US20080097446A1 - Prostheses, Systems and Methods for Replacement of Natural Facet Joints With Artificial Facet Joint Surfaces - Google Patents

Abstract

Cephalad and caudal vertebral facet joint prostheses and methods of use are provided. The prostheses provide an artificial facet joint structure including an artificial articular configuration unlike the preexisting articular configuration. The radii and material stress values of the prostheses are configured to sustain contact stress. The cephalad prosthesis provides for posterior-anterior adjustment. Both prostheses permit lateral adjustment and adjustment to accommodate interpedicle distance. Further, the prostheses may be customized to provide a pre-defined lordotic angle and a pre-defined pedicle entry angle.

Description

REFERENCE TO RELATED APPLICATIONS
• This application is a continuation of Ser. No. 11/197,679, filed Aug. 3, 2005 entitled “Implantable device for facet joint replacement”, which is a continuation of U.S. Non-Provisional application Ser. No. 10/158,563, filed May 30, 2002, entitled “Prostheses Systems and Methods for Replacement of Natural Facet Joints With Artificial Facet Joint Surfaces” (now U.S. Pat. No. 6,974,478), which is a Continuation-in-Part of application Ser. No. 10/067,137, filed Feb. 4, 2002, entitled “Facet Arthroplasty Devices and Methods”, (now U.S. Pat. No. 6,811,567) which is a Continuation-in-part of application Ser. No. 09/963,272, filed on Oct. 20, 2000, entitled, “Facet Arthroplasty Devices and Methods”, (now U.S. Pat. No. 6,610,091) which claims priority to U.S. Provisional Patent Application Ser. No. 60/160,891, filed Oct. 22, 1999, entitled “Facet Arthroplasty Devices and Methods,” which is incorporated herein by reference.
FIELD OF THE INVENTION
• This invention relates to prostheses for treating various types of spinal pathologies, as well as to methods of treating spinal pathologies.
BACKGROUND OF THE INVENTION
• I. Vertebral Anatomy
• AsFIG. 1 shows, the humanspinal column10 is comprised of a series of thirty-three stackedvertebrae12 divided into five regions. The cervical region includes sevenvertebrae12, known as C1-C7. The thoracic region includes twelvevertebrae12, known as T1-T12. The lumbar region contains fivevertebrae12, known as T1-T5. The sacral region is comprised of fivevertebrae12, known as S1-S5. The coccygeal region contains fourvertebrae12, known as Co1-Co4.
• FIG. 2 shows a normalhuman lumbar vertebra12. Although thelumbar vertebrae12 vary somewhat according to location, they share many features common tomost vertebrae12. Eachvertebra12 includes avertebral body14. Two short bones, thepedicles16, extend backward from each side of thevertebral body14 to form avertebral arch18.
• At the posterior end of eachpedicle16 thevertebral arch18 flares out into broad plates of bone known as thelaminae20. Thelaminae20 fuse with each other to form aspinous process22. Thespinuous process22 serves for muscle and ligamentous attachment. A smooth transition from thepedicles16 into thelaminae20 is interrupted by the formation of a series of processes.
• Twotransverse processes24 thrust out laterally on each side from the junction of thepedicle16 with thelamina20. Thetransverse processes24 serve as levers for the attachment of muscles to thevertebrae12. Four articular processes, two superior26 and two inferior28, also rise from the junctions of thepedicles16 and thelaminae20. The superiorarticular processes26 are sharp oval plates of bone rising upward on each side from the union of thepedicle16 with thelamina20. Theinferior processes28 are oval plates of bone that jut downward on each side.
• The superior and inferiorarticular processes26 and28 each have a natural bony structure known as a facet. The superior articular facet30 faces30, while the inferiorarticular facet31 faces downward. AsFIG. 3 shows, whenadjacent vertebrae12 are aligned, thefacets30 and31, capped with a smooth articular cartilage, interlock to form afacet joint32, also known as a zygapopysial joint.
• Thefacet joint32 is composed of a superior half and an inferior half. The superior half is formed by the vertebral level below thejoint32, and the inferior half is formed by the vertebral level above thejoint32. For example, in the L4-L5 facet joint, the superior portion of the joint is formed by bony structure on the L-5 vertebra (e.g., a superior articular surface and supporting bone on the L-5 vertebra), and the inferior portion of the joint is formed by bony structure on the L-4 vertebra (e.g., an inferior articular surface and supporting bone on the L-4 vertebra).
• As also shown inFIG. 3, anintervertebral disc34 between each pair ofvertebrae12 permits gliding movement betweenvertebrae12. Thus, the structure and alignment of thevertebrae12 permit a range of movement of thevertebrae12 relative to each other.
• II. Facet Joint Dysfunction
• Back pain, particularly in the “small of the back”, or lumbosacral (L4-S1) region, is a common ailment. In many cases, the pain severely limits a person's functional ability and quality of life. Such pain can result from a variety of spinal pathologies.
• Through disease or injury, the laminae, spinous process, articular processes, or facets of one or more vertebral bodies can become damaged, such that the vertebrae no longer articulate or properly align with each other. This can result in an undesired anatomy, loss of mobility, and pain or discomfort.
• For example, the vertebral facet joints can be damaged by either traumatic injury or by various disease processes. These disease processes include osteoarthritis, ankylosing spondylolysis, and degenerative spondylolisthesis. The damage to the facet joints often results in pressure on nerves, also called a “pinched” nerve, or nerve compression or impingement. The result is pain, misaligned anatomy, and a corresponding loss of mobility. Pressure on nerves can also occur without facet joint pathology, e.g., a herniated disc.
• One type of conventional treatment of facet joint pathology is spinal stabilization, also known as intervertebral stabilization. Intervertebral stabilization prevents relative motion between the vertebrae. By preventing movement, pain can be reduced. Stabilization can be accomplished by various methods.
• One method of stabilization is spinal fusion. Another method of stabilization is fixation of any number of vertebrae to stabilize and prevent movement of the vertebrae.
• Another type of conventional treatment is decompressive laminectomy. This procedure involves excision of the laminae to relieve compression of nerves.
• These traditional treatments are subject to a variety of limitations and varying success rates. Furthermore, none of the described treatments puts the spine in proper alignment or return the spine to a desired anatomy. In addition, stabilization techniques, by holding the vertebrae in a fixed position, permanently limit a person's mobility.
SUMMARY OF THE INVENTION
• There is a need for prostheses, systems, and methods that overcome the problems and disadvantages associated with current strategies and designs in various treatments for spine pathologies.
