GB2408938A

GB2408938A - Improved bearing compound of sandwich construction with layered stiffness for use in joint prosthesis

Info

Publication number: GB2408938A
Application number: GB0328777A
Authority: GB
Inventors: Roozbeh Shirandami
Original assignee: Individual
Current assignee: Individual
Priority date: 2003-12-12
Filing date: 2003-12-12
Publication date: 2005-06-15
Anticipated expiration: 2023-12-12
Also published as: GB2408938B; GB0328777D0

Abstract

The present design attempts to overcome some of the problems associated with existing bearing components used in total joint replacement procedures such as the accetabular cup in the total hip replacement and the bearing component in the total knee replacement. It addresses the mechanisms of load transfer and wear between the articulating surfaces of the prostheses and the bearing component by mimicking as closes as technologically possible the structural function, surface stiffness and properties of an intact articulating surfaces of a healthy joint, to prevent loosening of the prosthetic component and reduce wear of the articulating surfaces. The proposed design overcomes the problem associated with wear by utilising an intermediate bearing component with sandwich structure in which the bearing surfaces are comprised high surface stiffness to overcome the problems associated with wear while the inner core layer of the sandwich is formed of soft material to dampen the dynamic shock loads caused at the joint by every day human activity. With reference to Figure 1, in one embodiment, the proposed intermediate bearing component (10) is comprised of a first layer (11) and second layer (13) which define the outer surfaces of the said intermediate bearing-component (10) with elastic moduli and hardness E1, E2 and H1 and H2 respectively. Inner third layer (12) having elastic modulus and hardness E3 and H3 is sandwiched between the two layers (11 and 13) and bonded thereto at the inner surfaces (18 and 19) of the said layers (11 and 13) to form a layered sandwich structure such that E3 < E1 and E2, and H3 > H1 and H2. The said inner surfaces may be provided with shear keys (20 and 21) and/or have micro porous structure to provide mechanical key between the said surfaces and the inner core (12).

Description

Improved bearing component of sandwich construction with layered stiffness

for use in joint prosthesis One of the most significant factors leading to the revision of orthopaedic prostheses is the bone loss observed around failing implants caused by wear-derbies-activated macrophagic osteolytic mechanism. This is believed to be the universal failure mechanism for all arthroplasty components, irrespective of the fixation mode. The early descriptions of this bone-destruction process can be traced back to various clinical reports in the early 1 950s, which described failure of polymeric hemiarthoplasty implants such as Nylon and Polyethylene acetabular cup arthroplasties used in total hip replacement assemblies. Thus polymeric derbies produced by the bearing of the metallic femoral head on to the polymeric bearing surface of the prosthetic acetabular cup, and the associated macrophages are particularly reactive agents in bone Iyses phenomenon seen around contemporary total hip replacements. Similar problems are also observed in the tri-compartmental prosthesis used in total knee replacement in which the bearing surfaces of the metallic femoral and tibial components bear on to an; 2 intermediate Polyethylene bearing component. As a result there is a current trend for ' replacing the bearing surfaces of the prosthesis to eliminate the use polyethylene as the lo. :.

bearing surface and the problem associated with their wear particles. '.. . In an attempt to overcome some of the above problems the concept of ceramic on ceramic bearing has been developed. In the case of total hip replacement, the proposed concept utilises a ceramic femoral head articulating on a cemented ceramic cup and was once advocated for total hip replacement as providing virtually "zero wear" combination. However clinical experience in Europe has demonstrated that massive wear can also result from such ceramic/ceramic combinations as well as other complications such as three-body abrasive wear of ceramic particles, cup rim and cup fractures. In addition such designs are limited in their use to the hip prosthesis and cannot be adopted for use in knee prosthesis due to the complexity of geometry and degree of motion required for knee prosthesis.

A promising alternative to the aforementioned bearing designs is the use of metal on metal bearings. In the case total hip replacements these design rely on the bearing of a Page 1 of 18 metallic femoral head on to a metallic acetabular cup, which is cemented onto the acetabulum. The use of such bearings has reduced the wear rate from 100p m in the case of standard metal-on-plastic implants to 5'u m per year for metal-on-metal implants.

