WO2008128282A1

WO2008128282A1 - Acetabular prosthesis assembly with offset insert

Info

Publication number: WO2008128282A1
Application number: PCT/AU2008/000543
Authority: WO
Inventors: Ronald Sekel
Original assignee: Portland Orthopaedics Limited
Priority date: 2007-04-20
Filing date: 2008-04-20
Publication date: 2008-10-30

Abstract

An acetabular prosthesis assembly insertable in an acetabulum, the assembly comprising: a cup having a wall including a generally convex outer surface for engaging acetabular bone and a generally concave inner surface defining a recess, the wall including an apex and terminating at an equator; the assembly further comprising an insert capable of insertion inside the recess of the acetabular cup, the insert including an equator, an outer surface and a concave recess which receives a femora) head component; wherein, the insert is capable of location within the cup so that the equator of the cup and the equator of the insert lie at an angle to each other.

Description

ACETABULAR PROSTHESIS ASSEMBLY WTTH OFFSET INSERT

BACKGROUND

The present invention relates to acetabular assemblies and more particularly relates to an acetabular prosthesis assembly for implantation within an acetabulum and which includes a cup for fixation to acetabular bone and an insert which may be offset relative to the cup to facilitate correct anteveτ$ion without removal and resetting of the cup. More particularly, the invention relates to an acetabular assembly including a taper ring which has a geometry which allows the insert to be selectively orientated so its equator may be disposed at an angle relative to an equator of the cup. The invention further provides a taper ring in an acetabular assembly to receive an insert so that it is offset relative to an acetabular cup The insert includes a concave surface which receives a hip component, the offset geometry of the insert allowing setting of the hip component at a plurality of angles to enable correct antcversion.

PRlOR ART

There arc a wide variety of known acetabular components and assemblies used for restoration of and substitution for an internally damaged acetabulum. These assemblies are usually employed for local corrections in total hip replacement surgery where the pelvis other than the acetabulum is in a good state of repair Some of the current art is directed to various aspects of Acetabular cup fixation design and to the manner of engagement between an insert which receives a femoral head prosthesis and an acetabular cup.

Examples of known Acetabular cup arrangements incorporated herein by reference may be seen in United States patents 5,871,548, 5,782,929, 5,871,548, 5,326,368, 5,480,448, 5,370702. 5019,108.

The known acetabular assemblies generally comprise an acetabular metallic outer cup or shell, ( these terms can be used interchangeably) which overlies degraded acetabular bone. The cups are preferably made from chrome cobalt or titanium and include a generally convex outer surface and a concave inner surface together defining a cup wall. The wall maj include screw openings for fixation of the cup to bone. The convex outer surface is usually porous or roughened to promote bone growth thereabout when in situ. The cup typically receives inside a recess defined by the concave inner surface, a polyethylene, metal or ceramic insert which opposes a convex inner surface of the cup. The insert typically includes a recess which receives a femoral head component.

In a typical acetabular assembly there are two main wear interfaces, the first between a convex outer -surface of a cup insert and a concave inner surface of the cup ( also referred to as a shell) and the second between a head of a femoral prosthesis and a concave recess in the insert. At present, acetabular cups ( or shells) are provided with various diameters which receive and retain metal, ceramic and polyethylene inserts. As cup sizes typically range between 40 mm and 72mm, large inventories are required. Different cup designs are required for polyethylene inserts, metal and ceramic liners. Total hip replacement surgery requires accurate alignment and positioning of the acetabular and femoral components during the actual surgery. The acetabular component, is aligned to the pelvis three dirnensionally. Surgeons tend to orient the component with reference to two dimensions. These two dimensions are termed anteversion, and inclination. Anteversion is the foπwird tilt of the axis of the hemispherical cup. Inclination is the upward tilt of the component. In anatomic terms, the anteversion is forward towards the front of the patient in the transverse plane. Inclination is in an upward direction in the coronal plane. Anteversion is easier to estimate than inclination since there are taore landmarks to reference. The surgeon commonly estimates anteversion simply by referencing the coronal plane of the patient. Since the torso, shoulder, and pubis are generally visible or palpable, these landmarks can be utilized in the estimation of the coronal plane, and therefore the position of the socket in anteversion.