• The invention provides prostheses, systems, and methods designed to replace natural facet joints and/or part of the lamina at virtually all spinal levels including L1-L2, L2-L3, L3-L4, L4-L5, L5-S1, T-11-T12, and T12-L1. The prostheses, systems, and methods can restore a desired anatomy to a spine and give back to an individual a desired range of mobility. The prostheses, systems, and methods also can lessen or alleviate spinal pain by relieving the source nerve compression or impingement.
• For the sake of description, the prostheses that embody features of the invention will be called either “cephalad” or “caudal” with relation to the portion of a given natural facet joint they replace. As previously described, a given natural facet joint has a superior half and an inferior half. In anatomical terms, the superior half of the joint is formed by the vertebral level below the joint (which can thus be called the caudal portion of the facet joint, i.e., because it is near the feet). The inferior half of the joint is formed by the vertebral level above the joint (which can thus be called the cephalad portion of the facet joint, i.e., because it is near the head). Thus, a prosthesis that, in use, replaces the caudal portion of a facet joint (i.e., the superior half) will be called a “caudal” prosthesis. Likewise, a prosthesis that, in use, replaces the cephalad portion of a facet joint (i.e., the inferior half) will be called a “cephalad” prosthesis.
• One aspect of the invention provides a facet joint prosthesis to replace, on a vertebral body, a caudal portion of a natural facet joint (e.g., a superior articular surface and supporting bone structure on the vertebral body). The caudal prosthesis comprises a component sized to be fixed to the vertebral body, e.g., on or near a pedicle. The caudal prosthesis includes an artificial facet joint structure adapted to replace a caudal portion of the natural facet joint after its removal from the vertebral body. The removal of a caudal portion of the natural facet joint and its total replacement by the artificial facet joint structure of the caudal prosthesis frees the orientation of the prosthesis from anatomic constraints imposed by a preexisting articular configuration of the caudal portion of the natural facet joint. Furthermore, the artificial facet joint structure of the caudal prosthesis can comprise an artificial articular configuration that is unlike the preexisting articular configuration, so that a desired articulation or bony anatomy can be restored.
• This aspect of the invention also provides a method of replacing, on a vertebral body, a caudal portion of a natural facet joint. The method removes a caudal portion of the natural facet joint from the vertebral body, and, in its place, fixes a component to the vertebral body that includes an artificial facet joint structure adapted to replace the removed caudal portion of the natural facet joint. The artificial facet joint structure can include an artificial articular configuration unlike the preexisting articular configuration of the removed caudal portion of the natural facet joint.
• Another aspect of the invention provides a facet joint prosthesis to replace, on a vertebral body, a cephalad portion of a natural facet joint (e.g., an inferior articular surface and supporting bone structure on the vertebral body). The cephalad prosthesis comprises a component sized to be fixed to the vertebral body, e.g., on or near a pedicle, or on or near a lamina, or on or near a spinous process, or combinations thereof. The cephalad prosthesis includes an artificial facet joint structure adapted to replace a cephalad portion of the natural facet joint after its removal from the vertebral body. As with the removal and total replacement of a caudal portion of the natural facet joint, the removal of a cephalad portion of the natural facet joint and its total replacement by the artificial facet joint structure of the cephalad prosthesis makes possible the orientation of the prosthesis free from anatomic constraints imposed by a preexisting articular configuration of the cephalad portion of the natural facet joint. Furthermore, like the caudal prosthesis, the artificial facet joint structure of the cephalad prosthesis can comprises an artificial articular configuration that is unlike the preexisting articular configuration of the natural facet surface (which is removed), so that a desired articulation or bony anatomy can be totally restored.
• This aspect of the invention also provides a method of replacing, on a vertebral body, a cephalad portion of a natural facet joint. The method removes a cephalad portion of the natural facet joint from the vertebral body, and, in its place, fixes a component to the vertebral body that includes an artificial facet joint structure adapted to replace the removed cephalad portion of the natural facet joint. The artificial facet joint structure can include an artificial articular configuration unlike the preexisting articular configuration of the removed cephalad portion of the natural facet joint.
• Another aspect of the invention provides a prosthesis assembly and related method for replacing a natural facet joint between adjoining first and second vertebral bodies. The assembly and method utilize a first component sized to be fixed to the first vertebral body, which is superior to the second vertebral body. The first component includes a first artificial facet joint structure adapted to replace a cephalad portion of the natural facet joint on the first vertebral body after removal of the cephalad portion of the natural facet joint from the first vertebral body. The assembly and method also comprise a second component sized to be fixed to the second vertebral body. The second component includes a second artificial facet joint structure adapted to replace the caudad portion of the natural facet joint of the second vertebral body after removal of the caudad portion of the natural facet joint from the second vertebral body. Together, the first and second artificial facet joint structures comprise an artificial facet joint. The removal of both cephalad and caudal portions of a natural facet joint and their total replacement by the artificial facet joint structures of the first and second components allows the artificial facet joint to be installed without anatomic constraints imposed by a preexisting articular configuration of the natural facet joint. Furthermore, the artificial facet joint structures of either the first or second components, or both, can comprise create an artificial articular configuration for the artificial facet joint that is unlike the preexisting articular configuration of the removed natural facet joint, so that a desired articulation or bony anatomy can be completely restored.