As a result the risk associated with wear-derbies-activated macrophagic osteolytic mechanism is reduced. Furthermore compare to ceramic-on-ceramic and ceramic-on- polyethylene bearings, metal-on-metal bearings are inherently more resistant to fracture. Use of metal-on-metal bearing in implants however are not without their problems. The design relies on the use of large bearing surfaces, such that the diameter of the femoral head prosthesis and its associated metallic acetabular cup (typically around 37mm) are 40% bigger than in the case of metals-on-polyethylene counterparts (typically around 27mm). The larger diameter is required to reduce the bearing stresses between the cooperating surfaces as well as the shear forces to be resisted by the bone cup interface. The restricted geometry of the joint, result in the use of thin acetabular shell (typically 2-3 mm thick) that is fixed on to the acetabular either by means of Polymethylmetcrylate (PMMA) cement or Hydroxyapatite (HA) coating. In comparison for a Polyethylene bearing, the wall thickness must be greater than 4mm if attached to a metal or ceramic backing and greater than 6mm if there is no backing by FDA requirements (FDA Guidance for Testing of Acetabular Cups 1995). This reduction in thickness in the case of metal- on-metal bearing, coupled with increased stiffness of metallic cup compared to polymeric cup and impact loading created by the walking cycle, result in high stresses at the bone acetabular cup interface leading to its loosening as well as breakage of PMMA cement or HA, leading to three body wear at the joint.

The present design attempts to overcome some of the problems associated with existing bearing components used in total joint replacement procedures such as the acetabular cup in the total hip replacement and the bearing component in the total knee replacement. It addresses the mechanisms of load transfer and wear between the articulating surfaces of the prostheses and the bearing component by mimicking as closes as technologically possible the structural function, surface stiffness and properties of an intact articulating surfaces of a healthy joint, to prevent loosening of the prosthetic component and reduce wear of the articulating surfaces.

Page 2 of 18 In a preferred embodiment the present invention comprises an intermediate bearing- component used in total joint replacement between two bones comprising prosthesis of sandwich construction characterized by: 1) A first layer having Young's modulus E1 and hardness H1, forming first face of the said bearing component and adapted to engage with the bearing face of a prosthetic component implanted in the first bone on one side, and; 2) A second layer having Young's modulus E2 and hardness H2, forming second face of the said bearing component and adapted to engage with the bearing face of a prosthetic component implanted in the second bone on the other side, and; 3) A third inner layer sandwiched between and bonded to the said first and second layers with Young's modulus E3 and hardness H3 such that E3 is less than E1 and E2, and H3 is less than H1 and H2.

In a preferred embodiment the said first and second layers of the said intermediate bearing component and the co-operating bearing face of the said prostheses implanted in the first and second bones respectively, are comprised of biocompatible metals, and the said third inner layer is comprised of a biocompatible polymer. ' In a preferred embodiment the said first face of the intermediate bearing-component and . , corresponding face of the prosthesis in the first bone allow relative motion between their corresponding surfaces, while all motion between the second face of the said bearing component and corresponding face of the implant in the second bone is prevented.

In one embodiment the said intermediate bearing-component forms a ball and socket joint between the said two prostheses implanted in the first and second bone. Wherein the said first face forms a ball joint engaging the concave surface of first prosthesis implanted in the first bone on one side, forming a ball and socket joint allowing rotational movement between the engaging surfaces; while the said second face includes a tapered socket for rigidly engaging tapered end of the said second prosthesis implanted in the second bone, forming a rigid connection therebetween.

Page 3 of 18 In a preferred embodiment the depth of the said tapered socket is varied by means of an intermediate sleeve, which engages the said tapered socket on one side and the said tapered end of the said second prosthesis on the opposing side.

In a preferred embodiment of the said intermediate bearing-component the ball and socket joint assembly formed by the said proposed intermediate bearing-component and the said first prosthesis is run-in as matched pair prior implantation and attachment to the said second prosthesis.

In one embodiment, the said intermediate bearing-component forms the ball joint between the said first prosthesis implanted in the first prosthesis formed by the acetabular cup and the said second prosthesis formed by the femoral prosthesis of a tricompartmental hip prosthesis.

In another embodiment of the said intermediate bearing-component the said bearing faces of the intermediate bearing-component and the corresponding face of the prostheses implanted in the said first and second bones allows relative motion between both of the said engaging surfaces respectively. In a preferred embodiment the bearing surface formed by the second layer and corresponding bearing face of the said second prostheses implanted in the second bone is provided with means for keeping the said cooperating bearing surfaces in full contact while allowing relative motion therebetween.

