WO2006011028A1

WO2006011028A1 - Orthopaedic implants

Info

Publication number: WO2006011028A1
Application number: PCT/IB2005/002094
Authority: WO
Inventors: Charles Simon James Pickles
Original assignee: Element Six Limited; Donald, Heather, June
Priority date: 2004-07-20
Filing date: 2005-07-20
Publication date: 2006-02-02

Abstract

The prosthetic joint component, typically a hip joint component such as a ball (10) or a socket (12), consists generally of a substrate material (14) with a generally non-planar surface and a layer (20) of wear-resistant material on a surface thereof presenting a bearing or wear-resistant surface for the component. The substrate is tungsten metal, an alloy of tungsten, molybdenum, an alloy of molybdenum, or combinations thereof. The wear resistant material is preferably polycrystalline CVD diamond.

Description

ORTHOPAEDIC IMPLANTS

BACKGROUND OF THE INVENTION

THIS invention relates to an orthopaedic implant, in particular a prosthetic joint component.

Orthopaedic implants or prosthetic joints are used extensively in the replacement of damaged or destroyed human joints, including hip and knee joints, for example. Prosthetic joints suffer from limited life spans, typically in the order of 15 years or less, whilst the market requirement is for prosthetic joints that are viable for much greater periods.

For example, current prosthetic hip joints typically consist of a generally spherical ball formed of cobalt-chromium or titanium alloy, which is attached via a "stem" to a so-called long bone, and a hemispherical cup or socket that replaces the acetabular cup, and which is lined with ultra-high molecular weight polyethylene (UHMWPE). Such ball and socket arrangements, however, invariably result in frictional wear of the spherical ball and the UHMWPE lining, releasing very fine particles. The fine particles cause histiocytic reactions in the body in an attempt to eliminate these particles. As a consequence, osteolysis takes place as agents, which are released as a result of the histiocytic reactions, attack the neighbouring bone, causing joint loosening and ultimately joint failure. Various attempts have been made to address this and other problems associated with prosthetic joints.

For instance, attempts have been made to use metal balls and sockets made from biocompatible materials. However, the problems of wear debris being formed as a result of friction still occur, with the same consequences as previously mentioned.

Another alternative is to form the ball of a biocompatible ceramic, with the further option to coat the ceramic with a biocompatible non-wear material, such as diamond. Whilst this addresses the problems associated with wear debris, the potential for prosthetic joint failure due to other causes still exists. For example, the ceramic ball is generally attached to a stem having a tapered profile end, which is the industry standard stem used in joint replacements. Ceramics, although mechanically strong under compression, generally have low fracture strength under tension. Accordingly, stress concentrations, and in particular those associated with the taper joint, are prone to cause fractures in the ceramic and thus premature joint failure.

Thus for prosthetic hip joints and other prosthetic joints there is a market preference to avoid ceramic components and to use tougher and more robust materials. However, in attempting to use tougher and more robust materials, problems associated with biocompatibility and osteolysis via the production of wear debris, as well as a range of other practical considerations, remain. SUMMARY OF THE INVENTION

According to the invention a prosthetic joint component comprises a substrate formed of tungsten or an alloy of tungsten, molybdenum or an alloy of molybdenum, or combinations thereof, the substrate having a non- planar surface, and a layer of wear resistant material, preferably polycrystalline CVD diamond, located on at least a portion of the non-planar surface of the substrate, the layer of wear resistant material presenting a bearing or wear resistant surface for the component.

The wear resistant material is preferably in the form of a layer of polycrystalline CVD diamond synthesised onto the substrate.

In a preferred embodiment of the invention, the prosthetic joint component is characterised by having one or more of the following shape tolerances: .

sphericity of +/- 5 μm, radius of +/- 10 μm, and - roughness preferably <40 nm Ra and more preferably < 20 nm Ra.

The substrate is tungsten metal, or an alloy of tungsten containing at least 80%, more preferably at least 92% and most preferably at least 98%, tungsten by atomic fraction, or molybdenum, or an alloy of molybdenum containing at least 80%, more preferably at least 92% and most preferably at least 98%, molybdenum by atomic fraction.

The prosthetic joint component is preferably a hip joint component, which may be a ball component or a socket component for a hip joint, or a combination thereof.

