AU2014201658B2 - Ceramic metal composite for orthopaedic implants - Google Patents

Abstract

The invention relates to an orthopaedic implant made of a ceramic metal composite. The composite includes one phase that is a biocompatible metal or metal alloy and a second phase of ceramic particles examples of which include carbides, nitrides and/or oxides. In some embodiments, the implant comprises a homogeneous ceramic layer as part of a multilayered composition. In some embodiments, the multilayered composition comprises a homogeneous metal layer. WO 2007/121242 PCT/US2007/066434 10 -4

Description

CERAMIC METAL COMPOSITE FOR ORTHOPAEDIC IMPLANTS AND METHODS OF MAKING ORTHOPAEDIC IMPLANT

COMPONENTS

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of Australian Patent Application No. 2007238100, the entire contents of which are incorporated herein by reference.

[0001] This application claims priority to U.S. provisional patent application serial no. 60/790,918 filed April 11, 2006, which is incorporated by reference as though fully described herein.

TECHNICAL FIELD

[0002] The present invention relates to ceramic/metal composite materials for orthopaedic implants generally, and more specifically, to methods of making components for orthopaedic implants such as hip implants.

BACKGROUND OF THE INVENTION

[0003] For hip implants, the current standards of “hard-on-hard” bearings are ceramic-on-ceramic and metal-on-metal. The ceramics are inherently brittle in nature and are always associated with a finite risk of fracture. There are also limitations on the size of the ceramic components that can be made. Large size ceramic components (especially liners) have to be thin and may have higher risk of fracture. The metal-on-metal components have only insignificant fracture risk and larger size components can be made. The current standard material of metal-on-metal implants is high carbon Co-Cr (cobalt-chromium) alloy. The major concern with the metal-on-metal implant is the metal ion release from the joint and its unknown effects on the physiology of the human body. The advantage of metal-on-metal implants is that they can be used in larger sizes. The larger size of the implant allows greater range of motion. The metal-on-metal implants have also been shown to be useful for resurfacing types of applications where conservation of bone is desired. In such larger joints, conventional polyethylene or cross-linked polyethylene are not preferred as a counter-bearing surface and metal-on-metal is typically the only other alternative. This is due to the fact that the larger size requires a polyethylene liner to be thinner. A thinner liner may have less mechanical strength, may have increased creep, and may lead to increased wear and osteolysis and eventually to the failure of the implant.

[0004] The other commonly used hard-on-hard implant material is ceramic-on-ceramic. The current standard material of ceramic-on-ceramic implants is alumina. Metal ion release is typically not a concern for these implants. But due to limited toughness and the brittle nature of ceramics, it is difficult to make these implants in larger sizes. The ceramic components have finite probability of fracture thus leading to a potential joint failure and complications associated with the fracture of a joint.

[0005] It has been an object of much of the prior art to reduce the metal ion release and minimize the fracture risk by combining metal and ceramic components. One of the prior art approaches to reduce the risk of metal ion release is to use surface hardening of the head or liner or both using diffusion or plasma processes to incorporate nitrogen and/or carbon on the surface of the alloy. Another approach is to coat the metallic surface with ceramic coatings of nitrides (titanium nitride, chromium nitride, etc.), oxides (aluminum oxide, zirconium oxide, zirconium-alumina oxide, etc.) or diamond like carbon or diamond coatings. Another approach is to use a metal head on a ceramic liner or vice versa. In this approach, fracture risk is reduced along with the metal ion release. Another approach that has been used is the reduction of the diametrical clearance between the articulating components thereby forming a thick lubricating film which assists in the reduction of wear. Fisher et al (U.S. Patent Publication No. 2005/0033442) and Khandkar et al. (U.S. Patent No. 6,881,229) teach the use of a metal-on-ceramic articulation. Fisher et al teach that the difference in hardness between the metallic component and the ceramic component should be at least 4000 MPa. Khandkar et. al. specifically teach use of silicon nitride ceramic components for articulation against the metallic component. In both instances, the objective is to lower the wear of mating couples. But in both instances, the fracture risk of ceramic is still significant.