Inclination is much more difficult to assess than anteversion as the surgeon has few. if any, landmarks to spatially orient the pelvis in the sagittal plane. To rely on the visible bony anatomy of the acetabulum can be misleading as the majority of acetabular are inclined steeper than 45 degrees. Attempting to use the rim of the acetabulum to orient inclination varies with the size of the acetabular cup. In addition, the pelvis is known to move in different directions during the operation. This makes it extremely difficult, if not impossible, to reference the pelvis during the insertion of the cup.

There are in existence acetabular assemblies which include an acetabular cup, having an outer convex bone engajμng surface, an inner concave surface which receives an insert which is presented at an offset angle relative to a vertical axis so that a recess in the insert which receives a femoral head component is disposed at an angle to an equator of the cup. This angle is usually within the range 10 - 15 degrees. This allows for alternative orientation of the hip component compared to the non offset standard cup and insert which are both disposed along a central axis normal to an equator of the cup. The present applicant ha;; a cσ pending patent application which teaches a an acetabular assembly of the type including a cup and polyethylene insert in which the assembly includes a liner of preferably metallic material, but it may be ceramic or polyethylene, which provides a wear furface between the insert and a concave inner surface of the acetabular cup. The ace tabular cup has a generally convex outer surface for engaging acetabular bone and a generally concave inner surface together defining a cup wall which has an apex and an equator and an insert is capable of insertion inside the acetabular cup and includes a convex outer surface and a concave recess which receives a femoral head component. The assembly further includes a liner disposed between the concave inner surface of the cup and the convex outer surface of the insert, the liner providing a wear inhibiting surface.

According Io one embodiment, the insert described comprises a fust layer of polyethylene fused to the metallic liner which is preferably chrome cobalt or titanium alloy and which engages at least part of ihe inner concave surface of the cup. The circumferential wall of the cup is increased in length a predetermined circumferential distance beyond the equator This distance will vary but will preferably fall within the range of 2mm -5mm. The thickness of the liner at its apex influences the circumferential distance that the cup extends from its equatoj . The liner is in tight fitting engagement with part of the inner concave surface of the c up leaving a gap between an outer surface of the liner and the inner surface of the cup. The insert has a generally spherical outer surface opposing a generally spherical concave surface of the liner and the outer surface of the metallic liner includes a straight wall region extending from near its equator in the direction of the apex and an arcuate portion in the region of its apex., ihe arcuate portion opposing the inner concave surface of the cup. The axial distance from the apex of the acetabular cup to its equator is preferably less that the radial distance from the circumferential wall at the equator to its axis.

The straight wall region of the liner is disposed at an angle within the range of 15 -25 degrees , but not excluding other angles, from its axis of symmetry and changes to the arcuate portion at a location less than half the circumferential distance from its equator to its apex. The liner may comprises a circumferential ring which engages an inner surface of the cup when the insert is engaged with the cup. Acetabular assemblies having offset inserts and liners are known. One such assembly comprises an acetabular cup having a generally outer convex surface and an inner concave surface defining a recess, the recess receiving a retaining therein an insert. The known inserts are standard sym metrical and may be metal or ceramic. The cup includes a hood depending from an equal orial region of the cup which allows the insert to be located in the cup at an angle of around 15 degrees to an equator of the cup. The arrangement of the hood on the cup allows a singular orientation of the insert so that should the cup not be positioned in its correct orientation, this error cannot be 'corrected' by altering the orientation of the insert relative to the cup due to the restriction imposed by the hood. To achieve this, the shell inner taper alignment was altered to be at an angle relative to the outer geometry of the shell. The known offset insert and cup assembly can be put in one orientation onJy, that is fixed once the shell is inserted. The surgeon selects Hie desired orientation but is unable to adjust the insert position once the shell is implanted without changing the shell (cup) position as the design of the shell dictates the insert position and therefore orientation once the cup is fitted. Should the insert require adjustment that will require the surgeon to remove the already implanted shell in the acetabulum, readjust the position, rεimplant the shell, and then insert the liner hopefully in the correct position and orientation. The prior jut offset insert provides a 15 degree angle adjustment by interference fit implanting of the shell/cup at just the right angle. If that angle is incorrect, the shell has to be removed from the acetabular bone and then repositioned and again interference fit into the same bone before the liner is then reinserted in its fixed location within the cup so it will be at the correct orientation.

