WO2014087177A1

WO2014087177A1 - Acetabular cup, system and method

Info

Publication number: WO2014087177A1
Application number: PCT/GB2013/053238
Authority: WO
Inventors: Paul Mayon; Simon Collins
Original assignee: Corin Limited
Priority date: 2012-12-07
Filing date: 2013-12-09
Publication date: 2014-06-12
Also published as: GB2508654A; GB2508654B

Abstract

An acetabular cup (10) comprises an outer shell (12), an inner liner (14) receivable by the outer shell (12), and a two-part polygonal engagement element (16) at an interface between the outer shell (12) and the inner liner (14). Preferably, the two-part polygonal engagement element includes a Reuleaux polygon. An acetabular cup system utilising the said two-part acetabular cup, a second inner liner formed of a different material to the first inner liner, and a two- part taper engagement element at an interface between the outer shell and the second inner liner, along with a method of inhibiting rotational disassociation of an inner liner of an acetabular cup when engaged with an outer shell are also provided.

Description

Acetabular Cup, System And Method

The present invention relates to an acetabular cup, and more particularly for such a cup for a hip, to a system utilising such an acetabular cup along with a second inner liner, and to a method of inhibiting rotational and preferably also axial disassociation of an inner liner of such an acetabular cup when engaged with an outer shell.

Two-part acetabular cups are well known and are provided for use in total hip arthroplasty. Such acetabular cups provide a range of liners which are fitable in a common outer shell, and which utilise a variety of different materials and shapes. This enables a surgeon to intraoperatively choose a most appropriate combination for a particular patient, taking account of the patient's lifestyle and geometry of articulation.

It is common to provide a two-part acetabular cup, such as shown in international patent publication WO2012108610A1, having a metal outer shell and a plastics inner liner. The outer shell and inner liner are interengaged via a circular locking ring on an outer surface of the inner liner which snap-fits into a complementary circular channel on an inner surface of the outer shell. This prevents or limits liner disengagement during clinical use, and for example by direct push-out or by being levered apart.

To inhibit rotational disassociation of the inner liner and outer shell, rounded toothed projections on the inner liner are received in complementarily shaped recesses within the outer shell. This rotational feature can also be used to allow liner forms that reduce subsequent dislocation of the hip by limiting a range of motion. These are commonly termed extended posterior wall (EPW) liners.

The requirement of two different locking arrangements increases the complexity of the acetabular cup and therefore the manufacturing cost.

It is also problematic using toothed projections for 'thin-walled' acetabular cups. A thin-walled acetabular cup typically has a total wall thickness in the range of 2 to 5 mm, and is used to minimise the bone removed from the acetabulum during the reaming process. Since the material that must be removed from the outer shell to accommodate suitably dimensioned teeth is reasonably significant, undue weakening of the thin wall of the outer shell occurs and as such, this engagement feature cannot be readily accommodated by a thin-walled cup.

The present invention seeks to provide a solution to these problems.

According to a first aspect of the invention, there is provided an acetabular cup comprising an outer shell, an inner liner receivable by the outer shell, and a two-part polygonal engagement element at an interface between the outer shell and the inner liner, characterised in that an outwardly projecting vertex of at least one of the parts of the two-part polygonal engagement element is defined by curvilinear sides, each said curvilinear side being a constant distance from an opposing vertex. Preferable and/or optional features of the first aspect of the invention are set forth in claims 2 to 15, inclusive.

According to a second aspect of the invention, there is provided an acetabular cup system comprising an acetabular cup in accordance with the first aspect of the invention, a second inner liner formed of a different material to the first said inner liner, and a two- part taper engagement element at an interface between the outer shell and the second inner liner.

Preferable and/or optional features of the second aspect of the invention are set forth in claims 17 to 19, inclusive.

