Methods of manufacturing an acetabular cup

Abstract: There is described an acetabular cup (1) having an outer shell (3) and an inner liner (2) which inner liner (2) provides a bearing surface of metallic or ceramic material, wherein there is provided in a position around the rim (5) of the inner liner (2) a shoulder (4,10) formed of a material that is softer than that used for the head or neck of a femoral component with which the acetabular cup (1) is to be used, which shoulder (4,10) limits the motion of the femoral component during use. The shoulder (4,10) may be of a polymeric material such as polyethylene. There is also described an acetabular cup (1) having an inner liner (2) of metallic or ceramic material and an outer shell (3) which is manufactured form a bone cement material. There is further described a method of manufacturing the described acetabular cups (1) which method comprises directly moulding the polymeric shoulder (4,10) or outer shell (3) around the ceramic or metallic inner liner (2).

Description: The invention relates to components to be used in hip prostheses and in particular to acetabular cups and components therefor for use in hip prostheses and methods of making such components. In replacement hip surgery a femoral component is inserted into the prepared femur. The femoral component has a stem portion which projects into the femoral canal of the prepared femur and has an integral or separate modular head of substantially spherical shape. The ball-like head of the femoral component is received within an acetabular cup component which is implanted in the patient's hip socket, i.e. the acetabulum. The acetabular cup has a substantially hemi-spherical bearing surface for movement of the ball head of the femoral component during action of the joint. The acetabular cup is implanted into the prepared hip socket either with or without cement. Cementless types of acetabular cup may be secured in the prepared bone by a press fit or can be directly screwed in place or otherwise secured in place, for example by indirect means, eg. by the use of separate bone screws passing through apertures provided in the acetabular cup. Generally the femoral stem is of metal and the ball head is of metal or of a ceramic material. Various designs of acetabular cup are available and it is often a multi-piece component having at least a separate outer shell and an inner liner. Where the acetabular cup has an inner liner that inner liner is generally press-fitted into the outer shell. In some designs of hip prostheses the material of the bearing surface of the acetabular cup, e.g. its inner liner where present, is of the same material as that of the ball head, e.g. for a ceramic head a ceramic bearing surface is provided (a so-called ceramic-on-ceramic prothesis) and for a metal head a metal bearing surface is provided (a so-called metal-on-metal prothesis). In some other designs the acetabular bearing surface is of polyethylene as the acetabular cup is either provided with a polyethylene inner liner or the acetabular cup is a single component made entirely from polyethylene. The shape of the bearing surface into which the ball head is received affects the degree of movement available after implantation of the joint. It has been found that the use of polyethylene acetabular bearing surfaces in combination with metal or ceramic femoral ball heads leads to high wear rates of the polyethylene. Aseptic loosening of the separate components due to wear of the polyethylene is now emerging as a major factor in hip prosthesis longevity. As a result there has been a resurgence of interest in alternatives to the use of polyethylene acetabular bearing surfaces. The wear rate is substantially reduced for metal-on-metal prosthesis and ceramic-on-ceramic prosthesis have been shown in most cases to give an extremely low wear rate and generate very little wear debris. There is, however, in a significant minority of cases the possibility of wear debris, e.g. ceramic or metal particles, becoming trapped between the articulating surfaces which can create further debris by third body wear. This sets up an accelerating spiral resulting in massive wear and loosening of the implant. One of the ways in which this can be initiated is impingement of the head or neck of the femoral component on the rim of the acetabular cup. This is particularly a problem with ceramic ball heads articulating with a ceramic acetabular cup. This can generate ceramic debris which gets between the articulating surfaces. There is, therefore, a need for an improvement in acetabular cup design so as to provide a prosthesis having low wear characteristics. With conventional acetabular cups where the inner liner is simply press-fitted into the other acetabular shell the separate components can become loosened from each other and this can allow debris to come between the component parts. There is, therefore, also a need for improvements in the manufacture of acetabular shells. EP 0698382 A2 discloses a method of manufacture of a polymeric bearing element for an acetabular cup, which bearing element is then suitable to be held in a metal shell or cemented into the acetabulum (page 1, lines 23 to 25). The method disclosed involves the injection moulding of two polymeric components in order to form a bearing element which combines the desired "softness" of an elastomeric polyurethane with the necessary stiffness of polyethylene. According to the present invention, there is provided a method of manufacturing an acetabular cup as claimed in claim 1. An acetabular cup manufactured according to the present invention provides a good and much more secure fitting between the inner liner and outer shell. Any suitable moulding technique, such as injection or more preferably compression moulding, may be used. The polymeric material is preferably moulded from its powder form and the heat and other conditions required would be well known to the man skilled in the art. Some polymeric materials, e.g. PMMA, can also be cast and such a moulding process is also encompassed within the present invention. The outer acetabular shell can be moulded around the inner liner directly into its desired shape or it can be moulded onto the inner liner as a block and thereafter machined into the required shape after the moulding has taken place. The method according to the invention can be used to mould a shoulder or chamfer alone onto a metallic or ceramic inner liner or it can be used to mould an outer shell having such a shoulder or chamfer around a ceramic inner liner or it can be used to mould an outer shell having such a shoulder or chamfer around a ceramic of metallic inner liner and it may also be used for moulding an outer shell formed of a bone cement material around a metallic or ceramic inner liner. For a better understanding of the present invention and to show how the same may be put into effect reference will now be made, for the purposes of illustration only, to the accompanying drawings in which:- Figure 1 is a perspective view of an acetabular cup; Figure 2 is a cross-sectional view of the acetabular cup of Figure 1; Figure 3 is a cross-sectional view of another acetabular cup; Figure 4 is a cross-sectional view of a further acetabular cup; and Figure 5 is a cross-sectional view of a yet further acetabular cup. In Figures 1 and 2 there is shown an acetabular cup 1 having an inner ceramic or metallic liner 2 and an outer acetabular shell 3 of ultra high molecular weight polyethylene or polymethylmethacrylate. The outer shell 3 is provided with a tapering surface or shoulder 4 which extends around the outer edge 5 of the inner liner 2. The shoulder 4, which takes the form of a chamfer, provides a soft tapering surface acting as a limiting stop to prevent extreme ranges of motion of a femoral component received within the acetabular cup. A metallic or ceramic head or stem of a femoral component would impinge against that soft cushion 4. The acetabular cup shown in Figure 3 is substantially identical to that shown in Figure 1 except that its cushion or shoulder 6 has a soft rounded contour rather than a conical tapering surface. The acetabular cup shown in Figure 4 has an inner ceramic or metal liner 7 and an outer acetabular shell which may be of any material, e.g. a conventional metallic acetabular shell 8. Around the outer rim 9 of the inner liner 7 there is provided an annular shoulder 10 formed of polyethylene. The polyethylene shoulder 10 may be moulded on to the outer rim of the inner liner 7 or on to a suitably prepared surface of the outer acetabular shell 8. The acetabular shell shown in Figure 5 has an inner metallic or ceramic liner 11 an outer acetabular shell 12 of any material, e.g. metal, and an intermediate component 13 into which the inner liner 11 is recessed. The intermediate component 13 is provided with a shoulder 14 which extends around the outer edge 15 of the inner liner 11 and provides a soft shoulder as with the other embodiments. All the embodiments shown in the drawings can be manufactured by directly moulding the polymeric components, eg. shoulders, intermediate components or outer shells, onto and around the inner liners.

