Intervertebral prosthesis

Abstract

An intervertebral prosthesis for insertion between adjacent vertebrae includes upper and lower prosthesis plates locatable against respective vertebrae and having opposing, concavely curved recesses therein, and a core located between the plates. The core has opposed, convexly curved surfaces received in the recesses of the plates to allow the plates to slide in articulated manner over the core. The opposed surfaces of the core and the recesses of the plates have cooperating spherical curvatures. The recess of each plate surrounds a locating peg projecting centrally from the base of the recess and is bounded by an annular rim, such that the annular rims of the plates are arranged to contact one another at a predetermined limit of sliding movement of the plates over the core. The peg locates loosely in an opening located centrally in a curved surface of the core, whereby the plates can slide over the core in all directions while the peg holds the core captive.

Claims

1. A system for insertion of an intervertebral prosthesis between adjacent vertebrae, the system comprising: an intervertebral prosthesis comprising: a first prosthesis plate having a major surface to engage a first vertebra and a cooperating surface disposed opposite the major surface; a second prosthesis plate comprising a major surface to engage a second vertebra and a cooperating surface disposed opposite the major surface; a core located between the first and second plates, wherein the cooperating surfaces of the first and second plates are arranged to cooperate with the core to form an articulate joint; a peg projecting centrally from the cooperating surface of the first plate; a central opening in the core; wherein the peg locates loosely in the opening of the core, whereby the plates can slide over the core while the peg holds the core captive; wherein the cooperating surface of the second plate has no centrally projecting peg; a first annular rim disposed around the cooperating surface of the first plate, the first rim having an empty groove; a second annular rim disposed around the cooperating surface of the second plate, the second rim having an empty groove; and wherein each groove is adapted to facilitate manipulation and holding of the prosthesis with an appropriate instrument while the prosthesis is placed in the intervertebral space; and an instrument for placing the intervertebral disc, the instrument configured to hold the prosthesis at the grooves of the first and second annular rims.

2. The system according to claim 1, wherein the cooperating surface of the second plate is spherical and the core has a cooperating spherical surface.

3. The system according to claim 1, wherein the central opening in the core is a blind recess extending partway through the core.

4. The system according to claim 1, wherein the first and second prosthesis plates are each formed in one piece and configured to be received entirely within the disc space.

5. The system according to claim 1, wherein the first and second prosthesis plates include upstanding elongated projections on the major surfaces.

6. The system according to claim 1, wherein the cooperating surface of the first plate is complementary in shape to a curved surface of the core of the prosthesis.

7. The system according to claim 1, wherein the peg and the central opening are complementary in shape.

8. The system according to claim 1, wherein the first plate is an upper plate and the second plate is a lower plate, wherein only the lower plate includes the centrally projecting peg.

9. The system according to claim 1, wherein the major surfaces comprise machined serrated finishes.

10. The system according to claim 1, wherein the first and second plates are formed of titanium.

11. The system according to claim 1, wherein the core is formed of high molecular weight polyethylene.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings in which:

(2) FIG. 1 shows a cross-sectional view of a prosthesis plate used in a prosthesis according to the invention;

(3) FIG. 2 shows a plan view of the prosthesis plate seen in FIG. 1;

(4) FIG. 3 shows a cross-sectional view of a core used in a prosthesis according to the invention;

(5) FIG. 4 shows a plan view of the core seen in FIG. 3;

(6) FIG. 5 shows a cross-sectional view of an intervertebral prosthesis according to a first embodiment of the invention in a neutral condition before articulation has taken place;

(7) FIG. 6 shows the prosthesis of FIG. 5 articulated to a limit position;

(8) FIG. 7 shows a cross-sectional view of an intervertebral prosthesis according to a second embodiment of the invention; and

(9) FIG. 8 shows a cross-section of an intervertebral prosthesis according to a third embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

(10) The prosthesis plate 10 seen in FIGS. 1 and 2 may be formed from titanium, typically being formed in one piece of grade 5 titanium. Other suitable materials include titanium nitrides, cobalt chrome alloys, and ceramics. The prosthesis plate includes a major surface 12 which is machined to a coarse, serrated finish 14. Projecting from the surface 12 is an elongate fin 16 pierced by transverse holes 18. The opposite surface of the plate 10 is formed with a recess 20 surrounding a projecting conical peg 22 of round cross-section at the centre of the recess. The recess is bounded by an annular rim 24. The surface 26 of the recess is concavely, spherically curved and has a titanium nitride finish.

(11) A first embodiment of prosthesis of the invention, indicated generally, by the numeral 28 in FIGS. 5 and 6, includes a pair of identical prosthesis plates 10 arranged in opposition to one another. The prosthesis 28 also includes a core 30 illustrated in FIGS. 3 and 4. The core is made in one piece of a low friction plastics material, in this case a polyethylene known as Orthosol, but a variety of other high molecular weight polyethylenes might also find use. Alternatively, the core could be composed of a metal, such as cobalt chrome, or of a ceramic. The core is generally in the form of a circular, doughnut-shaped toroid and has identical upper and lower surfaces 32 which are convexly, spherically curved. The upper and lower surfaces 32 are formed centrally with conical openings 34, in this case forming the opposite ends of a circular cross-section passage 36 passing axially and centrally through the body of the core.

(12) The core surfaces 32 and recess surface 26 have the same radius of curvature which is, in this case, 18 mm.