In one embodiment, the said intermediate bearing-component forms the bearing component of a tricompartmental knee prosthesis wherein; the said first face forms a concave surface engaging the convex surface of the said first prosthesis formed by the femoral prosthesis; and the said second face engages the bearing surface of the said second prosthesis formed by tibial prosthesis while allowing rotational and/or translation movement between said bearing component and tibial prosthesis depending on the mode of engagement of the said bearing-component on to the tibial prosthesis.

In one embodiment, the said intermediate bearing-component forms the bearing component of a tricompartmental knee prosthesis wherein; the said intermediate bearing-component is engaged on to the bearing surface of the tibial prosthesis by Page 4 of 18 means of a single co-operating surface between the tibial plate and the said bearing component, and means for keeping theses surfaces together in the form of projecting arcuate end walls on the medial and lateral sides of the said bearing which engage the undercut arcuate walls on the medial and lateral ends of the tibial platform, allowing differing degree of motion of the said bearing component relative to the tibial plate depending on the relative curvature of the said projecting arcuate walls to that of undercut walls.

In a preferred embodiment, the said intermediate bearing-component forms the bearing component of a tricompartmental knee prosthesis wherein said projecting arcuate walls and the respective said undercut walls on the medial and lateral ends of the tibial plate are formed about common centres, allowing only axial rotation of the intermediate bearing component relative to the tibial plate.

In yet another embodiment, the said intermediate bearing-component forms the bearing A. component of a tricompartmental knee prosthesis wherein said projecting arcuate walls, and the respective said undercut walls on the medial and lateral ends of the tibial plate are formed about differing centres, allowing axial rotation and anterior-posterior ; " translation of the intermediate bearing component relative to the tibial plate. : In another embodiment, the said intermediate bearing-component forms the bearing.

component of a tricompartmental knee prosthesis wherein the said intermediate bearing component is provided with a cavity on the bearing surface opposing the bearing surface of the tibial platform engaging a central peg projecting from the co operating surface of the tibial plate and preventing separation of the said surfaces.

In preferred embodiment, the said cavity is circular, allowing only rotation of the said intermediate bearing-component relative to the tibial plate.

In yet another preferred embodiment the said cavity is elongated so as to allow rotational and translational movement of the said intermediate bearing-component relative to the bearing surface of the tibial plate.

Page 5 of 18 In a preferred embodiment the said bearing surfaces of the said intermediate bearing- component and the respective co-operating surfaces of the said prosthesis in the first and second bone allowing relative motion therebetween, are highly polished surfaces to reduce frictional resistance.

In yet another preferred embodiment the said bearing surfaces of the said intermediate bearing-component and the respective co-operating surfaces of the said prosthesis in the first and second bone allowing relative motion therebetween incorporate layer of wear resistant coating such as diamond like carbon.

In preferred embodiment the said inner third layer is reinforced by means of fibre reinforcement.

In yet another preferred embodiment the said fibre reinforcement is discontinuous with discrete length.

In addition to the typical ball and socket joint such ah the hip joint, broadly speaking this invention can be used in joint prostheses and more specifically in a joint prosthesis with area contact bearing engagement accommodating an axial rotational movement between the first bone and the second bone; a rotational movement perpendicular to the axis of second bone causing the flexion of the first bone about the second bone; and a translational movement of the second bone relative to the first bone in the plane of flexion, while the bearing surfaces are kept in contact under the action of the joint. The present invention may be applicable not only to a knee joint but also to other joints such as elbow, spinal, finger and wrist joints.

A possible method for the construction of the bearing component as described above using a biocompatible metal for the said first and second face and a polymer for the said third sandwich layer includes the steps of: a) Forming the first and second metallic layers by means of hydrofomming and machining to the desired shape, and Page 6 of 18 b) Injection moulding in the cavity between the said two first and second layer to form the said third polymer layer Alternatively the said metallic layers may be formed by means of hydro-forming a tubular cavity to form the said first and second metallic layers, which are interconnected.

The resulting bearing component is a bi-material composite component of sandwich construction in which the three layers are interlocked together through the bond between their common surfaces.