[0006] Ceramic-metal composites have also seen application in the prior art. U.S. Patent No. 6,620,523 discloses ice skating blades made from a metal matrix composite. The ice skating blade has a titanium core and a metal composite material cladding. The metal composite material may be comprised of titanium or zirconium. U.S. Patent Application Publication No. 2004/0243241 discloses an orthopaedic device, such as a spinal implant, formed of a metal matrix composite. The metal matrix composite includes a biocompatible metal alloy and a reinforcing component, such as a hard or refractory material.

[0007] However, there remains a need in the art for improved articulation joints that reduce the risk of metal ion release and the risk of monolithic ceramic fracture. There further remains a need in the art for improved articulating joints wherein one or both of the articulating surfaces are comprised of a metal matrix composite.

[0008] One particular advantage of the methods of the present invention is the reduction of the risks of fracture and metal ion release for orthopaedic implants. The risks are reduced by using a graded ceramic metal composite component with a reducing ceramic content away from the bearing articulation surface. As mentioned in the details of the invention, the graded ceramic metal composite formed by the present invention provides a solution to the above described problems pertaining to currently used hard-on-hard bearings. Although the present invention is particularly applicable to making hip implants, it is also useful for making orthopaedic implants generally. As examples of other embodiments of the invention, the methods described herein are applicable to making knee and spinal implants and other implants wherein hard-on-hard articulation is desired.

BRIEF SUMMARY OF THE INVENTION

[0009] The present invention is directed to methods of making orthopaedic implants generally using ceramic/metal composite materials. The parent patent AU 2007238100 is directed to orthopaedic implants generally having graded ceramic/metal composite materials.

[0010] In one aspect of the present invention, there is provided a method of making an orthopaedic implant component, comprising: mixing powders of ceramic components and metal components to desired ratios to form a first pourable powder; mixing powders of ceramic components and metal components to desired ratios to form a second pourable powder, the second pourable powder and the first pourable powder having different ratios of ceramic components and metal components; pouring said first pourable powder onto said second pourable powder to create a first layered powder; and sintering said first layered powder to form a graded composition that is a transition region between an exterior layer of material suitable for bearing articulation and an interior region of the orthopaedic articulation implant component, wherein the concentration of ceramic decreases from the exterior layer to the interior region.

[0011] In some embodiments of the present invention, the ceramic is selected from the group consisting of alumina, zirconia, chromium carbide, chromium nitride, silicon carbide, silicon nitride, titanium carbide, zirconium carbide, zirconium nitride, tantalum carbide, tungsten carbide, and any combination thereof.

[0012] In some embodiments of the present invention, the metallic species is selected from the alloy groups consisting of cobalt-chromium, titanium-aluminum-vanadium, zirconium-niobium and tantalum and any combination thereof.

[0013] In some embodiments of the present invention, the external layer is a surface layer of homogeneous ceramic. In some embodiments, the surface layer of homogeneous ceramic has a thickness of about 25 pm.

[0014] In some embodiments of the present invention, the surface layer is 100% alumina or 80% alumina by volume and 20% zirconia by volume.

[0015] In some embodiments of the first aspect of the invention at least one mixing step comprises mixing using ball milling.

[0016] In some embodiments of the first aspect of the invention the metal components in the first pourable powder and the second pourable powder have a Young’s modulus in the range of about 60 GPa to about 150 GPa.

[0017] In some embodiments of the first aspect of the invention, the method further comprises the step of altering the size and volume fraction of ceramic particles in a ceramic/metal mixture such that the average free distance between ceramic particles is less than the size of said ceramic particles. In some such embodiments, the step of altering the size and volume fraction of ceramic particles in a ceramic/metal mixture such that the average free distance between ceramic particles is less than the size of said ceramic particles comprises altering the size and volume fraction such that the average free distance between ceramic particles is less than half of the size of said ceramic particles.[0018] In some embodiments of the first aspect of the invention the method further comprises mixing powders to form one or more additional mixtures and pouring the one or more additional mixtures on the first layered structure; wherein sintering the first layered structure comprises sintering a combination of the first layered mixture and the one or more additional mixtures.