The polycrystalline CVD diamond coating typically has a thickness falling within the range of about 10 - 150 μm. The upper limit is preferably no more than about 120 μm, more preferably no more than about 100 μm, and -A-

even more preferably no more than about 80 μm. The lower limit is preferably at least about 30 μm, more preferably at least about 40 μm, even more preferably at least about 50 μm, and most preferably at least about 60 μm.

In a preferred embodiment of a ball component for a hip joint, the polycrystalline CVD diamond coating provides a continuous layer covering more than 2 π steradians, preferably more than 5/2 π steradians, more preferably more than 3 π steradians of the surface of the ball, and further it is preferable that the boundary between the coated and uncoated regions is substantially similar to a small circle.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be described in more detail, by way of example only, with reference to the accompanying drawing, which is a sectional side view of a prosthetic joint incorporating prosthetic joint components of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The prosthetic joint component of the invention consists generally of a substrate material with a generally non-planar surface and a layer of wear- resistant material on a surface thereof presenting a bearing or wear- resistant surface for the component. The substrate is tungsten metal, or an alloy of tungsten containing at least 80% (more preferably at least 92%, most preferably at least 98%) tungsten by atomic fraction, or molybdenum, or an alloy of molybdenum containing at least 80% (more preferably at least 92%, most preferably at least 98%) molybdenum. The wear resistant material is preferably polycrystalline CVD diamond. The prosthetic joint component is typically a component of a hip joint. In one version of the invention, it is generally a spherical ball having a recess that is suitable to receive the end of a stem, such as an industry standard stem used in joint replacement. A layer of wear-resistant material, in the form of polycrystalline CVD diamond, is formed on an outer surface of the ball presenting a bearing or wear-resistant surface for the ball. The polycrystalline CVD diamond coating preferably covers more than 2 π steradians of the surface of the ball.

In an alternative version of the invention, the prosthetic joint component is a socket or cup having an internal surface forming substantially a hemispherical surface. A layer of polycrystalline CVD diamond coats the internal surface of the socket or cup, or at least a substantial portion thereof, and presents a bearing or wear-resistant surface for the socket or cup.

A preferred version of the invention is a hip joint comprising a pair of complementary CVD polycrystalline diamond coated ball and socket components as described above. Such an arrangement is illustrated in the accompanying drawing.

Referring to the accompanying drawing, a prosthetic joint ball component 10 of the invention is receivable in a prosthetic joint socket component 12 to form a prosthetic joint. The ball component 10 comprises a generally spherical ball 14 mounted on a stem 16. The outer rounded surface 18 of the ball 14 is provided with a layer 20 of CVD diamond. The ball 14, in this case, is formed of tungsten metal and includes a recess or socket 22, which is shaped and sized to receive, in use, the tapered end 24 of the stem 16.

The socket component 12, which in this case is also formed of tungsten metal, is lined with a layer of polycrystalline CVD diamond 26 to reduce friction between the ball 10 and the socket 12, reducing wear and thereby reducing the risk of wear debris forming. In an alternative version of the invention, the socket component 12 may be a thick film CVD diamond socket or cup component.

Where the substrate is molybdenum or an alloy of molybdenum, the relevant surface thereof may be prepared with a thin coating of tungsten prior to coating with the polycrystalline CVD diamond.

The thickness of the polycrystalline CVD diamond coating falls within an optimum range, typically in the range of about 10 - 150 μm. The upper limit is preferably no more than about 120 μm, more preferably no more than about 100 μm, and even more preferably no more than about 80 μm. The lower limit is preferably at least 30 μm, more preferably at least 40 μm, even more preferably at least about 50 μm, and most preferably at least about 60 μm. Above the preferred range the layer is likely to fail by delamination under high or impact loading, whilst below the preferred range the layer is difficult to process to a suitable surface finish and is likely to fail by cracking of the film due to substrate deformation under high or impact loading.

Tungsten is well known as a substrate material for CVD polycrystalline diamond synthesis, with the diamond layer being released and used in a freestanding form after synthesis. The use of CVD diamond coated components for hip joints has previously been considered, but the combination of polycrystalline CVD diamond on tungsten or molybdenum, or tungsten coated molybdenum, has not previously been considered for prosthetic applications and in particular hip joints.

