PROSTHETIC DISC FOR INTERVERTEBRAL INSERTION

Abstract

A prosthetic disc for insertion between adjacent vertebrae includes a core having upper and lower curved surfaces, upper and lower plates, and peripheral restraining structure on at least one of the upper plate, the lower plate and the core. Each plate has an outer surface which engages a vertebra and an inner curved surface which slides over the curved surface of the core. The peripheral restraining structure serves to hold the core against a curved surface of at least one of the plates during sliding movement of the plates over the core.

Claims

1. A prosthetic disc for insertion between adjacent vertebrae, the prosthetic disc comprising: upper and lower plates, each plate having an outer surface which engages a vertebra and a curved inner bearing surface; a core having first and second outer surfaces which are each complementary in shape to, mate with and slide over the corresponding bearing surfaces of the upper and lower plates in an implantation configuration when the prosthetic disc is inserted between adjacent vertebrae; a retaining formation on the lower plate, the retaining formation including at least one inwardly facing groove at an edge of the curved inner bearing surface of the lower plate; at least one outwardly facing flange formed at an outer periphery of the core, the flange engaging the retaining formation on the lower plate to hold the core between the upper and lower plates as the core slides over the bearing surface of the lower plate; and wherein the entire core is movable with respect to the upper and lower plates in the implantation configuration.

2. The disc of claim 1, wherein the upper and lower plates are metallic and the core comprises a polymer.

3. The disc of claim 1, wherein the outwardly facing flange comprises a number of spaced flange segments.

4. The disc of claim 1, wherein at least one of the curved inner bearing surfaces and at least one of the first and second outer surface are cooperating spherical surfaces.

5. The disc of claim 1, wherein the outer surfaces of the upper and lower plates each comprise an upstanding central fin.

6. The disc of claim 5, wherein the outer surfaces of the upper and lower plates comprise a rough microfinish.

7. The disc of claim 6, wherein the outer surfaces of the upper and lower plates are coated with titanium plasma spray.

8. The disc of claim 2, wherein the core comprises polyethylene.

9. The disc of claim 1, wherein the core includes convex outer surfaces on both the upper and lower surfaces.

10. The disc of claim 1, wherein no retaining formation is provided on the upper plate.

11. The disc of claim 1, wherein no inwardly facing groove is provided on the upper plate.

12. A method of retaining a core in a prosthetic disc for insertion between adjacent vertebrae, the method comprising: providing upper and lower plates, each plate having an outer surface which engages a vertebra and a curved inner bearing surface; providing a retaining formation on the lower plate, the retaining formation including at least one inwardly facing groove at an edge of the curved inner bearing surface of the lower plate; inserting a core between the upper and lower plates, the core having first and second outer surfaces which are each complementary in shape to, mate with and slide over the corresponding bearing surfaces of the upper and lower plates in an implantation configuration when the prosthetic disc is inserted between adjacent vertebrae; holding the core between the upper and lower plates by providing at least one outwardly facing flange formed at an outer periphery of the core, the flange engaging the retaining formation on the lower plate to hold the core between the upper and lower plates as the core slides over the bearing surface of the lower plate, while allowing the upper and lower plates to move with respect to one another and with respect to the entire core.

13. The method of claim 12, wherein the upper and lower plates are metallic and the core comprises a polymer.

14. The method of claim 12, wherein the outwardly facing flange comprises a number of spaced flange segments.

15. The method of claim 12, further comprising providing the outer surfaces of the upper and lower plates each with an upstanding central fin for fixation.

16. The method of claim 15, wherein the outer surfaces of the upper and lower plates comprise a rough microfinish for further fixation.

17. The method of claim 15, wherein the outer surfaces of the upper and lower plates are coated with titanium plasma spray for further fixation.

18. The method of claim 13, wherein the core comprises polyethylene.

19. The method of claim 12, wherein no retaining formation is provided on the upper plate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 shows a cross-sectional anterior view of a prosthetic disc with the prosthesis plates and core in vertical alignment, according to one embodiment of the present invention;

[0023] FIG. 2 shows a side view of the prosthetic disc in FIG. 1 after sliding movement of the plates over the core;

[0024] FIG. 3 shows a side view of the prosthetic disc in FIG. 1 after translational movement of the plates relative to the core;

[0025] FIG. 4 shows a side view of the prosthetic disc in FIG. 1 with the prosthesis plates and core in vertical alignment;

[0026] FIG. 5 shows a plan view of a core of a prosthetic disc, according to one embodiment of the present invention; and

[0027] FIG. 6 shows a plan view of an upper plate of a prosthetic disc, according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0028] FIGS. 1 to 4 illustrate a prosthetic disc 10 for intervertebral insertion between two adjacent spinal vertebrae (not shown). The disc 10 comprises three components, namely an upper plate or shell 12, a lower plate or shell 14 and a core 16 located between the plates.

