US20050165487A1 - Artificial intervertebral disc - Google Patents

Abstract

An artificial intervertebral disc for at least partially replacing a diseased or damaged intervertebral disc. The artificial disc includes a concave-convex articulating surface. The artificial disc can be used in the cervical region of the spine, where a concave-convex articulating surface is advantageous for improved anatomical fit and region appropriate kinematics. The artificial disc of the present invention also includes an anchor for attachment to bone.

Description

FIELD OF THE INVENTION
• The present invention generally relates to an apparatus for the treatment of spinal disorders and, in particular, an artificial intervertebral disc, an implant that replaces a diseased or damaged intervertebral disc.
BACKGROUND OF THE INVENTION
• There are many painful disorders of the spine, many relating, at least in part, to diseased or damaged intervertebral discs. Disorders include Degenerative Disc Disease, generally an age related disorder where the intervertebral disc gradually loses its water content, resiliency, and height. With a loss in intervertebral disc height and associated loss of normal spacing between vertebrae, motion of the vertebrae can place pressure on the spinal cord or exiting nerve roots. The intervertebral disc itself can also be a source of pain. Spinal disorders, commonly referred to as disc herniation and bulging disc, place painful pressure on the spinal cord and exiting nerve roots. Abnormal bone growth, called osteophytes, can place pressure on nerves or the spinal cord. Often, a surgeon must at least partially remove an intervertebral disc to access and remove an osteophyte.
• A surgical approach to treating chronic spinal disorders relates to bony fusion of two adjacent vertebrae in a treatment called spine fusion. Following the achievement of appropriate spacing and alignment of the vertebral bodies, bone graft material and stabilization provide an environment for spine fusion. Implant systems, to include plate and rod systems and interbody devices, such as, interbody spacers and fusion cages can be used to support the spine during fusion. Concerns persist regarding spinal fusion treatment stemming from modest clinical success rates and the creation of rigid regions along an otherwise flexible spine.
• Artificial intervertebral discs, or simply artificial discs, are an alternative to spinal fusion and represent an emerging technology. These spinal implants are designed to restore or maintain the appropriate alignment and spacing of adjacent vertebral bodies. In addition, an artificial disc is also designed for kinematic behavior similar to a healthy natural disc. Known artificial disc concepts use numerous means for providing motion and stiffness similar to a natural healthy disc, to include the adaptation of elastomers, mechanical springs, and articulating surfaces.
• Prior art artificial discs often use articulating surfaces to create a joint between adjacent vertebrae. Disc implants using articulating surfaces rely on methodology and proven technology used in total joint arthroplasty of the hip, knee, and shoulder. Numerous prior art artificial discs resemble artificial hip and artificial knee joints. Numerous known artificial disc devices resemble variations of a ball-and-socket. Kuntz, in U.S. Pat. No. 4,349,921 (Sep. 21, 1982) discloses an artificial disc, with two components that articulate by means of a projection on one component pivotally engaging a depression on the second component. An artificial disc resembling an artificial knee joint has also been suggested. Shelokov, in U.S. Pat. No. 6,039,763 (Mar. 21, 2000) discloses an artificial spinal disc, similar in configuration to an artificial knee joint.
• Heggeness et. al., in U.S. Pat. No. 5,514,180 (May 1996) categorizes the shape or contours of vertebral endplates into five groups: “ramp”, “saddle”, “irregular”, “bowl”, and “hump”. Heggeness et. al., teaches the importance of endplate shape relating to fit and load distribution of a prosthetic devices within intervertebral disc spaces, but Heggeness et. al. does not discuss endplate shape relating to articulating surfaces or spinal kinematics.
• Spine kinematics and anatomical shapes vary by region of the spine (cervical, thoracic, and lumbar), and a need exists for artificial discs addressing specific regions of the spine, especially the unique geometry and kinematics of the cervical spine.
SUMMARY OF THE INVENTION
