WO2006136940A2

WO2006136940A2 - Ankle prosthesis

Info

Publication number: WO2006136940A2
Application number: PCT/IB2006/001726
Authority: WO
Inventors: Malan De Villiers
Original assignee: Malan De Villiers
Priority date: 2005-06-24
Filing date: 2006-06-26
Publication date: 2006-12-28
Also published as: ZA200704470B; WO2006136940A3

Abstract

The invention concerns a tibiotalar ankle replacement prosthesis (10) which includes a first metal plate (12) that can be fixed to the tibia, a second metal plate (14) that can be fixed to the talus and an intermediate element (16) of moulded plastics which can be located between the metal plates. The second metal plate has a superior surface (34) which curves convexly and relatively sharply in a sagittal plane and concavely and relatively gently in a frontal plane. The intermediate element has corresponding curvatures on its inferior surface to enable the element to slide congruently, in mutually orthogonal directions, over the second metal plate.

Description

"ANKLE PROSTHESIS"

BACKGROUND TO THE INVENTION

THIS invention relates to an ankle prosthesis.

Ankle replacement surgery is often used to address traumatic injuries as well as arthritis and other degenerative conditions affecting the ankle or tibiotalar joint between the tibia and talus. At this joint, the inferior or lower surface of the tibia is curved in an anterior/posterior or sagittal plane to articulate over a correspondingly curved superior or upper surface of the talus.

One known ankle replacement prosthesis or arthroplasty is the Scandinavian Total Ankle Replacement or STAR device. This device consists of an upper metal plate which is fixed to the tibia, a lower metal component which is fixed to the talus, and an intermediate element made of metal or polyethylene between the metal components. Cooperating monoplanar curvatures on the lower metal component and the intermediate element allow relatively free articulation of the tibia relative to the talus in the anterior/posterior or sagittal plane. However very little lateral misalignment of the upper and lower metal components, i.e. misalignment in the frontal plane, can be tolerated during implantation. This is because the lower metal component has a projection extending in the anterior/posterior direction which locates in a corresponding recess in the intermediate element. Misalignment of the lower metal component and the intermediate element in the lateral/medial sense means that the projection is not centralised in the recess. Loads which have to be transmitted through the ankle joint are then concentrated on a relatively small area of contact between the projection and the side of the recess, and it is perceived that this can lead to early failure of the replacement.

The present invention seeks to provide an alternative prosthesis for use in tibiotalar ankle joint replacement. SUMMARY OF THE INVENTION

According to the present invention there is provided a tibiotalar ankle replacement prosthesis comprising a first metal plate fixable to the tibia, a second metal plate fixable to the talus and an intermediate element for location between the metal plates, the second metal plate having a superior surface which curves convexly and relatively sharply in a sagittal plane and concavely and relatively gently in a frontal plane, and the intermediate element having corresponding curvatures on its inferior surface to enable the element to slide congruently, in mutually orthogonal directions, over the second metal plate.

The corresponding curvatures allow sliding movement to take place while congruent contact is maintained between the second metal plate and the intermediate element.

Preferably, the convex and concave curvatures of the superior surface of the second plate are continuous.

The first metal plate may have planar superior and inferior surfaces and it may carry projections on the superior surface which can be fixed into holes in an inferior surface of the tibia.

The second metal plate may have an inferior surface carrying projections thereon which can be fixed into holes in a superior surface of the talus.

In the preferred embodiments, the inferior surface of the second metal plate is curved in a manner similar to that of the superior surface.

The intermediate element may have a planar superior surface to mate with the inferior surface of the first plate.

Other features of the invention are described below and set forth in the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS

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

Figure 1 illustrates the components of a prosthesis according to the invention in an exploded perspective view;

Figure 2 shows a perspective view of the assembled prosthesis;

Figure 3 shows a cross-section at the line 3-3 in Figure 2; and

Figure 4 shows a cross-section at the line 4-4 in Figure 2.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

The prosthesis 10 seen in the drawings has three components, namely a first metal plate 12, a second metal plate 14 and an intermediate element 16.

The plate 12 is made of titanium. Its inferior surface has a titanium nitride wearing finish. Alternatively, this plate may be made of a cobalt chrome molybdenum (CCM) alloy. It is generally flat and relatively thin with an anterior or front edge 18, a posterior or rear edge 20, a medial or inner edge 22 and a lateral or outer edge 24. The plate tapers in width from the edge 20 to the edge 18. There are three projecting pegs 26 on the superior surface of the plate. In other embodiments, the pegs may be replaced by projecting fins.

In use, the lower end of the tibia 30 is modified as appropriate to have a generally flat surface and the plate 12 is fixed to this surface 'by anchoring the pegs 26 in holes 32 reamed for the purpose in the prepared end of the tibia, as shown in Figures 3 and 4. The pegs provide for initial anchorage and also for osseo-integration with the tibia with passage of time.