Claims: A method of manufacturing an acetabular cup (1) having an outer shell (3, 8, 12) and an inner liner (2, 7, 11), which acetabular cup either is provided around the inner liner with a shoulder (4, 6, 10) formed of a biomedically acceptable polymer or is provided with an outer shell (3) formed of a biomedically acceptable polymer, characterised in that the inner liner (2, 7, 11) provides a bearing surface of a metallic or ceramic material and in that the method comprises the step of directly moulding the polymeric shoulder (4, 6, 10) or outer shell (3) around the inner liner (2, 7, 11). A method as claimed in claim 1, characterised in that the shoulder (4, 6, 10) or outer acetabular shell (3) is moulded around the inner liner (2, 7, 11) immediately into its desired shape. A method as claimed in claim 1, characterised in that the shoulder (4, 6, 10) or outer acetabular shell (3) is moulded onto the inner liner (2, 7, 11) as a block and thereafter machined into the required shape after the moulding has taken place. A method as claimed in any one of the preceding claims, characterised in that the moulding technique comprises compression moulding. A method as claimed in any one of the preceding claims, characterised in that the polymeric material is moulded from its powder form.

Inventors: BATEMAN RONALD JAMES, SCOTT ROBERT ANDREW, BATEMAN, RONALD JAMES, SCOTT, ROBERT ANDREW
Applicants: BIOMET LTD, BIOMET LIMITED
Application year: 1996
Main classification: A61F2
IPC codes: A61F2, B29C70, B29C43