implantation to help approximate theupper endplate110 to the natural anatomy. The size and shape of thesuperior dome116 may be varied according to need. As shown inFIG. 13D, thesuperior dome116 is generally convex and has a maximum height (distance above the generally flat upper surface portion of the upper endplate) of approximately one-half the height of the anchoringfin111b. Thesuperior dome116 may be centered in the middle of theupper endplate110, as shown inFIG. 2D, or it may be shifted to one side or another, depending on need.
• With particular reference toFIG. 13F, apolymer film170 is sandwiched between the outer portion10aand inner portion10bof theupper endplate110, and anotherpolymer film170 is sandwiched between the outer portion120aandinner portion120bof thelower endplate120. Thepolymer films170 are adapted to tightly adhere, either mechanically or chemically, to thefibers140 wound through theslots114,124 formed in theupper endplate110 andlower endplate120.
• FIGS.13D-F provide additional detail concerning theannular capsule150. As shown there, theannular capsule150 seals the interior space between the upper and lower endplates. Theannular capsule150 is retained on the disc by a pair of retainingrings151 that engage a mating pair ofexternal facing grooves152 on the upper and lower endplates. (SeeFIG. 13F). Although the retaining rings may be of any suitable cross-section (e.g., round, triangular, square, etc.), the examples shown inFIG. 13F have a rectangular cross-section. The rectangular shape is believed to provide relatively better gasket retention and is more easily manufactured.
• FIGS. 13G and 13H illustrate still further examples of the one-piece structure embodiment of the prosthetic disc. In the examples shown there, theupper endplate110 includes an outer portion110aand aninner portion110b. Similarly, thelower endplate120 includes an outer portion120aand aninner portion120b. The twoportions110a-b,120a-bof each of the upper and lower endplates mate together to form the integratedupper endplate110 andlower endplate120. Preferably, the twoportions110a-b,120a-bof the upper andlower endplates110,120 are joined together by welding, e.g., laser welding or some similar process. An advantage that may be obtained with this structure is the ability to retain the annular capsule150 (not shown in FIGS.13G-H) without the need for a separate retaining ring. For example, the upper edge of the annular capsule may be captured and retained between the outer portion110aandinner portion110bof theupper endplate110 when they are attached to one another. Similarly, the lower edge of the annular capsule may be captured and retained between the outer portion120aandinner portion120bof thelower endplate120 when those components are attached to one another. In this manner, the annular capsule is held in place between the upper and lower endplates by the compression forces retaining the upper and lower edges of the annular capsule.
• An optional structure for retaining theannular capsule150 is illustrated in FIGS.13I-J. There, anupper endplate110 is shown including an outer portion110aand aninner portion110b. The upper surface of theinner portion110bof theupper endplate110 is provided with anannular groove117 that extends about the periphery of theinner portion110b. Theannular groove117 cooperates with the bottom surface of the outer portion110aof theupper endplate110 to create anannular space118. A similar structure, not shown in the drawings, may be provided on thelower endplate120. The annular capsule150 (not shown in FIGS.13I-J) may advantageously be formed having a bead, i.e., a ball-like termination about its upper and lower edge, (also not shown in the drawings), that occupies theannular space118 formed on the upper andlower endplates110,120. The cooperation of theannular space118 with the bead formed on theannular capsule150 creates a stronger and more secure retaining force for retaining the upper and lower edge of theannular capsule150 by the upper andlower endplates1110,120. Alternatively, the annular capsule may be retained by adhesives with or without the endplate compression already described.
• Another optional feature of the present invention is the placement of the fibers in a state of tensile fatigue upon fabrication so as to minimize long-term wear. For example, in the embodiment of FIGS.13I-J, amaterial131 such as a metal plate or a polymer film may be positioned withinspace119 of upper portion1110aof the endplate and between thefibers127 and the surface of the endplate. The material may initially be in a form, e.g., gel or emulsion, so as to coat and impregnate the fibers. With such material, the fibers are caused to impinge upon the endplate thereby reducing their susceptibility to movement during use of the disc. As an additional optional feature, each of the endplates may be made up of two plates that are selectively rotationally displaceable relative to each other. In this structure, a slight rotation of one of the plates relative to the other has the effect of changing the size and/or shape of the slots formed on the combined endplate. Thus, the user is able to select a desired set of dimensions of the slots.
• FIGS.13K-L illustrate another optional feature that may be incorporated in the one-piece structure embodiment of the prosthetic disc. In the examples shown there, aspring180 is located coaxially with thecore member130 between theupper endplate10 andlower endplate120. In this example, thecore member130 is in the form of a toroid, thus having a space at its center. Thespring180 is placed in the space at the center of thecore member130, with each being retained between theupper endplate110 andlower endplate120. Thespring180 provides a force biasing the two endplates apart, and having performance characteristics and properties that are different from those provided by thecore member130. Those characteristics may be varied by, for example, selecting aspring180 having different dimensions, materials, or a different spring constant. In this way, thespring180 provides an additional mechanism by which the performance of the prosthetic disc may be varied in order to approximate that of a natural disc.
• Turning to FIGS.50A-B, additional examples of the one-piece structure embodiment of the prosthetic discs are shown. The discs illustrated in FIGS.50A-B are particularly adapted in size and shape for implantation by minimally invasive surgical procedures, as described below. Aside from their size and shape, the structures of the examples shown in FIGS.50A-B are similar to those described above, including anupper endplate110,lower endplate120, acore member130, and anannular capsule150. Each of the upper andlower endplates110,120 is provided with an anchoringfin111,121 extending from its surface over most of the length of the endplate. Although not shown in the drawings, these examples also preferably includefibers140 wound between and connecting theupper endplate110 to thelower endplate120.
• In the example shown inFIG. 50A, a single elongated core member is provided, whereas the example structure shown inFIG. 50B has a dual core including two generallycylindrical core members130a,130b. It is believed that the dual core structure (FIG. 50B) better simulates the performance characteristics of a natural disc. In addition, the dual core structure is believed to provide less stress on thefibers140 relative to the single core structure (FIG. 50A). Each of the exemplary prosthetic discs shown in FIGS.50A-B has a greater length than width. Exemplary shapes to provide these relative dimensions include rectangular, oval, bullet-shaped, or others. This shape facilitates implantation of the discs by the minimally invasive procedures described below.
• The one-piece structure embodiment of the prosthetic disc is implanted by a surgical procedure. After removing the natural disc, grooves are formed in the superior and inferior vertebrae between which the prosthetic disc is to be implanted. The prosthetic disc is then inserted into the void, while aligning the anchoringfins111,121 with the grooves formed on the vertebral bodies. The anchoring fins cause the prosthetic disc to be secured in place between the adjacent vertebral bodies. The prosthetic disc has several advantages over prior art artificial discs, as well as over alternative treatment procedures such as spinal fusion. For example, the prosthetic discs described herein provide compressive compliance similar to that of a natural spinal disc. In addition, the motions in flexion, extension, lateral bending, and axial rotation are also restricted in a manner near or identical to those associated with a natural disc.
• B. Two-Piece Structure
• Representative prostheticintervertebral discs200 having two-piece structures are shown inFIGS. 16 through 20. The components and features included in the two-piece prosthetic discs are very similar to those of the one-piece disc described above. A primary difference between the devices is that the two-piece prosthetic disc contains two separable components, whereas the one-piece prosthetic disc contains a single, integrated structure. In particular, and as described more fully below, the lower endplate of the two-piece prosthetic disc is separated into an inner lower endplate220a, and an outerlower endplate220b(seeFIGS. 16-20), whereas there is only a singlelower endplate120 in the one-piece disc (seeFIGS. 12 and 13A-C).
• Turning toFIGS. 16-20, the two-piece prosthetic disc includes two primary, separable components: the outerlower endplate220b, and anupper subassembly205. In a first embodiment of the two-piece prosthetic disc, shown inFIGS. 16-18, theupper subassembly205 is constrained, i.e., it cannot freely rotate in relation to the outerlower endplate220b. In a second embodiment of the two-piece prosthetic disc, shown inFIGS. 19-20, theupper subassembly205 is unconstrained, i.e., it can substantially freely rotate in relation to the outerlower endplate220b.
• The upper subassembly includes the inner lower endplate220a, anupper endplate210, and acore member230 retained between theupper endplate210 and the inner lower endplate220a. One ormore fibers240 are wound around the upper and inner lower endplates to attach the endplates to one another. Thefibers240 preferably are not tightly wound, thereby allowing a degree of axial rotation, bending, flexion, and extension by and between the endplates. Thecore member230 is preferably pre-compressed. Anannular capsule250 is optionally provided in the space between the upper and inner lower endplates, surrounding thecore member230 and thefibers240. Alternatively, an outer ring or gasket (not shown in the drawings) may optionally be provided in place of theannular capsule250.
• Theupper endplate210 and outerlower endplate220bare generally flat, planar members, and are fabricated from a physiologically acceptable material that provides substantial rigidity. Examples of materials suitable for use in fabricating theupper endplate210 and outerlower endplate220binclude titanium, titanium alloys, stainless steel, cobalt/chromium, etc., which are manufactured by machining or metal injection molding; plastics such as polyethylene with ultra high molar mass (molecular weight) (UHMWPE), polyether ether ketone (PEEK), etc., which are manufactured by injection molding or compression molding; ceramics; graphite; and others. Optionally, the endplates may be coated with hydroxyapatite, titanium plasma spray, or other coatings to enhance bony ingrowth.
• As noted above, the upper and outer lower endplates typically have a length of from about 12 mm to about 45 mm, preferably from about 13 mm to about 44 mm, a width of from about 11 mm to about 28 mm, preferably from about 12 mm to about 25 mm, and a thickness of from about 0.5 mm to about 4 mm, preferably from about 1 mm to about 3 mm. The sizes of the upper and outer lower endplates are selected primarily based upon the size of the void between adjacent vertebral bodies to be occupied by the prosthetic disc. Accordingly, while endplate lengths and widths outside of the ranges listed above are possible, they are not typical.
• The upper surface of theupper endplate210 and the lower surface of the outerlower endplate220bare preferably each provided with a mechanism for securing the endplate to the respective opposed surfaces of the upper and lower vertebral bodies between which the prosthetic disc is to be implanted. For example, as shown inFIGS. 16 and 18-20, theupper endplate210 includes a plurality of anchoring fins211a-c. The anchoring fins211a-care intended to engage mating grooves that are formed on the surfaces of the upper and lower vertebral bodies to thereby secure the endplate to its respective vertebral body. The anchoring fins211a-cextend generally perpendicular from the generally planar external surface of theupper endplate210, i.e., upward from the upper side of the endplate as shown inFIG. 16. In theFIG. 16 embodiment, theupper endplate210 includes three anchoring fins211a-c. The first and third of the anchoring fins,211aand211c, are disposed near the external edges of the external surface of theupper endplate210 and have lengths that approximate the width of theupper endplate210. The second of the anchoring fins,211b, is disposed at the center of external surface of the upper endplate and has a relatively shorter length, substantially less than the width of theupper endplate210. Each of the anchoring fins211a-chas a plurality ofserrations212 located on the top edge of the anchoring fin. Theserrations212 are intended to enhance the ability of the anchoring fin to engage the vertebral body and to thereby secure theupper endplate210 to the vertebral body.
• The lower surface of the outerlower endplate220bincludes a plurality of anchoring spikes221. The anchoring spikes221 on the lower surface of the outerlower endplate220bare intended to engage the surface of the lower vertebral body, while the anchoring fins211a-con theupper endplate210 are intended to engage mating grooves on the upper vertebral body. Thus, theprosthetic disc200 is held in place between the adjacent vertebral bodies.