This is therefore a problem with the known acetabular offset insert assemblies in that once the cup is fitted to the acetabulum by the surgeon, since the offset insert is lockable in a specific location in trie cup and accommodated, its location dictated by a fixed hood, if the anteversion or inclination is incorrect, the cup has to be removed and reset requiring further but otherwise unnecessary bone erosion for fixation devices and additional surgery time. Removal and replacement of the shell may result in reduced shell interference fit during reimplantion leading to a poor surgical outcome.

There is a long felt want to provide an acetabular assembly including an σffsettable insert and which can be adjusted even when tbe cup in which the insert is fitted is fixed to bone to enable adjustments to the insert to obtain correct anteversion so that the femoral head component presents accurately to the recess in the insert.

INVENTION

The present invention is particularly concerned with improvements in the acetabular assemblies which employ inserts such as polyethylene inserts and more particularly to an acetabular assembly which includes an offset insert which allows a plurality of rotational adjustments which determine the disposition of the head component when an acetabular cup is secured to an acetabulum.

The invention further provides an acetabular assembly including a taper ring which has a geometry which allows the insert to be selectively orientated so its equator may be disposed at an angle relative to an equator of the cup even after the cup is fitted to the acetabulum. The inveniion further provides a taper ring in an acetabular assembly to receive an insert so that it is offset relative to an acetabular cup.

The present invention seeks to overcome the disadvantages of the prior art offset devices by providing an acetabular assembly including an offset insert which is adjustable within the cup even after the cup has been fixed to an acetabulum and which eliminates resetting of the cup if the initial cup setting position is incorrect. The invention further provides an insert for an acetabular cup which is offset at an angle relative to an equator of the cup and which is loωtable in the cup m an orientation through 360 degrees and while at an angle to an equator of the cup.

The invention provides an offset insert which may be ceramic, polyethylene, metal 5 or other material and which is re locatable in an acetabular cup once the cup is fitted to an acetabulum and without the need to reset the cup.

In one broad form the present invention comprises- an acetabular prosthesis jisserably insertable in an acetabulum, the assembly comprising: a cup having a wall including a generally convex outer surface for engaging acetabular

)0 bone and a generally concave inner surface defining a recess, the wall including an apex and terminating at an equator; the assembly further comprising an insert capable of insertion inside the recess of the acetabular cup, the insert including an equator, an outer surface and a concave recess which receives a femoral head component;

15 wherein, the insert is capable of selective location and rotation within the cup and so that the equator of tbe cup and the equator of the insert lie at an angle to each other. The assembly further comprises a ring having an internal taper and an external taper. The taper ring is disposed between the outer surface of the insert and the inner surface of the cup which enables the selective orientation of the insert. At least part of the outer surface0 of the taper ring has an outer surface which engages the wall of the cup and an inner surface which engages at least part of the outer surface of the insert. Preferably, the taper ring has a circumferentύil wall having a first part of the wall in which an outer surface is disposed at a first angle <ind a second part of the wall disposed at a second angle and a first equator which is co linear with the equator of the cup when inserted in the cup and a5 second equator which is disposed at an angle to the equator of the cup. The taper ring preferably has a central axis co linear with a central axis of the insert normal to the equator of the insert and a region which is co terminus with the cup equator and a region which extends beyond the cup equator. The orientation of the inner wall of the taper ring allows the insert when inserted in the ring to be rotatable at selected rotational orientations within the cup and offset such that a central axis of the insert is disposed at an angle to the central axis of the cup The ring has an outer surface which engages the inner surface of said cup and an inner surface which engages the outer surface of the insert. The taper ring is preferably manufactured from metallic material such as but not limited to titanium alloy or chrome cobalt and the insert may be manufactured from materials such as but not limited to either polyethylene, ceramic or metal. According to one embodiment, the taper ring is integral with the insert so the insert and ring can be uncoupled from the cup. Preferably the tapered ring has an internal geometry which places the equator of the insert at an angle to the equator of the cup. According to a preferred embodiment the insert is placed relative to the cup at an