According to a third aspect of the invention, there is provided a method of preventing or inhibiting rotational disassociation of an inner liner of an acetabular cup according to the first aspect of the invention when engaged with an outer shell, the method comprising the step of urging a polar axis of the inner shell to move radially relative to a polar axis of the outer shell when a rotational force is imparted to at least one of the inner liner and the outer shell. Preferable and/or optional features of the third aspect of the invention are set forth in claims 20 to 23, inclusive.

According to a fourth aspect of the invention, there is provided an acetabular cup comprising an outer shell, an inner liner receivable by the outer shell, and a Reuleaux polygonal engagement element at an interface between the outer shell and the inner liner.

According to a fifth aspect of the invention, there is provided an acetabular cup comprising an outer shell, an inner liner receivable by the outer shell, and a two-part polygonal engagement element at an interface between the outer shell and the inner liner.

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

Figure 1 shows a perspective view of a first embodiment of a two-part acetabular cup, shown with the outer shell and inner liner separated, and in accordance with the first aspect of the invention;

Figure 2 shows a partial cross-sectional view of the acetabular cup of Figure 1, shown with the outer shell and inner liner engaged;

Figure 3 is a side elevational view of the inner liner of the acetabular cup, shown in Figures 1 and 2;

Figure 4 is a perspective view of a second inner liner forming part of an acetabular cup system, in accordance with the second aspect of the invention;

Figure 5 shows a perspective cross-sectional view of a second embodiment of a two-part acetabular cup, shown with the outer shell and inner liner engaged, and in accordance with the first aspect of the invention;

Figure 6 is an elevational view of the cross-section, shown in Figure 5, showing the two-part acetabular cup;

Figure 7 is a perspective view of the inner liner of a third embodiment of a two- part acetabular cup, in accordance with the first aspect of the invention; Figure 8 is a perspective view of the outer shell of a third embodiment of a two- part acetabular cup, in accordance with the first aspect of the invention; Figure 9 is a diagrammatic top plan view of the inner liner, shown in Figure 7 and with dashed lines to aid understanding of the perimeter geometry; and

Figure 10 is an enlarged view of part of the inner liner shown in Figure 9, showing part of a Reuleaux polygon engagement element, being a shape of constant width or diameter and no fixed centre of rotation.

Referring firstly to Figures 1 to 3 of the drawings, there is shown a first embodiment of a two-part acetabular cup 10 which comprises an outer shell 12, an inner liner 14, and a two-part polygonal engagement element 16. An outer surface 18 of the outer shell 12 is at least in part part-spherical and is suitably dimensioned to fit a reamed acetabular opening of a patient.

Although the outer surface 18 is preferably at least in part part-spherical, it may be shaped such that the equatorial diameter of the outer surface 18 at a rim 20 is marginally greater than the polar diameter of the outer surface 18 which extends perpendicularly to the equatorial diameter. This shape is beneficial to aid press-fit and uncemented fixation of the outer shell 12 into a patient's acetabulum, which is generally reamed to the smaller polar diameter dimension.

The outer shell 12 also defines an inner cavity 22 dimensioned to receive as a close or tolerance fit the inner liner 14. The inner cavity 22 is preferably part-spherical at a region 25 which extends from a polar axis 24 of the outer shell 12 and to a point P partway towards the rim 20. A wall 26 of the inner cavity 22, from the part-spherical surface 28 to the rim 20, is preferably frusto-conical forming a first taper element 30 to enable a taper fit with the inner liner 14. However, in this embodiment, the frusto- conical surface 28 and taper fit engagement option are optional and not essential.

The outer shell 12 may, for example, be formed of, typically forged or cast, bi- compatible metal, and preferably cobalt chrome-alloy or titanium alloy and may be subjected to a heat treatment process to improve its mechanical properties. However, the outer shell could be formed of a ceramic material, such as alumina, zirconia, or a combination thereof. The outer surface 18 of the outer shell 12 is also preferably coated with Titanium plasma, Hydroxyapatite and/or a porous metallic bone-like structure, to aid fixation and bone growth.