• For the middle and lower regions of the cervical spine, the artificial disc of the present invention adapts an articulating surface with a concave-convex shape, also called a saddle shape. The artificial disc of the present invention is intended to fit substantially within the intervertebral space bound by adjacent vertebral bodies. A bone anchor for fixation of an artificial disc to vertebra is also disclosed.
• A first embodiment of the artificial disc of the present invention for the cervical spine includes a disc body having an articulating concave-convex surface secured to a base plate, which may incorporate a bone anchor. A second embodiment of the artificial disc of the present invention is comprised of an articulating concave-convex surface, bone anchor, and a disc body slidably attached to a base plate to permit additional motion. A third embodiment of the artificial disc of the present invention includes an upper disc body and a lower disc body cooperatively forming a saddle-joint. The upper disc body and lower disc body of the third embodiment are securely anchored to vertebral bodies using bone anchors. Further embodiments of the artificial disc of the present invention include an upper disc body and a lower disc body cooperatively forming a saddle-joint, and bone anchors adapted with a tension element to provide additional stability.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 shows a frontal view of a cervical spine motion segment comprised of two vertebrae and an intervertebral disc.
• FIG. 2A shows a cervical vertebra in a perspective view of the superior endplate.
• FIG. 2B shows an alternate perspective view of the cervical vertebra shown inFIG. 1A to emphasize the inferior endplate.
• FIG. 3 shows a first preferred embodiment artificial disc of the present invention for the cervical spine with an articulating concave-convex surface.
• FIG. 4A,FIG. 4B, andFIG. 4C show the artificial disc depicted inFIG. 3 in orthogonal views to include a front view, side view, and top view, respectively.
• FIG. 5 shows the artificial disc depicted inFIG. 3 in an exploded view to include adjacent vertebrae of the cervical spine.
• FIG. 6 shows a midsagittal sectional view of the first preferred embodiment artificial disc of the present invention depicted inFIG. 3 between adjacent cervical vertebrae.
• FIG. 7 shows a midsagittal sectional view of a first preferred embodiment artificial disc of the present invention depicted inFIG. 3 with a schematic of Instantaneous-Axis-of-Rotation (IAR) of a superior vertebra in flexion.
• FIG. 8 shows a second preferred embodiment artificial disc of the present invention with a disc body slidably attached to a base plate to permit rotational and translational sliding.
• FIG. 9 shows a third preferred embodiment artificial disc of the present invention to include an upper disc body and a lower disc body cooperatively forming a saddle-joint with concave-convex surfaces.
• FIG. 10 shows a fourth preferred embodiment artificial disc of the present invention to include an upper disc body and a lower disc body cooperatively forming a saddle-joint with concave-convex surfaces and a tension cable.
• FIG. 11 shows a fifth preferred embodiment artificial disc of the present invention to include an upper disc body and a lower disc body cooperatively forming a saddle-joint with concave-convex surfaces and a flexible plate.
• FIG. 12 shows a sixth preferred embodiment artificial disc of the present invention with an upper articulating concave-convex surface and a lower articulating surface.
DETAILED DESCRIPTION
• Consistent with common medical nomenclature, superior is nearer the head in relation to a specific reference point, whereas, inferior is nearer the feet in relation to a specific reference point. Anterior is forward in relation to a specific reference point and posterior is rearward in relation to a specific reference point. The midsagittal plane is an imaginary plane dividing the body into a right side and left side. A frontal plane is any imaginary vertical plane orthogonal to the midsagittal plane.