Offset angle within the range 1- 25 degrees.

The offset insert which is used in conjunction with a metal ring and which is releasably inserted into an outer shell, allows the shell to be implanted into the acetabular bone, followed by placement of the insert in the optimum orientation for stability of the articulating ball of a head component. The invention has the advantage of allowing the cup/shell to be implanted, and then the optimum position of the offset insert (within the ring) to be found withoict changing the setting of the outer cup/shell as occurred in the prior art assemblies'.

In another broad form the present invention comprises: Λ taper ring for an acetabular prosthesis assembly insertablc in an acetabulum, the assembly comprising: a cup having a wall including a generally convex outer surface for engaging acetabular bone and a generally concave inner surface defining a recess, the wall including an apex and terminating at an equator; the assembly further comprising an insert capable of insertion inside the recess of the acetabular cup, the insert including an equator, an outer surface and a concave recess which receives a femoral head component; wherein, the insert is located within the cup so that the equator of the cup and the equator of the insert lie at an anj;le to each other; wherein the insert is selectively releasable from the cup to adjust its orientation for correct anteversion when the cup is fixed in position on an acetabulum.

According to a preferred ernbodixnent the assembly further comprises a ring disposed about the outer surface of the insert, the ring having an outer surface which engages the inner surface of said cup and an inner surface which engages the insert and is disposed at the same angle as the outer surface of the insert and at the same angular orientation relative to said axis normal to the equator of said cup as the angle of repose of the outer surface of the insert. P;ιrt of the insert when in position extends a first distance from the cup equator and at least another part extends a second distance from the equator. Thus viewed from the side the insert will appear at an angle to the equator of the insert one side extending below an opposite side

Preferably the liner is a metallic material which opposes at least part of a concave inner surface of the cup and is integral with the insert. In its broadest for the present invention comprises an acetabular assembly comprising a cup having generally convex outer surface for engaging acetabular bone and a generally concave inner surface together defining an inner recess, the cup including a wall having an apex and an equator and an insert capable of insertion inside the acetabular cup, the insert including an outer surface which when the insert is inserted inside the cup, characterized in that the insert may be positioned in a plurality of orientations within the cup opposes the inner surface of the cup and a concave recess which receives a femoral head component; the outer surface of the insert having a circumferential wall which when the insert is inserted into the cup locates at an angle to the equator of said cup such that the recess in said liner is disposed at an angle to an axis normal to the equator; the insert geometry allowing the insert to be set at a selected orientations within the cup ,

Preferably, the liner is formed from a metallic material which opposes at least part of a concave inner surface of the cup and is integral with the insert.

The insert body is polyethylene and the metallic ring may be titanium alloy or chrome cobalt and when the insert is inserted by the cup, engages at least part of an inner surface of the acetabular cup. The liner is in tight fitting engagement with part of the inner surface of the ring.