In a preferred embodiment the surfaces of the metallic layers bonded to the polymer may incorporate mechanical keys to provide mechanical interlock between the two.

In one embodiment, the said mechanical key may be formed by means of micro-porous surface on the metallic surface engaging the polymer layer. , Embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: ..

Figure1 is a sectional view of the proposed intermediate bearingcomponent,;;; forming the ball joint in accordance with one embodiment of the present. . invention.

Figure 2 is a sectional view of a total hip prosthesis assembly with the said ball joint as of Figure 1.

Figure 3 is a sectional view of a total hip prosthesis assembly as of Figure 1 and 2 fitted with an adjustment sleeve.

Figure 4 is a sectional view of the adjustment sleeve as shown in Figure 3.

Figure 5 is a frontal elevation of tricompartmental knee prosthesis incorporating the proposed intermediate bearing-component.

Figure 6 is a plan elevation of the tibial prosthesis incorporating undercut walls along the medial and lateral ends of the tibial plate and proposed intermediate bearing-component incorporating arcuate shear key cooperating with undercut walls.

Page70fl8 Figure 7 is a sectional elevation along the medial-lateral plane A-A of the tibial prosthesis incorporating proposed intermediate bearing-component as shown in Figure 6.

Figure 8 is a sectional elevation along the anterior-posterior plane B-B of the tibial prosthesis incorporating proposed intermediate bearingcomponent as shown in Figure 7.

Figure 9 is a sectional elevation along the medial-lateral plane of the tibial prosthesis providing a central peg on the tibial plate, which engages a cavity, provided by the said intermediate bearing component.

Figure 10 is a plan elevation of a tibial prosthesis tatted with the proposed intermediate bearing having an elongated central opening and elongated sleeve.

Figure 11 is a sectional elevation along the medio-lateral plane C-C of a tibial prosthesis and intermediate bearing-component as presented in Figure 10.

Figure 12 is a plan elevation of the elongated sleeve as shown in Figure 10.

While the invention is susceptible to various modifications and alternative forms, specific embodiment thereof has been shown by way of examples. It should be understood that the examples and drawings are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling with in the spirit and scope of the invention as defined by the pending claims.

Figure 1 is a cross sectional view of an intermediate bearing component (10) forming the ball joint of a tricompartmental hip assembly as shown in Figure 2 in accordance to one aspect of the present invention. With reference to Figure 1, the proposed intermediate bearing component is comprised of a first layer (11) and second layer (13) which define the outer surfaces of the said intermediate bearing-component (10) with elastic moduli and hardness E1, E2 and H1 and H2 respectively. Inner third layer (12) having elastic modulus and hardness E3 and Ha is sandwiched between the two layers (11&13) and bonded thereto at the inner surfaces (18&19) of the said layers (11&13) to form a layered sandwich structure such that E3 c E1 and E2, and H3 H1 and H2. The Page 8 of 18 said inner surfaces may be provided with shear keys (20) and/or have micro porous structure to provide mechanical key between the said surfaces and the inner core (12).

In a preferred embodiment the outer layers (11&13) of the proposed intermediate bearing-component (10) are comprised of biocompatible metallic alloy and the inner core (12) is comprised of biocompatible polymers, which are inherently softer than the outer layers (11&13). In yet another embodiment the stiffness of the said inner core (12) may be controlled by means fibre reinforcement of the polymer material. This can be included during the injection moulding process resulting in short fibre reinforced polymeric inner layer (12).

With reference to Figure 2, the said intermediate bearing (10) forms the ball joint of a tricompartmental hip prosthesis assembly comprising an acetabular cup (6) formed buy a thin shell of biocompatible metallic alloy implanted in the in the first bone formed by the acetabulum (1) and hip prosthesis (3) implanted in the second bone formed by the femur (2). In a preferred embodiment, the said acetabular cup (6) may be secured to the acetabulum by means of biocompatible cement (5) such as PMMA. The convex surface (9) of the acetabular cup (6) may include porous surface treatment or shear ..

keys in the form of rings so as to allow mechanical fixation of the prosthesis to the A. PMMA cement. The hip prosthesis (3) however may be secured to the femoral bone (2) by means of cement-less fixation employing biocompatible coatings such as Hydroxyapatite or PMMA in the case of cemented prostheses.

