GB2191402A

GB2191402A - Improvements relating to hip implants

Info

Publication number: GB2191402A
Application number: GB08614460A
Authority: GB
Inventors: Michael Antony Tuke; Michael Alexander Reyk Freeman
Original assignee: FINSBURY
Current assignee: FINSBURY
Priority date: 1986-06-13
Filing date: 1986-06-13
Publication date: 1987-12-16
Anticipated expiration: 2006-06-13
Also published as: GB2191402B; GB8614460D0

Abstract

An artificial acetabulum, intended for implantation in a living body, which comprises a socket-forming member with a curved lining surface to receive the ball head of an artificial femural implant for rotation within the socket. An adjacent peripheral lip is arranged about the periphery of the socket lining surface, the lip being attached at its base to the socket-forming member, but separated at its edge opposite the base from the socket lining surface. <IMAGE>

Description

SPECIFICATION Improvements relating to hip implants This invention relates to hip implants.

Hip replacement operations are commonly performed to replace diseased or deformed hip joints with, on the whole, successful and beneficial results. In the usual operation, the ball of the femur and some of the adjacent bony neck is resected and an implant, for example of titanium, surmounted by a ball is inserted in a suitably formed aperture in the femur. The acetabulum is also replaced by a synthetic cup, providing a socket for the ball of the femur implant.

It is important for the artificial joint to provide as good a range of motion as possible.

At the same time it is clearly necessary to provide a stable long lasting joint which does not have the disadvantages of ready dislocation, loosening, or wear after a comparatively short time. Such a combination of desirable factors is by no means easy to achieve.

Thus to give a good range of movement with minimal danger of the neck of the implant hitting the edge of the acetabulum and causing loosening, the acetabulum should provide a relatively shallow socket and the ball should be comparatively large. However, larger heads tend to produce more debris, and the risk of dislocation becomes greater with a shallow socket. As the artificial joint is used, the ball tends to sink deeper into the socket and some debris is produced. This debris tends to be displaced from the socket and can find its way between the implant and the bone giving an artificial joint of uncertain longevity. Small heads and shallow sockets provide less debris, but the risk of dislocation is enhanced. Larger heads produce more undesirable debris.A deep acetabulum helps to contain the ball against dislocation but the consequent risk of impingement between the neck of the implant and the acetabulum can lead to loosening of the artificial joint.

The optimum dimensions of acetabulum and femoral implant for any one patient must be a careful balance bearing in mind these factors.

The present invention seeks to provide an improved artificial acetabulum.

According to this invention, we provide an artificial acetabulum, intended for implantation in a living body, in the form of a socketforming member, having a curved socket lining surface capable of receiving the ball head of an artificial femoral implant for rotation within the socket, the periphery of the socket lining surface having an adjacent peripheral lip, attached at its base to the socket-forming member, but separated at its edge opposite the base from the adjacent periphery of the lining surface.

The provision of the peripheral lip allows flexing movement of the socket edge in the plane of the socket lining surface but resists flexion from compressive loads on the socket.

Preferably the acetabulum is of synthetic plastics material such as ultra high molecular weight polyethylene and the lip is integral with the remainder of the socket-forming member, being separated from the socket lining surface by a peripheral cut.

Alternatively the socket-forming member may be of a material such as ceramics with the lip constructed of a separate material such as plastics, for example polyethylene, attached by any suitable bonding or assembly means.

One form of the invention will now be described with reference to the accompanying drawing, which is a side view, partly in section, of an acetabulum and associated femoral implant.

The drawings shows the upper portion of a femoral implant comprising a portion 2 of flattened section which extends laterally from the implant stem (not shown) and tapers to a neck 4 which is surmounted by a ball head 6.

This femoral implant may be of any suitable material compatible with the body. The ball head 6 can rotate within an artificial acetabulum 10 for example of ultra high molecular weight polyethylene. The outwardly facing surface 11 of the acetabulum 10 can be contoured as desired to key to the hip bone of the patient. The inwardly facing socket lining surface 12 provides a bearing surface for the ball head 6. Around the periphery of surface 12, separated therefrom by a wedge section peripheral cut out 14, is a peripheral lip 16, integral at its base with the rest of the acetabulum 10. The lip 16 may however be of a different material from the rest of the acetabulum, for example it may be of plastics such as polyethylene while the remaining acetabulum is of ceramics.

In use, the lip 14 acts in a manner analogous to that of the labrum (also known as the cotyloid ligament) which provides a fibro-cartilaginous rim to a natural acetabulum which serves to deepen the acetabulum socket while at the same time protecting the edges of the bone and mating snugly against the ball head of the femur. Thus the lip 14 can flex under the influence of forces substantially normal to the side of the lip as illustrated by arrow 18.

However the lip is still of sufficient strength so as not to adversely affect the load bearing properties of the ball and socket joint in a substantially radial direction as shown by arrow 19. The flexing of the lip 14 reduces the risk of damage if the movement of the ball within the socket is sufficient to cause neck 4 to hit the acetabulum 10. Thus a deeper socket for a particular size of ball can be employed with reduced risk of dislocation of the ball and reduced tendency to loosen the joint.

We have found, for example, that a ball head of 32 mm diameter can be employed with an acetabulum having an inner surface, lip to lip, which extends over an arc of approximately 1700.

Claims

1. An artificial acetabulum, intended for implantation in a living body, comprising a socket-forming member having a curved socket lining surface capable of receiving the ball head of an artificial femural implant for rotation within the socket, the periphery of the socket lining surface having an adjacent peripheral lip attached at its base to the socketforming member, but separated at its edge opposite the base from the adjacent periphery of the lining surface.

2. An acetabulum according to claim 1, wherein the socket forming member is of synthetic plastics material.

3. An acetabulum according to claim 2, wherein the plastics material is ultra high molecular weight polyethylene.

4. An acetabulum according to any one of the preceding claims wherein the lip is integral with the remainder of the socket forming member, being separated from the socket lining surface by a peripheral cut.

5. An acetabulum according to claim 4 wherein the cut has a wedge-shaped cross section.

6. An acetabulum according to claim 1, wherein the lip is of a different material from the socket-forming member.

7. An acetabulum according to claim 6 wherein the socket-forming member is of ceramic material and the lip of plastics material.

8. An artificial acetabulum substantially as shown in the accompanying drawings and described herein with reference thereto.