In another broad form thi; present invention comprises: an acetabular cup assembly for repair of an acetabulum; the assembly comprising an acetabular cup generally defining a hemisphere having an exterior convex surface and an interior at least partially concave surface, an apex and an equator; an insert for engagement with said cup and which receives a femoral head; the insert arranged inside the cup so that its axis of symmetry is disposed at an angle to an axis normal to an equator of the cup and so that the insert may be placed in one of a plurality of available orientations to achieve correct anteversion. In another broad form of the method aspect the present invention comprises: a method of assembly oFaxi acetabular cup assembly comprising the steps of ; a) taking an acetabular cup having a generally convex outer surface for engaging acetabular bone and a generally concave inner surface together defining a cup wall which has an apex and an equator; b) taking an insert capable of insertion inside the acetabular cup; c) inserting a ring inside the cup so that it locates between the insert and inner wall of the cup ; d) selecting an orientation for the insert within the cup; the insert arranged insido the cup so that its axis of symmetry is disposed at an angle to an axis normal to an equator of the cup and so that the insert may be placed in one of a plurality of available orientations to achieve correct anteversϊon.

The insert may be manufactured from ceramic, metal, polyethylene or other insert materials, not just ceramic in the ring. Typically an axis of symmetry of the insert will when the insert is located in the cup be disposed at an angle to an axis normal to the cup equator and at an angle other than normal to the equator. Unlike the prior art this geometry allows 360 degree orieniation of the insert as required after the cup has been fitted into position. The central or normal axis of the cup /shell is disposed at an angle to the corresponding central axis of the insert disposed in the ring. The angle adjustability of the insert allows optimal head cover, and thus optimal stability against dislocations of the hip.

This allows the surgeon to obtain say 15 degree of angled offset of the liner to be accomplished without changing the already implanted (in acetabular bone) outer shell. The present invention eliminates the requirement to remove the shell and reset if it is found upon insertion of tbe insert that it is not correctly located.

U DETAILED DESCRIPTION

The present invention will now be described in more detail according to a preferred but non limiting embodiment and with reference to the accompanying illustrations; wherein

Figure 1 shows a side elevation view of a standard ceramic insert located in a taper ring with respective axes of symmetry co linear.

Figure 2 shows, an elevation view of a standard taper ring .

Figure 3 show?, a cross sectional view of the taper ring of figure 2. Figure 4 shows a side elevation view of an assembly including an insert located in an offset taper ring so that an axis of symmetry of the insert is disposed at an angle to a transverse axis of the taper ring according to one embodiment.

Figure 5 shows a shows an elevation view of an offset taper ring.

Figure 6 shows a cross sectional view of the offset taper ring of figure 5.

Figure 7 shows a perspective view of an assembly according to a preferred embodiment having an insert located in an offset taper ring disposing the axis of symmetry of the insert at an angle to an equator of the cup.

Figure 8 shows a side elevation view of the arrangement of Figure 7 with corresponding numbering.

Referring to figure 1 there is shown a side elevation view of a standard acetabular assembly I including a ceramic insert 2 located in a tapered ring 3 ( liner) with respective axes of symmetry 4 co linear so that the insert 2 and ring 3 are in alignment and their normal axes co linear. Insert 2 and tapered ring liner 3 are insertable in an acetabular cup

( not shown). In the assembly of figure 1 equator 2a of insert 2 is parallel with equator 3a of tapered ring 3. Thi^ arrangement causes the axis of symmetry 4a of insert 2 and axis of symmetry 4b ( see figures 2 and 3), of liner 3 to be co linear.

Figure 2 shows an elevation view of a standard taper ring 3 shown in figure 1 exploded from insert 2. Liner ring 3 is symmetrical about a central vertical axis 4b and has a tapered outer surface 5 whose jjeometry is consistent about the periphery of the ring 3. Tapered surface 5 engages an inner concave surface of a corresponding acetabular cup ( not shown) in a known manner.

Figure 3 shows a cross sectional view of the taper ring 2 of figure 2 from which it may be seen that the circumferential tapered outer surface 5 and corresponding tapered inner surface 5a are symmetrical about axis 4b. This geometry will place liner 3 and insert 2 inside an acetabular cup so that respective equators of the liner 3 and insert 2 are disposed parallel with an equator of an acetabular cup. The aforesaid describes a known arrangement

Figure 4 shows according to one embodiment, a side elevation view of an acetabular assembly 6 including an insert 7 located in an offset taper ring 8. Insert 7 has an axis of symmetry 9 which, when insert 7 is coupled with taper ring 8, is disposed at an angle to a transverse axis 10 of the taper ring 8. Insert 7 is a conventional shape comprising a generally convex outer surface 1 1 and a concave inner surface 12 defining a recess 13 which receives and retains therein a head component of a hip prosthesis ( not shown).