In a preferred embodiment the convex surface (16) of said intermediate bearing- component (10) and the concave cooperating surface (7) of the acetabular cup (6) are both made of biocompatible metals or materials of similar stiffness with low wear properties such as ceramics, and are highly polished to reduce wear of these cooperating surfaces (16&7). In a preferred embodiment, the said surfaces (16&7) are coated by wear resistant coating (15&8) such as diamond like carbon.

With reference to figures 1&2 while the said first layer (11) and acetabular cup (6) provide congruent surfaces (7&16) allowing unrestricted rotational movement of the joint, the said second layer (13) is provided with a cavity (14) for rigidly engaging the said intermediate component (10) on to a corresponding end of the femoral prosthesis Page 9 of 18 neck (4). In a preferred embodiment the said cavity (14) and the corresponding end of the femoral prosthesis neck (4) are tapered inwardly by 1-10 degrees to form Morse- taper type of connection which is common practice in orthopaedic prostheses.

Compared to the use of Ultra High Molecular Weight Polyethylene (UHMWPE) based acetabular cups and metallic femoral ball joint forming a metal on polymer bearing, the proposed intermediate bearing-component (10) forming a metal on metal bearing as indicated in Figure 2 has the following advantages: It eliminates the aforementioned problems with UHMWPE wear particles associated with metal on Polyethylene bearing design.

The minimum recommended thickness of UHMWPE that can be used in bearing design in 6mm in accordance with FDA requirements (FDA Guidance for Testing of Acetabular Cups 1995). Given the lack of bone block in the acetabulum in the . majority of cases, this will result in difficulties in the fixing of the acetabular cup. :.

The proposed design reduces the thickness of the acetabular cup (6) to a minimum (2-3mm) while achieving the same degree of shock absorbency as of,. .

Polyethylene bearings now provided by the inner core layer (12) of the proposed ë intermediate bearing (10). .... :...

There is a current trend to use metal on metal bearing design, which uses a metallic acetabular and metallic femoral ball joint. Although such designs overcome the problems associated with UHMWPE wear particles as in the case of said metal on plastic bearing designs, there is a lack of shock absorbency in these designs due to the removal of the plastic bearing component and replacing it with all metal assembly which is significantly stiffer. This lack of shock absorbency result in the breakage of the implant bone interface in the case of HA coated prostheses, or the cement layer both at the femoral prosthesis and acetabular cup bone interfaces. In addition the HA and cement particles broken way from the implant interface find their way back into the joint capsule where they result in significant three body wear. The proposed intermediate bearing-component (10) resolves the above problem by reintroducing the shock absorbency at the joint by using a layered bearing design in which although the outer layers (11&13) are of biocompatible metallic alloy or other mechanically stiff materials Page lOofl8 (i.e. high skin stiffness), the inner core layer (12) is of polymeric compound (i.e. low core stiffness) to provide means for absorbing the shock load produced at the joint by the cyclic nature of human activities.

An important aspect of current practice in total hip arthroplasty is to achieve the correct neck length and therefore correct strain in the surrounding soft tissue. In current designs this is achieved by use of femoral ball joints, which have differing depth of cavity. With reference to Figures 3 and 4, the said intermediate bearing component (10) forming the ball joint component of the said total hip prosthesis assembly may be fitted with a sleeve (30) so as to adjust the neck length (F) of the femoral prosthesis (3) by having sleeves of differing internal length G and wall thickness K while keeping the overall length of the sleeve L the same, therefore resulting with interchangeable sleeves. The sleeve (30) is provided with surfaces (31) and (32) to match the said tapered surfaces of the said cavity (14) provided in the second layer (13) and femoral neck (4) so as to rigidly Ax all three components (3,30&10) together using the said Morse-taper type connection. The use of the sleeve (30) in achieving the variation in the over all neck length (F) has the advantage that the intermediate bearing-component (10) forming the ball joint remains the same for different neck lengths, unlike the current designs for the femoral heads with differing cavity depth. Therefore the acetabular cup (5) and the said bearing-component (10) can form a "matched pair" which has been run- in prior to implantation to reduce the initial wear between their co- operating surfaces immediately after implantation.