• Alternatively, theupper endplate210 and outerlower endplate220bof the two-piece prosthetic disc may employ one of the alternative securing mechanisms shown in FIGS.13A-C. As described above, inFIG. 13A, each of theupper endplate110 andlower endplate120 is provided with asingle anchoring fin111,121. The anchoringfins111,121 are located along a center line of the respective endplates, and each is provided with a plurality ofserrations112,122 on its upper edge. Thesingle anchoring fins111,121 are intended to engage grooves formed on the opposed surface of the upper and lower vertebral bodies, as described above. InFIG. 13B, each of theupper endplate110 andlower endplate120 is provided with three anchoringfins111a-c,121a-c. TheFIG. 13B prosthetic disc is the same as the prosthetic disc shown inFIG. 12, but it is shown in perspective rather than cross-section. Thus, the structure and function of the anchoringfins111a-cand121a-care as described above in relation toFIG. 12. Finally, inFIG. 13C, each of theupper endplate110 andlower endplate120 is provided with a plurality ofserrations113,123 over a portion of the exposed external surface of the respective endplate. Theserrations113,123 are intended to engage the opposed surfaces of the adjacent vertebral bodies to thereby secure the endplates in place between the vertebral bodies. Theserrations113,123 may be provided over the entire external surface of each of the upper and lower endplates, or they may be provided over only a portion of those surfaces. For example, inFIG. 13C, theserrations113 on the upper surface of theupper endplate110 are provided over three major areas, a first area113anear a first edge of theupper endplate110, asecond area113bnear the center of theupper endplate110, and a third area near a second edge of theendplate113c.
• Turning toFIG. 54, in an optional embodiment, the anchoringfins111 are selectively retractable and extendable by providing adeployment mechanism160 that is associated with theupper endplate110. A similar mechanism may be used on thelower endplate120. The deployment mechanism includes aslider161 that slides within achannel162 formed in theupper endplate110. Thechannel162 includes a threadedregion163, and theslider161 includes matchingthreads164, thereby providing a mechanism for advancing theslider161 within thechannel162. As theslider161 is advanced within thechannel162, a taperedregion165 engages the bottom surface of adeployable fin166. Further advancement of theslider161 causes thedeployable fin166 to be raised upward within aslot167 on the upper surface of theupper endplate110. Reversing thedeployment mechanism160 causes thefin166 to retract. Thedeployment mechanism160 may also be used in conjunction with spikes, serrations, or other anchoring devices. In an alternative embodiment, the threadedslider161 of the deployment mechanism may be replaced with a dowel pin that is advanced to deploy thefin166. Other advancement mechanisms are also possible.
• Returning toFIG. 18, theupper endplate210 contains a plurality ofslots214 through which thefibers240 may be passed through or wound, as shown. The actual number ofslots214 contained on the endplate is variable. Increasing the number of slots will result in an increase in the circumferential density of the fibers holding the endplates together. Additionally, the fibers may be wound multiple times within the same slot, thereby increasing the radial density of the fibers. In each case, this improves the wear resistance and increases the torsional and flexural stiffness of the prosthetic disc, thereby further approximating natural disc stiffness. In addition, thefibers240 may be passed through or wound on each slot, or only on selected slots, as needed.
• As described above, the purpose of thefibers240 is to hold theupper endplate210 and lower endplate220 together and to limit the range-of-motion to mimic the range-of-motion of a natural disc. Accordingly, the fibers preferably comprise high tenacity fibers with a high modulus of elasticity, for example, at least about 100 MPa, and preferably at least about 500 MPa. By high tenacity fibers is meant fibers that can withstand a longitudinal stress of at least 50 MPa, and preferably at least 250 MPa, without tearing. Thefibers240 are generally elongate fibers having a diameter that ranges from about 100 μm to about 500 μm, and preferably about 200 μm to about 400 μm. Optionally, the fibers may be injection molded with an elastomer to encapsulate the fibers, thereby providing protection from tissue ingrowth and improving torsional and flexural stiffness.
• Thefibers240 may be fabricated from any suitable material. Examples of suitable materials include polyester (e.g., Dacron®), polyethylene, polyaramid, poly-paraphenylene terephthalamide (e.g., Kevlar®), carbon or glass fibers, polyethylene terephthalate, acrylic polymers, methacrylic polymers, polyurethane, polyurea, polyolefin, halogenated polyolefin, polysaccharide, vinylic polymer, polyphosphazene, polysiloxane, and the like.
• Thefibers240 may be terminated on an endplate by tying a knot in the fiber on the superior surface of an endplate. Alternatively, thefibers240 may be terminated on an endplate by slipping the terminal end of the fiber into a slot on an edge of an endplate, similar to the manner in which thread is retained on a thread spool. The slot may hold the fiber with a crimp of the slot structure itself, or by an additional retainer such as a ferrule crimp. As a further alternative, tab-like crimps may be machined into or welded onto the endplate structure to secure the terminal end of the fiber. The fiber may then be closed within the crimp to secure it. As a still further alternative, a polymer may be used to secure the fiber to the endplate by welding. The polymer would preferably be of the same material as the fiber (e.g., PE, PET, or the other materials listed above). Still further, the fiber may be retained on the endplates by crimping a cross-member to the fiber creating a T-joint, or by crimping a ball to the fiber to create a ball joint.
• Thecore member230 is intended to provide support to and to maintain the relative spacing between theupper endplate210 and inner lower endplate220a. Thecore member230 is made of a relatively compliant material, for example, polyurethane or silicone, and is typically fabricated by injection molding. A preferred construction for thecore member230 includes a nucleus formed of a hydrogel and an elastomer reinforced fiber annulus. For example, the nucleus, the central portion of thecore member230, may comprise a hydrogel material such as tecophilic water absorbing polyurethane, polyvinyl alcohol (PVA), polyethylene oxide (PEO), polyvinylpyrrolidone (PVP), polyacrylamide, silicone, or PEO based polyurethane. The annulus may comprise an elastomer, such as silicone, polyurethane or polyester (e.g., Hytrel®), reinforced with a fiber, such as polyethylene, polyethylene terephthalate, or poly-paraphenylene terephthalamide (e.g., Kevlar®).
• The shape of thecore member230 is typically generally cylindrical or bean-shaped, although the shape (as well as the materials making up the core member and the core member size) may be varied to obtain desired physical or performance properties. For example, thecore member230 shape, size, and materials will directly affect the degree of flexion, extension, lateral bending, and axial rotation of the prosthetic disc.
• Theannular capsule250 is preferably made of polyurethane or silicone and may be fabricated by injection molding, two-part component mixing, or dipping the endplate-core-fiber assembly into a polymer solution. Alternatively, an outer ring or gasket (not shown in the drawings) may optionally be provided in place of theannular capsule250.
• Theupper subassembly205 is configured to be selectively attached to the outerlower endplate220b. As shown, for example, inFIGS. 3 and 6, theedges225 of the inner lower endplate220ahave a size and shape adapted to engageslots226 formed on the upper surface of the outerlower endplate220b. Accordingly, theupper subassembly205 will slide onto the outerlower endplate220b, with the inner lower endplate edges225 engaging the outerlower endplate slots226.
• At this point, the differences between the constrained, semi-constrained and unconstrained embodiments of the two-piece prosthetic disc will be described. Turning first to the constrained embodiment shown inFIGS. 16-18, once theupper subassembly205 is fully advanced onto the outerlower endplate220b—i.e., once theleading edge225 of the inner lower endplate220aengages the back portion of theslot226 of the outerlower endplate220b—atab261 on the bottom surface of the inner lower endplate220aengages anotch262 on the top surface of the outerlower endplate220b(seeFIG. 18), thereby locking theupper subassembly205 to the outerlower endplate220b. Thetab261 and notch262 are squared surfaces, thereby preventing relative rotation between the inner lower endplate220aand outerlower endplate220b. Additionally, theedges225 of the inner lower endplate220aand themating slots226 of the outerlower endplate220binclude matingstraight portions227 and228, respectively, which also tend to inhibit rotation of the inner lower endplate220arelative to the outerlower endplate220b.
• Turning next to the unconstrained embodiment shown inFIGS. 19-20, the outerlower endplate220bis provided with a raisedlip271. The raisedlip271 is slightly downwardly displaceable, i.e., the raisedlip271 will deflect downwardly when force is applied to it. Accordingly, when the upper subassembly is being attached to the outerlower endplate220b, the raised lip will displace downwardly to allow theedges225 of the inner lower endplate220ato engage theslots226 of the outerlower endplate220b. Once theupper subassembly205 is fully advanced onto the outerlower endplate220b—i.e., once theleading edge225 of the inner lower endplate220aengages the back portion of theslot226 of the outerlower endplate220b—the raisedlip271 snaps back into place, as shown inFIG. 20, thereby locking theupper subassembly205 to the outerlower endplate220b. Notably, the raisedlip271 and inner lower endplate220ainclude rounded surfaces, thereby allowing relative rotation between the inner lower endplate220aand outerlower endplate220b. Additionally, theedges225 of the inner lower endplate220aand themating slots226 of the outerlower endplate220bdo not include the matingstraight portions227,228 of the constrained embodiment. Thus, in the unconstrained embodiment of the two-piece prosthetic disc, as shown inFIGS. 19 and 20, theupper subassembly205 is capable of substantially free rotation relative to the outerlower endplate220b.
• The two-piece structure embodiment of the prosthetic disc is implanted by a surgical procedure. After removing the natural disc, the outerlower endplate220bis placed onto and anchored into the inferior vertebral body within the void between the two adjacent vertebral bodies previously occupied by the natural disc. Next, grooves are formed in the superior vertebral body. Theupper subassembly205 of the prosthetic disc is then inserted into the void, while aligning the anchoring fins211 with the grooves formed on the superior vertebral body, and while sliding the inner lower endplate220ainto the outerlower endplate220bin a manner that theedges225 of the inner endplate220aengage theslots226 of theouter endplate220b. The anchoring fins cause the prosthetic disc to be secured in place between the adjacent vertebral bodies.
• The two-piece prosthetic disc has several advantages over prior art artificial discs, as well as over alternative treatment procedures such as spinal fusion. For example, the two-piece prosthetic discs described herein provide compressive compliance similar to that of a natural spinal disc. In addition, the motions in flexion, extension, lateral bending, and axial rotation are also restricted in a manner near or identical to those associated with a natural disc.
• C. Three-Piece Structure
• A representative prostheticintervertebral disc300 having a three-piece structure is shown inFIGS. 21 through 23. The prosthetic disc includes anupper endplate310, alower endplate320, and acore assembly330 retained between theupper endplate310 and thelower endplate320.
• Theupper endplate310 andlower endplate320 are generally flat, planar members, and are fabricated from a physiologically acceptable material that provides substantial rigidity. Examples of materials suitable for use in fabricating theupper endplate310 andlower endplate320 include titanium, titanium alloys, stainless steel, cobalt/chromium, etc., which are manufactured by machining or metal injection molding; plastics such as polyethylene with ultra high molar mass (molecular weight) (UHMWPE), polyether ether ketone (PEEK), etc., which are manufactured by injection molding or compression molding; ceramics; graphite; and others. Optionally, the endplates may be coated with hydroxyapatite, titanium plasma spray, or other coatings to enhance bony ingrowth.
• As noted above, the upper and lower endplates typically have a length of from about 12 mm to about 45 mm, preferably from about 13 mm to about 44 mm, a width of from about 11 mm to about 28 mm, preferably from about 12 mm to about 25 mm, and a thickness of from about 0.5 mm to about 4 mm, preferably from about 1 mm to about 3 mm. The sizes of the upper and lower endplates are selected primarily based upon the size of the void between adjacent vertebral bodies to be occupied by the prosthetic disc. Accordingly, while endplate lengths and widths outside of the ranges listed above are possible, they are not typical. The upper surface of theupper endplate310 and the lower surface of thelower endplate320 are preferably each provided with a mechanism for securing the endplate to the respective opposed surfaces of the upper and lower vertebral bodies between which the prosthetic disc is to be implanted. For example, inFIGS. 21 and 23, theupper endplate310 includes an anchoringfin311. The anchoringfin311 is intended to engage a mating groove that is formed on the surface of the upper vertebral body to thereby secure the endplate to the vertebral body. The anchoringfin311 extends generally perpendicularly from the generally planar external surface of theupper endplate310, i.e., upward from the upper side of the endplate as shown inFIGS. 21 and 23. As shown in the Figures, the anchoringfin311 is disposed at the center of external surface of theupper endplate310 and has a length that is slightly shorter than the width of theupper endplate310. Although not shown in the Figures, the anchoringfin311 may be provided with a plurality of serrations located on the top edge of the anchoring fin. The serrations are intended to enhance the ability of the anchoring fin to engage the vertebral body and to thereby secure theupper endplate310 to the spine.