Outer convex surface 1 1 comprises a pole region 14, a region of increasing diameter 15 and a tapered region 16. Tapered region 16 has a gradually increasing taper (distally) which engages inner surfaces 17 and 18 of ring 8. Ring 8 includes a hood portion 19 which engages part of tapered region 16. Figure 5 shows a shows an elevation view of an offset taper ring 8 in isolation from insert 7. Figure 6 show* a cross sectional view of the offset taper ring 8 of figure 5. Ring 8 comprises hood region 19. Outer circumferentiaj surface 21 of wall 22 is disposed at a first taper angle 23. Λt hood region 19 surface 21 of wall 22 changes its angular orientation along surface 24. Ring 8 further comprises an inner surface defined by a first taper 17 and a second laper 18. In comparison to ring 2 of figure 1, tapered ring 8 is asymmetric about axis 25. Outer surfaces 21 and 24 are arranged to engage a cup concentrically so that hood region 19 extends beyond an equator of a cup in which ring 8 is inserted. Inner surfaces 17 and 18 are arranged so that when an insert is placed within ring 8 axis of symmetiy 25 of the ring is disposed at an angle to an equator of the as shown in figure 4.

Figure 7 shows a fully assembled perspective view of an assembly 30 according to a preferred embodiment. Assembly 30 comprises acetabular cup/shell 31 having a wall 32 terminating in an equator 33. Wall 32 defines a recess 34 which receives offset tapered ring 35. Tapered ring 35 includes a hood 36 so that an equator 37 of ring 35 at location

37a is co terminus with uquator 33 of cup 31 and at location 38 equator 37 of ring 35 sits proud of equator 33 of oup 31. Ring 35 receives insert 39 which includes an internal recess 40 and wall 41 terminating in equator 42. Insert 39 includes recess 36 for receiving a femoral head component ( not shown).Equator 37 of tapered ring 35 and equator 42 of insert 39 are parallel but both are disposed at an angle relative to equator 33 of cup 31.

This causes an angular offset between insert 39 and cup 31. Figure 8 shows a side elevation view of the arrangement of Figure 7 .

Cup 31 typically will include apertures ( not shown) which receive fixation sercws ( not shown) for additional fixation of acetabular cup 31 to acetabular bone. Convex cup surface 43 includes a porous coating to promote acetabular bone growth about the cup enhancing fixation. Recess 40 has radii measured from its axis of symmetry to its circumference which may typically fall between 22 and 32 mm. Insert 39 is press fit to taper ring 35. As best shown in figure 6, ring 35 has non parallel axes . This arrangement with an angled inner geometry of ring 35 aDows alteration of the position of insert 39 and therefore its articular surface without alteration of the position of cup 33.

The insert 39 is according to one embodiment made from polyethylene and may be sealed to the chrome cobalt ( or titanium ) metal ring 35 layer or by pressure or injection moulding. Thus, by use of the embodiment described, any micro wear is confined to metal on metallic ( ring to convex inner surface of cup 31) thereby eliminating wear induced polyethylene micro particles. Any micro movement between the insert and cup will therefore result in metallic rather than polyethylene wear. Metallic wear is preferred to polyethylene, wear as metallic wear particles are non toxic.