Figure 5, represents another embodiment of the present invention in which the proposed intermediate bearing component (60) forms the bearing component of a tricompartmental knee prosthesis (40) comprising a femoral prosthesis (50) implanted in the femur (41), the first bone on one side, and a tibial prosthesis (80) implanted in the tibial bone (42), the second bone, on the other, via a stem (83).

GB patent GB2312377 (Shirandami) discloses knee prosthesis incorporating undercut medial and lateral end wall and projecting arcuate nibs which engage the said under cut walls to ensure that the bearing surface formed by the tibial plate (80) and intermediate bearing-component (60) are kept in full contact. With reference to figures 6, 7 and 8, Page 11 of 18 the said bearing faces of the intermediate bearing-component formed by the said first and second layers (61&62), and the corresponding face of the first and second prostheses formed by femoral and tibial prosthesis (50&80) implanted in the first and second bone, formed by the femur (41) and tibia (42), allow relative motion between both of the engaging surfaces. In addition to prevent the dislocation of the bearing surface formed by the said second layer (62) and the corresponding bearing surface of the tibial plate (81) is provided with means for keeping the said cooperating surfaces in full contact while allowing relative motion therebetween, in the form of arcuate undercut walls (82) and projecting end walls (64).

Figures 7 and 8 present sectional elevation of a knee prosthesis as described in the aforementioned patent GB2312377 fitted with proposed intermediate bearing- component (60) in which the first layer (61) forms the bearing surface for the femoral prosthesis (50) and the said second layer (62) forms the bearing surface for the tibial..

prosthesis (80) formed by the tibial plate (81). Depending on manufacturing preferences, the projecting nibs (64) may be formed by either the first layer (61) or.....

second layer (62). Altematively both first and second layers (61&62) and the projecting.. . nibs (64) may be formed in a single step of hydroforming a thin hollow section. In both ë e.

cases the space depicted between said two layers (61&62) is filled with a material with significantly lower mechanical properties than the said outer layers (61&62) to form a soft inner core layer (63), which is bonded to the said outer layers (61&62). In addition the said inner surfaces (65&66) of the first and second layer (61&62) may be provided with porous structure and/or shear keys (67) to improve the bond between the inner layer (63) and the said first and second layers (61 &62).

With reference to Figure 6, an embodiment of the said intermediate bearing component may incorporate the proposed design as described in UK Patent GB2312377 (Shirandami) wherein the said projecting arcuate end walls (64) and the respective undercut walls (82) are formed about a common centres so as to only allow axial rotation of the said intermediate bearing-component (60) relative to the tibial plate (81).

Alternatively the said projecting end walls (64) and respective undercut walls (82) are formed about non-concentric centres so as to allow anterior-posterior translation of the said bearing component (60) relative to the tibial plate (81).

Page 12 of 18 With reference to Figure 9, an alternative to the aforementioned arcuate tongue and grove joint described in UK patent GB2312377 (Shirandami) is to locate the proposed intermediate bearingcomponent (60) on to the tibial component by means of a central opening (68) provided on the said second layer (62) which engages a circular peg (84) projecting from the tibial tray bearing (81) as described in the French patent FR-A-22663536 (Jouan Jean Paul et al.), and the European patents EP0551793A1 (Heinz), EP0636353A1 (Cramascoli). For a circular opening (in plan), the bearing is only allowed to rotate relative to the tibial plate while for an elongated opening in the anterior-posterior plane, the intermediate bearing component can rotate as well as translate in the anterior-posterior direction relative to the tibial tray (81). In the case of a elongated opening the said circular pin (84) and the walls of opening (68) form a line contact surface which result in high contact stresses leading to increased wear and plastic deformation of the bearingunder the medio-lateral loads at the joint . The proposed design over comes this shortcoming by enclosing the soft polymer inner :.

core (63) by the said second metallic layer (62). Therefore eliminating the problems,, associated with plastic flow of UHMWPE bearing as well as the aforementioned wear.. . particles.

Figures 10 to 12 present an alternative in which the said elongated opening (68) can be.. '.: fitted with an elongated sleeve (90) the length of which (S) controls the extent of translational movement of the said intermediate bearing component (60) relative to the tibial plate (81) while allowing free rotation of the said bearing (60) about the central pin (85). The use of the proposed sleeve (90) results in surface contact on to the walls, reducing the bearing stresses between the central peg and cavity wall, therefore eliminating the aforementioned problems associated with line contact between the circular peg and elongated cavity.