• Similarly, the lower surface of thelower endplate320 includes an anchoringfin321. The anchoringfin321 on the lower surface of thelower endplate320 is identical in structure and function to the anchoringfin311 on the upper surface of theupper endplate310, with the exception of its location on the prosthetic disc. The anchoringfin321 on thelower endplate320 is intended to engage a mating groove formed on the lower vertebral body, whereas the anchoringfin311 on theupper endplate310 is intended to engage a mating groove on the upper vertebral body. Thus, theprosthetic disc300 is held in place between the adjacent vertebral bodies.
• Alternatively, theupper endplate310 andlower endplate320 of the three-piece prosthetic disc may employ one of the alternative securing mechanisms shown in FIGS.13A-C. As described above in relation to the one-piece prosthetic device shown inFIG. 13A, each of theupper endplate110 andlower endplate120 is provided with asingle anchoring fin111,121. The anchoringfins111,121 are located along a center line of the respective endplates, and each is provided with a plurality ofserrations112,122 on its upper edge. Thesingle anchoring fins111,121 are intended to engage grooves formed on the opposed surface of the upper and lower vertebral bodies, as described above. InFIG. 13B, each of theupper endplate110 andlower endplate120 is provided with three anchoringfins111a-c,121a-c. TheFIG. 13B prosthetic disc is the same as the prosthetic disc shown inFIG. 12, but it is shown in perspective rather than cross-section. Thus, the structure and function of the anchoringfins111a-cand121a-care as described above in relation toFIG. 12. Finally, inFIG. 13C, each of theupper endplate110 andlower endplate120 is provided with a plurality ofserrations113,123 over a portion of the exposed external surface of the respective endplate. Theserrations113,123 are intended to engage the opposed surfaces of the adjacent vertebral bodies to thereby secure the endplates in place between the vertebral bodies. Theserrations113,123 may be provided over the entire external surface of each of the upper and lower endplates, or they may be provided over only a portion of those surfaces. For example, inFIG. 13C, theserrations113 on the upper surface of theupper endplate110 are provided over three major areas, a first area113anear a first edge of theupper endplate110, asecond area113bnear the center of theupper endplate110, and a third area near a second edge of theendplate113c.
• Turning toFIG. 54, in an optional embodiment, the anchoringfins111 are selectively retractable and extendable by providing adeployment mechanism160 that is associated with theupper endplate110. A similar mechanism may be used on thelower endplate120. The deployment mechanism includes aslider161 that slides within achannel162 formed in theupper endplate110. Thechannel162 includes a threadedregion163, and theslider161 includes matchingthreads164, thereby providing a mechanism for advancing theslider161 within thechannel162. As theslider161 is advanced within thechannel162, a taperedregion165 engages the bottom surface of adeployable fin166. Further advancement of theslider161 causes thedeployable fin166 to be raised upward within aslot167 on the upper surface of theupper endplate110. Reversing thedeployment mechanism160 causes thefin166 to retract. Thedeployment mechanism160 may also be used in conjunction with spikes, serrations, or other anchoring devices. In an alternative embodiment, the threadedslider161 of the deployment mechanism may be replaced with a dowel pin that is advanced to deploy thefin166. Other advancement mechanisms are also possible.
• Thecore assembly330 is intended to provide support to and to maintain the relative spacing between theupper endplate310 andlower endplate320. Thecore assembly330 provides compressive compliance to the three-piece prosthetic disc, as well as providing limited translation, flexion, extension, and lateral bending by and between theupper endplate310 andlower endplate320. Thecore assembly330 further provides substantially unlimited rotation by and between theupper endplate310 and thelower endplate320.
• Thecore assembly330 includes atop cap331, abottom cap332, asidewall333, and acore center334 held by and retained between thetop cap331,bottom cap332, andsidewall333. Thetop cap331 andbottom cap332 are generally planar, and are fabricated from a physiologically acceptable material that provides substantial rigidity. Examples of materials suitable for use in fabricating thetop cap331 andbottom cap332 include titanium, titanium alloys, stainless steel, cobalt/chromium, etc., which are manufactured by machining or metal injection molding; plastics such as polyethylene with ultra high molar mass (molecular weight) (UHMWPE), polyether ether ketone (PEEK), etc., which are manufactured by injection molding or compression molding; ceramics; graphite; and others. Thecore center334 is made of a relatively compliant material, for example, polyurethane or silicone, and is typically fabricated by injection molding. The shape of thecore center334 is typically generally cylindrical or bean-shaped, although the shape (as well as the materials making up the core center and the core member size) may be varied to obtain desired physical or performance properties. For example, thecore member334 shape, size, and materials will directly affect the degree of flexion, extension, lateral bending, and axial rotation of the prosthetic disc.
• Thetop cap331 andbottom cap332 each preferably includes a generallyconcave indentation336 formed at a center point of the cap. Theindentations336 are intended to cooperate with a pair of retainers formed on the internal surfaces of the endplates to retain thecore assembly330 in place between the retainers, as described more fully below.
• Thetop cap331 andbottom cap332 preferably contain a plurality ofslots335 spaced radially about the surface of each of the caps. One ormore fibers340 are wound around thetop cap331 andbottom cap332 through theslots335 to attach the endplates to one another. Thefibers340 preferably are not tightly wound, thereby allowing a degree of axial rotation, bending, flexion, and extension by and between thetop cap331 andbottom cap332. Thecore center334 is preferably pre-compressed. The actual number ofslots335 contained on each of thetop cap331 andbottom cap332 is variable. Increasing the number of slots will result in an increase in the circumferential density of the fibers holding the endplates together. Additionally, the fibers may be wound multiple times within the same slot, thereby increasing the radial density of the fibers. In each case, this improves the wear resistance and increases the torsional and flexural stiffness of the prosthetic disc, thereby further approximating natural disc stiffness. In addition, thefibers340 may be passed through or wound on each slot, or only on selected slots, as needed.
• The purpose of thefibers340 is to hold thetop cap331 andbottom cap332 together and to limit the range-of-motion to mimic the range-of-motion of a natural disc. Accordingly, the fibers preferably comprise high tenacity fibers with a high modulus of elasticity, for example, at least about 100 MPa, and preferably at least about 500 MPa. By high tenacity fibers is meant fibers that can withstand a longitudinal stress of at least 50 MPa, and preferably at least 250 MPa, without tearing. Thefibers140 are generally elongate fibers having a diameter that ranges from about 100 μm to about 500 μm, and preferably about 200 μm to about 400 μm. Optionally, the fibers may be injection molded with an elastomer to encapsulate the fibers, thereby providing protection from tissue ingrowth and improving torsional and flexural stiffness.
• Thefibers340 may be fabricated from any suitable material. Examples of suitable materials include polyester (e.g., Dacron®), polyethylene, polyaramid, poly-paraphenylene terephthalamide (e.g., Kevlar®), carbon or glass fibers, polyethylene terephthalate, acrylic polymers, methacrylic polymers, polyurethane, polyurea, polyolefin, halogenated polyolefin, polysaccharide, vinylic polymer, polyphosphazene, polysiloxane, and the like.
• Thefibers340 may be terminated on an endplate by tying a knot in the fiber on the superior surface of an endplate. Alternatively, thefibers340 may be terminated on an endplate by slipping the terminal end of the fiber into a slot on an edge of an endplate, similar to the manner in which thread is retained on a thread spool. The slot may hold the fiber with a crimp of the slot structure itself, or by an additional retainer such as a ferrule crimp. As a further alternative, tab-like crimps may be machined into or welded onto the endplate structure to secure the terminal end of the fiber. The fiber may then be closed within the crimp to secure it. As a still further alternative, a polymer may be used to secure the fiber to the endplate by welding. The polymer would preferably be of the same material as the fiber (e.g., PE, PET, or the other materials listed above). Still further, the fiber may be retained on the endplates by crimping a cross-member to the fiber creating a T-joint, or by crimping a ball to the fiber to create a ball joint.
• Thesidewall333 is preferably made of polyurethane or silicone and may be fabricated by injection molding, two-part component mixing, or dipping the core assembly into a polymer solution. Alternatively, an outer ring or gasket (not shown in the drawings) may optionally be provided in place of thesidewall333.
• As noted above, thecore assembly330 is selectively retained between theupper endplate310 and thelower endplate320. A preferred mechanism for retaining thecore assembly330 between the two endplates is illustrated inFIGS. 21 through 23. For example, theupper endplate310 is provided with aretainer313 formed on the interior surface of theupper endplate310. Theretainer313 is a convex body formed at the center of the internal surface of theupper endplate310 that extends into the space between theupper endplate310 andlower endplate320 when the endplates are implanted into the patient. Asimilar retainer323 is formed on the opposed internal surface of thelower endplate320. Each of theretainers313,323 is preferably of generally semi-spherical shape, and each is preferably formed from the same material used to fabricate the upper andlower endplates310,320.
• As shown, for example, inFIG. 23, theretainers313,323 formed on the internal surfaces of the endplates cooperate with theindentations336 formed on the external surfaces of thetop cap331 andbottom332 of thecore assembly330 to hold the core assembly in place between the endplates. The amount of retaining force holding thecore assembly330 in place will depend on several factors, including the materials used to form the endplates and the core assembly, the size and shape of the core assembly, the distance separating the two endplates, the size and shape of each of the retainers and indentations, and other factors. Any one or all of these factors may be varied to obtain desired results. Typically, the retaining force will be sufficient to hold the core assembly in place, while still allowing each of the endplates to rotate substantially freely relative to the core assembly.
• Turning to FIGS.24A-C, three embodiments of thecore assembly330 are illustrated. In a first embodiment, shown inFIG. 24A, thecore assembly330 is provided with a throughhole337, i.e., the central portion of thecore assembly330 is hollow. In this embodiment, although there are noindentations336, the throughhole337 creates ashoulder338 on each of thetop cap331 andbottom cap332. Theshoulders338 have a size selected to suitably engage theretainers313,323 formed on the endplates. In a second embodiment, thecore assembly330 is provided withindentations336 and thecore center334 extends throughout the internal volume of the core assembly. Finally, in a third embodiment, thecore assembly330 is provided withindentations336, but thecore center334 occupies only a central portion of the internal volume of thecore assembly330.
• Turning to FIGS.25A-C, the core assembly may optionally include a plurality of reinforcingfibers360 distributed throughout the body of the core assembly. Thefibers360 may be fabricated from any suitable material. Examples of suitable materials include polyester (e.g., Dacron), polyethylene, polyaramid, carbon or glass fibers, polyethylene terephthalate, acrylic polymers, methacrylic polymers, polyurethane, polyurea, polyolefin, halogenated polyolefin, polysaccharide, vinylic polymer, polyphosphazene, polysiloxane, and the like. The reinforcingfibers360 provide additional strength to the core assembly. The fiber reinforcement is made by injecting core center material around the fibers formed in the shape of the core center. Exemplary core shapes are shown in FIGS.25A-C, and include acore assembly330 having a through hole337 (FIG. 25A), acore assembly330 havingindentations336 on each of the top and bottom surfaces (FIG. 25B), and acore assembly330 having a toroidal shape (FIG. 25C).
• Thefibers360 may be terminated on an endplate by tying a knot in the fiber on the superior surface of an endplate. Alternatively, thefibers360 may be terminated on an endplate by slipping the terminal end of the fiber into a slot on an edge of an endplate, similar to the manner in which thread is retained on a thread spool. The slot may hold the fiber with a crimp of the slot structure itself, or by an additional retainer such as a ferrule crimp. As a further alternative, tab-like crimps may be machined into or welded onto the endplate structure to secure the terminal end of the fiber. The fiber may then be closed within the crimp to secure it. As a still further alternative, a polymer may be used to secure the fiber to the endplate by welding. The polymer would preferably be of the same material as the fiber (e.g., PE, PET, or the other materials listed above). Still further, the fiber may be retained on the endplates by crimping a cross-member to the fiber creating a T-joint, or by crimping a ball to the fiber to create a ball joint.