• Various other aspects of the invention provide caphalad and/or caudal prostheses that readily adapt to or physically change the specific anatomy of an individual. For example, a cephalad prosthesis can be capable of being adjusted in either an anterior or posterior direction relative to a vertebra. As another example, a cephalad prosthesis and/or a caudal prosthesis can provide for lateral (left and right) adjustment, to accommodate or create variances in the distance between the right and left pedicles of a single vertebra. Furthermore, a cephalad prosthesis and/or a caudal prosthesis can provide vertical (up and down) adjustment, to accommodate or create variations in interpedicle distance between adjacent vertebra. Or, as another example, a cephalad prosthesis and a caudal prosthesis can together create a desired lordotic angle between adjacent vertebral bodies, or create a pre-defined pedicle entry angle for mounting each prosthesis on a given vertebral body. And, as yet another example, the configuration of articulating artificial facet joint structures on cooperating caphalad and caudal prostheses can be matched, taking into account the material(s) from which they are made, to minimize contact stress.
• Another aspect of the invention provides an intermediate prosthesis that, together with the cephalad and caudal prostheses, makes possible multiple-level facet joint replacement.
• Other features and advantages of the inventions are set forth in the following Description and Drawings, as well as in the appended Claims.
DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a lateral elevation view of a normal human spinal column;
• FIG. 2 is a superior view of a normal human lumbar vertebra;
• FIG. 3 is a lateral elevation view of a vertebral lumbar facet joint;
• FIG. 4 is a perspective view of a cephalad prosthesis for replacing the inferior half of a natural facet joint;
• FIG. 5 is a perspective view of a caudal prosthesis for replacing the superior half of a natural facet joint;
• FIG. 6 is a perspective view illustrating the cephalad prosthesis shown inFIG. 4 in articulation with the caudal prosthesis shown inFIG. 5;
• FIG. 7 is an exploded perspective view of an alternative embodiment of a caudal prosthesis for replacing the superior half of a natural facet joint, the prosthesis having a multiple-piece construction;
• FIG. 8 is an assembled perspective view of the caudal prosthesis shown inFIG. 7;
• FIG. 9 is a rear view of the caudal prosthesis shown inFIG. 8;
• FIG. 10 is a side elevation view of the caudal prosthesis shown inFIG. 8, illustrating, in phantom lines, anterior-posterior adjustment of the arm components;
• FIG. 11 is a top view of the caudal prosthesis shown inFIG. 8;
• FIG. 12 is a front view of a cephalad prosthesis for replacing the inferior half of a natural facet joint, illustrating horizontally-elongated openings that accommodate lateral adjustment of the prosthesis;
• FIG. 13 is a front view of a caudal prosthesis for replacing the superior half of a natural facet joint, illustrating horizontally-elongated openings that accommodate lateral adjustment of the prosthesis;
• FIG. 14 is a front view of a cephalad prosthesis for replacing the inferior half of a natural facet joint, illustrating vertically-elongated openings that accommodate adjustment of the prosthesis to accommodate varying interpedicle distances;
• FIG. 15 is a front view of caudal prosthesis for replacing the superior half of a natural facet joint, illustrating vertically-elongated openings that accommodate adjustment of the prosthesis to accommodate varying interpedicle distances;
• FIG. 16 is a side view of the cephalad prosthesis shown inFIG. 4 in articulation with the caudal prosthesis shown inFIG. 5, illustrating the orientation of the fixation openings relative to a superior-inferior axis to provide a pre-defined lordotic angle;
• FIG. 17 is a top view of the cephalad prosthesis shown inFIG. 4 in articulation with the caudal prosthesis shown inFIG. 5, illustrating the orientation of the fixation openings relative to a lateral axis to provide a pre-defined pedicle entry angle;
• FIG. 18 is an exploded perspective view of a multiple-level prosthesis system comprising a cephalad prosthesis for replacing the inferior halves of natural facet joints on a first superior vertebral body, a caudal prosthesis for replacing the superior halves of natural facet joints on a second inferior vertebral body, an intermediate prosthesis for replacing both inferior and superior halves of natural facet joints on a third vertebral body between the first and second vertebral bodies;
• FIG. 19 is an assembled perspective view of the multiple-level prosthesis system shown inFIG. 18;
• FIG. 20 is a perspective view of a representative embodiment of a cephalad prosthesis for replacing the inferior half of a natural facet joint;
• FIG. 21 is a front view of the cephalad prosthesis shown inFIG. 20;
• FIG. 22 is a top view of the cephalad prosthesis shown inFIG. 20;
• FIG. 23 is a side view of the cephalad prosthesis shown inFIG. 20;
• FIG. 24 is a rear view of the cephalad prosthesis shown inFIG. 20;
• FIG. 25 is a perspective view of a representative embodiment of a caudal prosthesis for replacing the superior half of a natural facet joint;
• FIG. 26 is a rear view of the caudal prosthesis shown inFIG. 25;
• FIG. 27 is a front view of the caudal prosthesis shown inFIG. 25;
• FIG. 28 is a side view of the caudal prosthesis shown inFIG. 25;
• FIG. 29 is a top view of the caudal prosthesis shown inFIG. 25;
• FIG. 30 is a perspective view of the cephalad prosthesis shown inFIG. 20 in articulation with the caudal prosthesis shown inFIG. 25;
• FIG. 31 is a front view of the articulated prostheses shown inFIG. 30;
• FIG. 32 is a side view of the articulated prostheses shown inFIG. 30;
• FIG. 33 is a rear view of the articulated prostheses shown inFIG. 30;
• FIG. 34 is a top view of the articulated prostheses shown inFIG. 30;
• FIG. 35 is a posterior perspective view of the natural left and right facet joints between two lumbar vertebrae;
• FIG. 36 is a posterior perspective view of the lumbar vertebrae shown inFIG. 35, showing the surgical removal of the natural inferior processes and related bony structure of the superior vertebra and the surgical removal of the natural superior processes and related bony structure of the inferior vertebra;
• FIG. 37 is a posterior perspective view of the lumbar vertebrae shown inFIGS. 35 and 36, after removal of the inferior and superior halves of the natural facet joints, illustrating the fixation of a caudal prosthesis onto the inferior vertebra for replacing the superior halves of the natural facet joints that have been removed; and
• FIG. 38 is a posterior perspective view of the lumbar vertebrae shown inFIG. 37, after removal of the inferior and superior halves of the natural facet joints and the fixation of the caudal prosthesis onto the inferior vertebra, illustrating the fixation of a cephalad prosthesis on the superior vertebra for replacing the inferior halves of the natural facet joints that have been removed, the caudal and cephalad prostheses articulating to provide an artificial articulation that can be unlike the natural articulation of the removed natural facet joints.
• The invention may be embodied in several forms without departing from its spirit or essential characteristics. The scope of the invention is defined in the appended claims, rather than in the specific description preceding them. All embodiments that fall within the meaning and range of equivalency of the claims are therefore intended to be embraced by the claims.
DETAILED DESCRIPTION