[0019] According to a second aspect of the invention there is provided a method of making an orthopaedic articulation implant component, comprising: mixing powders of ceramic components and metal components to desired ratios to form a first pourable powder; mixing powders of ceramic components and metal components to desired ratios to form a second pourable powder; pouring said first pourable powder onto a layer of ceramic powder and pouring said second pourable powder onto said first pourable powder to create a first layered powder; and sintering said first layered powder to form a graded composition, the graded composition having a ceramic material suitable for bearing articulation at an exterior layer of the graded composition, wherein the concentration of ceramic decreases from the exterior layer of the graded composition to an interior of the graded composition.

[0020] In some embodiments of the second aspect of the invention the method further comprises diffusing one or more of boron, nitrogen, oxygen and carbon into the exterior layer.

[0021] In some embodiments of the second aspect of the invention the method further comprises coating the exterior layer with a carbide, nitride, boride or crystalline coating by physical vapour deposition or chemical vapour deposition.

[0022] According to a third aspect of the present invention there is provided a method of making an orthopaedic articulation implant component, comprising: mixing powders of ceramic components and metal components to desired ratios to form a first powder; mixing powders of ceramic components and metal components to desired ratios to form a second powder, the second powder mixture and the first powder mixture having different ratios of ceramic components and metal components; pouring said first pourable powder onto said second pourable powder to create a first layered powder; and sintering said first layered powder to form a graded composition, the graded composition having an exterior layer of material suitable for bearing articulation, wherein the sintered second pourable powder forms the exterior layer, and wherein the concentration of ceramic decreases from the exterior layer to an interior of the graded composition.

[0023] In some embodiments, the metal components in the first pourable powder and the second pourable powder have a Young’s modulus in the range of about 60 GPa to about 150 GPA.

[0024] In some embodiments, the method further comprises the step of altering the size and volume fraction of ceramic particles in a ceramic/metal mixture such that the average free distance between ceramic particles is less than the size of said ceramic particles. In some such embodiments, the step of altering the size and volume fraction of ceramic particles in a ceramic/metal mixture such that the average free distance between ceramic particles is less than the size of said ceramic particles comprises altering the size and volume fraction such that the average free distance between ceramic particles is less than half of the size of said ceramic particles.

[0025] According to a fourth aspect of the present invention there is provided a method of making an orthopaedic articulation implant component, comprising: mixing powders of ceramic components and metal components to desired ratios to form a first pourable powder; mixing powders of ceramic components and metal components to desired ratios to form a second pourable powder, the second pourable powder and the first pourable powder having different ratios of ceramic components and metal components; laying said first pourable powder on said second pourable powder to create a first layered powder, wherein laying said first pourable powder on said second pourable powder to create the first layered powder comprises pouring said first pourable powder and said second pourable powder into a mould; and sintering said first layered powder to form a graded composition that is a transition region between an exterior layer of material suitable for bearing articulation and an interior region of the orthopaedic articulation implant component, wherein the concentration of ceramic decreases from the exterior layer to the interior region.

[0026] In some embodiments, at least one mixing step comprises a ball milling process.

[0027] According to a fifth aspect of the present invention, there is provided a method of making an orthopaedic articulation implant component, comprising: mixing powders of ceramic components and metal components to desired ratios to form a first pourable powder; mixing powders of ceramic components and metal components to desired ratios to form a second pourable powder; laying said first pourable powder on a layer of ceramic powder and laying said second pourable powder on said first pourable powder to create a first layered powder, wherein laying said first pourable powder on said second pourable powder to create the first layered powder comprises pouring said first pourable powder and said second pourable powder into a mould; and sintering said first layered powder to form a graded composition, the graded composition having a ceramic material suitable for bearing articulation at an exterior layer of the graded composition, wherein the concentration of ceramic decreases from the exterior layer of the graded composition to an interior of the graded composition.