• Turning next toFIGS. 26, 27, and28A-C, the core assembly may optionally be formed of stacks of reinforcing fabric having no silicone, polyurethane, or other polymeric component. As shown inFIG. 26, wovenfibers370 are formed into sheets of fabric that are compressed into a stack to form a core body. Thewoven fibers370 may be formed of materials such as polyester (e.g., Dacron), polyethylene, polyaramid, carbon or glass fibers, polyethylene terephthalate, acrylic polymers, methacrylic polymers, polyurethane, polyurea, polyolefin, halogenated polyolefin, polysaccharide, vinylic polymer, polyphosphazene, polysiloxane, and the like.FIG. 27 is a cross-sectional view of a woven fiber core body.FIG. 28A illustrates a wovenfiber core body330 having a throughhole337 similar to the structure described previously. Similarly,FIG. 28B illustrates a wovenfiber core body330 havingindentations336 on its upper and lower surfaces. Finally,FIG. 28C illustrates a wovenfiber core body330 having a toroidal shape.
• The three-piece structure embodiment of the prosthetic disc is implanted by a surgical procedure. After removing the natural disc, grooves are formed in the superior and inferior vertebrae between which the prosthetic disc is to be implanted. Theupper endplate310 andlower endplate320 are then each implanted into the void, while aligning the anchoring fins311321 with the grooves formed on the vertebral bodies. The anchoring fins cause the prosthetic disc to be secured in place between the adjacent vertebral bodies. After theupper endplate310 andlower endplate320 are implanted, thecore assembly330 is engaged between the endplates to complete the implantation.
• The three-piece prosthetic disc has several advantages over prior art artificial discs, as well as over alternative treatment procedures such as spinal fusion. For example, the prosthetic discs described herein provide compressive compliance similar to that of a natural spinal disc. In addition, the motions in flexion, extension, lateral bending, and axial rotation are also restricted in a manner near or identical to those associated with a natural disc.
• D. Four-Piece Structure
• Representative prostheticintervertebral discs400 having four-piece structures are shown inFIGS. 29 through 35. The prosthetic discs include anupper endplate410, alower endplate420, and a two-piece core assembly430 retained between theupper endplate410 and thelower endplate420.
• Theupper endplate410 andlower endplate420 are generally flat, planar members, and are fabricated from a physiologically acceptable material that provides substantial rigidity. Examples of materials suitable for use in fabricating theupper endplate410 andlower endplate420 include titanium, titanium alloys, stainless steel, cobalt/chromium, etc., which are manufactured by machining or metal injection molding; plastics such as polyethylene with ultra high molar mass (molecular weight) (UHMWPE), polyether ether ketone (PEEK), etc., which are manufactured by injection molding or compression molding; ceramics; graphite; and others. Optionally, the endplates may be coated with hydroxyapatite, titanium plasma spray, or other coatings to enhance bony ingrowth.
• As noted above, the upper and lower endplates typically have a length of from about 12 mm to about 45 mm, preferably from about 13 mm to about 44 mm, a width of from about 11 mm to about 28 mm, preferably from about 12 mm to about 25 mm, and a thickness of from about 0.5 mm to about 4 mm, preferably from about 1 mm to about 3 mm. The sizes of the upper and lower endplates are selected primarily based upon the size of the void between adjacent vertebral bodies to be occupied by the prosthetic disc. Accordingly, while endplate lengths and widths outside of the ranges listed above are possible, they are not typical
• The upper surface of theupper endplate410 and the lower surface of thelower endplate420 are preferably each provided with a mechanism for securing the endplate to the respective opposed surfaces of the upper and lower vertebral bodies between which the prosthetic disc is to be implanted. For example, as shown inFIGS. 30 and 32, theupper endplate410 includes an anchoringfin411. The anchoringfin411 is intended to engage a mating groove that is formed on the surface of the upper vertebral body to thereby secure the endplate to the vertebral body. The anchoringfin411 extends generally perpendicularly from the generally planar external surface of theupper endplate410, i.e., upward from the upper side of the endplate as shown inFIGS. 30 and 32. As shown in the Figures, the anchoringfin411 is disposed at the center of external surface of theupper endplate410 and has a length that is slightly less than the width of theupper endplate410. Although not shown in the Figures, the anchoringfin411 may be provided with a plurality of serrations located on its top edge. The serrations are intended to enhance the ability of the anchoring fin to engage the vertebral body and to thereby secure theupper endplate410 to the spine.
• Similarly, the lower surface of thelower endplate420 includes an anchoringfin421. The anchoringfin421 on the lower surface of thelower endplate420 is identical in structure and function to the anchoringfin411 on the upper surface of theupper endplate410, with the exception of its location on the prosthetic disc. The anchoringfin421 on thelower endplate420 is intended to engage a mating groove formed on the lower vertebral body, whereas the anchoringfin411 on theupper endplate410 is intended to engage a mating groove on the upper vertebral body. Thus, theprosthetic disc400 is held in place between the adjacent vertebral bodies.
• Alternatively, theupper endplate410 andlower endplate420 of the three-piece prosthetic disc may employ one of the alternative securing mechanisms shown in FIGS.13A-C. As described above in relation to the one-piece prosthetic device shown inFIG. 13A, each of theupper endplate110 andlower endplate120 is provided with asingle anchoring fin111,121. The anchoringfins111,121 are located along a centerline of the respective endplates, and each is provided with a plurality ofserrations112,122 on its upper edge. Thesingle anchoring fins111,121 are intended to engage grooves formed on the opposed surface of the upper and lower vertebral bodies, as described above. InFIG. 13B, each of theupper endplate110 andlower endplate120 is provided with three anchoringfins111a-c,121a-c. TheFIG. 13B prosthetic disc is the same as the prosthetic disc shown inFIG. 12, but it is shown in perspective rather than cross-section. Thus, the structure and function of the anchoringfins111a-cand121a-care as described above in relation toFIG. 12. Finally, inFIG. 13C, each of theupper endplate110 andlower endplate120 is provided with a plurality ofserrations113,123 over a portion of the exposed external surface of the respective endplate. Theserrations113,123 are intended to engage the opposed surfaces of the adjacent vertebral bodies to thereby secure the endplates in place between the vertebral bodies. Theserrations113,123 may be provided over the entire external surface of each of the upper and lower endplates, or they may be provided over only a portion of those surfaces. For example, inFIG. 13C, theserrations113 on the upper surface of theupper endplate110 are provided over three major areas, a first area113anear a first edge of theupper endplate110, asecond area113bnear the center of theupper endplate110, and a third area near a second edge of theendplate113c.
• Turning toFIG. 54, in an optional embodiment, the anchoringfins111 are selectively retractable and extendable by providing adeployment mechanism160 that is associated with theupper endplate110. A similar mechanism may be used on thelower endplate120. The deployment mechanism includes aslider161 that slides within achannel162 formed in theupper endplate110. Thechannel162 includes a threadedregion163, and theslider161 includes matchingthreads164, thereby providing a mechanism for advancing theslider161 within thechannel162. As theslider161 is advanced within thechannel162, a taperedregion165 engages the bottom surface of adeployable fin166. Further advancement of theslider161 causes thedeployable fin166 to be raised upward within aslot167 on the upper surface of theupper endplate110. Reversing thedeployment mechanism160 causes thefin166 to retract. Thedeployment mechanism160 may also be used in conjunction with spikes, serrations, or other anchoring devices. In an alternative embodiment, the threadedslider161 of the deployment mechanism may be replaced with a dowel pin that is advanced to deploy thefin166. Other advancement mechanisms are also possible.
• FIG. 29 illustrates yet another alternative mechanism for securing the upper and lower endplates to the vertebral bodies. As shown in the Figure, theupper endplate410 may be provided with a plurality of anchoringspikes419 spaced over the external surface of the endplate. The anchoring spikes419 are adapted to engage the internal surface of the vertebral body. Although not shown inFIG. 29, the external surface of thelower endplate420 may optionally be provided with similar anchoring spikes to secure the lower endplate to the internal surface of the inferior vertebral body.
• The core assembly430 is intended to provide support to and to maintain the relative spacing between theupper endplate410 andlower endplate420. The core assembly430 provides compressive compliance to the four-piece prosthetic disc, as well as providing limited translation, flexion, extension, and lateral bending by and between theupper endplate410 andlower endplate420. The core assembly430 further provides substantially unlimited rotation by and between theupper endplate410 and thelower endplate420.
• The core assembly430 includes anupper core member430aand alower core member430b,430c. Two embodiments of the core assembly430 of the four-piece prosthetic disc are shown inFIGS. 29 through 35. In the first embodiment, shown inFIGS. 29 through 32, both theupper core member430aand thelower core member430binclude a core structure having top and bottom caps, slots, fibers, a core center, and an annular capsule. In the second embodiment, shown inFIGS. 33 through 35, theupper core member430ais identical to that of the first embodiment, but thelower core member430cis, instead, a solid structure. These structures are described more fully below.
• Theupper core member430aincludes atop cap431a, abottom cap432a, a sidewall433a, and a core center434aheld by and retained between thetop cap431a,bottom cap432a, and sidewall433a. Thetop cap431aandbottom cap432aare generally planar, and are fabricated from a physiologically acceptable material that provides substantial rigidity. Examples of materials suitable for use in fabricating thetop cap431aandbottom cap432ainclude titanium, titanium alloys, stainless steel, cobalt/chromium, etc., which are manufactured by machining or metal injection molding; plastics such as polyethylene with ultra high molar mass (molecular weight) (UHMWPE), polyether ether ketone (PEEK), etc., which are manufactured by injection molding or compression molding; ceramics; graphite; and others. The core center434ais made of a relatively compliant material, for example, polyurethane or silicone, and is typically fabricated by injection molding. The shape of the core center434ais typically generally cylindrical or bean-shaped, although the shape (as well as the materials making up the core center and the core member size) may be varied to obtain desired physical or performance properties. For example, the core member434ashape, size, and materials will directly affect the degree of flexion, extension, lateral bending, and axial rotation of the prosthetic disc.
• Thebottom cap432apreferably includes a generallyconvex retainer437aformed at a center point of thebottom cap432a. Theretainer437ais intended to cooperate with anindentation436b,436cformed on the upper surface of thelower core member430b,430cto create an engagement between theupper core member430aand thelower core member430b,430c, as described more fully below.
• Thetop cap431aandbottom cap432apreferably contain a plurality ofslots435aspaced radially about the surface of each of the caps. One ormore fibers440 are wound around thetop cap431aandbottom cap432athrough theslots435ato attach the top and bottom caps to one another. Thefibers440 preferably are not tightly wound, thereby allowing a degree of axial rotation, bending, flexion, and extension by and between thetop cap431aandbottom cap432a. The core center434ais preferably pre-compressed. The actual number ofslots435acontained on each of thetop cap431aandbottom cap432ais variable. Increasing the number of slots will result in an increase in the circumferential density of the fibers holding the endplates together. Additionally, the fibers may be wound multiple times within the same slot, thereby increasing the radial density of the fibers. In each case, this improves the wear resistance and increases the torsional and flexural stiffness of the prosthetic disc, thereby further approximating natural disc stiffness. In addition, thefibers440 may be passed through or wound on each slot, or only on selected slots, as needed.
• The purpose of thefibers440 is to hold thetop cap431aandbottom cap432atogether and to limit the range-of-motion to mimic the range-of-motion of a natural disc. Accordingly, the fibers preferably comprise high tenacity fibers with a high modulus of elasticity, for example, at least about 100 MPa, and preferably at least about 500 MPa. By high tenacity fibers is meant fibers that can withstand a longitudinal stress of at least 50 MPa, and preferably at least 250 MPa, without tearing. Thefibers440 are generally elongate fibers having a diameter that ranges from about 100 μm to about 500 μm, and preferably about 200 μm to about 400 μm. Optionally, the fibers may be injection molded with an elastomer to encapsulate the fibers, thereby providing protection from tissue ingrowth and improving torsional and flexural stiffness.