(13) In the assembled prosthesis 28, the plates 10 are arranged in opposition to one another with the core 30 located between them. The pegs 22 of the plates locate in the openings 34, i.e. in the ends of the passage 36. The combined length of the pegs is less than the length of the passage 36, so the inner ends of the pegs are spaced apart from one another by a short distance 38, as shown in FIG. 5. The transverse dimension of each peg is, at all points along the length of the peg, considerably less than the diameter of the passage 36 at the corresponding point along the length of the passage.

(14) The cooperating spherical surfaces 26 and 32 and the relative dimensions of the pegs 22 and the passage 36 allow the plates 10 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.

(15) At a predetermined limit of articulated movement of the plates relative to the core, the rims 24 of the plates contact with one another as indicated by the numeral 40 in FIG. 6. Further articulation of the prosthesis 28 beyond this point is not possible. At the limit of articulation, the pegs 22 also come into abutment with the side of the passage 36, as also illustrated in FIG. 6.

(16) It will also be noted in FIG. 6 that the openings 34 at the ends of the passage 36 are defined by similar cone angles to the pegs 22, so that contact between the pegs and the sides of the openings takes place complementally over virtually the entire length of each peg.

(17) Throughout the range of possible articulation, the pegs 22 remain in the passage 36 and prevent the core from separating laterally from the plates 10. In other words, the core is held captive by the pegs during all possible articulations which can take place. In the illustrated embodiment, the plates 10 are limited to 12° of articulation before the rims 24 abut one another, i.e. 12° is the maximum articulation which can take place.

(18) In the prosthesis 28 described above, the pegs 22 locate in a passage 32 which passes right through the core 30. It will however be understood that in other embodiments, the pegs could merely locate in blind recesses or openings in the opposite surfaces of the core without such openings actually being joined to one another to form a continuous passage.

(19) FIG. 7 illustrates a prosthesis according to a second embodiment of the invention in a view similar to that of FIG. 5. In FIG. 7, components corresponding to components illustrated in the earlier Figures are indicated by the same reference numerals.

(20) The lower prosthesis plate 10 in FIG. 7 is identical to the prosthesis plates of the first embodiment. However, the upper prosthesis plate, designated 10.1, has no central peg. Instead, the surface 26 is continuously, spherically curved. In all other respects the plate 10.1 is identical to the plate 10.

(21) In the assembled prosthesis 50, the core is held captive by the action of the single peg 22 carried by the lower prosthesis plate 10. The core 30 is identical to the core described previously. In the assembled prosthesis 50.1 of FIG. 8, the core 30.1 has a blind recess 36.1 in its downwardly facing, curved surface only to receive the single peg 22.

(22) The prosthesis 28, 50 is surgically implanted between adjacent spinal vertebrae in place of a damaged disc. Those skilled in the art will understand that the adjacent vertebrae are forcibly separated from one another to provide the necessary space for insertion. The plates 10, 10.1 are slipped laterally into place between the vertebrae with their fins 16 entering slots cut in the opposing vertebral surfaces to receive them.

(23) After insertion of the core between the opposing plates, the vertebra are allowed to move together to hold the assembled prosthesis in place.

(24) The surfaces 12 of the plates 10, 10.1 locate against the opposing vertebrae and, with passage of time, firm connection between the plates and the vertebrae will be achieved as bone tissue grows over the serrated finish. Bone tissue growth will also take place about the fins 16 and through the holes 18 therein, further enhancing the connection which is achieved.

(25) Referring to FIGS. 3 and 4, the core 30 used in the embodiments described above is formed with narrow, angularly spaced, blind passages which accommodate titanium pins 42. The core itself is transparent to X-radiation and so would normally be invisible in a post-operative X-ray examination. The pins 42 serve as radiographic markers and enable the position of the core 30 to be ascertained during such examination.

(26) Annular grooves 44 are provided in the plates 10, 10.1 to facilitate holding and manipulation of the prosthesis by appropriate instruments during placement into the intervertebral disc space.

(27) Compared to known prostheses, the prostheses 28, 50 described above have a number of advantages, as follows:

(28) 1. The peg or pegs 22 hold the core captive and prevent it from slipping out sideways.

(29) 2. At a predetermined maximum limit of articulation, the rims 24 contact one another to prevent further articulation. At the same time, the peg or pegs 22 contact the sides of the openings 34, also preventing further articulation.

(30) 3. An acceptable bearing area of the plates on the core is obtained with the central peg/central opening configurations described above. This is particularly advantageous compared to prior art prostheses where the core has peripheral channels on its upper and lower surfaces to receive peripheral rims on the plates at the limit of articulation.

(31) In practice, it may happen that there is imperfect alignment of the prosthesis plates. In the case of very poor alignment, the dual peg configuration of FIGS. 5 and 6 gives rise to the possibility that the pegs 22 are laterally offset from one another by a substantial distance. This can in turn lead to the undesirable consequence that the range of articulation which can be accommodated is unduly limited, and attempts to articulate past the limit could result in damage to the core by the misaligned pegs. In the embodiment of FIG. 7 the single peg can be received centrally by the core, thereby avoiding this potential problem.

(32) The principles of the invention are applicable to prosthetic disc implants for lumbar vertebrae as well as cervical vertebrae. In the latter case, the fins 16 will typically be omitted.