[0028] In some embodiments, the metal components in the first pourable powder and the second pourable powder have a Young’s modulus in the range of about 60 GPa to about 150 GPA.

[0029]According to a sixth aspect of the present invention there is provided a method of making an orthopaedic articulation implant component, comprising:mixing powders of ceramic components and metal components to desired ratios to form a first powder; mixing powders of ceramic components and metal components to desired ratios to form a second powder, the second powder mixture and the first powder mixture having different ratios of ceramic components and metal components; laying said first pourable powder on said second pourable powder to create a first layered powder, wherein laying said first pourable powder on said second pourable powder to create the first layered powder comprises pouring said first pourable powder and said second pourable powder into a mould; and sintering said first layered powder to form a graded composition, the graded composition having an exterior layer of material suitable for bearing articulation, wherein the sintered second pourable powder forms the exterior layer, and wherein the concentration of ceramic decreases from the exterior layer to an interior of the graded composition.

[0030] In some embodiments, the method further comprises the step of altering the size and volume fraction of ceramic particles in a ceramic/metal mixture such that the average free distance between ceramic particles is less than the size of said ceramic particles.

[0031] In some such embodiments, the step of altering the size and volume fraction of ceramic particles in a ceramic/metal mixture such that the average free distance between ceramic particles is less than the size of said ceramic particles comprises altering the size and volume fraction such that the average free distance between ceramic particles is less than half of the size of said ceramic particles.

[0032] In some embodiments, of the methods of the invention, the methods further comprise forming at least a portion of the orthopaedic articulation implant component by selectively melting a region of the orthopaedic articulation implant component and alloying the melted region with ceramic particles.

[0033] In some embodiments of the methods of the invention, the sintering comprises electric field assisted sintering.

[0034] In some embodiments of the methods of the invention, the sintering comprises spark plasma sintering.

[0035] (not used) [0036] (not used) [0037] (not used) [0038] Further features, aspects, and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.

[0039] The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions of embodiments, given by way of example only, taken in conjunction with the accompanying drawings, in which: [0041] FIG. 1 illustrates a hip prosthesis formed by one embodiment of the present invention; [0042] FIG. 2 shows a cross-sectional view of an example of a multilayered structure formed by one embodiment of the present invention; and [0043] FIG. 3 shows a cross-sectional view of an example of a multilayered structure formed by one embodiment of the present invention.

[0044] (not used)

DETAILED DESCRIPTION OF THE INVENTION

[0045] As used herein, “a” or “an” means one or more, unless otherwise indicated. The singular encompasses the plural and the plural encompasses the singular. Thus, where reference is made to “a ceramic” or to a “ceramic component”, this means one or more ceramics and one or more ceramic components, respectively. Where reference is made to “a metal” or to a “metallic species”, this means one or more metals and one or more metallic species, respectively. A metallic species may be one metal or a metal alloy.

[0046] As used herein the term “homogenous ceramic” encompasses, but is not limited to, 100% of one specific ceramic (for example, 100% alumina). In this way, the term “homogenous ceramic” also encompasses a mixture of ceramics (for example a mixture of zirconia and alumina and possibly other ceramics) so long as it is substantially free of non-ceramic components. Similarly, the term “homogenous metal” or “homogenous metallic component” covers, but is not limited to, 100% of one specific metal (for example, 100% zirconium). In this way, the term “homogenous metal” or “homogenous metallic component” also encompasses a mixture of metals (for example a mixture of zirconium and hafnium and possibly other metals) so long as no non-metal is present.

[0047] As used herein, the term “orthopaedic implant component” is defined as a portion of an entire orthopaedic implant, or an entire orthopaedic implant.

[0048] Where compositions are recited herein in percentages, the percentages are volume percentages, unless otherwise indicated.