• Thefibers440 may be fabricated from any suitable material. Examples of suitable materials include polyester (e.g., Dacron®), polyethylene, polyaramid, poly-paraphenylene terephthalamide (e.g., Kevlar®), carbon or glass fibers, polyethylene terephthalate, acrylic polymers, methacrylic polymers, polyurethane, polyurea, polyolefin, halogenated polyolefin, polysaccharide, vinylic polymer, polyphosphazene, polysiloxane, and the like.
• Thefibers440 may be terminated on an endplate by tying a knot in the fiber on the superior surface of an endplate. Alternatively, thefibers440 may be terminated on an endplate by slipping the terminal end of the fiber into a slot on an edge of an endplate, similar to the manner in which thread is retained on a thread spool. The slot may hold the fiber with a crimp of the slot structure itself, or by an additional retainer such as a ferrule crimp. As a further alternative, tab-like crimps may be machined into or welded onto the endplate structure to secure the terminal end of the fiber. The fiber may then be closed within the crimp to secure it. As a still further alternative, a polymer may be used to secure the fiber to the endplate by welding. The polymer would preferably be of the same material as the fiber (e.g., PE, PET, or the other materials listed above). Still further, the fiber may be retained on the endplates by crimping a cross-member to the fiber creating a T-joint, or by crimping a ball to the fiber to create a ball joint.
• The sidewall433ais preferably made of polyurethane or silicone and may be fabricated by injection molding, two-part component mixing, or dipping the core assembly into a polymer solution. Alternatively, an outer ring or gasket (not shown in the drawings) may optionally be provided in place of the sidewall433a.
• As shown, for example, inFIGS. 29 through 35, thetop cap431aof theupper core member430aincludes anedge member438athat is adapted to engage agroove416 formed on the perimeter of the internal surface of theupper endplate410 to provide an engagement mechanism for attaching theupper core member430ato theupper endplate410. For example, theupper core member430ais slid into theupper endplate410, as shown by the arrows inFIG. 29. After theupper endplate430ais fully advanced, i.e., once the leading edge of theedge member438acontacts the interior of thegroove416 on theupper endplate410, a tab461aon the upper surface of thetop cap431aengages aslot462aon the lower surface of the upper endplate410 (seeFIGS. 30 and 33), thereby locking theupper core member430ain place within theupper endplate410.
• Turning to the first embodiment of thelower core member430b, shown inFIGS. 29 through 32, thelower core member430bincludes atop cap431b, abottom cap432b, asidewall433b, and acore center434bheld by and retained between thetop cap431b,bottom cap432b, andsidewall433b. Thetop cap431bandbottom cap432bare generally planar, and are fabricated from a physiologically acceptable material that provides substantial rigidity. Examples of materials suitable for use in fabricating thetop cap431bandbottom cap432binclude titanium, titanium alloys, stainless steel, cobalt/chromium, etc., which are manufactured by machining or metal injection molding; plastics such as polyethylene with ultra high molar mass (molecular weight) (UHMWPE), polyether ether ketone (PEEK), etc., which are manufactured by injection molding or compression molding; ceramics; graphite; and others. Thecore center434bis made of a relatively compliant material, for example, polyurethane or silicone, and is typically fabricated by injection molding. The shape of thecore center434bis typically generally cylindrical or bean-shaped, although the shape (as well as the materials making up the core center and the core member size) may be varied to obtain desired physical or performance properties. For example, thecore member434bshape, size, and materials will directly affect the degree of flexion, extension, lateral bending, and axial rotation of the prosthetic disc.
• Thetop cap431bpreferably includes a generallyconcave indentation436bformed at a center-point of thetop cap431b. Theindentation436bis intended to cooperate with theretainer437aformed on the lower surface of theupper core member430ato create an engagement between theupper core member430aand thelower core member430b, as described more fully below.
• Thetop cap431bandbottom cap432bpreferably contain a plurality ofslots435bspaced radially about the surface of each of the caps. One ormore fibers440 are wound around thetop cap431bandbottom cap432bthrough theslots435bto attach the top and bottom caps to one another. Thefibers440 preferably are not tightly wound, thereby allowing a degree of axial rotation, bending, flexion, and extension by and between thetop cap431bandbottom cap432b. Thecore center434bis preferably pre-compressed. The actual number ofslots435bcontained on each of thetop cap431bandbottom cap432bis variable. In addition, thefibers440 may be passed through or wound on each slot, or only on selected slots, as needed.
• The purpose of thefibers440 is to hold thetop cap431bandbottom cap432btogether. Accordingly, the fibers preferably comprise high tenacity fibers with a high modulus of elasticity, for example, at least about 100 MPa, and preferably at least about 500 MPa. By high tenacity fibers is meant fibers that can withstand a longitudinal stress of at least 50 MPa, and preferably at least 250 MPa, without tearing. Thefibers440 are generally elongate fibers having a diameter that ranges from about 100 μm to about 500 μm, and preferably about 200 μm to about 400 μm.
• Thefibers440 may be fabricated from any suitable material. Examples of suitable materials include polyester (e.g., Dacron), polyethylene, polyaramid, carbon or glass fibers, polyethylene terephthalate, acrylic polymers, methacrylic polymers, polyurethane, polyurea, polyolefin, halogenated polyolefin, polysaccharide, vinylic polymer, polyphosphazene, polysiloxane, and the like.
• Thesidewall433bis preferably made of polyurethane or silicone and may be fabricated by injection molding, two-part component mixing, or dipping the core assembly into a polymer solution. Alternatively, an outer ring or gasket (not shown in the drawings) may optionally be provided in place of thesidewall433b.
• As shown, for example, inFIGS. 29 through 32, thebottom cap432bof thelower core member430bincludes anedge member438bthat is adapted to engage agroove426 formed on the perimeter of the internal surface of thelower endplate420 to provide an engagement mechanism for attaching thelower core member430bto thelower endplate420. For example, thelower core member430bis slid into thelower endplate420, as shown by the arrow inFIG. 16. After thelower endplate430bis fully advanced, i.e., once the leading edge of theedge member438bcontacts the interior of thegroove426 on thelower endplate420, atab461bon the lower surface of thebottom cap432bengages aslot462bon the upper surface of the lower endplate420 (seeFIG. 30), thereby locking thelower core member430bin place within thelower endplate420.
• Turning to the second embodiment of thelower core member430c, shown inFIGS. 33 through 35, thelower core member430cis formed of a solid structure having none of the top and bottom caps, sidewall, core center, or fibers that are included in the first embodiment of thelower core member430b. The second embodiment of thelower core member430chas an identical external shape and size to that of the first embodiment of thelower core member430b, including having anedge member438cthat engages thegroove426 on the internal surface of thelower endplate420. A tab461cis configured to selectively engage thenotch462cformed on the upper internal surface of thelower endplate420. Anindentation436cis formed on the central upper surface of thelower core member430c, and is adapted to engage theretainer437aformed on theupper core member430a.
• Examples of materials suitable for use in fabricating the second embodiment of thelower core member430cinclude titanium, titanium alloys, stainless steel, cobalt/chromium, etc., which are manufactured by machining or metal injection molding; plastics such as polyethylene with ultra high molar mass (molecular weight) (UHMWPE), polyether ether ketone (PEEK), etc., which are manufactured by injection molding or compression molding; ceramics; graphite; and others.
• The four-piece structure embodiment of the prosthetic disc is implanted by a surgical procedure. After removing the natural disc, grooves are formed in the superior and inferior vertebrae between which the prosthetic disc is to be implanted (only in the situation where the endplates are provided with anchoring fins). Theupper endplate410 andlower endplate420 are then each implanted into the void, while aligning the anchoringfins411,421 with the grooves formed on the vertebral bodies. The anchoring fins cause the prosthetic disc to be secured in place between the adjacent vertebral bodies. After theupper endplate410 andlower endplate420 are put in position, the core assembly430 is engaged between the endplates to complete the implantation.
• The four-piece prosthetic disc has several advantages over prior art artificial discs, as well as over alternative treatment procedures such as spinal fusion. For example, the prosthetic discs described herein provide compressive compliance similar to that of a natural spinal disc. In addition, the motions in flexion, extension, lateral bending, and axial rotation are also restricted in a manner near or identical to those associated with a natural disc.
• E. Fabric Tubes
• The one-piece, two-piece, three-piece, and four-piece structures of the prosthetic discs described above include upper and lower endplates that are attached to each other by fibers wound around the endplates. In an alternative embodiment, the fiber component is provided in a fabric cylinder or tubing of woven or knitted form, rather than as individual fibers. The fabric tubing extends between and structurally connects the upper and lower endplates.
• In a first example, a single fabric tube may be provided in place of the wound fibers. The fabric tube may be attached at its upper edge to the upper endplate, and at its lower edge to the lower endplate. For example, through-holes may be provided in each of the endplates to allow the fabric tubing (or individually woven fibers) to pass through and to be secured by knots or crimping on the external surfaces of the endplates. Alternatively, the fabric tube may be attached to each endplate by a peripheral metal or plastic ring that is fixed to the interior surfaces of the endplates.
• In another example, two or more tubes of fabric may be provided between and interconnecting the upper and lower endplates. The two or more fabric tubes may be attached to the endplates by through-holes, as described above, or by press-fit, adhesion, weld, or injection molding integration to a progressively smaller metal or plastic ring with tubing circumferentially affixed to it. This structure creates an assembly of two or more concentric layers of fabric tubing. Alternatively, the concentric tubes may be terminated by collecting each tubing end together and crimping or sewing them together, then fixing the collected ends to the upper and lower endplates. As a still further alternative, an injection molded lid may be fabricated in a manner in which the lid captures the terminal ends of each of the fabric tubes during the injection molding process.
• In a particularly preferred embodiment, multiple concentric fabric tubes are provided. Each of the fabric tubes may be formed from a fabric of material different from the other tubes (e.g, PET, PE, PTFE, Polyamide, etc.), or from a fabric having different material properties. This provides the ability to construct prosthetic discs having a range of performance characteristics.
• As an alternative, the tubing may be comprised of a fiber reinforced elastomeric material rather than a fabric alone. For example, a polyurethane, PDMS, polyester, or other elastomer may be integrated with a fabric or with individual fibers to create a tubing that attaches and interconnects the upper and lower endplates.
• F. Anti-Creep Compression Member
• Turning toFIG. 53, an optionalanti-creep compression member135 is shown. Theanti-creep member135 is intended to prevent “creeping” of the core due to vertical compression and lateral expansion of the core, which occurs due to extended from wear. Theanti-creep member135 is preferably used in connection with a toroidal shapedcore member130,230,330,430 of any of the one-, two-, three-, or four-piece structures of the prosthetic disc. Theanti-creep element135 includes apost136 extending downward from theupper endplate110,210,310,410 and a mating receptacle orcup137 extending upward from thelower endplate120,220,320,420. Alternatively, thepost136 may extend upward from the lower endplate and thereceptacle137 may extend downward from the upper endplate. Aspring138 is located within thereceptacle138. Thepost136 is slightly conical in shape, and thereceptacle137 has a slightly larger diameter than thepost136 in order to receive thepost136 within thereceptacle137. The slightly conical shapes of thepost136 andcup137 are preferred in order to accommodate lateral bending (side-to-side), flexion (forward), and extension (backward) of the upper and lower endplates relative to one another. The spring is pre-loaded to provide a force biasing the two endplates apart.
• G. Advantages of the Present Prosthetic Intervertebral Discs
• It is evident from the above discussion and results that the present invention provides significantly improved prosthetic intervertebral discs. Significantly, the subject discs closely imitate the mechanical properties of the fully functional natural discs that they are intended to replace.
• More specifically, the modes of spinal motion may be characterized as compression, shock absorption (i.e., very rapid-compressive loading and unloading), flexion (forward) and extension (backward), lateral bending (side-to-side), torsion (twisting), and translation and sublaxation (motion of axis). The prosthetic discs described herein semi-constrain each mode of motion, rather than completely constrain or allow a mode to be unconstrained. In this manner, the present prosthetic discs closely mimic the performance of natural discs. The tables below provide data that illustrates this performance.