[0029] The upper plate 12 includes an outer surface 18 and an inner surface 24 and may be constructed from any suitable material or combination of materials, such as but not limited to cobalt chrome molybdenum, titanium (such as grade 5 titanium) and/or the like. In one embodiment, typically used in the lumbar spine, the upper plate 12 is constructed of cobalt chrome molybdenum, and the outer surface 18 is treated with aluminum oxide blasting followed by a titanium plasma spray. In another embodiment, typically used in the cervical spine, the upper plate 12 is constructed of titanium, the inner surface 24 is coated with titanium nitride, and the outer surface 18 is treated with aluminum oxide blasting. An alternative cervical spine embodiment includes no coating on the inner surface 24. In some embodiments, it may be useful to couple two materials together to form the inner surface 24 and the outer surface 18. For example, the upper plate 12 may be made of an MRI-compatible material, such as titanium, but may include a harder material, such as cobalt chrome molybdenum, for the inner surface 24. Any suitable technique may be used to couple materials together, such as snap fitting, slip fitting, lamination, interference fitting, use of adhesives, welding and/or the like. Any other suitable combination of materials and coatings may be employed in various embodiments of the invention.

[0030] In some embodiments, the outer surface 18 is planar. Oftentimes, the outer surface 18 will include one or more surface features and/or materials to enhance attachment of the prosthesis 10 to vertebral bone. For example, the outer surface 18 may be machined to have a serrations 20 or other surface features for promoting adhesion of the upper plate 12 to a vertebra. In the embodiment shown (FIG. 6), the serrations 20 extend in mutually orthogonal directions, but other geometries would also be useful. Additionally, the outer surface 18 may be provided with a rough microfinish formed by blasting with aluminum oxide microparticles or the like. In some embodiments, the outer surface may also be titanium plasma sprayed to further enhance attachment of the outer surface 18 to vertebral bone.

[0031] The outer surface 18 may also carry an upstanding, vertical fin 22 extending in an anterior-posterior direction. The fin 22 is pierced by transverse holes 23. In alternative embodiments, the fin 22 may be rotated away from the anterior-posterior axis, such as in a lateral-lateral orientation, a posterolateral-anterolateral orientation, or the like. In some embodiments, the fin 22 may extend from the surface 18 at an angle other than 90°. Furthermore, multiple fins 22 may be attached to the surface 18 and/or the fin 22 may have any other suitable configuration, in various embodiments. In other embodiments, the fin 22 In some embodiments, such as discs 10 for cervical insertion, the fins 22, 42 may be omitted altogether.

[0032] The inner, spherically curved concave surface 24 is formed at a central, axial position with a circular recess 26 as illustrated. At the outer edge of the curved surface 24, the upper plate 12 carries peripheral restraining structure comprising an integral ring structure 26 including an inwardly directed rib or flange 28. The flange 28 forms part of a U-shaped member 30 joined to the major part of the plate by an annular web 32. The flange 28 has an inwardly tapering shape and defines upper and lower surfaces 34 and 36 respectively which are inclined slightly relative to the horizontal when the upper plate 12 is at the orientation seen in FIG. 1. An overhang 38 of the U-shaped member 30 has a vertical dimension that tapers inwardly as illustrated.

[0033] The lower plate 14 is similar to the upper plate 12 except for the absence of the peripheral restraining structure 26. Thus, the lower plate 14 has an outer surface 40 which is planar, serrated and microfinished like the outer surface 18 of the upper plate 12. The lower plate 14 optionally carries a fin 42 similar to the fin 22 of the upper plate. The inner surface 44 of the lower plate 14 is concavely, spherically curved with a radius of curvature matching that of the inner surface 24 of the upper plate 12. Once again, this surface may be provided with a titanium nitride or other finish.

[0034] At the outer edge of the inner curved surface 44, the lower plate 14 is provided with an inclined ledge formation 46. Alternatively, the lower plate 14 may include peripheral restraining structure analogous to the peripheral restraining structure 26 on the upper plate 12.

[0035] The core 16 of the disc 10 is made of a low-friction material, such as polyethylene (Chirulen™). In alternative embodiments, the core 16 may comprise any other suitable material, such as other polymers, ceramics or the like. The core 16 has identical upper and lower spherically curved convex surfaces 48, 50. The radius of curvature of these surfaces matches the radius of curvature of the inner surfaces 24, 44 of the upper and lower plates 12, 14. The curved surfaces are accordingly complementary. For wear resistance, the surface zones of the core may be hardened by an appropriate cross-linking procedure.

[0036] The core 16 is symmetrical about a central, equatorial plane 52 which bisects it laterally. (Although in other embodiments, the core 16 may be asymmetrical.) Lying on this equatorial plane is an annular recess or groove 54 which extends about the periphery of the core. The groove 54 is defined between upper and lower ribs or lips 56. When the plates 12, 14 and core 16 are assembled and in the orientation seen in FIG. 1, the flange 28 lies on the equatorial plane and directly aligned with the groove 54. The outer diameter 58 of the lips 56 is preferably very slightly larger than the diameter 60 defined by the inner edge of the flange 28. Assembly of the core and upper plate may involve pressing the core through the circular aperture defined by the flange 28, with the inherent resilience of the core allowing the minor deformation of the upper rib 56, or that the core be introduced at an inclination. In other less preferred embodiments of the invention (not shown), the diameter 58 may be equal to or even slightly less than the diameter 60.