The proposed elongated sleeve (90) is engaged on to the pin (85) by placing it on to the shoulders (69) provided on the elongated opening (68) and rotating the bearing component (60) and said sleeve (90) to align the shear keys (86) with the elongated opening in the sleeve (92) on the said sleeve and then rotating the assembly to engage the said bearing component on to the tibial plate (81). In a preferred embodiment the Page 13 of 18 said elongated sleeve (90) is comprised of a highly polished biocompatible metallic alloy.

In a preferred embodiment of the proposed intermediate bearing-component forming the bearing component of a tricompartmental knee prosthesis, said first and second layers (61&62) are made of biocompatible metallic alloys or ceramics and the inner third layer (63) is made of biocompatible polymers while the femoral and tibial prostheses are comprised of biocompatible metallic alloy. The tricompartmental knee prosthesis results in a metal-on-metal bearing design at the co-operating surfaces of the said layer (61&62) and the respective bearing surfaces of the femoral and tibial prostheses. In a preferred embodiment the said co-operating surfaces are highly polished or may be coated with wear resistant coatings such as diamond like carbon.

Compared to the exiting designs which use a UHMWPE intermediate bearing component and rely of metaln-plastic design the proposed design eliminates the problems associated with polymer wear particles by changing the design to a metal-on- metal bearing while achieving the same shock absorbency by using a polymer third layer (63) which forms an inner core to the proposed intermediate bearing component (60).

Page 14 of] 8

Claims

Claims I claim: 1. An intermediate bearingomponent used in total joint

replacement between two bones comprising a prosthesis of sandwich construction characterized by: a first layer having Young's modulus E1 and hardness H1, forming first face of the said bearing component and adapted to engage with the bearing face of a prosthetic component implanted in the first bone on one side, and; a second layer having Young's modulus E2 and hardness H2, forming second face the said bearing component and adapted to engage with the bearing face of a prosthetic component implanted in the second bone on the other side; and a third inner layer sandwiched between and bonded to the said first and second layers with Young's modulus E3 and hardness H3 such that E3 is less than E1 and E2, and H3 is less than H1 and H2. . -
2. An intermediate bearing-component as claimed in claim 1 wherein the said first and second layers of the said intermediate bearing-component and the co- operating bearing face of the said prostheses implanted in the first and second bones respectively, are comprised of biocompatible metals, and the said third inner layer is comprised of a biocompatible polymer.
3. An intermediate bearingomponent as claimed in claim 1 and 2 wherein the said first face of the intermediate bearing-component and corresponding face of the implant in the first bone, allow relative motion between the engaging surfaces, while all motion between the second face of the said bearing and corresponding face of the implant in the second bone is prevented.
4. An intermediate bearing-component as claimed in claims 1 to 3 in the form of a ball and socket between the said two prosthesis implanted in the first and second bone wherein; the said first face forms the ball joint engaging the concave surface of first prosthesis implanted in the first bone on one side, forming a ball and socket joint allowing rotational movement between the engaging surfaces; and the said second face includes a tapered socket for rigidly engaging tapered end of the said second prosthesis implanted in the second bone, forming a rigid connection therebetween.

Page 15 of 18
5. An intermediate bearing-component as claimed in claim 4 wherein the depth of the said tapered socket is varied by means of an intermediate sleeve, which engages the said tapered socket on one side and the said tapered end of the said second prosthesis on the opposing side.
6. An intermediate bearing-component as claimed in claims 4 and 5 in which the ball and socket joint assembly formed by the said proposed intermediate bearing- component and the said first prosthesis is run-in as matched pair prior implantation and attachment to the said second prosthesis.
7. An intermediate bearing-component as claimed in claims 4 to 6 in the form of tricompartmental hip prosthesis wherein the said intermediate bearing component forms the ball joint between the said first prosthesis implanted in the first prosthesis formed by the acetabular cup and the said second prosthesis formed by the femoral prosthesis. . . ,
8. An intermediate bearing-component assembly as claimed in claim 7 wherein the said acetabular cup is formed of a concave metallic shell implanted in the, acetabulum. .. .
9. An intermediate bearing-component as claimed in claims 1 and 2 wherein the....