When an acetabular assembly of the type described is to be inserted the surgeon prepares the acetabular cavity by reaming with a spherical reamer. The spherical reamer will allow formation of a cavity in the acetabulum of predetermined depth and diameter which neatly accommodates outer convex surface 43 of cup 31. Although cup 31 is for the most part spherical, the cup includes a slightly flattened apex region 44.. This flattening or abbreviation is introduced to ensure the cup surface 43 does not preferentially bottom on the apex ( not shown) of a reamed acetabular surface to which the assembly is to be fitted before the assembly is properly located in its predesignated position. In the absence of this geometry, the assembly may undergo unwanted withdrawal from the acetabular cavity. The objective is to ensure that the cup positively engages the circumferential wall of the reamed acetabulum. This ensures preferential press fit engagement between cup 31 and an acetabular side wall rather than prematurely with aa acetabular bone apex. Once Cup 31 is set in position, both insert 41 and ring 35 may be selectively removed from the cup, rotated and reset within the cup to achieve an alternative orientation. Thus the cup orientation once fixed to the acetabulum, does not dictate the orientation of the insert as the cup does not have a fixed hood as in the prior art which restricts the capacity to alter the insert position once the cup is set in position. This allows the surgeon wider flexibility for orientation of the insert and hence the achievement of the correct anteversion with femoral component after the cup is set in position and eliminates completely the need to re set the cup should the cup not be in an optimal initial orientation. Although the assembly has been described with reference to tapered interengagements between the insert and tapered ring and between tapered ring and acetabular cup. it will be appreciated that alternative forms of interlocking engagement may be introduced. For instance, locking of the insert and integral tapered ring, may employ a ratchet mechanism within the ring facets or teeth which allows fine adjustments of the ring location with in the cup or the insert with in the tapered ring. Also the shape of the ring and insert may be other than circular such as hexagonal or polygonal. ft is an objective of the present invention to provide an alternative to the known acetabular assemblies having an offset insert and which allows adjustment of an orientation of an insert after fixation of an acetabular cup. It will be recognized by persons skilled in the art that numerous variations and modifications may be made to the invention broadly described herein without departing from the overall spirit and scope of the invention.

>e

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1 An acetabular prosthesis assembly inscrtablc in an acetabulum, the assembly comprising: a cup having a wall .including a generally convex outer surface for engaging acetabular bone and a generally concave inner surface defining a recess, the wall including an apex and terminating at an equator; the assembly further comprising an insert capable of insertion inside the recess of the acetabular cup, the insert including an equator, an outer surface and a concave recess which receives a femora) head component; wherein, the insert is cjupable of selective location and rotation within the cup such that the equator of the cup and the equator of the insert lie at an angle to each other.

2 An assembly according to claim 1 further comprising a taper ring disposed between the outer surface of the insert and the inner surface of the cup which enables the selective orientation of the insert.

3 An assembly according to claim 2 wherein the taper ring has an outer surface at least part of which engages the wall of the cup and an inner surface which engages at least part of the outer surface of tho insert.

4 An assembly according to claim 3 wherein the taper ring has a circumferential wall having a first part of the wall in which an outer surface is disposed at a first angle and a second part of the wall disposed at a second angle.

5 An assembly according to claim 4 wherein the taper ring has a first equator which is co linear with the equator of the cup when inserted in the cup and a second equator whichs disposed at an angle to the equator of the cup. 6 ΛJΠL assembly according to claim 5 wherein the taper πng has a central axis co linear with a central axis of the- insert normal to the equator of the insert.

7 An assembly according to claim 6 wherein the taper ring has a region which is co terminus with the cup equator and a region which extends beyond the equator.

5 8 An assembly according to claim 7 wherein the orientation of the inner wall of the taper ring allows the insert when inserted in the ring to be offset such that a central axis of the insert is disposed at *tn angle to the central axis of the cup.

9 An assembly according to claim 8 wherein the offset allows the insert to be set at selected rotational orientations within the cup . 10 10 An assembly according to claim 9 wherein the ring has an outer surface which engages the inner surfacυ of said cup and an inner surface which engages the outer surface of the insert.

1 1 An assembly according to claim 10 wherein the taper ring liner is manufactured from a metallic material, i s 12 An assembly according to claim 1 1 wherein, the insert is manufactured from either polyethylene, ceramic or metal.