• FIG. 1 shows a cervicalspinal motion segment1 characteristic of the middle and lower cervical spine comprised of asuperior vertebra10, anintervertebral disc7, and aninferior vertebra10′.Superior vertebra10 is divided into two regions to include superiorvertebral body11 and superiorposterior elements17. Similarly,inferior vertebra10′ is divided into two regions to include inferiorvertebral body11′ and inferiorposterior elements17′. Although separated by theintervertebral disc7 to permit motion, the superiorvertebral body11 and the inferiorvertebral body11′ are anatomically interlocking due to the reciprocal reception of the generally saddle-shapedinferior endplate23 of the superiorvertebral body11 with thesuperior endplate13′ of inferiorvertebral body11′. Although the spinal motion segments are not a synovial joint, the most closely related synovial joint to the middle and lower cervical spine motion segments is the carpometacarpal joint of the thumb, which is formed by articulating surfaces in the form of a saddle-joint. In contrast, the vertebral bodies of other regions of the spine (thoracic and lumbar) are not anatomically interlocking.
• FIG. 2A andFIG. 2B showcervical vertebra10 ofFIG. 1 viewed at different perspectives to highlightsuperior endplate13 andinferior endplate23, respectively. Consistent with established medical nomenclature,cervical vertebra10 includesvertebral body11, and the bony structures attached tovertebral body11 areposterior elements17. Thesuperior endplate13 is generally concave, or at least partially concave, as indicated by the superiorfrontal surface line14. The superiormidsagittal surface line15 is substantially straight but may be slightly concave or convex, or at least partially concave or convex. The inferior concave-convex endplate23 is substantially convex in the medial-lateral direction and substantially concave in the anterior/posterior direction to form a saddle surface, as indicated by inferiorconvex surface line24 and inferiorconcave surface line25.
• Primarily addressing the middle and lower cervical spine, the present invention uses a generally concave-convex articulating surface. As will become apparent in subsequent discussion, a concave-convex articulating surface is an essential element of the present invention, providing anatomical and biomechanical advantages. Referring now toFIG. 3, a first preferred embodiment,artificial disc100, is comprised of adisc body130,base plate138, andbone anchor150. Concave-convex articulatingsurface133 is further revealed by considering two example reference lines formed from convenient planer reference planes,concave surface line134 and midsagittalconvex surface line135. The curvature of concave-convex articulatingsurface133 may be formed of simple radii or variable radii, and may be further expressed mathematically, at least in part, as a hyperbolic paraboloid, saddle-surface, or surface with negative curvature As will become apparent in subsequent discussion, a concave-convex articulating surface is an essential element of the present invention, providing anatomical and biomechanical advantages.Disc body130 andbase plate138 are shown as separate components, although manufacture as a single component is also contemplated.Disc body130 has a height appropriate for spacing of the vertebral bodies and may be of various shapes and sizes in order to substantially fill the surgically prepared space between vertebral bodies.Base plate138 may have planer surfaces and uniform thickness, as shown inFIG. 3, although variable thickness and curved surfaces are also contemplated.Bone anchor150 andbase plate138 are adapted for fixation to bone.Bone anchor150 is comprised ofvertical anchor web152,anchor body154, andanchor hole156. As shown inFIG. 3,vertical anchor web152 has a vertical height greater than width. In addition,vertical anchor web152 may take other various forms, to include, but not be limited to, a web with variable thickness.Anchor body154 is a protuberance with a width substantially greater than the thickness ofvertical anchor web152. As shown inFIG. 3,anchor body154 is cylindrical, but other shapes are also contemplated, to include, but not limited to, a box shape. A use ofanchor hole156 is to releasably attach a surgical instrument during insertion ofartificial disc100.FIG. 4A,FIG. 4B, andFIG. 4C showartificial disc100 in orthogonal views to include a front view, side view, and top view, respectively
• FIG. 5 shows the first preferred embodimentartificial disc100 of the present invention depicted inFIG. 3 in an exploded perspective view between superiorcervical vertebra110 and inferiorcervical vertebra110′. Inferiorvertebral body111′ includes a surgically created vertebral body key-hole128′ intended to receivebone anchor150.Bone anchor150 provides secure interlocking fixation with inferiorvertebral body111′ and requires a minimal amount of bone removal.