The prosthetic industry has recognised the benefits of using various forms of diamond as the wear surface on prosthetic joints, in particular focusing on the application of diamond to existing metal and alloy systems already used in prosthetics without such coatings.

Polycrystalline CVD diamond synthesis involves the use of a substrate onto which the diamond nucleates and grows. The substrate must be capable of withstanding the growth conditions, in particular a temperature of typically 600-1000°C, and provide a surface suitable for diamond growth (many carbide forming elements are not compatible), a surface suitable for preparation for diamond nucleation, and a surface to which the diamond when grown can adhere well in application. This last aspect requires two issues to be considered, adhesion of the diamond layer to the substrate and low stress between the diamond and the substrate. Stress arises because of the thermal mismatch between the diamond and the substrate as the coated substrate is cooled from the growth temperature to the temperature of storage and use. In addition, for application in a prosthetic device the substrate material is required to be biocompatible.

On first inspection, tungsten and molybdenum are unlikely candidates for elements of a prosthetic device. Tungsten and molybdenum are not commonly considered in applications where biocompatibility plays an important role, and are also not generally used for diamond coated wear applications. However, in terms of the present invention, it has been found that tungsten can be used in components for a joint provided there is a further layer or component providing the contact of the component to the bone.

Further, whilst tungsten is generally thought of as a relatively brittle refractory metal, it has substantially higher fracture toughness than the ceramic alternative, lying typically in the range 10-20 MPam⁰⁵ depending on the details of preparation. Accordingly, it has been found that tungsten is more than adequate for use as a substrate material in a prosthetic joint component. In addition, it has the particular advantage of providing a very high Young's modulus and so provides good stiff support to a diamond coating.

Molybdenum is less common as a substrate material for CVD diamond synthesis because it is believed to form complex carbides which, unless the process is carefully controlled, can give unpredictable adhesion. This invention demonstrates that this can be overcome by synthesis at low temperatures or by use of a tungsten interlayer, to provide adequate adhesion for the application. Low temperature synthesis is also beneficial in that it reduces the thermal mismatch stresses generated.

Molybdenum has a higher fracture toughness than tungsten, lying typically in the range 15-90 MPam⁰⁵ depending on the details of preparation and so is a more robust substrate material, for example in terms of the taper joint or for high impact loads. It does, however, have a lower Young's modulus (193 GPa for molybdenum compared with 411 GPa for tungsten) and thus does not provide quite as good a support for the diamond coating on the surface. Again, whilst not generally considered biocompatible, molybdenum can be used in non-bone contacting applications.

A key requirement for replacement prosthetics is reliability and robustness in use. The prosthetics must ideally provide a suitable level of performance against all criteria rather than merely excelling in one or two and failing in others. Clearly the failure of an implanted device is at the very least a costly inconvenience to the user, but often requires major and potentially life threatening surgery to rectify. Consequently the market does not tolerate device failures.

In a system utilising coatings, a key requirement is for reliability and longevity of the coating such that delamination or cracking of the coating does not occur. Thus, whilst the CVD polycrystalline diamond coating provides a surface which is wear resistant, the substrate to which it is bonded must provide sufficient rigidity to support the coating, sufficient toughness to withstanding high impact loadings without itself failing, suitable mechanical properties to receive the taper or other means of connecting to other mechanical components of the joint, good and reliable adhesion, and low interface stresses since interface stresses act to assist in adhesion failure. In addition, for thin film diamond coatings, the diamond needs to be grown directly onto the substrate so that the substrate material needs to be compatible with CVD diamond synthesis. Further, a moderately high thermal conductivity is advantageous to assist in obtaining a uniform deposition temperature and thus uniform deposition, and consequently uniform adhesion and other properties.

Key to the development of the polycrystalline CVD diamond coated components of the invention has been the study of the failure modes of CVD diamond coatings used in prosthetic devices under the loading and movements experienced typically and under extreme conditions. This has shown that cobalt chrome, stainless steel and zirconium, for example, are incompatible with diamond synthesis, and that zirconium and titanium have thermal expansion coefficients that are too high.