13 An assembly according to claim 12 wherein and the metallic ring is manufactured from titanium alloy or chrome cobalt.

14 An assembly according to claim 12 wherein, the taper ring is integral with the insert.0 J 5 An assembly according to claim 14 wherein a central axis of the insert normal to its equator is when fitted in the cup disposed at an angle to a central axis normal to the equator of the cup.

16 An assembly according to claim 15 wherein the tapered ring has an internal geometry which places the equator of the insert at an angle to the equator of the cup. 5 17 An assembly according to claim 16 wherein the tapered ring allows the insert to be

JS placed relative to the cup at an offset angle within the range 1- 25 degrees.

18 An assembly according to claim wherein the insert is press fit to the taper ring. .

19 An acetabular prosthesis assembly comprising a cup having a wall including a generally convex outer surface for engaging acetabular bone and a generally concave

S inner surface defining a lecess, the wall including an apex and terminating at an equator; the assembly further comprising an insert locatable inside the recess of the acetabular cup, wherein, the insert is including an equator, an outer surface and a concave recess which receives a femoral head c-omponent; wherein, the engagement between the insert and cup is such that it allows selective ] 0 orientation of the insert relative to the cup so that the equator of the cup and the equator of the insert lie at an angle Io each other.

20 An acetabular prosthesis assembly according to claim 18 wherein, the insert may be placed in one of a plurality of available orientations to enable correct anteversion between the insert recess and a femoral head component.

15 21 An acetabular prosthesis assembly according to claim 20 wherein the insert and taper ring are releasable from ihe cup when the cup is fitted to an acetabulum, thereby allowing alternative orientation of the insert without adjustment of the cup .

22 A taper ring for an acetabular prosthesis assembly insertable in an acetabulum, the assembly comprising: 0 a cup having a wall including a generally convex outer surface for engaging acetabular bone and a generally concave inner surface defining a recess, the wall including an apex and terminating at an equator; the assembly further comprising an insert capable of insertion inside the recess of the acetabular cup, the insert including an equator, an outer surface and a concave recess5 which receives a femoral head component;

/9 wherein, the insert is loc ated within the cup so thai the equator of the cup and the equator of the insert lie at an angle to each other; wherein the insert is selectively releasable from the cup to adjust its orientation for correct anteversion when the cup is fixed in position on an acetabulum. 23 An insert for an acetabular cup assembly; lhe insert including a generally convex outer surface and a concave inner surface defining a body including a recess which receives a femoral head component; the convex outer surface of the insert body receives thereon a ring which has an outer surface which opposes an inner surface of the cup; .wherein, the insert is placed in an optimum orientation for stability of an articulating ball of a head component, the orientation of the insert being at an angle to an axis normal to an equator of the cup so that when the cup is implanted an optimum position of the offset insert within the ring may be set without changing the orientation of the cup .

24 An acetabular assembly comprising a cup having generally convex outer surface for engaging acetabular bone and a generally concave inner surface together defining an inner recess, the cup including a wall having an apex and an equator and an insert capable of insertion inside the acetabular cup, the insert including an outer surface which when the insert is. inserted inside the cup, characterized in that the insert may be positioned in a plurality of orientations within the cup opposing the inner surface of the cup and a concave recess which receives a femoral head component; the outer surface of the insert having a circumferential wall which when the insert is inserted into a taper ring locates the insert at ati angle to the equator of said cup, the ring geometry allowing the insert to be set at a selected orientations within the cup without adjustment of the cup.

25 A method of assembly of an acetabular cup assembly comprising the steps of ; a) taking an acetabular cup having a generally convex outer surface for engaging acetabular bone and a generally concave inner surface together defining a cup wall which has an apex and an equal or; b) taking an insert capable of insertion inside the acetabular cup; c) inserting a ring inside the cup so that it locates between the insert and inner wall of the cup ; d) selecting an orientatioα for the insert within the cup;

the insert arranged inside- the cup so that its axis of symmetry is disposed at an angle to an axis normal to an equator of the cup and so that the insert may be placed in one of a plurality of available orientations to achieve correct anteversiσn without removal of the cup once implanted