• FIG. 6 shows a midsagittal sectional view of the first preferred embodimentartificial disc100 depicted inFIG. 3 between superiorcervical vertebra110 and inferiorcervical vertebra110′.Superior vertebra110 is comprised of superiorvertebral body111 and superiorposterior elements117. Similarly,inferior vertebra110′ is divided into two regions to include inferiorvertebral body111′ and inferiorposterior elements117′. Concave-convex articulatingsurface133 is intended to slide with respect to inferior concave-convex endplate123 provided the cartilaginous inferior concave-convex endplate123 remains substantially intact following treatment of the patient's pathology. The contour of concave-convex articulatingsurface133 is substantially similar to the contour of inferior concave-convex endplate123, although some mismatch and variability is expected. It is expected that a patient's endplate can modify and generally adapt to the shape of an implant during healing.Bone anchor150 is positioned within vertebral body key-hole128′.Base plate138 may have additional features for bony in-growth into inferiorvertebral body111′ through the use of established porous materials or surface treatments. The anatomically aligned shape ofartificial disc100 results in a near complete fill of the intervertebral disc space; the resulting construct is substantially free of large voids that are potentially susceptible to eventual tissue encroachment. Significant tissue encroachment into an artificial disc could potentially interfere with movement of an artificial disc. Finally, the anatomically aligned shape ofartificial disc100 is intended to fit within the intervertebral disc space with minimal bone removal and endplate preparation during surgery, reducing surgical time and preserving otherwise healthy tissue.
• The anatomically aligned shape of the present invention also has biomechanical benefits associated with natural kinematics of the spine. To assist with the analysis of spine kinematics, Panjabi and White established the Instantaneous-Axis-of-Rotation (IAR) for planer motion analysis of vertebrae. Planar motion of vertebrae is fully described by the position of the IAR and the angle of rotation about the IAR. The IAR is an instantaneous measure and therefore may shift within a region through a range-of-motion, such as, but not limited to, flexion/extension range-of-motion. The present inventors have discovered a relation between the shape of a vertebra's inferior endplate and the natural motion of the same vertebra.
• Flexion/extension is the most commonly considered degree-of-freedom when evaluating cervical spine kinematics.FIG. 7 shows the first preferred embodiment,artificial disc100 depicted inFIG. 3, in a midsagittal sectional view betweensuperior vertebra110 andinferior vertebra110′.Superior vertebra110 is shown in a neutral position with locator point A1 and locator point B1 and in a flexed position with locator point A2 and locator point B2. Locator points may be any two unique points on a vertebra, and convenient anatomical landmarks are often used for these reference points. The selected frame of reference isinferior vertebra110′, so the instantaneous-axis-of-rotation IAR is ofsuperior vertebra110 with respect toinferior vertebrae110′. Continuing to refer toFIG. 7, instantaneous-axis-of-rotation IAR is established by determining the intersection of line LA and line LB, where line LA and line LB are bisected normal lines of translation vector A1A2 and translation vector B1B2, respectively. Concave-convex articulatingsurface133 has been adapted to have a substantially similar shape to inferior concave-convex endplate123. Continuing to refer toFIG. 7, inferior concave-convex endplate123 sliding with respect to concave-convex articulatingsurface133 establishes the motion in flexion. The general region of Instantaneous-Axis-of-Rotation IAR is within the posterior region of inferiorvertebral body111′. Concerning the middle and lower cervical spine, the Instantaneous-Axis-of-Rotation IAR in flexion/extension ofartificial disc100, shown inFIG. 7, is consistent with recent scientific research demonstrating the IAR of a superior vertebra in flexion/extension generally lay within a posterior region of an inferior vertebral body (DiAngelo et. al., proceedings of Cervical Spine Research Society Meeting 2000). Further, the mathematical equations defining the shape of concave-convex articulatingsurface133, generally in the form of a hyperbolic paraboloid, can be developed to substantially replicate the complex natural motions of the spine, to include, but not limited to, flexion/extension, lateral bending, and torsional motion.
