GB2306330A

GB2306330A - Acetabular cup

Info

Publication number: GB2306330A
Application number: GB9521737A
Authority: GB
Inventors: Peter Jeremy Gibson; George John James Cremore
Original assignee: Corin Medical Ltd
Current assignee: Corin Medical Ltd
Priority date: 1995-10-24
Filing date: 1995-10-24
Publication date: 1997-05-07
Also published as: GB9521737D0

Abstract

The cup comprises a backing member 10 and an insert 12 which, in use, is allowed to move relative to the backing member. The insert may be retained within the backing member by a flange 24 and may have an upper planar surface 30 which contacts a similar surface 20 of the backing member. Unconstrained rotation of the insert is thus permitted whilst relative gliding movement of the components is also allowed, the contacting surfaces of such components being highly polished.

Description

ACETABULAR CUP The invention relates to an acetabular cup.

Hip prostheses commonly consist of an acetabular cup and a femoral stem having a ball joint designed to be held inside a socket in the acetabular cup. The acetabular cup consists of a generally hemispherical metal backing member designed to be immovably fixed in a pelvic cavity and an insert made from an implant grade plastics material and designed to fit immovably inside the backing member. Many inserts are designed to be snap-fitted into the metal backing member. In all cases, the outer diameter of the insert is substantially identical to the inner diameter of the backing member so that, in use, the outer surface of the insert and the inner surface of the backing member are in constant contact.This can lead to undesirably high levels of wear and can also result in loosening forces being transmitted through the prosthesis to the interface between the backing member and the pelvic bone.

It is an object of the present invention to provide a hip prosthesis which overcomes the aforementioned disadvantages.

The invention provides an acetabular cup as set out in claim 1. Advantageous features are set out in the subsidiary claims.

Allowing relative movement between the backing member and the insert reduces wear between the said components. It also helps to reduce the loosening forces transmitted through the prosthesis to the interface between the backing member and the pelvic bone which decreases the risk of the prosthesis becoming dislodged, thus increasing the life of the prosthesis.

An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawing wherein Figure 1 is a vertical cross section through an acetabular cup according to the invention.

The figure shows a backing member 10 and an insert 12 which, together with a femoral component, form a hip prosthesis. The femoral component is of standard design and essentially comprises a stem for introduction into a femoral cavity and a femoral head dimensioned so as to fit into a socket 14 formed in the insert 12. The socket can be of any desired diameter to accommodate femoral heads of differing sizes. The connection and interaction between the femoral component and the insert 12 are well known and form no part of the present invention. They will therefore be described no further here.

The backing member 10 is generally hemispherical in shape. Fixing means (not shown) for fixing the backing member 10 into a prepared cavity in the pelvic bone are provided in the usual manner. The fixing means are arranged on the outer surface 16 of the backing member 10 and can take the form of, for example, screws, pegs, spikes, a flange, porous surface texturing for bone ingrowth or shapings for receiving cement.

The inner surface 18 of the backing member 10 is generally hemispherical save for a portion 20 remote from the periphery of the hemispherical walls. The portion 20 takes the form of a generally circular planar surface extending perpendicular to the longitudinal axis 22 of the backing member 10. The inner surface 18 and the planar surface 20 are both highly polished.

Around the periphery of the hemispherical walls is an inwardly extending flange 24 whose function will be described below.

The insert 12 is also generally hemipsherical in shape with the socket 14 for receiving the femoral head being located substantially centrally of the otherwise planar lower surface 26. The outer walls 28 of the insert 12 are complimentary to the inner walls 18 of the backing member 10 but the outer diameter of the insert 12 is substantially 4mm less than the inner diameter of the backing member 10. Alternative differences in diameter are equally possible. An upper planar surface 30 is also located remote from the lower surface 26 at a distance therefrom equal to the distance between the flange 24 and the planar surface 20 of the backing member 10.

In use, the insert 12 is located inside the backing member 10 and is retained therein by means of the flange 24. The dimensions are such that the upper planar surface 30 of the insert 12 is maintained in contact with the planar surface 20 of the backing member 10.

However, the reduced diameter of the outer walls 28 of the insert 12 provides 2mm of play in all directions between the insert 12 and the backing member 10. This allows the upper surface 30 of the insert 12 to glide across the planar surface 20 of the backing member 10 in any direction defined by the interface between the two surfaces 20,30 until contact between the part-hemispherical walls of the insert 12 and the backing member 10 prevents further movement. In this way the gliding relative movement is constrained.

However, rotation of the insert 12 with respect to the backing member 10 about the longitudinal axis 22 is permitted and is unconstrained. Constraining means can be provided if desired.

The relative movement described above between the insert 12 and the backing member 10 is advantageous in that there is very little wear between the two relevant components 10,12. Indeed, contact between the components 10,12 is reduced. Although relative movement between the components in the direction of the longitudinal axis 22 is prevented, the freedom of constrained movement in the other two possible directions is highly advantageous. In particular, forces which would otherwise be transmitted through the prosthesis, such as torsional forces which tend to loosen the fixing of the backing member in the pelvic bone, are not transmitted or are transmitted to a lesser extent through this arrangement.

Very commonly, hip prostheses consist of a metal backing member, a metal femoral component and an insert made of an implant grade plastics material such as ultra high molecular weight polyethylene. As usual, the head of the femoral component will be highly polished to allow substantially uninhibited rotational movement between the head and the socket. In the present arrangement, the interior walls of the backing member will be highly polished to allow uninhibited gliding movement between the planar surfaces 20,30 and reduced wear at the other surfaces 18,28. The insert can, as an alternative, be made from ceramics with highly polished surfaces or, if desired, from metal, again with highly polished surfaces to allow the desired freedom of movement.

It will be appreciated that the invention is not limited to the exact features of the embodiment illustrated above. For example, alternative means for retaining the insert inside the backing member could be employed. Equally, the amount of play between the components could be varied as desired. Further variations and modifications will be apparent to a reader skilled in the relevant art.

Claims

1. An acetabular cup comprising a backing member and an insert, wherein, in use, relative movement is allowed between the insert and the backing member.

2. An acetabular cup as claimed in claim 1, wherein the insert is allowed to rotate with respect to the backing member about a longitudinal axis thereof.

3. An acetabular cup as claimed in claim 1 or 2, wherein the insert is allowed to move with respect to the backing member in a plane perpendicular to the longitudinal axis thereof.

4. An acetabular cup as claimed in claim 3, wherein the backing member and insert each have a polished planar surface extending, in use, perpendicular to the longitudinal axis of the backing member.

5. An acetabular cup as claimed in claim 3 or 4, wherein the outer diameter of the insert is substantially smaller than the inner diameter of the backing member.

6. An acetabular cup as claimed in claim 5, wherein the outer diameter of the insert is between 2mm and 8mm smaller than the inner diameter of the backing member.

7. An acetabular cup as claimed in claim 6, wherein the outer diameter of the insert is substantially 4mm smaller than the inner diameter of the backing member.

8. An acetabular cup as claimed in any one of the preceding claims, wherein, in use, the insert is held in place in the backing member by means of an inwardly-extending flange.

9. An acetabular cup as claimed in any one of the preceding claims1 wherein all of the surfaces of the backing member which are contacted by the insert are highly polished.

10. An acetabular cup as claimed in any one of the preceding claims, wherein all of the surfaces of the insert which are contacted by the backing member are highly polished.

11. An acetabular cup substantially as hereinbefore described with reference to the accompanying drawing.