One compatibility problem is the use of carbide forming materials, such as cobalt or iron, which cause graphitisation at the diamond/substrate interface and weaken adhesion or degrade the entire diamond layer. A potential solution to this problem and others, such as adhesion, is the use of interlayers, but this adds additional materials to be considered for biocompatibility and has generally met with little overall success. It has to be reiterated that in this context, success is denoted by a zero failure rate in application testing rather than a majority successful at the fabrication stage.

The issue resulting from too high a thermal expansion coefficient is that CVD polycrystalline diamond synthesis generally takes place at temperatures in excess of 600⁰C, and more typically in excess of 700⁰C, particularly when high performance of thin layers is required in a wear application. In contrast the application temperature is normal body temperature. Consequently, thermal mismatch stresses between the substrate and the coating arise from the difference in the respective thermal expansion coefficients integrated over the temperature range between synthesis and application, and can cause adhesion failure of the coating.

The use of a coating rather than a bonded thick film component is advantageous in that it provides the possibility of covering more than a hemisphere of the ball component in a hip joint. Limiting the wear surface of the ball to a hemisphere results in limited mobility of the joint, the risk of additional wear mechanisms operating at the edge of the prepared wear surface, and excessive loading on the edge of the prepared wear surface causing additional failure modes such as cracking. The need to release thick film components from their original substrate, and bond them onto the spherical component of the joint, limits such components to no larger than a hemisphere, terminating in a great circle. However, thin film coatings can extend beyond a hemisphere point, ideally to cover about 270° of the circle seen in cross-section, thus providing for a continuously covered region of the sphere greater than 2 π steradians preferably bounded by a small circle. In the ideal circumstances then the edge of the coating on the ball component will not enter into contact with the corresponding wear surface of the socket component, removing this potential source of wear and failure.

An alternative solution to enable coverage of more than 2 π steradians is to use more than one component to cover the ball. One possibility would be two or three thick diamond film components. Another option that has been revealed in the prior art is to use multiple individual plates of diamond or diamond compact, which may or may not make contact. The prime disadvantage with this general approach is that the edges of elements of the ball wear surface now move against the wear surface of the socket component. It is impractical to ensure such edges do not cause additional local wear or loading of the joint, and thus degrade the performance of the joint. A typical problem is edge chipping forming additional wear debris.

Thus, the use of molybdenum or tungsten as the substrate, with the additional option in the case of molybdenum of using a tungsten interface layer, and then a CVD diamond coating on top, provides a system which has greater wear resistance than other options using a non-diamond wear layer, a greater toughness at the taper joint and under impact or high static loads, the ability to cover more than a hemisphere on the ball element of a ball and socket type joint such as the hip (or more than 180° in cross- section of other substantially curved surfaces), and avoids the problems of having edges at interfaces or on individual isolated components which are liable to chip and exacerbate loading and wear problems.

The advantages of using a diamond coating in the present application are clear, but there are certain challenges in the use thereof in practice. The quality and thickness of the CVD diamond polycrystalline coating must be sufficiently uniform over the whole of the operational area of the coating, posing a significant problem in providing a suitable synthesis method. Furthermore, whilst molybdenum and tungsten have a lower thermal mismatch problem than alternative materials, it is still essential to maximise adhesion and minimise thermal mismatch stresses. Adequate adhesion is obtained by careful preparation of the surface finish of the metal components prior to coating, and careful control of the initial coating conditions. Thermal mismatch stress is minimised by selecting low temperature growth conditions compatible with other aspects of the requirements. Furthermore, the post growth processing and in particular the final surface finish of the diamond coating must also be to specification and sufficiently uniform over the contact region. Surface finish of the uncoated metal (W or Mo) must also be controlled, as this can affect biocompatibility issues.

The general method of fabrication will now be described, with particular reference to the production of the ball component of a hip prosthetic although those skilled in the art will understand that this does not limit the generality of the invention.

The ball of the selected metal (tungsten or molybdenum) is first prepared by either lapping in a molybdenum cup using Element Six 350 XF diamond grit until the surface is uniformly processed, or wet-blasted using 90/100 mesh alumina grit in a water suspension. Final tolerances are typically: - sphericity +/- 5 μm, radius +/- 10 μm. The surface of the ball is then seeded using diamond grit using methods known in the art. The ball is then thoroughly cleaned using isopropanol. Within the growth chamber the ball is mounted vertically upon a molybdenum post using the tapered hole. The fit is such that a good thermal path is maintained between the ball and post. The other end of the post is water cooled.