• Within the scope of the present invention, multiple components may be allowed to articulate to address multiple degrees-of-freedom associated with spinal motion, to include, but not limited to, torsional and translational degrees-of-freedom. Accordingly,
• FIG. 8 shows a second preferred embodiment of the present invention,artificial disc200 in an exploded perspective view.Artificial disc200 is comprised of adisc body230, concave-convex articulatingsurface233,base plate238, andbone anchor250.Bone anchor250 is comprised ofvertical web252 andanchor body254.Base plate protrusion240 inserts intodisc body socket236, while discbody articulating surface237 and baseplate articulating surface239 allow sliding rotation ofdisc body230 aboutaxis X-X245. Alternately,disc body socket236 may take various shapes and sizes relative tobase plate protrusion240 to allow planer translation betweendisc body230 andbase plate238. For example,disc body socket236 may take the form of an elongated slot. Although, discbody articulating surface237 and baseplate articulating surface239 are shown as planer surfaces, curved surfaces are also contemplated. A cylindrical shape forbase plate protrusion240 is shown inFIG. 8, however, other shapes are also envisioned within the scope of the present invention.
• For a number of reasons, to include anatomical variation, the inferior endplate of a superior vertebra may not be suitable as an articulating surface. During treatment of a cervical spine disorders, endplates are often partially or completely removed in order to access and remove offending soft tissue (e.g., extruded disc nucleus) or offending hard tissue (e.g., posterior bone spurs). A total joint artificial disc is often warranted. Accordingly,FIG. 9 shows a third preferred embodiment of the present invention,artificial disc300, comprised oflower disc body330 andupper disc body370.Lower disc body330, attached tolower base plate338, has a lower body concave-convex articulatingsurface333.Lower bone anchor350 is comprised of lowervertical web352 andlower anchor body354.Upper disc body370 is comprised ofupper base plate378, upper body concave-convex articulatingsurface373, andupper bone anchor380.Upper bone anchor380 is comprised of uppervertical web382 andupper anchor body384.Lower anchor hole356 andupper anchor hole386 can be used to releasably attach a surgical instrument during implantation ofartificial disc300. Lower body concave-convex articulatingsurface333 is further defined byconcave surface line334 and midsagittalconvex surface line335.Lower base plate338 andlower bone anchor350 are adapted for fixation to an inferior vertebra. Similarly,upper bone anchor380, comprised of uppervertical anchor web382 andupper anchor body384 are adapted for body fixation to a vertebra. The reciprocal reception of lower body concave-convex articulatingsurface333 with upper body concave-convex articulatingsurface373 forms a saddle-joint. The complement of lower bodyconcave surface line334 is upper bodyconvex surface line374, and the complement of lower bodyconvex surface line335 is upper bodyconcave surface line375. Surface contours of lower body concave-convex articulatingsurface333 and upper body concave-convex articulatingsurface373 may be dimensionally matched. Or, surface contours of lower body concave-convex articulatingsurface333 and upper body concave-convex articulatingsurface373 may be cooperatively aligned, but dimensionally mismatched to give the saddle-joint additional freedom (“toggle”) or to establish selected regions of surface contact within manufacturing tolerances. A “gentle braking” occurs in torsion between thelower disc body330 and theupper disc body370 as the lower body concave-convex articulatingsurface333 and upper body concave-convex articulatingsurface373 interact during a torsional motion away from a neutral position, reflecting a more natural resistance to torsional loading. In addition, the surface geometry defining the articulating interaction of lower body concave-convex articulatingsurface333 and upper body concave-convex articulatingsurface373 may be developed to substantially replicate the complex natural motions of the spine, to include, but not limited to, flexion/extension, lateral bending, and torsional motion.