• Although the disclosure hereof is detailed and exact to enable those skilled in the art to practice the invention, the physical embodiments herein disclosed merely exemplify the invention that may be embodied in other specific structure. While the preferred embodiment has been described, the details may be changed without departing from the invention, which is defined by the claims.
I. VERTEBRAL PROSTHESES
• FIGS.4 to6 illustrate various prostheses for replacing inferior and/or superior portions of natural facet joints. The prostheses are desirably fixed to vertebral bodies following the surgical removal of the respective natural facet joint portions from the vertebral bodies.
• FIG. 4 shows acephalad prosthesis36 for replacement of the natural inferior half of a facet joint following removal of the natural inferior half of the facet joint.FIG. 5 shows acaudal prosthesis38 for replacement of the natural superior half of a facet joint following removal of the natural superior half of the facet joint.
• Eitherprosthesis36 or38 can be used by itself. Theprostheses36 and38 each enables bilateral facet joint replacement (both left and right sides of a given vertebral body), or unilateral facet joint replacement (one side of a given vertebral body).
• As shown inFIG. 6, the prostheses are desirably used in articulated association between a given pair of vertebral bodies. AsFIG. 6 shows, the caudal andcephalad prostheses36 and38 form an articulated system that permits total (superior and inferior) facet joint replacement of one or more natural facet joints32. The system can provide a succession of entirely artificial facet joint structures between two vertebral bodies or along a length of thespinal column10.
• As shown inFIG. 6, the caudal andcephalad prostheses36 and38 cooperate in combination to provide an artificial articular configuration. Since the inferior and superior halves of the natural facet joint are removed, the artificial articular configuration need not be constrained by, and can be unlike, the preexisting articulation of the natural facet joint prior to the removal of the inferior and superior halves of the natural facet joint.
• A. The Cephalad Prosthesis
• Theprosthesis36 shown inFIG. 4 is designated “cephalad” because it provides one or more artificial facetjoint structures40 for the inferior half of a natural facet joint32. Theprosthesis36 allows for the removal of injured, diseased and/or deteriorating natural inferiorarticular surfaces28 and supporting bony structure on thevertebra12 above the facet joint32. Theartificial structures40 serve to replace the naturalinferior processes28 and supporting bone of the vertebral body, which have been desirably removed prior to mounting theprosthesis36 on the vertebral body, as will be described in greater detail later.
• The artificial facetjoint structures40 articulate with the superior half of the facet joint32. The superior half can comprise the natural superior portions of the facet joint32 (i.e., the natural superiorarticular surfaces26 and supporting bony structure on the vertebral body below the facet joint32). Desirably, however, the superior half comprises an artificial facetjoint structure54 formed by a caudaljoint replacement prosthesis38, shown, e.g., inFIG. 6.
• Thecephalad prosthesis36 is sized to extend across thelaminae20 of a vertebral body. In the illustrated embodiment, the caphalad prosthesis comprises achimney42 and left andright arm components44.
• Thechimney42 is configured to receive thespinous process22 of the vertebral body. In this manner, thechimney42 serves to support and stabilize theprosthesis36. Thechimney42 desirably includes a lamina hook46 (best shown inFIG. 7) that rests under thelaminae20 of the vertebral body to further support theprosthesis36. If desired, thespinous process22 may be fixed within thechimney42 with a trans-spinous process screw48. In some instances, when a significant portion of thelaminae20 is removed, it may be desirable to omit thechimney42 entirely.
• Thechimney42 carries right and leftarms44 in association with thechimney42. Eacharm44 carries an artificial facetjoint structure40.
• As seen inFIGS. 4 and 6, thearms44 comprise an inferior-to-superior diverging geometry, with a greater lateral width at the superior end than the inferior end, facilitate mounting to the pedicles of a vertebral body. However, other configurations can be used.
• Eacharm44 additionally carries at least oneopening50 configured to receive afixation element52 for fixing theprosthesis36 to the vertebral body. It should be understood that the number and location ofopenings50 andfixation elements52 could vary.
• InFIG. 4, two openings40 (right and left) serve to receive two fixation elements52 (right and left). In the illustrated embodiment, thefixation elements52 take the form of pedicle screws or nails. The right and leftfixation elements52 are adapted to extend into the right and leftpedicles16 respectively of the vertebral body and serve to anchor theprosthesis36 in place.
• As shown inFIG. 4, thecephalad prosthesis36 can be of unitary construction, in which thechimney42 andarms44 are a single piece. Alternatively, as shown inFIG. 7, theprosthesis36 can be of multiple-piece construction, in which thearms44 are configured to be selectively detached from thechimney42, as will be discussed in greater detail later with respect toFIG. 7.
• Thecephalad prosthesis36 may be formed of a material commonly used in the prosthetic arts including, but not limited to, polyethylene, rubber, titanium, chrome cobalt, surgical steel, bony in-growth sintering, sintered glass, artificial bone, ceramics, or a combination thereof.
• B. The Caudal Prosthesis
• Theprosthesis36 shown inFIG. 5 is designated “caudal” because it creates one or more artificial facetjoint structures54 for the superior half of a natural facet joint. Thecaudal prosthesis38 allows for the removal of injured, diseased and/or deteriorating natural superiorarticular surfaces32 and supporting bony structure on the vertebral body below the facet joint32. Theartificial structures54 serve to replace the naturalsuperior processes26 and supporting bone of the vertebral body, which have been desirably removed prior to mounting theprosthesis38 on the vertebral body. This aspect will be described in greater detail later.
• In use, the artificial facetjoint structure54 articulates with the inferior half of the facet joint32. The inferior half can comprise the natural inferior portions of the facet joint32 (i.e., the natural inferior articular surfaces and supporting bony structure on the vertebral body above the facet joint32). Desirably, however, the inferior half comprises an artificial facetjoint structure40 formed by a cephaladjoint replacement prosthesis36, asFIG. 6 shows.
• In the illustrated embodiment, thecaudal prosthesis38 is a bar-like member sized to extend across thelaminae20 of the vertebral body. WhileFIG. 4 illustrates a unitary construction, theprosthesis38 can be constructed multiple-parts that are joined together for use.
• Like thecephalad prosthesis36, thecaudal prosthesis38 carries at least oneopening56 configured to receive afixation element58 for fixing theprosthesis38 to the vertebral body and at least one artificial facetjoint structure element54. It is to be understood that the number and location of openings and fixation elements can vary.