The key elements of synthesis are to:

1) Control the initial nucleation of the diamond film as it forms on the ball, in order to ensure reproducible adhesion.

2) Ensure sufficiently uniform deposition over the whole area of interest, where this uniformity extends to thickness, grain size and morphology, and other properties.

The details of how these objectives may be achieved vary with the method of synthesis. Synthesis itself may use a variety of techniques which are generally known in the art, including microwave plasma CVD, CVD jet techniques, and hot filament CVD. Hot filament CVD is generally preferred and will be described more fully. The filament arrangement in the hot filament reactor comprises one or more filament arrays consisting of one or more filaments each. Individual filaments may be wires or flat strips of a refractory metal. Providing uniform growth over a three dimensional object may then be achieved by careful arrangement of the individual filaments, or of the filament arrays, around the ball, or by relative motion of the filaments and ball, or by using gas injection nozzles to distribute the species produced at the filament. A particular form of filament which provides good three dimensional coverage is a mesh filament. The surface area of the filaments and their distribution around and separation from the ball substrate is selected to provide sufficient activation of the gases, and to conform to the requirements of the heat balance required to hold the substrate at the correct temperature during synthesis. This heat balance can also be controlled by means such as a heater element in the post onto which the ball is mounted. This technique is advantageous to preheat the ball when shutters are used to protect the ball surface from deposition during the start up phase of the filaments. Prior to the synthesis stage the filaments are pre-carburised, typically by running them at 1700 - 1800⁰C with a high methane level (e.g. 100 seem CH₄ in 1000 seem H₂) for at least one hour growth.

During deposition the gases are pre-mixed before injection, with a typical composition of 0.5-5% methane, 0.5-5% Ar, balance hydrogen. The total flow is typically in the range 500 to 3000 seem. Growth pressures are in the range 1 x 10³ Pa to 6 x 10³ Pa. The filament temperature is in the range 1900-2420⁰C. The distance from the filament arrays to the substrate is set such that under growth conditions the ball maintains a temperature of 750 - 1050⁰C as measured by pyrometry.

In a preferred embodiment of deposition the gases are pre-mixed before injection and typical flows are 1000 seem H₂, 25-45 seem CH₄ and 30-100 seem Ar. Growth pressures are in the range 1 x 10³ Pa to 6 x 10³ Pa. The filament temperature is in the range 2300-2420⁰C. The distance from the filament arrays to the substrate are set such that under growth conditions the ball maintains a temperature of 850 - 95O⁰C as measured by pyrometry.

Synthesis continues until a layer in excess of the target thickness by about 10 - 120 μm is produced, and then the ball removed from the synthesis chamber. Subsequently the ball is processed using a range of techniques such as mechanical processes or hot metal processes, which are well known in the art, to produce the final form. Tolerances in final form are typically:- sphericity +/- 5 μm, radius +/- 10 μm, roughness < 40 nm Ra.

In a preferred embodiment, synthesis continues until a layer in excess of the target thickness by about 20 - 40 μm is produced, and then the ball removed from the synthesis chamber. Subsequently the ball is processed using a range of techniques such as mechanical processes or hot metal processes, which are well known in the art, to produce the final form. Tolerances in final form are preferably :- sphericity +/- 5 μm, radius +/- 10 μm, roughness < 20 nm Ra. In a further preferred embodiment, a microwave plasma CVD technique is used to deposit the diamond layer. The substrate is prepared as previously described. Diamond deposition is performed in a typical microwave CVD deposition system using a cavity with a resonant frequency equal to one of the standard microwave heating frequencies, for example 2.54 GHz, 896 MHz or 915 MHz. Total gas flows in the range 300 to 6000 seem are used with typical volumetric gas compositions of 0.5-10% methane, 0-10% Argon, 0-5% oxygen, balance hydrogen. The chamber is typically maintained at a pressure of between 3.9 x 10³ Pa and 3.9 x 10⁴ Pa. The substrate is maintained at a temperature of between 750°C and 1050⁰C as measured by optical pyrometry. Deposition typically continues until a diamond layer in the range 10 to 150 μm has been deposited. Subsequently the ball is processed using a range of techniques, such as mechanical processes or hot metal processes, which are well known in the art, to produce the final form. Tolerances in final form are typically:- sphericity +/- 5 μm, radius +/- 10 μm, roughness < 40 nm Ra.