The plate 14 is also made of titanium and also has a titanium nitride or other smooth and hard wearing finish on its superior or upper surface 34. The plate could alternatively be made of a CCM alloy. The superior surface has a continuous, relatively sharp convex curvature in the sagittal plane, as will be evident from the cross-section of Figure 3. It also has a continuous, somewhat more gentle concave curvature in the frontal plane, as will be evident from the cross-section of Figure 4.

By the term "continuous curvature" is meant that the curvature proceeds in a generally uniform manner from one edge of the plate to the opposite edge. Thus, in the case of the anterior/posterior direction, the convex curvature proceeds continuously and in a generally uniform manner from the anterior or front edge 36 to the posterior or rear edge 38 and in the lateral direction the concave curvature proceeds continuously and in a generally uniform manner from one lateral edge to the other.

Projecting from the inferior or lower surface of the plate 14 is a pair of pegs 40 with roughened outer surfaces. In use, after the superior surface of the talus has been modified as necessary, holes are formed therein at a spacing matching that of the pegs 40. The plate is then fixed down onto the talus by locating the pegs in the holes and cementing them there as shown in Figures 3 and 4.

The intermediate element 16 is moulded in a hard plastics material with a suitably low coefficient of sliding friction, such as HDPE (high density polyethylene) although other suitable polymers may also be used. It is also envisaged that the element 16 could be metallic, possibly a CCM alloy suitable for metal on metal contact. It has a flat superior surface 42 and an inferior .surface 44 which is relatively sharply, concavely curved in the sagittal direction, as will be evident from Figure 3 and concavely and somewhat more gently curved in the transverse direction as will be evident from Figure 4. The convex and concave curvatures of the surface 44 are again continuous between opposite edges and they are complemental to the curvatures of the surface 34 of the plate 14.

The intermediate element 16 is in use positioned between the plates 12 and 14 as shown in Figures 3 and 4.

The main flexural movement of the foot at the tibiotalar ankle joint is in the anterior/posterior plane. Such movement is accommodated by relative sliding movement of the intermediate element 16, in the sagittal plane, over the metal plate 14.

An important advantage of the illustrated embodiment is its ability to accommodate some inaccuracy in the positioning of the plates 12 and 14 not only in the sagittal direction but also laterally!. It will be understood that such accommodation comes about because of the ability of the intermediate element 16 to slide over the plate 14 in both directions. Also, because the curvatures of the relevant surfaces are continuous, there are no undue stresses transferred from one component to another. This may be contrasted with the situation pertaining in the case of known prostheses, such as the STAR prosthesis, which do not have a continuous lateral curvature and in which the undesirably high stress concentrations can be created by non-centralisation of the components relative to one another attributable to the aforementioned projection and recess configuration.

Claims

1.

A tibiotalar ankle replacement prosthesis comprising a first metal plate fixable to the tibia, a second metal plate fixable to the talus and an intermediate element for location between the metal plates, the second metal plate having a superior surface which curves convexly and relatively sharply in a sagittal . plane and concavely and relatively gently in a frontal plane, and the intermediate element having corresponding curvatures on its inferior surface to enable the element to slide congruently, in mutually orthogonal directions, over the second metal plate.

2.

A prosthesis according to claim 1 wherein the convex curvature of the superior surface of the second plate is continuous.

3.

A prosthesis according to either one of the preceding claims wherein the concave curvature of the second plate is continuous.

4.

A prosthesis according to any one of the preceding claims wherein the first metal plate has planar superior and inferior surfaces and carries projections on the superior surface which can be fixed into holes in an inferior surface of the tibia.

5.

A prosthesis according to any one of the preceding claims wherein the second metal plate has an inferior surface carrying projections thereon which can be fixed into holes in a superior surface of the talus.

6.

A prosthesis according to claim 5 wherein the inferior surface of the second metal plate is curved in a manner similar to that of the superior surface.

7.

A -prosthesis according to any one of the preceding claims wherein the intermediate element has a planar superior surface to mate with the inferior surface of the first plate.

8.

A prosthesis according to any one of the preceding claims wherein the intermediate element is of moulded plastics construction.

9.

A prosthesis according to claim 8 wherein the intermediate elements is made of HDPE (high density polyethylene).

10.

A prosthesis according to any one of claims 1 to 7 wherein the intermediate element is metallic.

11.

A prosthesis according to claim 10 wherein the intermediate element is made of a CCM (cobalt chrome molybdenum) alloy.

12.

A prosthesis according to any one of the preceding claims wherein the first plate is made of titanium and has an inferior surface with a titanium nitride finish thereon.

13.

A prosthesis according tδ any one of claims 1 to 11 wherein the first plate is made of CCM (cobalt chrome molybdenum) alloy.

14.

A prosthesis according to any one of the preceding claims wherein the second plate is made of titanium and the superior surface thereof has a titanium nitride finish.

15.

A prosthesis according to any one of claims 1 to 13 wherein the second plate is made of CCM (cobalt chrome molybdenum) alloy.