GB2116848A

GB2116848A - Element for prosthetic joint

Info

Publication number: GB2116848A
Application number: GB08305361A
Authority: GB
Inventors: James Michael Sheehan; Ian Andrew Brown
Original assignee: Zimmer Deloro Surgical Ltd
Current assignee: Zimmer Deloro Surgical Ltd
Priority date: 1982-03-19
Filing date: 1983-02-25
Publication date: 1983-10-05
Also published as: GB2116848B; GB8305361D0

Abstract

An element for a prosthetic joint assembly is provided which is intended for engagement with another element having a forked end each of the furcations of which have an inwardly directed arcuate flange. The element of the invention has two grooves in the head to engage the rolling surface of the other element, and, between the grooves, a retaining stud to engage the flanges of the other element. The bearing surface of the element is a layer formed from a high molecular plastics material and is supported on an underlying metallic base on the bearing surface of which are shaped two grooves corresponding to the grooves of the final construction and therebetween a metallic web extending into the stud. The metal web provides lateral bending strength and the overall construction greater strength to the prosthetic element. <IMAGE>

Description

SPECIFICATION Element for a prosethetic joint This invention relates to an element for a prosthetic joint.

In U.K. Patent Specification No. A 1 333 412 there is described a prosthetic joint suitable for use as an artificial knee joint or for other joints such as an elbow or finger joint.

Such joint must be suitable for either implantation in the body or else for use in an artificial limb mechanism. In the Specification No. A 1 333 412 there is described a joint comprising two members one of which has a forked end, each furcation of the forked end being of plate-like construction and forming on its edge an arcuate rolling surface there being an inwardly directed arcuate flange on each furcation. The other element has a head including a pair of grooves, each groove engaging one of the arcuate rolling surfaces and having a retaining stud integral with the head between the grooves which stud projects outwardly on each side to embrace the flanges.

In particular the arcuate surfaces and grooves are so formed that the relative movement therebetween simulates the movement of the femural condyle.

This prosthesis has been very successful but the need has arisen to provide further strength in the stud and it would also be desirable to provide a construction in which a greater portion of the head could be formed from materials such as titanium or titanium alloy which are difficult to forge in a complex shape. Metal bases supporting plastics material surfaces have been suggested but primarily as support for the articular surfaces (Insall-Burstein Knee System Brochure of Zimmer Inc. Warsaw Indiana USA 1979).

According to the invention there is provided an element for a prosthetic joint for engagement with another element which has a forked end in which each furcation has an inwardly directed arcuate flange, comprising on the head of the element a pair of grooves and a retaining stud with projections on each side to embrace the flanges, the grooves engaging rolling surfaces of the other element, the bearing surface of the element being a high molecular weight plastics material layer disposed over a main body of high strength metal having on the end surface thereof grooves corresponding to the engaging grooves in the bearing surface and therebetween a tongue extending into the stud.

In a preferred construction the tongue comprises a web extending along the stud and having one or more recessed portions particularly openings through the thickness of the web to engage tongues of the plastics material so as to assist in retaining or keying the plastics material layer for engagement with the metallic web. A further means of retaining the plastics layer for engagement with the metallic body is at least a partially circumferential groove spaced a short distance from the edge of the outer end of the main body into which a skirt or annular section of plastics material depending from the main portion of plastics material extends and is engaged thereto.The invention will be more clearly understood within the following description of a preferred construction of the knee joint according to the invention given by way of example with reference to the accompanying drawings of the tibial element for a knee joint as generally described in U.K. Patent Specification No. A 1 333 412 sold as the Sheehan Knee Prosthesis.

In the accompanying drawings: Figure 1 is a cross-section of a tibial element according to the invention; Figure 2 is a side-view (with partial crosssection) of the element; Figure 3 is a plan view of the head of the element.

This element is intended for engagement with a femural limb element from a Sheehan Knee Prosthesis. Such element has a forked end having two plate-like furcations the inner surfaces of which have inwardly directed flanges.

The tibial element comprises a head generally 1 and a stem 13.