The inner liner 14 shown in the drawings is of plastics, typically Ultra High Molecular Weight Polyethylene, comprising an exterior surface 32 which is at least in part abuttable with the inner surface 34 of the outer shell 12. The inner liner 14 also includes a part-spherical, typically hemispherical or substantially hemispherical, articulating surface 36 which extends from a polar region 38 of the inner liner 14. A diameter of the articulating surface 36 is appropriate for captively or non-captively receiving a natural or prosthetic femoral head. The exterior surface 32 of the inner liner 14 is preferably complementarily shaped to match or substantially match the inner surface 34 of the outer shell 12. The exterior surface 32 includes a part-spherical portion 40 which extends from the polar region 38 of the inner liner 14, and a frusto-conical wall 42 having a diverging taper in a direction of a rim 44 of the inner liner 14 forming a second taper element 46. The taper of the second taper element 46 is substantially the same as that of the first taper element 30 of the outer shell 12.

As above, the frusto-conical wall 42 may be dispensed with, and in this case the part- spherical portion 40 of the exterior surface 32 may extend to or substantially to the rim 44, as required. Although a frusto-conical wall may be preferred, the wall may be a plane section instead of being tapered, and in this case a circlip or other retaining means may be utilised to retain the liner within the outer shell.

The inner liner 14 may be one of a plurality of differently dimensioned inner liners available to a surgeon for intraoperative selection. The two-part polygonal engagement element 16 comprises a projecting male engagement part 48 on the exterior surface 32 of the inner liner 14 and a recessed female engagement part 50 on the inner surface 34 of the outer shell 12. The projecting male engagement part 48 and the recessed female engagement part 50 include a plurality of sides 52, 54 defining complementary polygons 56, 58. In this particular embodiment, the projecting male engagement part 48 and the recessed female engagement part 50 are considered endless, since there are no breaks in the rings that they define. However, it is feasible that one or both of the projecting male engagement part and the recessed female engagement part may be discontinuous, and therefore not considered to be endless.

In this embodiment, each said polygon 56, 58 is a Reuleaux polygon having an odd number of sides 52, 54, each of which has a curving longitudinal extent. A lateral extent of each side 52, 54 is preferably straight, but also may be curved or chamfered to aid introduction of the projecting male engagement part 48 with the recessed female engagement part 50.

The Reuleaux polygon 56, 58 has a constant diameter at any given point measured through a centre of a circle. Consequently, this provides a constant surface area of engagement between the projecting male engagement part 48 and the recessed female engagement part 50 at any point, thus improving push-out or lever-out resistance of the inner liner 14 from the outer shell 12.

Furthermore, a diameter of each Reuleaux polygon 56, 58 is not based on a fixed centre, and therefore rotation of the inner liner 14 about a fixed centre, such as the polar axis 24 of the outer shell 12 is prevented or limited. During the impartation of a relative rotational force between the inner liner 14 and the outer shell 12 about respective polar axes 24, 60, the curvilinear sides 52, 54 of the projecting male engagement part 48 and the recessed female engagement part 50 attempt to urge the polar axis 60 of the inner liner 14 laterally or radially relative to the polar axis 24 of the outer shell 12. Since the inner liner 14 is held captive by the outer shell 12, this lateral or radial movement of the polar axes 24, 60 is prevented or limited, and therefore relative rotation of the two parts is also prevented or limited.

Consequently, the two-part polygonal engagement element 16 not only provides for resistance against push-out and lever-out of the inner liner 14 from the outer shell 12, but also provides for anti-rotation between the inner liner 14 and outer shell 12. The two-part polygonal engagement element 16 is positioned to extend in a plane which is at or adjacent to an equatorial plane of the outer shell 12. It is also preferable that the two-part polygonal engagement element 16 extends perpendicularly or substantially perpendicularly to the polar axis 24 of the outer shell 12. The two-part polygonal engagement element 16 is preferably spaced slightly from the rim 20, thereby enabling the recessed female engagement part 50 to extend both over and under a received corner portion 62 of the projecting male engagement part 48. However, the polygonal engagement element may be at any position between a polar region of the outer shell and/or inner liner and the associated rim. In this embodiment, the projecting male engagement part 48 and the recessed female engagement part 50 are integrally formed as one-piece with the inner liner 14 and the outer shell 12, respectively. The projecting male engagement part 48 essentially defines a polygonal ring 64 which extends continuously around the exterior surface 32 of the inner liner 14, in this case at the frusto-conical wall 42. The curvilinear sides 52 meet or substantially meet the frusto-conical wall 42 at or substantially at their midpoints, and the corners 66 defined between contiguous neighbouring sides 52 project.