• Using an anterior approach to the cervical spine, the anterior longitudinal ligament is at least partially resected with associated loss of stability, especially in extension. An artificial disc with a tension element provides stability during extension. Referring now toFIG. 10, a fourth preferred embodiment is shown,artificial disc400, comprised oflower disc body430 andupper disc body470.Lower disc body430 is joined tolower base plate438.Lower disc body430 is also comprised of a lower body concave-convex articulatingsurface433.Lower bone anchor450, comprised of lowervertical web452 andlower anchor body454, is adapted for fixation to bone.Upper disc body470 comprised ofupper base plate478, upper body concave-convex articulatingsurface473 andupper bone anchor480.Upper bone anchor480 is comprised of uppervertical web482 andupper anchor body484. One use oflower anchor hole456 andupper anchor hole486 is to releasably attach a surgical instrument during insertion ofartificial disc400. A saddle-joint is formed by the reciprocal reception of lower body concave-convex articulatingsurface433 with upper body concave-convex articulatingsurface473.Tension cable490, to includecable knot491, is secured through thelower anchor eyelet458 and theupper anchor eyelet488.Tension cable490 provides additional stability, restricting motion, especially during extreme extension.
• FIG. 11 shows a fifth preferred embodiment,artificial disc500, comprised oflower disc body530 andupper disc body570.Lower disc body530 includes lower body concave-convex articulatingsurface533.Lower disc body530 is attached tolower base plate538.Lower bone anchor550, adapted for interlocking connection to bone, is comprised of lowervertical web552 andlower anchor body554.Upper disc body570 is comprised ofupper base plate578 and upper body concave-convex articulatingsurface573.Upper bone anchor580 is comprised of uppervertical web582 andupper anchor body584. The reciprocal reception of lower body concave-convex articulatingsurface533 with upper body concave-convex articulatingsurface573 forms a saddle-joint.Flexible plate590 is secured tolower anchor body550 andupper anchor body580 bylower fastener594′ andupper fastener594, respectively.Flexible plate590 is intended to provide greater stability in extension. To allow specified range-of-motion in extension and flexion whereflexible plate590 is not substantially under load,lower fastener594′ andupper fastener594 are intended to slide with respect tolower plate slot592′ andupper plate slot592, respectively.
• Within the scope of the current invention, an artificial disc may have an upper articulating surface and a lower articulating surface for sliding interaction with vertebral bodies.FIG. 12 shows a sixth preferred embodiment,artificial disc600, comprised ofdisc body630, upper articulating concave-convex surface633 and lower articulatingsurface673. Curvature of upper articulating concave-convex surface633 is further defined byconcave surface line634 and midsagittalconvex surface line635.Lower surface line674 is at least generally convex and lower midsagittalplane surface line675 is generally straight, but may also have curvature.
• The present invention, to include, but not limited to the aforementioned embodiments, can be constructed of established orthopaedic materials. Established wear resistant materials for components with articulating surfaces include metals (e.g., stainless-steel, cobalt-chrome, and titanium), plastics (e.g., Ultra-High-Molecular-Weight-Polyethylene), and ceramics (e.g., alumina and zironia). Non-articulating features of an artificial disc of the present invention may have features or material characteristics to facilitate rigid attachment to bone. An artificial disc of the present invention may be adapted additional features known for attachment to bone including, but not limited to, spikes, screws, serration, and plate-like appendages generally exterior to the intervertebral disc space. Components may be made, in part, constructed of substantially porous materials for bone in-growth, yet have smooth non-porous regions for articulating surfaces. Surface treatments commonly practiced, such as beaded-coatings, may also be used for attachment to bone through eventual bone in-growth. In addition, within the scope of the current invention, components may be given flexibility through the use of geometry and materials to replicate the cushioning characteristics of the natural intervertebral disc. Additional components, such as, springs might be added to provide flexibility. In addition, portions of the patient's annulus may remain intact, such that the present invention augments a patient's existing intervertebral disc. Although the utility of the disclosed artificial disc is best achieved in the cervical region of the spine, adaptations for the thoracic and lumbar regions of the spine are also within the scope and spirit of the present invention.