said bearing faces of the intermediate bearing-component and the corresponding face of the prostheses implanted in the said first and second bones allow relative motion between both of the said engaging surfaces respectively.
10. An intermediate bearing-component as claimed in claims 1, 2 and 9 wherein bearing surfaces formed by the second layer and corresponding bearing face of the said second prostheses implanted in the second bone is provided with means for keeping the said cooperating bearing surfaces in full contact while allowing relative motion therebetween.
11. An intermediate bearing-component as claimed in claims 1, 2, 9 and 10 in the form of bearing component of a tricompartmental knee prosthesis wherein; the said first face form a pair of concave surfaces engaging the convex surfaces of the said first prosthesis formed by the femoral prosthesis; and the said second face engages the bearing surface of the said second prosthesis formed by tibial prosthesis while allowing rotational and/or translation movement between said Page 16 of 18 '^ bearing-component and tibial prosthesis depending on their mode of engagement on to the tibial prosthesis.
12. An intermediate bearing-component as claimed in claims 9 to 11 in the form of bearing component of a tricompartmental knee prosthesis wherein; the said intermediate bearing-component is engaged on to the bearing surface of the tibial prosthesis by means of a single co-operating surface between the tibial plate and the said bearing component, and means for keeping theses surfaces together in the form of projecting arcuate end walls on the medial and lateral sides of the said bearingomponent which engage the undercut arcuate walls on the medial and lateral ends of the tibial platform, allowing differing degree of motion of the said bearing component relative to the tibial plate depending on the relative curvature of the said walls.
13. An intermediate bearing-component as claimed in claims 12 in the form of bearing component of a tricompartmental knee prosthesis wherein said projecting arcuate walls and the respective said undercut walls on the medial and lateral ends of the tibial platform are formed about common centres, allowing only axial rotation of the intermediate bearing component relative to the tibial plate.
14. An intermediate bearing-component as claimed in claims 12 in the form of bearing component of a tricompartmental knee prosthesis wherein said projecting arcuate walls and the respective said undercut walls on the medial and lateral ends of the tibial platform are formed about differing centres, allowing axial rotation and anterior-posterior translation of the intermediate bearing component relative to the tibial plate.
15. An intermediate bearing-component as claimed in claims 1, 2, 9 and 10 in the form of bearing component of a tricompartmental knee prosthesis wherein the said intermediate bearing component is provided with a cavity on the bearing surface opposing the bearing surface of the tibial platform, engaging a central peg projecting from the co-operating surface of the tibial plate, preventing separation of the said surfaces.
16. An intermediate bearing-component as claimed in claim 15 in the form of bearing component of a tricompartmental knee prosthesis wherein the said cavity is Page 17 of 18 circular, allowing only rotation of the said intermediate bearing component relative to the tibial platform.
17. An intermediate bearing-component as claimed in claim 12 in the form of bearing component of a tricompartmental knee prosthesis wherein the said cavity is elongated so as to allow rotational and translational movement of the said intermediate bearing-component relative to the bearing surface of the tibial plate.
18. An intermediate bearing-component as claimed in claim 17 in the form of bearing component of a tricompartmental knee prosthesis wherein the said elongated cavity may be fitted with an elongated sleeve to adjust the translational movement between the said intermediate bearing- component and the tibial plate.
19. An intermediate bearing-component as claimed in any of the above claims . . . wherein the said bearing surfaces of the said intermediate bearing-component and the respective co-operating surfaces of the said prosthesis in the first and, second bone allowing relative motion therebetween, are highly polished surfaces. . ..

to reduce frictional resistance. ....
20. An intermediate bearing-component as claimed in any of the above claims wherein the said bearing surfaces of the said intermediate bearing-component and the respective co-operating surfaces of the said prostheses in the first and second bone allowing relative motion therebetween incorporate layer of wear resistant coating.
21. An intermediate bearing-component as claimed in claim 20 wherein the said wear resistant coating is diamond like carbon.
22. An intermediate bearing-component as claimed in any of the above claims wherein the said inner third layer is reinforced by means of fibre reinforcement.
23. An intermediate bearing-component as claimed in claim 22 wherein the fibre reinforcement is discontinuous with discrete length.
24. An intermediate bearing-component of sandwich construction forming a bearing component used in total joint replacement substantially as hereinbefore described with reference to the accompanying drawings.

Page 18 of 18