[0037] In some embodiments, the inner surface of the groove 54 may be provided, for wear resistance, with a lining of pure titanium or titanium impregnated with cobalt chrome, titanium nitride, other titanium alloy or the like.

[0038] The central axis of the disc 10 (the axis passing through the centers of curvature of the curved surfaces) is indicated with the reference numeral 62. As shown in FIG. 1, the disc 10 may be symmetrical about a central anterior-posterior plane containing the axis 62. Referring to FIG. 4, in some embodiments the axis 62 is posteriorly disposed, i.e. is located closer to the posterior limit of the disc than the anterior limit thereof

[0039] In use, the disc 10 is surgically implanted between adjacent spinal vertebrae in place of a damaged disc. The adjacent vertebrae are forcibly separated from one another to provide the necessary space for insertion. The disc is inserted, normally in a posterior direction, into place between the vertebrae with the fins 22, 42 of the plates 12, 14 entering slots cut in the opposing vertebral surfaces to receive them. After insertion, the vertebrae, facets, adjacent ligaments and soft tissues are allowed to move together to hold the disc in place. The serrated and microfinished surfaces 18, 40 of the plates 12, 14 locate against the opposing vertebrae. The serrations 20 and fins 22, 42 provide initial stability and fixation for the disc 10. With passage of time, enhanced by the titanium surface coating, firm connection between the plates and the vertebrae will be achieved as bone tissue grows over the serrated surface. Bone tissue growth will also take place about the fins 22, 40 and through the transverse holes 23 therein, further enhancing the connection which is achieved.

[0040] Referring to FIG. 5, the core 16 may be formed with narrow, angularly spaced, blind passages 61 which accommodate titanium pins 64. In many embodiments, the core 16 itself is transparent to X-radiation and so is invisible in a post-operative X-ray examination. The pins 64 serve as radiographic markers and enable the position of the core 16 to be ascertained during such examination.

[0041] In the assembled disc 10, the complementary and cooperating spherical surfaces of the plates and core allow the plates to slide or articulate over the core through a fairly large range of angles and in all directions or degrees of freedom, including rotation about the central axis 62. FIGS. 1 and 4 show the disc 10 with the plates 12 and 14 and core 16 aligned vertically with one another on the axis 62. FIG. 2 illustrates a situation where maximum anterior flexion of the disc 10 has taken place. At this position, the upper rib 56 has entered the hollow 38 of the U-shaped member 30, the lower surface of the rib 56 has moved into contact with the upper surface 34 of the flange 28, the flange having moved into the groove 54, and the lower surface 36 of the flange has moved into contact with the upper surface of the ledge formation 46, as will be seen in the encircled areas 69. Abutment between the various surfaces prevents further anterior flexure. The design also allows for the inner extremity of the flange 28 to abut against the base of the groove 54, thereby limiting further relative movement between the core and plate. A similar configuration is achieved in the event of maximum posterior flexure of the plates 12, 14 over the core, such as during spinal extension and/or in the event of maximum lateral flexure.

[0042] FIG. 3 illustrates how the disc 10 can also allow for translational movement of the plates relative to the core. In the illustrated situation there has been lateral translation of the plates relative to the core. The limit of lateral translation is reached when the inner extremity of the flange 28 abuts the base of the groove 54 as indicated by the numeral 70.

[0043] The flange 28 and the groove 54 defined between the ribs 56, prevent separation of the core from the plates. In other words, the cooperation of the retaining formations ensures that the core is held captive between the plates at all times during flexure of the disc 10.

[0044] In an alternative embodiment, the continuous annular flange 28 may be replaced by a retaining formation comprising a number of flange segments which are spaced apart circumferentially. Such an embodiment could include a single, continuous groove 54 as in the illustrated embodiment. Alternatively, a corresponding number of groove-like recesses spaced apart around the periphery of the core could be used, with each flange segment opposing one of the recesses. In another embodiment, the continuous flange or the plurality of flange segments could be replaced by inwardly directed pegs or pins carried by the upper plate 12. This embodiment could include a single, continuous groove 54 or a series of circumferentially spaced recesses with each pin or peg opposing a recess.

[0045] In yet another embodiment, the retaining formation(s) could be carried by the lower plate 14 instead of the upper plate, i.e. the plates are reversed. In some embodiments, the upper (or lower) plate is formed with an inwardly facing groove, or circumferentially spaced groove segments, at the edge of its inner, curved surface, and the outer periphery of the core is formed with an outwardly facing flange or with circumferentially spaced flange segments.

[0046] Although the foregoing is a complete and accurate description of the invention, any of a number of modifications, additions or the like may be made to the various embodiments without departing from the scope of the invention. Therefore, nothing described above should be interpreted as limiting the scope of the invention at it is described in the claims.