A diameter of the polygonal sides 52 is constant, but the radius varies. Due therefore to the curvature and number of sides, a projecting extent can be defined. This latter feature aids a surgeon in controlling the push-out or lever-out resistance between the inner liner 14 and outer shell 12.

The recessed female engagement part 50 essentially defines a complementary polygonal channel 68 which extends continuously around the inner surface 34 of the outer shell 12, again in this case at the frusto-conical wall 26. The curvilinear sides 54 meet or substantially meet the frusto-conical wall 26 at or substantially at their midpoints, and the corners 70 defined between contiguous neighbouring sides 54 are recessed to receive the projecting corners 66 of the male engagement part 48.

Although it is preferred that the two-part polygonal engagement element includes complementary Reuleaux polygons or polygons having arcuate sides of constant width, the polygonal sides may have straight or linear longitudinal extents. In this case, rotational resistance is not as pronounced since there is a constant centre of rotation whereby the polar axes of the inner liner and outer shell remain at their fixed relative positions.

It may be feasible to provide the two-part polygonal engagement element having complementary engaging polygons with one or more arcuate sides and one or more linear sides. For example, the projecting male engagement part and the recessed female engagement part may each have one side with an arcuate longitudinal extent, as with a Reuleaux polygon, but the remaining sides may have straight or linear longitudinal extents. This would require a specific relative orientation between the inner liner and the outer shell. However, during attempted relative rotation, the arcuate sides would attempt to offset or move the polar axis of the inner liner relative to the polar axis of the outer shell, thus causing the required resistance and therefore the anti-rotation characteristic.

It is also feasible that the projecting male engagement part of the two-part polygonal engagement element could be provided on the inner surface of the outer shell, whereas the recessed female engagement part could be provided on the exterior surface of the inner liner. However, a benefit of the first described arrangement is that the outer shell, by having no projecting elements on its inner surface, can be utilised with other kinds of inner liner. For example, a ceramic inner liner 72, as shown in Figure 4, only typically requires a taper-fit engagement. Consequently, by providing the inner surface 34 of the outer shell 12 with the required tapering frusto-conical wall 42, as described above, even though the recessed female engagement element 50 extends therearound, a ceramic or taper-only fit inner liner 72 being devoid of the projecting male engagement part 48 can still be utilised and engaged with the outer shell 12 via the first and second taper elements 30, 46 forming a self-locking two-part taper engagement element 74 at the interface between the inner liner 14 and the outer shell 12. As such, a greater range or variety of selectable inner liners, not only having different dimensions but also being formed of different bio-compatible materials, can be utilised with the outer shell 12 of the present invention.

Although a ceramic inner liner 72 is suggested above, other suitable bio-compatible materials may be considered. As shown in Figures 5 and 6, to simplify manufacture of the outer shell 12 with the recessed female engagement part 50 of the polygonal engagement element 16, the polygonal part of the recessed female engagement part 50 may be formed separately as an independent member 50a. In this case, the inner surface 34 of the outer shell 12 is formed with a ring-like circular channel 50b, and at least one locating element 50c, such as a notch or tab, is provided therein. The female engagement part 50 can thus be considered to comprise two main separate independent elements 50a, 50b.