[0049] The invention relates to methods of making an orthopaedic implant comprising a subsurface ceramic metal composite. The composite includes a first phase that is a biocompatible metal species such as alloy and a second phase that is ceramic such as carbide, nitride, boride and/or oxide. The ceramic phase can be present from about 1 % to about 100 % by volume (preferably from about 1% to about 95% by volume) in the metal. The volume fraction of the ceramic phase is graded from a surface of the composite material towards a non articulating substrate. The surface of the implant component is suitable for bearing articulation and may comprise the outer surface of the graded ceramic metal composite. It may be 100% homogenous ceramic. The composite may be further hardened by allowing diffusion of nitrogen, oxygen, carbon or any combination thereof. The ceramic composite may be further coated with hard coatings of carbides, nitrides, oxides, or diamond-like carbon or polycrystalline-nanocrystalline diamond coatings or amorphous diamond coatings.

[0050] The approach of the present invention is to use a subsurface ceramic metal composite where the ceramic is part of the metal matrix. The ceramic provides surface hardening to the metal, reduces metal ion release because the ceramic particles articulate against each other instead of metallic particles articulating against each other, and also mitigates the fracture risk associated with monolithic ceramics. The primary goal is to reduce metal ion release in metal-on-metal type articulation but particularly a hardened surface can also be used for articulation against other materials.

[0051] Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.

[0052] The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. Most of the following discussion focuses on hip implants for illustrative purposes, but it should be clear that the present invention is also applicable to other orthopaedic implants.

[0053] FIG. 1 illustrates a hip prosthesis 10. The hip prosthesis 10 includes a stem 12, a femoral head 14, a liner 16, and a shell 18. The femoral head 14 is operatively connected to the stem 12, and the liner 16 is assembled to the shell 18. The stem 12 is adapted for mounting in a femur (not shown), and the shell 18 is adapted for mounting to an acetabulum (not shown). The femoral head 14 articulates against the liner 16.

[0054] In some embodiments of the present invention, the femoral head 14 and/or the liner 16 are made of a ceramic metal composite where the ceramic phase is part of the metal matrix. In one embodiment, a surface layer of 100% ceramic of thickness of ca. 5 to 200 pm is followed by a graded layer of ceramic/metal composite having an initial composition of 100% ceramic and decreasing to pure metal or to a steady state level of ceramic. In another embodiment, the surface composition may comprise ceramic from about 1% to nearly 100% (the surface layer having a small amount of metal). The ceramic phase can be present from about 1 % to preferably about 95% by volume in the metal. The ceramic phase provides surface hardening to the metal and the presence of ceramic particles articulating against each other reduces the metal ion release and also mitigates the fracture risk associated with monolithic ceramics. In the hip implant 10 embodiment depicted in FIG. 1, both the femoral head 14 and the liner 16 of the acetabular component 18 are made from the ceramic metal composite, but those skilled in the art would understand that in some instances only one of the components need comprise the ceramic metal composite. For example, the ceramic metal composite of the femoral head 14 may articulate against a polyethylene liner, monolithic ceramic liner, or a metal liner. Alternatively, a ceramic metal composite liner 16 may articulate against a polymeric femoral head (such as one made of polyethylene), monolithic ceramic femoral head, or a metal femoral head. As those skilled in the art would understand, the term “polyethylene” includes both conventional polyethylene and cross-linked polyethylene. Also in the hip implant 10 embodiment of FIG. 1, a portion of the femoral stem 12 is illustrated for proper context. Although FIG. 1 illustrates the application of the present invention to hip implants, it should be understood that this is merely one illustrative example and that the present invention is applicable to all orthopaedic implants, such as, but not limited to, knee implants, shoulder implants, elbow implants, and other implants.