  TABLE 1
  One-Piece Structure Lumbar Prosthetic Disc Compared
  to Natural Human Disc and Ball & Socket Design
  Prosthetic Disc Compressive Mode of Motion

  			Prosthetic	Prosthetic
  	Human	Ball & Socket	Disc Core	Disc Core &
  Properties	Spine	Design	Only	Fiber

  Stiffness	≈1288	Very large	800-1600	850-1650
  (N/mm)
  ROM (mm)	0.50	≈0	0.61	0.50
  Ult. Load (N)	3952	>5900	>5900	>5900

• TABLE 2
  One-Piece Structure Cervical Prosthetic Disc Compared
  to Natural Human Disc and Ball & Socket Design Prosthetic
  Disc Compressive Mode of Motion

  			Prosthetic	Prosthetic
  	Human	Ball & Socket	Disc Core	Disc Core &
  Properties	Spine	Design	Only	Fiber

  Stiffness	≈737	Very large	100-950	150-1000
  (N/mm)
  ROM (mm)	0.70 +/− 0.03	≈0	≈0.87	≈0.60
  Ult. Load	≈1600	>5900	>9000	>9000
  (N)

• The subject discs exhibit stiffness in the axial direction, torsional stiffness, bending stiffness in the saggital plane, and bending stiffness in the front plane, where the degree of these features can be controlled independently by adjusting the components of the discs. The interface mechanism between the endplates and the core members of several embodiments of the described prosthetic discs enables a very easy surgical operation. In view of the above and other benefits and features provided by the subject inventions, it is clear that the subject inventions represent a significant contribution to the art.
• II. Implantation Apparatus and Methods
• A. Conventional (Non-Minimally Invasive Method)
• The prosthetic intervertebral discs may be implanted into a patient's spine using the apparatus and methods described herein. This description will focus on use of apparatus to implant one- and two-piece prosthetic discs, although the apparatus may also be used to implant other embodiments of the prosthetic disc with little or no modification, as will be appreciated by a person of skill in the art. In addition, and as described below, the method may incorporate less than all of the apparatus components described below.
• The prosthetic discs are implanted surgically between two adjacent vertebrae, an upper vertebra and a lower vertebra, in a patient's spinal column. The vertebrae to be treated are exposed using conventional surgical procedures. After exposure, the natural vertebral disc is removed, leaving a void space between the two adjacent vertebrae. The prosthetic intervertebral disc is then implanted using the apparatus and methods described below.
• 1. Implantation Tools
• In a first embodiment, and in reference toFIGS. 36-38, the implantation tools include aspacer810, a two-sided chisel830, and a holder850.
• Turning first to FIGS.36A-B, thespacer810 includes aproximal handle812, ashaft814, and ahead portion816. Thehandle812 is adapted to be easily grasped by the user during the implantation procedure. Theshaft814 is preferably cylindrical and smaller in cross-section than the handle. Thehead portion816 has a size and shape adapted to perform its function of being inserted between and separating two adjacent vertebral bodies. In the embodiment shown in the Figures, thehead portion816 has a generally trapezoidal shape when viewed from above or below, with aleading edge817 being generally parallel to, but having a shorter length than the trailingedge818. Other shapes may be used. The head portion has a thickness “h”. The thickness “h” may be varied according to need, i.e., the thickness “h” will impact the ability of the user to insert thehead portion816 between the two vertebral bodies, and also the amount by which thehead portion816 will be able to separate the two bodies. Thus, aspacer810 with ahead portion816 of relatively large or small thickness “h” may be used depending on the need. Theedges819 of thehead portion816 are generally rounded to allow thehead portion816 to be more easily inserted between the two vertebral bodies.
• Turning next to FIGS.37A-B, the two-sided chisel830 includes ahandle832, ashaft834, and ahead portion836. Thehandle832 is adapted to be easily grasped by the user during the implantation procedure. Theshaft834 is preferably cylindrical and smaller in cross-section than the handle. Thehead portion836 has a size and shape adapted to perform its function of being inserted between and creating grooves on the two adjacent vertebral bodies. In the embodiment shown in the Figures, thehead portion836 has a generally trapezoidal shape when viewed from above or below, with aleading edge837 being generally parallel to, but having a shorter length than the trailingedge838. Other shapes may be used. The head portion has a thickness “h”. The thickness “h” may be varied according to need, i.e., the thickness “h” will impact the ability of the user to be able to insert thehead portion836 between the two vertebral bodies and to cut grooves on the two bodies. Thus, achisel830 with ahead portion836 of relatively large or small thickness “h” may be used depending on the need.
• Thechisel830 includes a plurality of wedge-shapedblades839 formed on the upper and lower surfaces of thehead portion836. Theblades839 of thechisel830 are adapted to create grooves in the lower surface of the upper vertebra and on the upper surface of the lower vertebra being treated. In the embodiment shown in the Figures, thechisel830 includes threeblades839 on each of the upper and lower surfaces. More or fewer blades may be provided. Optimally, the number, shape, and orientation of theblades839 on the surfaces of thechisel830 are selected to match those of the anchoring fins provided on the surfaces of the prosthetic disc to be implanted.
• Turning next to FIGS.38A-B, the holder850 includes ahandle852, ashaft854, and ahead portion856. Thehandle852 is adapted to be easily grasped by the user during the implantation procedure. Theshaft854 is preferably cylindrical and smaller in cross-section than the handle. Thehead portion856 has a size and shape adapted to perform its function of retaining the prosthetic disc on an end thereof in order to implant the disc between the two adjacent vertebral bodies.
• Thehead portion856 of the holder850 includes aproximal body portion857 and two arms858a-bextending distally from thebody portion857. Thebody portion857 has a generally square shape, and its distal end includes a slightlyconcave section859 at its center that provides a space for receiving a portion of the prosthetic disc. Each of the arms858a-balso includes a slightly recessed portion860a-bthat is adapted to engage the side surfaces of the prosthetic disc in order to facilitate holding the disc in place during the implantation procedure. The body portion also includes engagement pins861 on its distal surface, which engagement pins861 are adapted to engage mating holes provided on the prosthetic disc.
• In an alternative embodiment, and in reference toFIGS. 39-42, the implantation tools include aguide500, alower pusher520 connected to afirst chisel540, anupper endplate holder560, and asecond chisel580.
• Turning first toFIG. 39, theguide500 serves the purposes of, first, positioning and retaining thelower endplate220bin place on the lower of the two adjacent vertebrae being treated, and, second, guiding one or more of the other implantation tools to their proper locations for performing their functions. In the preferred embodiment, theguide500 comprises a generally flat,elongated member501 having afirst end502, asecond end503, and a pair of raisedsides504,505. Each of the raisedsides504,505 includes an inwardly facingportion504a,505athat extends back over theelongated member501 on a plane slightly above that of the elongated member. Each of the inwardly facingportions504a,505aof the pair of raisedsides504,505 thereby forms agroove506,507 that extends along the length of theguide500. As described below, thegrooves506,507 may be used to guide one or more of the other implantation tools in cooperation with a flange provided on those other tools.
• Extending from thefirst end502 of theguide500 are a pair oflower endplate rods508,509. Each of thelower endplate rods508,509 is a generally cylindrical rod that extends outward from thefirst end502 of theguide500 in the plane of theelongate member501 or parallel to that plane. The sizes of thelower endplate rods508,509—e.g., lengths, cylindrical diameters—are not critical, provided that the rods are of sufficient size to be capable of performing the function of engaging and retaining thelower endplate220b, as described more fully below.
• Turning toFIG. 40, in the preferred embodiment, aspacer tool570 includes a combination of alower pusher520 and afirst chisel540 attached to abase member530. Thelower pusher520 includes a pair oflower pusher rods521a,521b. Each of thelower pusher rods521a,521bis connected at a first end to thebase530. At a second end, a cross-member522 extends between and connects to each of the pair oflower pusher rods521a,521b. The twolower pusher rods521a,521bare thus held in a generally parallel relation to one another and extend outward from thebase530. At the end opposite thebase530, each of thelower pusher rods521a,521bis attached to alower endplate insert523. Thelower endplate insert523 includes aflange524 along its edge that is adapted to engage thematching slot226 found on an outerlower endplate220bof a two-piece prosthetic disc, such as those described herein.
• In the preferred embodiment, each of thelower pusher rods521a,521band the cross-member522 are generally cylindrical rods. The cross-sectional shape and size of the rods are not critical, such that thelower pusher rods521a,521bare capable of advancing the lower endplate during the implantation procedure, as described more fully below.
• In the preferred embodiment illustrated inFIG. 40, thebase530 includes a block-shapedbottom portion531. Thebottom portion531 of thebase530 is the portion of the base to which thepusher rods521a,521bof thelower pusher520 are attached. Thebottom portion531 shown inFIG. 40 has a generally block-shaped body, although the size and shape of thebottom portion531 are not critical.
• Extending upward from the top surface of the bottom portion are two flanges, atall flange532 and ashort flange533. Apivot pin534 is located at the upper end of thetall flange532. Thepivot pin534 extends through a hole in the upper end of thetall flange532, and is able to rotate around its pivot axis. A pair ofupper pusher rods541a,541bare attached to thepivot pin534, with one of the twoupper pusher rods541aattached to a first end of thepivot pin534, and the otherupper pusher rod541battached to the opposite end of thepivot pin534. At the end of theupper pusher rods541a,541bopposite thepivot pin534, theupper pusher rods541a,541bare attached to afirst chisel540. In addition, a cross-member542 attaches to and interconnects the pair ofupper pusher rods541a,541bnear the end to which thefirst chisel540 is attached.
• Aratchet key535 is extends through a hole in theshort flange533. Theratchet key535 is able to rotate around its longitudinal axis within the hole in the short flange. Theratchet key535 includes a graspingportion536 extending from one side of theshort flange533, and a gear portion (not shown in the Figures) extending from the opposite side of theshort flange533. Anelongated guide rail537 extends beneath the gear portion of theratchet key535 and generally between the pair ofupper pusher rods541a,541band the pair oflower pusher rods521a,521b. Theguide rail537 includes a plurality of teeth538 formed on its upper side, which teeth are adapted to engage the gear portion of theratchet key535. Thus, by rotating theratchet key535, a user is able to advance or withdraw theguide rail537.
• Aseparator515 is attached to an end of theguide rail537. Theseparator515 is a generally flat member that is disposed generally transversely to theguide rail537. A pair ofupper grooves516a,516bare formed on the top edge of theseparator515. Theupper grooves516a,516bhave a size and are located so as to slidably engage theupper pusher rods541a,541b. Similarly, a pair oflower grooves517a,517bare formed on the bottom edge of theseparator515. Thelower grooves517a,517bhave a size and are located so as to slidably engage thelower pusher rods521a,521b. Thus, as shown inFIG. 40, the separator is able to be advanced or withdrawn along the lengths of theupper pusher rods541a,541bandlower pusher rods521a,521bby turning theratchet key535. Turning theratchet key535 causes the gear portion of theratchet key535 to engage the teeth538 on theguide rail537. With reference to the perspective illustrated inFIG. 40, rotating the ratchet key clockwise will cause theguide rail537 and theseparator515 to withdraw, i.e., to draw nearer to thebase530. Alternatively, rotating theratchet key535 counter-clockwise will cause theguide rail537 and the separator to advance, i.e., to move away from thebase530.
• As best seen in the illustration inFIG. 40, theseparator515 has a partial height, h, that is defined as the distance between the bottom edge of theupper grooves516a,516band the top edge of thelower grooves517a,517b. The partial height h of theseparator515 is less than the distance separating theupper pusher rods541a,541bandlower pusher rods521a,521bat the point that they attach to thebase530. The partial height h of the separator is greater than the height of the prosthetic disc or, stated otherwise, the partial height h of the spacer is greater than the post-operative distance separating the two adjacent vertebrae being treated. Thus, as explained more fully below, theseparator515 has a partial height h that is suitable for expanding the distance separating thefirst chisel540 and thelower endplate insert523 as theseparator515 is advanced during the implantation procedure.
• Thefirst chisel540 is attached to the ends of each of theupper pusher rods541a,541bopposite thetall flange532. Thefirst chisel540 includes a generallyflat plate portion543 and one or more wedge-shapedblades544 extending upward from theflat plate portion543. Theblades544 of the first chisel are adapted to create grooves in the lower surface of the upper vertebra being treated. Theflat plate portion543 of the first chisel is preferably relatively thin in relation to the height of the prosthetic disc, thereby allowing the first chisel to be inserted between the two adjacent vertebrae after the natural disc has been removed.
• Turning toFIG. 41, theupper endplate holder560 includes apusher block561 attached to the end of apush rod563. Thepusher block561 has a generally flatfront surface562 that is adapted to engage the trailing surface of the upper endplate of the prosthetic disc, as described more fully below. In addition, theupper endplate holder560 includes a pair of outer engagement pins564 extending outward from thefront surface562, and acenter engagement pin565 also extending outward from the front surface. The outer engagement pins564 andcenter engagement pin565 are each generally cylindrical in shape, and relatively short in length relative to the size of thepush rod563. The engagement pins564,565 are intended to engage and retain the upper endplate of the prosthetic disc during the implantation procedure, as explained more fully below.