• InFIG. 5, two openings56 (right and left) serve to receive two fixation elements58 (right and left). In the illustrated embodiment, thefixation elements58 take the form of pedicle screws or nails. The right and leftfixation elements58 are adapted to extend into the right and left pedicles respectively of the vertebral body and serve to anchor theprosthesis38 in place.
• Thecaudal prosthesis38 may be formed of a material commonly used in the prosthetic arts including, but not limited to, polyethylene, rubber, titanium, chrome cobalt, surgical steel, bony in-growth sintering, sintered glass, artificial bone, ceramics, or a combination thereof.
II. ADDITIONAL FEATURES OF THE PROSTHESES
• Either or both of the cephalad andcaudal prostheses36 and38 can incorporate a variety of additional features, which adapt theprosthesis36 or38 to the specific anatomy encountered or desired. These adaptive features further enhance the restoration of a desired anatomy and/or the alleviated of pain, as will be described in greater detail later. As will become apparent to one skilled in the art, any of the following features can be used alone or in combination with any other feature or features, to “customize” aprosthesis36 or38 to a given vertebral location and a specific individual.
• A. Posterior-Anterior Adjustment
• As shown in FIGS.7 to11, thecephalad prosthesis36 can comprise a multiple-piece construction. The multiple-piece construction permits posterior and anterior (i.e., “front and back”) adjustment of theprosthesis36 relative to the vertebral body. Either symmetric or asymmetric posterior-anterior mounting arrangements are thereby enabled.
• In the embodiment shown in FIGS.7 to11, theprosthesis36 is a three-piece assembly comprising a center member42 (which can comprise the chimney already discussed) and right andleft arm components44. Eacharm44 is a separate piece that is selectively detachable from thecenter member42. Thearms44 can be coupled to thecenter member42 by a variety of means, including, but not limited to, a slotted joint between thechimney42 and therespective arm44, a screw attachment, a hook attachment, or a snap-fit engagement.
• A slotted joint or the like allows for relative sliding movement between the respective arm and center member. As best seen inFIG. 10, this arrangement permits independent posterior and anterior adjustment (represented by phantom lines) of the right and leftarms44. The adjustment allows theprosthesis36 to accommodate the asymmetric anterior-posterior anatomy of a particular vertebral body.
• In an alternative construction (not shown), thecenter member42 can comprise one piece and thearms44 are integrally attached to form a second, discrete piece that is selectively attachable and detachable from themember42. This arrangement would similarly permit posterior and anterior adjustment of thearms44, but would not permit independent adjustment of the right and leftarms44.
• The orientation of theprosthesis36 is thus not dictated by the natural posterior-anterior anatomy encountered. Instead, theprostheses36 can be fixed in position between two vertebral bodies in an orientation that corresponds to existing natural anatomy or that establishes a desired posterior-anterior anatomy unlike the natural anatomy.
• B. Lateral Adjustment
• As seen inFIGS. 12 and 13, any or all of theopenings50 and56 of thecephalad prosthesis36 andcaudal prosthesis38 can be horizontally-elongated, i.e., oriented transverse the superior-inferior axis of theprosthesis36 or38. The horizontal configuration permits lateral adjustment (i.e., “right to left”) of theprosthesis36 or38 relative to a vertebral body (see alsoFIGS. 1-3). Thus, this arrangement allows for variance in distance between the right and leftpedicles16 of asingle vertebra12.
• The horizontal configuration allows thefixation elements52 and58 on opposite lateral sides of theprostheses36 and38 to be placed anywhere between an “A” position (illustrated by solid lines inFIGS. 12 and 13) and a “B” position (illustrated by phantom lines inFIGS. 12 and 13).
• The lateral orientation of the artificial facet joint structures of theprostheses36 or38 is thus not dictated by the natural lateral anatomy (i.e., intrapedicular distance) encountered. Instead, the position of the artificial facet joint structures of theprostheses36 and38 can be changed relative to the position of the pedicles, either medial to or lateral to the pedicles (establishing a desired lateral anatomy unlike the natural anatomy), or in an orientation that corresponds to an existing natural anatomy.
• C. Adjustment of Interpedicle Distance
• Referring now toFIGS. 14 and 15, any or all of theopenings50 and56 of either or both of the cephalad andcaudal prostheses36 and38 can be vertically-elongated along the superior-inferior axis of theprosthesis36 or38. The vertical arrangement permits superior and inferior (i.e., “up and down”) adjustment of theprosthesis36 or38 relative to a vertebral body (see alsoFIGS. 1-3).
• The vertical configuration allows thefixation elements52 and58 to be placed anywhere from an “A” position (illustrated by solid lines inFIGS. 14 and 15) and a “B” position (illustrated by phantom lines inFIGS. 14 and 15). This arrangement permits the distances between thepedicles16 of adjacent vertebral bodies to be varied. It therefore accommodatesasymmetric pedicle16 orientation (i.e., lateral alignment of the pedicle16) while maintaining vertical alignment of theprosthesis36 or38.
• The orientation of theprostheses36 and38 is not dictated by the natural interpedicular distances encountered. Instead, theprostheses36 and38 can be fixed in position between two vertebral bodies in an orientation that corresponds with the existing natural anatomy or that establishes a desired interpedicular distance unlike the natural preexisting interpedicle distance. Theprostheses36 and38 thereby serve to create a desired interpedicular distance for the vertebral bodies consistent with a desired anatomy.
• D. Lordotic Angle Adjustment
• As shown inFIG. 1, the cervical and lumbar regions of thespinal column10 normally have an anteriorly convex curvature, known as lordosis. The curvature defines a lordotic angle between adjacent vertebral bodies, which is defined by the superior/inferior orientation of the end plates of adjacent vertebral bodies. The lardotic angle varies between adjacent vertebral bodies along the spine. A desired anatomy results by maintaining desired lordotic angles along the spine, which assures desired ligament distention and posture.
• In the illustrated embodiment (seeFIG. 16), theopenings50 and56 of adjacent cephalad andcaudal prostheses36 and38 are mutually oriented in non-parallel planes along the inferior-superior axis. The non-parallel orientation of the planes defines between thefixation elements52 and58, when supported by theopenings50 and56, an angle that results a desired lordotic angle. The mutual orientation and the resulting angle defined depends upon the intended location of theprostheses36 and38 along thespinal column10.
• The defined angle is designated angle “L” inFIG. 16. InFIG. 16, the angle L is defined by orienting the plane of theopening50 of thecephalad prosthesis36 generally parallel to the inferior-superior axis, while tilting the plane of theopening56 of thecaudal prosthesis38 generally downward at an acute inferior angle relative to the inferior-superior axis. The resulting defined angle L between thefixation elements52 and58 inFIG. 16 (about 15.degree.) achieves a desired lordotic angle for the L4-L5 level.