[0055] The composite includes a first phase that is preferably a biocompatible alloy (such as cobalt chromium based, titanium based, zirconium based, niobium based and/or tantalum based) and a second phase that is ceramic particles of carbides, nitrides, borides and oxides, such as but not restricted to chromium carbide, titanium carbide (TiC), titanium carbo-nitride (TiCN), zirconium carbide (ZrC), tantalum carbide (TaC), niobium carbide (NbC), tungsten carbide, aluminum oxide (AI2O3), and zirconium oxide (ZrC>2). The second phase can also include polycrystalline, amorphous and or nano-crystalline diamond grit. The composite may include one type of ceramic phase or a combination of more than two or more types of ceramic phases.

[0056] The hardness of the femoral head 14 and the liner 16 may be the same or the hardness of each component may be different. For example, the ceramic composite femoral head 14 may articulate against a monolithic ceramic liner 16 in which the hardness of the ceramic particle in the composite is different from the monolithic ceramic. As another example, the ceramic composite femoral head 14 may articulate against a ceramic composite liner 16 in which the hardness of the ceramic particle in the femoral head is different from that of the liner. The volume fraction of the ceramic phase can be varied from about 1% to about 100% in order for each of the components to have a different hardness, Alternatively, both components can be made in such a way that the volume fraction of the ceramic phase is graded from the surface of the implant component to a substrate. For example, there may be approximately 100% of the ceramic phase at the surface, with the volume fraction of the ceramic phase gradually decreasing to less than about 1 percent at the substrate. The objective of the graded composite is not to significantly affect the fracture toughness of the substrate material.

[0057] In accordance with the invention, this graded structure in at least one of the implant components can be achieved using powder metallurgical processes. In such processes the powders of the ceramic component and the metal component of the composite are mixed and sintered. The mixing can be achieved using a ball milling process where a uniform distribution of the ceramic phase is required in a layer of the composite. Such processes are known to those of ordinary skill in the art. To obtain the graded structure, powders can be applied in the form of layers and then sintered together. The sintering of powders can be achieved using a high temperature sintering process or using lower temperature electric field assisted sintering. Such sintering methods, known to those of ordinary skill in the art, as well as other sintering methods, are useful in the present invention. The temperature of the sintering will depend on the alloy composition and type of structure desired. Other alloying techniques, such as electron beam alloying of the surface, can also be done to achieve such graded structures. In one non-limiting example of the use of a powder metallurgical process, a composite structure of alumina and CoCr is made. CoCr powder is mixed with alumina powder in volume fractions ranging from 25% to 100%. These mixtures were then poured in a graphite mould as different layers. The first layer is 100% CoCr. The second layer is CoCr with 25% alumina. The third layer is 50% CoCr and 50% alumina. The fourth layer is 75% alumina and 25% CoCr. The top (surface) layer was 100% alumina. This layered structure was then sintered using a spark plasma sintering process at 1200°C. Spark plasma sintering is another sintering technique known in the art. The surface is 100% alumina and the fraction of alumina decreases towards the substrate. The alumina powder had a large particle size range that resulted in formation of big discontinuities in the matrix. It will be understood by those skilled in the art that the powder size and distribution can further be optimized. In such composite structure the 100% alumina is expected to be the articulating surface. Such surface is expected to eliminate the metal ion release. The graded composition ensures that the adhesion of alumina to the substrate CoCr is adequate. In the example described, the change in graded composition may be relatively abrupt. It will be understood by those skilled in the art that this change in composition can be changed from step function (more abrupt change) to linear or to any other shape desired. It will be understood by those skilled in the art, that sintering of such powder mixtures can be achieved using conventional sintering processes. For a cooperating component not formed in accordance with the invention, such structure could be formed by selectively melting the surface and then alloying it with ceramic particles. Alternatively, ceramic particles may be precipitated during solidification of the molten alloy. Alternatively again, the ceramic particles may be dispersed in a molten bath of alloy and then the mixtures is cooled to obtain uniform distribution of ceramic particles.