EXAMPLE 1

A molybdenum ball of diameter 32 mm was first prepared by lapping in a molybdenum cup using Element Six 350 XF diamond grit until the surface was uniformly processed, giving a sphericity of better than +/- 4 μm, and a radius within 8 μm of the target value which was 50 μm less than the 32 mm target for the final coated object. Before installation in the growth reactor the ball is seeded over the entire surface using methods known in the art. The ball was then thoroughly cleaned using isopropanol.

Within the growth chamber the ball was mounted vertically upon a molybdenum post using the tapered hole.

The growth process used a tantalum mesh filament preformed to an approximately hemispherical shape extending into the four corners of the original flat mesh from which it was formed with these corners providing the points of electrical attachment. This filament configuration was precarburised for 1 hr at 1750⁰C, at a pressure of 3.3 x 10³ Pa, and with gas flows of 100 seem CH₄ and 1000 seem H₂, pre-mixed before entering the chamber. Chamber gas flow was configured to move from the apex of this dome to the open flat base, assisting in transfer of growth species to the remainder of the ball.

The growth process was run under the following conditions: 1000 seem H₂, 30 seem CH₄ and 100 seem Ar and a pressure of 3.3 x 10³ Pa with the ball substrate at a temperature of 850⁰C. This produced a uniform coating 130 μm thick over the 270° of the ball as viewed in side profile.

The ball was removed from the synthesis chamber. Subsequently the ball was processed using mechanical processing techniques to produce the final form which was a coating 105 μm thick with tolerances of:- sphericity +/- 4 μm, radius + 5 μm, and a surface roughness < 20 nm Ra.

EXAMPLE 2

The general process described in Example 1 was repeated using a Mo ball of 28 mm coated with a tungsten interface layer. After mechanical lapping to form the correct ball surface and the ultrasonic cleaning in isopropyl alcohol, the interface layer was applied by sputtering using a tungsten target in an Ar atmosphere at 5 x 10^~1 Pa to produce a layer about 2 μm thick.

In the growth phase the mesh filament was replaced by a series of flat strip filaments positioned round in a ring, alternative filaments lying on a vertical tube and on a cone with a 40° included angle at the apex to improve uniformity over the top of the ball, with the filaments operating at just over 2200^°C. Gas flows of 1000 seem H₂, 40 seem CH₄ and 50 seem Ar were used with the chamber at 4.6 x 10³ Pa. Under these conditions the ball settled at a growth temperature just over 1000^°C. Processing of the ball after synthesis was similar to that in Example 1 , achieving a ball with a 85 μm thick diamond layer and an external surface with tolerances of :- sphericity +/- 5 μm, radius -3 μm (from the target 28 mm), and a surface roughness < 20 nm Ra.

EXAMPLE 3

The preparation of a substrate was completed as described in Example 1 using a tungsten ball 32 mm in diameter. This was then mounted in a 2.45 GHz microwave chamber with eight nozzle radial gas injection from the wall of the reaction chamber positioned at the height of the centre of the ball. The growth process was run under the following conditions: 1200 seem H₂, 35 seem CH₄ and 100 seem Ar and a pressure of 13 x 10³ Pa with the ball substrate at a temperature of 875°C. This produced a sufficiently uniform coating about 130 μm thick over the 270° of the ball as viewed in side profile.

Post growth processing used the technique of hot metal thinning, using a two stage process. Each stage took place in a modified vacuum furnace at 1000⁰C with oxygen excluded, using a low pressure (approx. 10 Pa) inert atmosphere of argon in stage 1 and a mildly reducing atmosphere of 5% H₂ in Ar in stage 2. In stage 1 a mild steel hemispherical cup with substantial wall thickness and an internal radius of 60 μm greater than the coated ball was brought into contact with the diamond coating on the ball with the ball then rotated whilst the cup was oscillated up and down the side of the ball to cover the whole coated area of the ball. Stage 2 repeated this process with a 316 stainless steel hemispherical cup with an internal radius 20 μm greater that the final target radius of the ball, using the dwell time at each position around the ball to produce the required radius on the ball. This processing achieved a ball with a 60 μm thick diamond layer and an external surface with tolerances of:- sphericity +/- 3 μm, radius +8 μm (from the target 32 mm), and a surface roughness < 20 nm Ra.