• The orientation of theprostheses36 and38 is not dictated by preexisting natural lordotic angle between two vertebral bodies. Instead, theprostheses36 and38 can be fixed in position between two vertebral bodies in an orientation that establishes a desired lordotic angle unlike the natural preexisting angle. Theprostheses36 and38 thereby serve to create a desired lordotic angle for the vertebral bodies consistent with a desired anatomy.
• Changes in the thickness of and/or orientation of the artificial facet joint structures on either or bothprostheses36 and/or38 can also affect a desired lardotic angle between adjacent vertebral bodies.
• E. Adjustment of Pedicle Entry Angle
• As shown inFIG. 2, thepedicles16 extend from a vertebral body at an angle. With reference toFIG. 17, to fix thefixation elements50 and58 securely within thepedicle16, it is desirable for thefixation elements52 and58 to enter thepedicle16 at an angle approximating the natural angle of thepedicle16, e.g., about 15.degree. at the L4 and L5 level.
• To achieve the desired angle, theopenings50 and56 of adjacent cephalad andcaudal prostheses36 and38 are mutually tilted inwardly to define between a lateral axis and thefixation elements52 and58, when supported by theopenings50 and56, an angle that approximates a desired pedicle entry angle P1/P2. In the illustrated embodiment (FIG. 17), theright fixation element52 or58 extends at a first angle (designated “P1” inFIG. 17) and theleft fixation element52 or58 extends at a second angle (designated “P2” inFIG. 17). In the illustrated embodiment, P1 is generally the same as P2. However, the angles P1 and P2 can differ.
• While the illustrated embodiment shows cephalad andcaudal prostheses36 and38 having the same pedicle entry angles (i.e., P1 and P2 are the same for the cephalad andcaudal prostheses36 and38), it is to be understood that the cephalad andcaudal prostheses36 and38 can be formed to have different pedicle entry angles.
• F. Multiple-Level Replacement
• As shown inFIGS. 18 and 19, the cephalad andcaudal prostheses36 and38 can be coupled by anintermediate prosthesis60, permitting multiple-level facet joint replacement. Thecephalad prosthesis36 is placed on a most superior vertebral body, e.g., L3. Theintermediate prosthesis60 is placed on the next adjacent inferior vertebral body, e.g., L4. Thecaudal prosthesis38 is placed on the next adjacent inferior vertebral body, e.g., L5.
• Theintermediate prosthesis60 is similar to thecephalad prosthesis36 previously described, having achimney42 and two openings (right and left)50 that receivefixation elements52. Right and leftarms44 provide a first and second pairs of artificial facetjoint structures64 and66. Thefirst pair64 is configured to replace thesuperior processes26 and related bony structure of the middle vertebral body, and to articulate with the artificial facetjoint surfaces40 of thecephalad prosthesis36. Thesecond pair66 is configured to replace theinferior processes28 and related bony structure of the middle vertebral body, and to articulate with the artificial facetjoint surfaces54 of thecaudal prosthesis38.
• G. Ability to Sustain Contact Stress
• In theprostheses36 and38, each artificial facetjoint structure40/54 creates a bearing surface having a configuration that facilitates articulation with the bearing surface of another artificial facet joint structure. The particular geometry for the bearing surface configuration for a given artificial facetjoint structure40/54 can vary. It can, for example, be concave, convex, or flat. It may also include a hybrid of curved and flat bearing surface designs, i.e., Miniscal, hinge, etc.
• The radii of two articulating bearing surface configurations are desirably selected and matched, taking into account the material from which the surfaces are formed, to minimize contact stress during articulation.
• For example, in the embodiment illustrated inFIG. 16, thecephalad prosthesis36 includesartificial facet structures40 employing generallyconcave surfaces68, forming socket-like artificial facet joint structures. In this arrangement, thecaudal prosthesis38 includesartificial facet structures54 employing generally complementaryconvex surfaces70, forming hemisphere-like artificial facet joint structures that articulate with the socket-like artificial facet joint structures. It should be appreciated that the articulating surfaces can be reversed, with theartificial facet structures40 of thecephalad prosthesis36 employing generally hemispherical-like surfaces, and theartificial facet structures54 of thecaudal prosthesis38 employing generally socket-like surfaces.
• Alternatively, a Miniscal bearing design could be employed, utilizing a conformal curved surface as one artificial facet joint structure, with the bearing side of the opposed artificial facet joint structure having an essentially flat surface. A hemiarthroplasty design could also alternatively be employed, in which one surface of the opposing surfaces does not incorporate the use of an artificial facet joint structure.
• In another arrangement, one surface of an artificial facet joint structure can have bearing articulation on both sides of the component and have opposing articulation with a receiving artificial facet joint structure with having opposing mating bearing surfaces.
• A variety of materials are suitable for the artificial facet joint structures. Ceramic or ceramic in opposition with a chrome alloy can be used. Suitable stainless steel, including 3161, or titanium alloys, with or without the use of surface hardening and overlay, or hard surface coatings, including zirconia and alumina, can also be employed. The metal surfaces can be made from cast, wrought, hot-forged, or powder-metal consolidated sintered materials. Any of these metals or combination of metals and ceramics can be used in articulation with each other: Biocompatible polymers, e.g., polyethylene, can also be used in articulation with the metals, ceramic, and surface-hardened metals just described. Ultra High Molecular Weight Polyethylene can further be gamma-irradiated, as-molded or as-machined.
• The radii of articulating artificial facet joint structures are desirably closely matched to provide contact stress values less than a given threshold value. The desired contact stress value changes with the material employed.
• For example, the contact stress value for metal-to-metal bearing combinations is desirably less than about 25,000 psi, and preferably less than 12,000 psi. For polymer surfaces bearing against a metal, ceramic, or surface-hardened metal counter bearing surface, the contact stress value is desirably less than 10,000 psi, and preferably less than 5,000 psi.
• For a given material to achieve a desired contact stress value less than the threshold value, the appropriate radii must be chosen. Thus, the radii chosen will change as material changes.
III. REPRESENTATIVE EMBODIMENTS
• A. Cooperating Caphalad and Caudal Prostheses
• FIGS.20 to24 show a representative embodiment of acephalad prosthesis36 that embody features previously described.