The head 1 comprises a retaining stud 2 with projections 3 on each side to embrace the flanges of the forked element and a pair of troughs or grooves 4 which grooves engage the rolling surfaces of the forked element and which are so formed that the relative movement therebetween and the surfaces of the forks of the femural limb simulate the movement of the femural condyle when in use. The element comprises a main structural body 5 which is of a suitable metal and in the end surface portion of which are two grooves 6 corresponding to the engaging grooves 4. The grooves in the metallic portion generally correspond to the grooves in the plastics layer although not necessarily identical thereto.By the metal curvature of the grooved portions 6 mirroring the radii of curvature of the engaging grooves 4 i.e. the articular surfaces, the grooved portions 6 act as an internal support for the articular surfaces. Between the grooved portions 6 a metal tongue 7 extends from the main body 5 into the stud 2.

The bearing surface layer 8 of the element comprises a high molecular plastics material and on either side of the stud forms the projections 3 which embrace the flanges on the femural element. Preferably the tongue extends as a web 7 along the length of the stud to provide lateral bending strength to the stud. Disposed along the length of the web are one or more indentations or interstices 9 which can be openings which extend through the thickness of the web and into which tongues of the plastics material extend so as to ensure retention of the plastics bearing surface on the element. The entry portions to these openings can be of slightly greater diameter than the central bores which join the entry portions. This assists entry of the plastic into the openings.The plastics material layer extends from the stud over the end of the element to form the engaging grooves 4 and then edges or lips 10. These raised lips stabilise movement of the joint. The bearing layer can have an annular portion or skirt extending around the edge of the end surface of the element. About the metal head there can be, slightly below the outer edge of the main metallic body thereof, a circumferential groove 11 into which tongue portions from the annular section or skirt of the plastics upper surface extend so as to ensure further retention of the plastics bearing layer to the main metallic element. The lip 12 of this groove nearest the outer end is segmented or discontinuous.

Preferably when implantation in the body is envisaged the main body of the element is manufactued from a suitable metal such as chrome cobalt alloy or a stainless steel or more preferably a titanium or titanium alloy which can be forged. The plastics layer can be formed from a suitable bearing material such as a biologically compatible plastics material.

A particularly suitable biologically compatible plastics material is a high density polyethylene material or a fibre reinforced ultra high molecular weight polyethylene particularly carbon fibre reinforced material. Fibre composite metal supported structures help reduced plastic deformation of the plastic articulating surface.

The stem 13 has three flats 14 to prevent rotation within the cement bed which holds the stem in the tibial shaft and minimise stress concentration at the prosthesis-cement interface.

Rotation is further resisted by flats 15 on the opposite sides of the bulbous part of the main body 5 which is embedded in cement in the proximal tibia.

The element is constructed by moulding the plastics layer onto the metallic body. This is in contrast to the prior art structure where a substantial body of polymeric material formed the head and, because of the difficulty of shaping, generally required machining from a block. A moulded layer has a superior finish for the surface.

Claims

1. An element for a prosthetic joint for engagement with another element which has a forked end in which each furcation has an inwardly directed arcuate flange and comprising on the head of the element a pair of grooves and a retaining stud with projections on each side to embrace the flanges, the grooves engaging rolling surfaces of the other element, the bearing surface of the element being a high molecular weight plastics material layer disposed over a main body of high strength metal having on the end surface thereof grooves corresponding to the engaging grooves in the bearing surface and therebetween a tongue extending into the stud.

2. An element according to claim 1, wherein the metal tongue is a web extending along the length of the stud.

3. An element according to either of claims 1 and 2, wherein the web is provided with one or more indentations along the length of the web to provide keying points for the plastics material layer.

4. An element according to claim 3, wherein the indentations are openings through the web.

5. An element according to claim 4, wherein the entries into the openings are greater in diameter than the interior of the opening.

6. An element according to any one of claims 1 to 5, wherein the circumferential edge of the polymeric layer has a skirt portion extending around the metallic main body.

7. An element according to claim 6, wherein the lower edge of the skirt portion extends into a circumferential groove around the head of the metallic main body.

8. An element according to claim 7, wherein the upper lip of the groove is discontinuous.

9. An element according to any one of claims 1 to 8, wherein the metallic body is forged titanium or titanium alloy.

10. An element according to any one of claims 1 to 9, wherein the plastics material of the bearing surface layer is a fibre reinforced high molecular weight polymeric material.

11. An element according to claim 10, wherein the layer is a carbon fibre reinforced ultra high molecular weight polyethylene.

12. An element according to claim 1, substantially as hereinbefore described.