The separate female engagement part is formed as a ring 50a, and this may be a split ring to aid insertion into the circular channel 50b of the outer shell 12. The ring-like female engagement part 50a preferably comprises a smooth or substantially smooth radially outer surface, and the polygonal radially inner surface. If the two-part polygonal engagement element 16 is utilising the Reuleaux polygons, then the sides of the polygonal radially inner surfaces are arcuate of constant radius to be complementary with those of the projecting male engagement part 48. A further locating element or elements to complementarily match the first said locating element or elements in the circular channel of the outer shell 12 is/are formed on the separate female engagement part.

The separate female engagement part 50a can thus be received in the circular channel 50b of the outer shell 12, and held against relative angular displacement by the interlocking locating elements 50c. The inner liner 14 can then be inserted into the outer shell 12, whereby the projecting male engagement part 48 interengages with the independent recessed female engagement part.

As above, although the separate female engagement part may utilise an odd number of curvilinear polygonal sides, it is feasible that an even number of polygonal sides can be utilised and/or the polygonal sides or a proportion thereof may be linear. However, in these latter cases and as explained above, the push-out or lever-out resistance characteristics in combination with the anti-rotational characteristics may not be as great.

The acetabular cup having the inner liner and the outer shell may thus comprise a male polygonal engagement element and a female polygonal engagement element, wherein in this case the female polygonal engagement element includes two intercooperating members, one of which is independent of the outer shell and the inner liner.

It is also possible that the inner liner may be introduced into the outer shell prior to engagement of the polygonal engagement element. To this end, for example, the projecting male engagement part and/or the recessed female engagement part may be a separate independent part. This separate independent part may thus be inserted following insertion of the inner liner, for example, by dropping the separate independent part into the interface at or adjacent to the respective rims. A circlip or other retaining means may thus then be utilised to axially retain the separate independent part in place, thereby enabling the engagement of the polygonal engagement element.

As suggested previously, there is no requirement for the male and/or female engagement parts 48, 50 to be continuous about the exterior surface 32 of the inner liner 14 or the inner surface 34 of the outer shell 12. With this in mind, a third embodiment of the inner liner 114 and the outer shell 12 of acetabular cup 10 is shown in Figures 7 to 10, respectively.

In this embodiment, there is provided a plurality of discrete projecting male engagement parts 148 on the outer surface 132 of the inner liner 114, forming one part of the two- part polygonal engagement element 116, the other being the recessed female engagement part 150. The male engagement parts 148 in the embodiment are formed as equi-angularly spaced-apart discrete arcuate-sided triangular protrusions, with the outermost corners or vertices 166 of the protrusions, being equivalent to the corners 66 of the continuous male engagement part 48 in the first embodiment of the invention. The corners 166 define a polygon 156, shown using a dashed line in Figure 7, equivalent to the complementary polygon 56 defined by the corners 66 of the continuous male engagement part 48 in the first embodiment.

The extent of the polygon 156 can be seen in Figures 9 and 10. The extent of a regular, odd-sided polygon is defined by the outermost corners 166 of the male engagement parts 148. Arcs, depicted as further dashed lines Al, A2 in Figure 9 are defined as having a distance R, defined as being the distance from a first outermost corner 166, indicated as CI in Figure 9, to either of the opposing outermost corners 166, indicated as C2 and C3.

In the first embodiment of the invention, these arcs combine to form the outermost extent of the inner liner 14, and the complementary polygon 56 thus defines a complete Reuleaux polygon. This is because the distance R between opposing vertices is greater than the diameter of the inner liner 14.

In this third embodiment of the invention, distance R is of a similar magnitude to the diameter of the inner liner 114. Therefore, although a Reuleaux polygon is formed by the combination of the arcs to define a polygon 156, only the outermost corners or vertices 166 and parts of the sides defining the vertices of the polygon 157 protrude from the outer surface 132 of the inner liner 114.

Therefore, the polygonal interface of the polygon 156 is actually highly irregular. However, the outermost corners 166 of the male engagement parts 148 interengage with the recessed female engagement part 150 in an equivalent manner to that of a full Reuleaux polygon. Therefore, the third embodiment of the invention displays the same push-out or lever-out resistance as demonstrated in the first embodiment of the invention.