• Theprosthesis36 comprises right andleft arm components44 joined to achimney42 in a single-piece, unitary construction. Eacharm44 includes an artificial facet joint structure40 (right and left). As best seen inFIG. 23, each artificial facetjoint structure40 comprises aconcave surface70, forming a socket-like bearing surface.
• In the illustrated embodiment (seeFIG. 24), thefixation openings50 are vertically-elongated, thereby permitting adjustment of theprosthesis36 to create a desired interpedicle distance. As best seen inFIG. 22, both the right and leftopenings50 are also oriented inward, to provide a pre-defined pedicle entry angle for thefixation elements52.
• FIGS.25 to29 show a representative embodiment of acaudal prosthesis38 that embody features previously described and that is intended to be used in articulation with thecephalad prosthesis36 shown in FIGS.22 to24. FIGS.31 to34 show thecaudal prosthesis38 in articulation with thecephalad prosthesis36.
• Theprosthesis38 includes a pair of artificial facet joint structures54 (right and left). Each artificialfacet structure element54 includes aconvex surface68, forming hemaspherical-like bearing surface. Thesurfaces68 are intended, in use, to articulate with the socket-shaped bearing surfaces40 on the cephalad prosthesis36 (seeFIG. 32).
• In the illustrated embodiment, theopenings56 are vertically-elongated, thereby permitting adjustment of theprosthesis38 to create a desired interpedicle distance. Thevertical openings50 and56 on theprostheses36 and38 permit eachprosthesis36 and38 to be independently adjusted to create a desired interpedicle distance.
• AsFIG. 28 best shows, theopenings56 are also oriented inward and downward. The inward orientation establishes a pre-defined pedicle entry angle for thefixation elements58. The downward orientation of thefixation element58 for thecaudal prosthesis38, in combination with the different, non-parallel orientation of thefixation element52 for thecephalad prosthesis36, establishes a desired lordotic angle.
• B. Total Facet Replacement Using the Cephalad and Caudal Prostheses
• With reference now principally to FIGS.35 to38, both the superior and inferior portions of the natural facet joint32 are removed and replaced by thecephalad prosthesis36 and thecaudal prosthesis38. More particularly, theinferior lamina20 and the inferior portion of the natural facet joint32 (e.g., the articulatedinferior processes28 and its supporting bone of thevertebral body14 above the facet joint) is removed. The lamina may additionally be cut for a wide decompressive laminectomy along a decompressive superior-to-inferior resection line on one or both sides of the vertebral body. The removed natural anatomy is replaced with thecephalad prosthesis36. The superior portion of the natural facet joint32 (e.g., the articulatedsuperior process26 and its supporting bone of the targeted vertebral body14) is also removed. Desirably, the mamillary process, the accessory process, a portion of the transverse process, and a portion of the pedicle is removed by being rongeured or reamed. The removed natural anatomy is replaced with thecaudal prosthesis38.
• In one embodiment, a surgical procedure exposes thespinous process22,lamina20, andfacet joints32 at a desired level of thespine10 using any method common to those of skill in the medical arts.FIG. 35 shows the exposedspinous process22,lamina20, and facet joint32 of the L4-L5 joint.
• The inferior portion of the facet joint32 is cut at or near a selected resection line. Most of thelamina20 is desirably preserved, as is the facet joint capsule, which may be opened and folded back. The facet joint capsule may be cut perpendicular to its direction. The natural inferior portion of the facet joint32 may then be retracted from the superior portion. Once the inferior and superior portions of the facet joint are separated, the cut inferior bone, e.g., the inferiorarticular process28 and its supporting bone, of the upper joint (e.g., the cut inferior portion of the L4 vertebra in the L4-L5 joint) may be removed, as depicted by phantom lines inFIG. 36. Alternatively, it may be possible to remove the cut inferior bone while simultaneously separating the facet joint32.
• Prominent bone of the superior portion of the natural facet joint, e.g., the superiorarticular process26 and its supporting bone, may be also removed, as depicted by phantom lines inFIG. 36, using any means common in the field. The superior portion of the natural facet joint32 may also be trimmed to decompress the adjacent nerve root. A reamer or any other instrument that is useful for grinding or scraping bone, may be used to ream the superior portion of the facet joint32 into thepedicle16, to reach the geometry shown inFIG. 36, which is suitable for receiving thecaudal prosthesis38.
• With reference toFIG. 37, thecaudal prosthesis38 as described above can be directly screwed or tapped into thevertebral body14 using pedicle screws orother fixation elements58.
• With reference now toFIG. 38, thecephalad prosthesis36, as described above, can be installed over thespinous process22 and over thelamina20, either before or after placement of thecaudal prosthesis38. Thecephalad prosthesis36 can be directly screwed or tapped using pedicle screws orother fixation elements52 to thelamina20 and to eachpedicle16. Thecephalad prosthesis36 can also be further attached to thespinous process22 with a trans-spinous-process screw48 to provide additional stability, as also previously described.
• As shown inFIG. 38, articulation between the artificial facetjoint structures40/54 of the cephalad prosthesis and thecaudal prosthesis36 and38 is established, to provide an artificial articular configuration in place of the preexisting natural articular configuration.
• Because both the superior and inferior portions of the natural facet joint32 and surrounding bone structures have been removed, the artificial facetjoint structures40/54 of thecephalad prosthesis36 and thecaudal prosthesis38 can be installed in desired positions and orientations, free of anatomic constraints imposed by the preexisting articular configuration of either the inferior or superior portions of the natural facet joint32. Furthermore, the artificial facetjoint structures40/54 of thecephalad prosthesis36 andcaudal prosthesis38 can create an artificial articular configuration that is unlike the pre-existing natural articular configuration in terms of, e.g., interpedicle distance, lardotic angle, and contact stress, so that a desired articulation or bony anatomy can be totally restored. At the same time a desired bone anatomy is restored, decompression of the adjacent nerve root can be maintained to eliminate pain.
• Further details of surgical procedures suitable for installing the prostheses are described in co-pending U.S. patent application Ser. No. 09/693,272, filed Oct. 20, 2000, and entitled “Facet Arthroplasty Devices and Methods,” which is incorporated herein by reference.
• The above described embodiments of this invention are merely descriptive of its principles and are not to be limited. The scope of this invention instead shall be determined from the scope of the following claims, including their equivalents.