• Turning toFIG. 42, thesecond chisel580 includes a generallyflat plate portion583 attached to the end of apush rod582. One or more wedge-shapedblades584 attach to and extend upward from the top surface of theflat plate portion583. Similar to theblade544 of thefirst chisel540, theblades584 of the second chisel are adapted to create grooves in the lower surface of the upper vertebra being treated. Theflat plate portion583 of the second chisel is preferably thicker than theflat plate portion543 of thefirst chisel540, and is generally about the same thickness as the height of the prosthetic disc. Aflange585 extends outward from the bottom of thesecond chisel580. Theflange585 has a size and is oriented such that it will engage thegrooves506,507 on theguide member500 during the implantation procedure.
• 2. Implantation Procedures
a. First Embodiment
• A preferred implantation procedure utilizes thespacer810,chisel830, and holder850 shown inFIGS. 36-38. As discussed above, the procedure described herein is in relation to implantation of a one-piece prosthetic disc. This description is intended to illustrate the apparatus and methods described herein, however, and is not intended to be limiting.
• A first step of the procedure is to expose the two adjacent vertebrae to be treated by conventional surgical procedures and to remove the natural disc. Once the natural disc has been removed, thespacer810 is advanced and itshead portion816 is placed between the two adjacent vertebrae in order to separate them. After the vertebrae are adequately separated, thespacer810 is withdrawn.
• The two-sided chisel830 is then advanced and itshead portion836 is placed between the vertebral bodies. Because of the size of thehead portion836 relative to the axial space between the vertebrae, the wedge-shapedblades839 engage the inward-facing surfaces of the vertebrae, creating grooves on those surfaces simultaneously. After the grooves are formed as needed, the two-sided chisel is withdrawn.
• A prosthetic disc is then installed on the distal end of the holder850. Optimally, the arms858a-bof the holder850 engage the side surfaces of the prosthetic disc, and the proximal side of the disc butts up against the distal face of thebody portion857 of the holder850. In this position, the holder is able to retain the prosthetic disc and hold it in place. The prosthetic disc is then advanced by the holder into the disc space between the two vertebrae. Optimally, the anchoring fins on the external surfaces of the prosthetic disc are aligned with the grooves formed in the upper and lower vertebrae as the disc is implanted. Once the disc has been satisfactorily located, the holder850 is withdrawn, leaving the disc in place.
b. Second Embodiment
• An alternative implantation procedure is illustrated inFIGS. 43 through 49. The preferred procedure utilizes the implantation tools described above in relation toFIGS. 39-42. As discussed above, the procedure described herein is in relation to implantation of a two-piece prosthetic disc. This description is intended to illustrate the apparatus and methods described herein, however, and is not intended to be limiting.
• Turning first to FIGS.43A-B, after the two adjacent vertebrae to be treated are exposed by conventional surgical procedures and the natural disc is removed, theguide member500,lower pusher520, andupper pusher rods541a,541bare advanced in the direction of arrow “A” toward the void space between the twoadjacent vertebrae601,602 until the outerlower endplate220bandfirst chisel540 are located between the twoadjacent vertebrae601,602 (seeFIG. 43B).
• At this point in the procedure, the distance “d” between thevertebrae601,602 is insufficient to accommodate the prosthetic disc. Accordingly, as shown in FIGS.44A-B, a force is applied to separate thefirst chisel540 and the outerlower endplate220b, e.g., as represented by arrows “B” in FIGS.44A-B. The separating force is applied by advancing theseparator515 away from the base member in the apparatus shown inFIG. 40 by the method described above. The upward force by thefirst chisel540 causes the wedge-shapedblades544 of the first chisel to embed in the lower surface of theupper vertebra601, creating grooves in that surface. Similarly, the downward force by thelower endplate insert523 and outerlower endplate220bcause theanchor fins221 on the lower surface of the outerlower endplate220bto embed in the upper surface of thelower vertebra602. Thus, by advancing theseparator515 between theupper pusher rods541a,541bandlower pusher rods521a,521b, the user is able to implant the outerlower endplate220bonto thelower vertebra602 and to create a set of grooves in theupper vertebra601 that will accommodate the anchoring fins211 on the upper endplate of the prosthetic disc.
• After the separating forces are applied as described above, the first chisel apparatus is withdrawn, as shown in FIGS.45A-B. More particularly, thelower pusher520 is withdrawn, thereby withdrawing thelower endplate insert523 from the outerlower endplate220b, leaving the outerlower endplate220bimplanted onto thelower vertebra602. Also, theupper pusher rods541a,541bare withdrawn, thereby withdrawing thefirst chisel540, leaving one or more grooves formed on the upper vertebra601 (seeFIG. 45B). Theguide member500 remains in place to facilitate additional procedures described below.
• After the first chisel apparatus is withdrawn, thesecond chisel580 is advanced into the space between the twovertebrae601,602, as shown in FIGS.46A-B. Preferably, the second chisel is advanced (see arrows “A”) into the void space by advancing thepush rod583. Upon entry into the void space, the wedge-shapedblades584 on the top surface of the second chisel engage the grooves formed in the lower surface of theupper vertebra601 by thefirst chisel540. Advantageously, theflange585 on the bottom surface of thesecond chisel580 engages and rides in thegrooves506,507 on theguide member500 as the second chisel is being advanced, thereby guiding thesecond chisel580 into place.
• As noted above, thesecond chisel580 preferably has a thickness that is similar to the height of the upper endplate assembly of the two-piece prosthetic disc. Thus, advancing thesecond chisel580 into the void space between the twoadjacent vertebrae601,602 ensures that the void space is adequately prepared for implanting the remaining portion of the prosthetic disc. In addition, if thesecond chisel580 has a snug fit within the void space, this will further confirm that a prosthetic disc of the appropriate size and shape is being used.
• After thesecond chisel580 has been advanced and engages the lower surface of theupper vertebra601, it is withdrawn, once again leaving behind the outerlower endplate220bimplanted onto thelower vertebra601 and theguide member500 engaged with the outerlower endplate220b. (See FIGS.47A-B).
• Once the pair ofvertebrae601,602 have been adequately prepared for implantation of the remaining portions of the prosthetic disc, theupper subassembly205 of the prosthetic disc is implanted using theupper endplate holder560. (See FIGS.48A-B). Theupper endplate holder560 is advanced in the direction “A” by thepush rod563 until theupper subassembly205 engages the outerlower endplate220band is locked in place by thetab261 and notch (not shown). At this point, the anchoring fins211 on the upper subassembly engage the grooves formed on the lower surface of theupper vertebra601, thereby helping to retain the prosthetic disc in place between the twoadjacent vertebrae601,602. Turning to FIGS.49A-B, theupper endplate holder560 and theguide member500 are then withdrawn, leaving the prosthetic disc in place.FIG. 49A provides additional detail showing the manner by which the engagement pins564,565 of the upper endplate holder engages a set of mating holes206 formed in the trailing edge of theupper subassembly205. Similarly,FIG. 49A shows the manner by which thelower endplate rods508,509 engage the mating holes215 formed on the trailing edge of the outerlower endplate220b.
• In an alternative method particularly adapted for implanting the one-piecestructure prosthetic discs100 described herein, implantation of the prosthetic disc is achieved without using theguide member500, through use of only thesecond chisel580, thespacer tool570, and a modifiedupper endplate holder560′. Thespacer tool570 is used, as described above, to separate the adjacent vertebral bodies to provide space for the prosthetic disc. Thesecond chisel580 is also used in the manner described above to provide grooves on the internal surface of the vertebral bodies to accommodate the fins on the prosthetic disc. The modifiedupper endplate holder560′ has a similar structure to theendplate holder560 shown inFIG. 41, but is provided with an additional set of engagement pins564′,565′ for engaging mating holes provided on thelower endplate120 of the one-piece prosthetic disc100. The modifiedupper endplate holder560′ is used to advance theprosthetic disc100 into place between the adjacent vertebrae, then is withdrawn.
• B. Minimally Invasive Implantation
• A minimally invasive surgical implantation method is illustrated inFIG. 51. The minimally invasive surgical implantation method may be performed using a posterior approach, rather than the anterior approach used for conventional lumbar disc replacement surgery.
• Turning toFIG. 51, the a pair ofcannulas700 are inserted posteriorly to provide access to the spinal column. More particularly, an small incision is made and a pair of access windows are created through thelamina610 of one of the vertebrae on each side of the vertebral canal to access the natural vertebral disc to be replaced. Thespinal cord605 andnerve roots606 are avoided or mobilized to provide access. Once access is obtained, each of thecannulas700 is inserted. Thecannulas700 may be used to remove the natural disc by conventional means. Alternatively, the natural disc may have already been removed by other means prior to insertion of the cannulas.
• Once the natural disc has been removed and thecannulas700 located in place, a pair of prosthetic discs are implanted between adjacent vertebral bodies. In the preferred embodiment, the prosthetic discs have a shape and size adapted for the minimally invasive procedure, such as the elongated one-pieceprosthetic discs100 described above in relation to FIGS.50A-B.A prosthetic disc100 is guided through each of the two cannulas700 (see arrows “C” inFIG. 51) such that each of the prosthetic discs is implanted between the two adjacent vertebral bodies. In the preferred method, the twoprosthetic discs100 are located side by side and spaced slightly apart between the two vertebrae. Optionally, prior to implantation, grooves are created on the internal surfaces of one or both of the vertebral bodies in order to engage anchoring fins located on theprosthetic discs100. The grooves may be created using a chisel tool adapted for use with the minimally invasive procedure.
• Optionally, a third prosthetic disc may be implanted using the methods described above. The third prosthetic disc is preferably implanted at a center point, between the twoprosthetic discs100 shown inFIG. 51. The third disc would be implanted prior to the two discs shown in the Figure. Preferably, the disc would be implanted by way of either one of the cannulas, then rotated by 90.degree. to its final load bearing position between the other two prosthetic discs. The first twoprosthetic discs100 would then be implanted using the method described above.
• An alternative minimally invasive implantation method and apparatus is illustrated schematically in FIGS.52A-B. In this alternative implantation method, asingle cannula700 is used. The cannula is inserted on one side of the vertebral canal in the manner described above. Once the cannula is inserted, a chisel is used to create agroove701 having a 90.degree. bend on the interior surfaces of the two adjacent vertebral bodies. Thus, the terminal portion of thegroove702 is perpendicular to the axis defined by theinsertion cannula700.
• Turning toFIG. 52B, adual prosthetic disc710 structure is shown. Thedual disc710 includes a pair of one-piecestructure prosthetic discs100a-bidentical in structure to those described above in relation to FIGS.50A-B. The twoprosthetic discs100a-bof thedual disc710 are joined by aseparating mechanism711. Theseparating mechanism711 is accessed remotely by the surgeon after thedual disc710 has been implanted into a patient's spinal column, and is adapted to drive the twoprosthetic discs100a-bapart once they are implanted. Theseparating mechanism711 may be a screw device such as a worm screw, a ratcheting mechanism, a spring, or any other mechanism suitable for providing the capability of applying a separating force between the twoprosthetic discs100a-bof thedual disc710. Preferably, an anchoringfin111 is provided on only one of theprosthetic discs100a. Thus, when thedual disc710 is implanted, the anchoringfin111 of thefirst prosthetic disc100awill retain thefirst disc100ain place while theratcheting mechanism711 causes thesecond disc100bto be separated spatially from thefirst disc100a, as shown by the arrow “D”.
• The subject devices and systems may be provided in the form of a kit for performing the methods of the present invention. The kits may include instructions for using the various devices and systems.
• Part C
• I. Information Concerning the Descriptions Contained Herein
• It is to be understood that the inventions that are the subject of this patent application are not limited to the particular embodiments described, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims.
• Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which these inventions belong. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present inventions, the preferred methods and materials are herein described.
• All patents, patent applications, and other publications mentioned herein are hereby incorporated herein by reference in their entireties. The patents, applications, and publications discussed herein are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the present invention is not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided may be different from the actual publication dates which may need to be independently confirmed.
• As will be apparent to those of skill in the art upon reading this disclosure, each of the individual embodiments described and illustrated herein has discrete components and features which may be readily separated from or combined with the features of any of the other several embodiments without departing from the scope or spirit of the present inventions.
• The preceding merely illustrates the principles of the invention. It will be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. Furthermore, all examples and conditional language recited herein are principally intended to aid the reader in understanding the principles of the invention and the concepts contributed by the inventors to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents and equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure. The scope of the present invention, therefore, is not intended to be limited to the exemplary embodiments shown and described herein. Rather, the scope and spirit of present invention is embodied by the appended claims.