As such, the present invention is characterised in that an outwardly projecting vertex C3 of at least one of the parts 148 of the two-part polygonal engagement element 16 is defined by curvilinear sides 157, each said curvilinear side 157 being a constant distance R from either opposing vertex CI or C2.

It will be appreciated that the corners or vertices 66, 166 of the first part of the male engagement part 48 or parts 148 provide the necessary engagement with the female engagement part 50, 150. As such, the polygons 56, 156 of the two-part polygonal engagement element 16 do not necessarily have to be complementary in shape. Both polygons 56, 156 could be Reuleaux polygons, or alternatively, the polygon 56 defined by the female engagement part 50, 150 could be a straight- sided regular polygon, accepting a Reuleaux or interrupted Reuleaux polygon defined by the male engagement part 48 or parts 148. As with the first embodiment of the invention, it will be appreciated that the male engagement parts 148 could equally be positioned on the inner surface 34 of the outer shell 12, with the female engagement part 150 being positioned on the outer surface 32 of the inner liner 14. It is thus possible to provide a two-part acetabular cup having an inner liner which can be held captive in an outer shell and which can be indexed relative thereto as required, and which is also able to resist push-out and lever-out forces, in addition to rotational forces whilst in use through the use of a single two-part polygonal engagement element. The use of separate features or mechanisms to prevent or limit axial separation as well as relative angular displacement is therefore dispensed with. It is also possible to utilise a two-part polygonal engagement element which utilises complementary Reuleaux or otherwise interengaging polygons, whereby relative rotational disassociation is resisted due to the attempted lateral offset of a polar axis of the inner liner relative to the polar axis of the outer shell. The invention can also be conveniently utilised by so-called 'thin walled' acetabular cups where a wall thickness of the outer shell is reduced to maximise an articulating surface dimension of the inner liner, since a receiving depth of the projecting male engagement part within the recessed female engagement element can be controlled by a number of polygonal sides utilised.

The embodiments described above are provided by way of examples only, and various other modifications will be apparent to persons skilled in the field without departing from the scope of the invention as defined by the appended claims.

Claims

1. An acetabular cup (10) comprising an outer shell (12), an inner liner (14; 114) receivable by the outer shell (12), and a two-part polygonal engagement element (16) at an interface between the outer shell (12) and the inner liner (14; 114), characterised in that an outwardly projecting vertex (C2) of at least one of the parts (48; 148) of the two-part polygonal engagement element (16) is defined by curvilinear sides (56; 156), each said curvilinear side (56; 156) being a constant distance (R) from an opposing vertex (CI).

2. An acetabular cup (10) as claimed in claim 1, wherein the polygonal engagement element (16) includes a Reuleaux polygon.

3. An acetabular cup (10) as claimed in claim 2, wherein the Reuleaux polygon includes discontinuous sides.

4. An acetabular cup (10) as claimed in any one of claims 1 to 3, wherein the polygonal engagement element (16) extends in a plane which is at or adjacent to an equatorial plane of the outer shell.

5. An acetabular cup (10) as claimed in any one of the preceding claims, wherein the polygonal engagement element (16) extends in a plane which is partway between a polar region of the outer shell and/or the inner liner (38) and the associated rim (20, 44).

6. An acetabular cup (10) as claimed in any one of the preceding claims, wherein the polygonal engagement element (16) extends in a plane which is parallel or substantially parallel with an equatorial plane of the outer shell and/or the inner liner.

7. An acetabular cup (10) as claimed in any one of the preceding claims, wherein the polygonal engagement element (16) extends in a plane which is parallel or substantially parallel with a rim of the outer shell and/or the inner liner (20, 44).