Claims (17)

1. A multi-level spinal stabilization system, comprising: at least two X-shaped members, each X-shaped member having a central member with a superior pair of arms extending from opposed lateral sides thereof and an inferior pair of arms extending from the opposed lateral sides thereof, the superior pair of arms of a first X-shaped member being coupled to an inferior pair of arms of a second X-shaped member.

2. The multi-level spinal stabilization system ofclaim 1, wherein the at least two X-shaped members are of a unitary construction.

3. The multi-level spinal stabilization system ofclaim 2, wherein the at least two X-shaped members are formed from an elastomeric material having multiple durometers.

4. The multi-level spinal stabilization system ofclaim 2, wherein at least a portion of the at least two X-shaped members are formed from a polymeric material.

5. The multi-level spinal stabilization system ofclaim 1, further comprising first and second connectors adapted to couple the superior pair of arms of the first X-shaped member to the inferior pair of arms of the second X-shaped member.

6. The multi-level spinal stabilization system ofclaim 5, wherein the first and second connectors each comprise first and second plates pivotally coupled to one another, each plate being adapted to engage a terminal end of one arm of the superior and inferior pair of arms.

7. A multi-level spinal stabilization system, comprising: at least two X-shaped portions having a unitary configuration, each X-shaped portion having a central spacer adapted to be positioned between posterior elements of adjacent vertebrae, and first and second pairs of arms adapted to couple to adjacent vertebrae.

8. The device ofclaim 7, wherein the X-shaped portions are formed form an elastomeric material have multiple durometers.

9. The device ofclaim 8, wherein the at least two X-shaped portions, when coupled to adjacent vertebrae, are adapted to provide resistance to movement of the adjacent vertebrae.

10. A method for stabilizing multiple adjacent vertebrae; positioning a central spacer of a first X-shaped member between posterior elements of a first vertebra and an adjacent second vertebra;

coupling opposed superior arms of the first X-shaped member to the first vertebra; coupling opposed inferior arms of the first X-shaped member to the second vertebra; positioning a central spacer of a second X-shaped member between posterior elements of the second vertebra and an adjacent third vertebra, the second X-shaped member having opposed superior arms that are coupled to the opposed inferior arms of the first X-shaped member; and coupling opposed inferior arms of the second X-shaped member to the third vertebra.

11. The method ofclaim 10, wherein the opposed superior arms of the second X-shaped member are coupled to the opposed inferior arms of the first X-shaped member by a connector.

12. The method ofclaim 10, wherein the first and second X-shaped members are of a unitary construction.

13. The method ofclaim 10, wherein the first and second X-shaped members are formed from an elastomeric material having multiple durometers.

14. A method for stabilizing multiple adjacent vertebrae, comprising: coupling an implant to a posterior portion of at least three adjacent vertebrae, the implant having a unitary configuration and being formed from an elastomeric material with multiple durometers, and the implant being adapted to provide resistance to movement of the at least three adjacent vertebrae.

15. The method ofclaim 14, wherein the implant includes at least two central spacers, each central spacer being disposed between posterior elements of two adjacent vertebrae and adapted to limit extension of the adjacent vertebrae.

16. The method ofclaim 14, wherein each central spacer includes a plurality of arms extending therefrom, and wherein the central spacers are connected to one another by the arms.

17. The method ofclaim 14, wherein the implant comprises a plurality of X-shaped members coupled to one another.

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