Claims

1. A prosthetic joint component, comprising a substrate formed of tungsten or an alloy of tungsten, molybdenum or an alloy of molybdenum, or combinations thereof, the substrate having a non-planar surface, and a layer of wear resistant material located on at least a portion of the non- planar surface of the substrate, the layer of wear resistant material presenting a bearing or wear resistant surface for the component.

2. A prosthetic joint component according to claim 1 , wherein the wear resistant material comprises a layer of polycrystalline CVD diamond synthesised onto the substrate.

3. A prosthetic joint component according to claim 1 or claim 2, characterised by having one or more of the following shape tolerances:

- sphericity of +/- 5 μm, radius of +/- 10 μm, and roughness < 40 nm Ra.

4. A prosthetic joint component according to claim 3, characterised in that it has a roughness of less than about 20 nm Ra.

5. A prosthetic joint component according to any one of the preceding claims, wherein the substrate is tungsten metal, or an alloy of tungsten containing at least 80% tungsten by atomic fraction, or molybdenum, or an alloy of molybdenum containing at least 80% molybdenum by atomic fraction.

6. A prosthetic joint component according to claim 5, wherein the substrate is an alloy of tungsten containing at least 92% tungsten by atomic fraction or an alloy of molybdenum containing at least 92% molybdenum by atomic fraction.

7. A prosthetic joint component according to claim 6, wherein the substrate is an alloy of tungsten containing at least 98% tungsten by atomic fraction or an alloy of molybdenum containing at least 98% molybdenum by atomic fraction.

8. A prosthetic joint component according to any one of the preceding claims, wherein the prosthetic joint component is a hip joint component.

9. A prosthetic joint component according to claim 8, wherein the prosthetic joint component is a ball component or a socket component for a hip joint, or a combination thereof.

10. A prosthetic joint component according to any one of claims 2 to 9, wherein the polycrystalline CVD diamond coating has a thickness falling within the range of about 10 - 150 μm.

11. A prosthetic joint component according to claim 10, wherein the thickness of the polycrystalline CVD diamond coating is no more than about 120 μm.

12. A prosthetic joint component according to claim 11 , wherein the thickness of the polycrystalline CVD diamond coating is no more than about 100 μm.

13. A prosthetic joint component according to claim 12, wherein the thickness of the polycrystalline CVD diamond coating is no more than about 80 μm.

14. A prosthetic joint component according to any one of claims 10 to 13, wherein the thickness of the polycrystalline CVD diamond coating is at least about 30 μm.

15. A prosthetic joint component according to claim 14, wherein the thickness of the polycrystalline CVD diamond coating is at least about 40 μm.

16. A prosthetic joint component according to claim 15, wherein the thickness of the polycrystalline CVD diamond coating is at least about 50 μm.

17. A prosthetic joint component according to claim 16, wherein the thickness of the polycrystalline CVD diamond coating is at least about 60 μm.

18. A prosthetic joint component according to any one of claims 2 to 17, wherein the prosthetic joint component is a ball component for a hip joint, the polycrystalline CVD diamond coating providing a continuous layer covering more than 2 π steradians of the surface of the ball.

19. A prosthetic joint component according to claim 18, wherein the polycrystalline CVD diamond coating covers more than 5/2 π steradians of the surface of the ball.

20. A prosthetic joint component according to claim 19, wherein the polycrystalline CVD diamond coating covers more than 3 π steradians of the surface of the ball.

21. A prosthetic joint component according to any one of claims 2 to 17, wherein the prosthetic joint component is a socket or cup having an internal surface forming substantially a hemispherical surface, the layer of polycrystalline CVD diamond coating at least a substantial portion of the internal surface of the socket or cup, which presents a bearing or wear- resistant surface.

22. A prosthetic joint component according to any one of the preceding claims, wherein the substrate is formed of molybdenum or an alloy of molybdenum, the outer surface thereof being prepared with a thin coating of tungsten prior to coating with the polycrystalline CVD diamond.