8. An acetabular cup (10) as claimed in any one of the preceding claims, wherein the polygonal engagement element (16) comprises a projecting male engagement part (48; 148) on one of the inner liner (14) and the outer shell (12) and a recessed female engagement part (50; 150) for receiving the projecting male engagement part (48; 148) on the other of the inner liner (14) and the outer shell (12), the projecting male engagement part (48; 148) and the recessed female engagement part (50; 150) including a plurality of sides (52, 54) defining two polygons (56; 156, 58).

9. An acetabular cup (10) as claimed in any one of claims 1 to 7, wherein sides (52, 54) of the polygonal engagement element (16) are straight.

10. An acetabular cup (10) as claimed in any one of claims 1 to 7, wherein sides (52, 54) of the polygonal engagement element (16) are arcuate.

11. An acetabular cup (10) as claimed in any one of the preceding claims, wherein a number of sides (52, 54) of the polygonal engagement element (16) is odd.

12. An acetabular cup (10) as claimed in any one of the preceding claims, wherein the two parts (48; 148, 50; 150) of the polygonal engagement element (16) are integrally formed as one-piece with the outer shell (12) and the inner liner (14), respectively.

13. An acetabular cup (10) as claimed in any one of claims 1 to 10, wherein at least one said part (48; 148, 50; 150) of the polygonal engagement element (16) is independent of the outer shell (12) and inner liner (14), the said at least one part (48; 148, 50; 150) being engagable with the outer shell (12) or inner liner (14).

14. An acetabular cup (10) as claimed in any one of the preceding claims, wherein the outer shell (12) is a biocompatible metal, and the inner liner (14) is a biocompatible plastics.

15. An acetabular cup (10) as claimed in claim 8, wherein the male engagement part (148) is formed as a plurality of discrete arcuate-sided triangular protrusions, the corners (166) of the protrusions defining one of the polygons (156).

16. An acetabular cup system comprising an acetabular cup (10) as claimed in any one of the preceding claims, a second inner liner (72) formed of a different material to the first said inner liner (14), and a two-part taper engagement element (74) at an interface between the outer shell (12) and the second inner liner (72).

17. An acetabular cup system as claimed in claim 16, wherein the two-part taper engagement element (74) includes a first taper element (30) on an inner surface

(34) of the outer shell (12), and a second taper element (46) on an outer surface of the second inner liner (72).

18. An acetabular cup system as claimed in claim 17, wherein the first and second taper elements (30, 46) extend from the rims of the outer shell (20) and the second inner liner, respectively.

19. An acetabular cup system as claimed in any one of claims 16 to 18, wherein the second inner liner (72) is ceramic.

20. A method of inhibiting rotational disassociation of an inner liner (14) of an acetabular cup (10) as claimed in any one of claims 1 to 18 when engaged with an outer shell (12), the method comprising the step of urging a polar axis (60) of the inner shell (14) to move radially relative to a polar axis (24) of the outer shell (12) when a rotational force is imparted to at least one of the inner liner (14) and the outer shell (12).

21. A method as claimed in claim 20, wherein the said relative movement of the polar axes (60, 24) on impartation of the said rotational force is caused by a plurality of contiguous curvilinear sides (52, 54) which abut each other at an interface between the outer shell (12) and the inner liner (14).

22. A method as claimed in claim 20 or claim 21, wherein the said relative movement of the polar axes (60, 24) on impartation of the said rotational force is caused by a two-part polygonal engagement element (16) at an interface between the outer shell (12) and the inner liner (14).

23. A method as claimed in any one of claims 20 to 22, wherein the said relative movement of the polar axes (60, 24) on impartation of the said rotational force is caused by at least one Reuleaux polygon (56; 156, 58) at an interface between the outer shell (12) and the inner liner (14).

24. An acetabular cup (10) comprising an outer shell (12), an inner liner (14) receivable by the outer shell (12), and a Reuleaux polygonal engagement element (16) at an interface between the outer shell (12) and the inner liner (14).

25. An acetabular cup (10) comprising an outer shell (12), an inner liner (14) receivable by the outer shell (12), and a two-part polygonal engagement element (16) at an interface between the outer shell (12) and the inner liner (14).