Claims (18)

1. A prosthetic disc comprising:

A disc body, having a disc body base surface having a center point having a base surface central normal vector extending therefrom; and a disc body articulating surface opposite the disc body base surface, the articulating surface being concave with respect to a first disc body plane parallel to the base plate central normal vector, and being convex with respect to a second disc body plane, parallel to the base plate central normal vector and orthogonal to the first disc body plane.

2. The prosthetic disc ofclaim 1 wherein the second disc body plane is the midsagittal plane.

3. The prosthetic disc ofclaim 1 wherein the curve of the articulating surface in the first disc body plane is parabolic.

4. The prosthetic disc ofclaim 1 wherein the curve of the articulating surface in the first disc body plane is hyperbolic.

5. The prosthetic disc ofclaim 1 wherein the curve of the articulating surface in the first disc body plane follows a radius.

6. The prosthetic disc ofclaim 1 wherein the curve of the articulating surface in the second disc body plane is parabolic.

7. The prosthetic disc ofclaim 1 wherein the curve of the articulating surface in the second disc body plane is hyperbolic.

8. A prosthetic disc comprising:

A disc body, having a disc body base surface and a disc body articulating surface opposite the disc body surface, wherein at least a substantial region of said disc body articulating surface is concave-convex.

9. The prosthetic disc ofclaim 9 wherein the disc body articulating surface is a hyperbolic paraboloid.

10. The prosthetic disc ofclaim 9 wherein the curve of the articulating surface in a first disc body plane is parabolic.

11. The prosthetic disc ofclaim 9 wherein the curve of the articulating surface in a first disc body plane is hyperbolic.

12. The prosthetic disc ofclaim 9 wherein the curve of the articulating surface in a first disc body plane follows a radius.

13. The prosthetic disc ofclaim 12 further comprising a base plate secured to the disc body base surface.

14. The prosthetic disc ofclaim 13 wherein the base plate comprises an anchor.

15. A method of installing a prosthetic disc comprising the steps of:

removing a portion of an intervertebral disc, thereby creating an intervertebral disc space, and

placing a prosthetic disc substantially within said intervertebral disc space;

wherein the prosthetic disc comprises:

a disc body, having a disc body base surface having a center point having a base surface central normal vector extending therefrom; and

a disc body articulating surface opposite the disc body base plate surface, the articulating surface being concave along a first disc body plane parallel to the base plate central normal vector, and being convex along a second disc body plane, parallel to the base plate central normal vector and orthogonal to the first disc body plane.

16. A prosthetic disc comprising:

A first disc body, having a first disc body base surface having a first vector extending normally therefrom, and a first disc body articulating surface opposite the first disc body base surface, the articulating surface being concave along a first disc body first plane parallel to the first vector, and being convex along a first disc body second plane, parallel to the first vector and orthogonal to the first disc body first plane, the outer regions of the first disc body articulating surface along and adjacent to the first disc body first plane defining a first disc body concave region and the outer regions of the first disc body articulating surface along and adjacent to the first disc body second plane defining a first disc body convex region;

a second disc body, having a second disc body base surface having a second vector extending normally therefrom, and a second disc body articulating surface opposite the second disc body base surface, the articulating surface being concave along a second disc body first plane parallel to the second vector, and being convex along a second disc body second plane, parallel to the second vector and orthogonal to the second disc body first plane, the outer regions of the second disc body articulating surface along and adjacent to the second disc body first plane defining a second disc body concave region and the outer regions of the second disc body articulating surface along and adjacent to the second disc body second plane defining a second disc body convex region;

the first disc body articulating surface and the second disc body articulating surface being disposed in abutting relationship, and oriented such that at least a portion of the first disc body concave region is mated to at least a portion of the second disc body convex region or at least a portion of the first disc body convex region is mated to at least a portion of the second disc body concave region.

17. An artificial disc suitable for placement between adjacent vertebra comprising:

a first disc body, having a first disc body base surface and a first disc body articulating surface opposite said first disc body surface, wherein at least a substantial region of said first disc body articulating surface is a hyperbolic paraboloid; and

a second disc body, having a second disc body base surface and a second disc body articulating surface opposite said second disc body surface, wherein at least a substantial region of said second disc body articulating surface is a hyperbolic paraboloid, and wherein said second disc body articulating surface is substantially reciprocal to first disc body articulating surface, wherein said first disc body articulating surface and said second disc body articulating surface being disposed in abutting relationship and cooperatively form a saddle-joint.

18. An artificial disc suitable for placement between adjacent vertebra comprising:

an upper body having an upper body base surface and an upper body concave-convex articulating surface opposite said first disc body base surface; and

a lower body, having a lower body base surface and a lower body concave-convex articulating surface opposite said lower body base surface;

the upper body and lower body forming a saddle joint by the reciprocal reception of the lower body concave-convex articulating surface with the upper body concave-convex articulating surface.

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