Claims (15)

1. A prosthetic intervertebral disc comprising:

a first substantially rigid, physiologically acceptable end plate,

a second substantially rigid, physiologically acceptable end plate,

a compressible core member positioned between said first and second end plates, said core member comprising at least one fiber extending between and engaged with said first and second end plates.

2. The prosthetic intervertebral disc ofclaim 1 wherein said at least one fiber extends through said first end plate and through said second end plate.

3. The prosthetic intervertebral disc ofclaim 2 wherein said at least one fiber extending through said first end plate and through said second end plate is a single fiber.

4. The prosthetic intervertebral disc ofclaim 1 wherein said at least one fiber is oblique to said first end plate and said second end plate.

5. The prosthetic intervertebral disc ofclaim 1 wherein the core member and the at least one fiber extending between and engaged with said first and second end plates are configured to permit relative rotation of said first end plate relative to said second end.

6. The prosthetic intervertebral disc ofclaim 1 wherein the first end plate comprises a material is selected from the group consisting of titanium, titanium alloys, stainless steel, cobalt/chromium, ultrahigh molecular weight polyethylene (UHMW-PE), polyetheretherketone (PEEK), ceramics, and graphite.

7. The prosthetic intervertebral disc ofclaim 1 wherein the second end plate comprises a material is selected from the group consisting of titanium, titanium alloys, stainless steel, cobalt/chromium, ultrahigh molecular weight polyethylene (UHMW-PE), polyetheretherketone (PEEK), ceramics, and graphite.

8. The prosthetic intervertebral disc ofclaim 1 further comprising a plurality of fibers, wherein each of said plurality of fibers extends through said first end plate and through said second end plate.

9. The prosthetic intervertebral disc ofclaim 8 wherein said plurality of fibers define two or more layers of fibers of said core member.

10. The prosthetic intervertebral disc ofclaim 9 wherein two or more different fibers define the two or more layers of fibers of said core member.

11. The prosthetic intervertebral disc ofclaim 8 wherein each of said plurality of fibers defines a layer of fibers of said core member.

12. The prosthetic intervertebral disc ofclaim 8 wherein the fibers of a first layer and the fibers of a second layer exhibit the same tension.

13. The prosthetic intervertebral disc ofclaim 8 wherein the fibers of a first layer and the fibers of a second layer exhibit different tensions.

14. The prosthetic intervertebral disc ofclaim 8 wherein said fibers of a first layer extend at a first angle and said fibers of a second layer extend at a second angle, wherein said first angle is different than said second angle.

15. The prosthetic intervertebral disc ofclaim 1 wherein at least one of said at least one fibers comprises a member selected from the group consisting of elastomers, metals, and polymers and said at least one of said at least one fibers is further is selected from multifilament fibers, monofilament fibers, and encapsulated fibers.

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