[0058] In some embodiments, the ceramic metal composite is formed using low modulus alloys, such as but not limited to titanium, zirconium, or niobium. The Young’s modulus of these alloys typically ranges from about 60 to about 150 GPa. The lower modulus alloy leads to lower contact pressure and allows for the formation of thicker fluid film lubrication. The thicker fluid film reduces the wear of the articulating components. The low modulus alloy is also selected to help in getting larger clearances between the articulating components, which lowers the chances of seizure of the joint. In one aspect of invention the diametrical clearance between the mating components of the hip joint is maintained above 150 micron.

[0059] In some embodiments, the composite is engineered in such a way that the average free distance between the ceramic particles is less than the average size of the ceramic particles. This is achieved by altering the average diameter (size) of the ceramic particles and their volume fraction in the metal. The volume fraction of particles (Vp), the mean free distance between these particles (λ), and the average size of the particles expressed as mean intercept length (L3) can be related to each other as provided below [Metals Handbook Ninth Edition, Volume 9, Metallography and Microstructures, ASM, 1989):

[0060] For a known particle size (L3) and desired distance between the particles (λ) a suitable volume fraction of particles (Vp) can be chosen as a starting point. This mixture then can be further evaluated analytically by known and standard techniques to verify the above expressed relationship. In preferred embodiments, the average free distance (λ) is less than mean intercept length (L3) by at least 10%. The above process can be repeated iteratively to optimize the mean distance between the particles. The average diameter may be calculated by measuring the particle diameter of the particles through the centroid of the particles and averaging the measured diameters. For the case of non-spherical particles, the diameter may be measured through the centroid of the particles to different locations on the perimeter of the particle. For example, for an ellipsoid shaped particle, the diameter will be determined at different locations of the particle and the average of same may be determined. Standard stereological software, familiar to those of skill in the art can be used for such a purpose. In general particles can range from sub-micron sized (< 1 micron) to as large as about 200 microns. Based on the particle size optical microscope or scanning electron microscope can be used to measure the particle sizes. Preferably, at least 10 to 15 fields at of view at lOOOx chosen randomly are inspected. It may be required to inspect more fields of view if a magnification higher than lOOOx is used. It will be understood by those skilled in the art that the accuracy and precision of such measurements can be increased by inspecting a greater number of fields. Standard guidelines known to those of ordinary skill in the art (such as, for example, the ASTM guidelines) can be used for this purpose. By controlling the average free distance between the ceramic particles, the risk of the harder ceramic articulating against the softer substrate can be reduced. In such composite, a 100% ceramic is not always necessary, although it is within the scope of the invention and may be used. In each of two or more composite components articulating against each other in an orthopaedic implant, the following can be controlled independently: (1) the type of ceramic particles, (2) the volume fraction of the ceramic particles, (3) the size of the ceramic particles, (4) the size distribution of the ceramic particles, (5) the shape of the ceramic particles, and (6) the metallic substrate material. One or more types of ceramic particles with independently controlled size distributions and volume fractions can be used in the composite.

[0061] By controlling the average free distance between the ceramic particles, the risk of the harder ceramic articulating against the softer substrate can be reduced. With such a composite the surface of the implant component need not be 100% ceramic. In some embodiments, the composite is engineered in such as way that the average free distance between the ceramic particles is less than half of the size of the ceramic particles.

[0062] If the surface of the composite is not 100% ceramic then it can be further hardened by allowing diffusion of nitrogen, oxygen, carbon, or any combination thereof, for example using plasma or ion implantation type of processes to gain additional advantage of the hardened layer. The composite is further hardened by allowing diffusion of reactive species such as boron, carbon, nitrogen and oxygen to gain additional advantage of the hardened layer in reducing the wear of the component. The diffusion processes can be accompanied by conventional high temperature diffusion processes of reactive species or by ion implantation or by plasma assisted process. The diffusion hardened depth of such composites can range from about 1 micron to about 1000 microns.

[0063] In some embodiments, the ceramic composite is further coated with hard coatings of carbides (TiC, ZrC, TaC etc) or nitrides (TiN, ZrN, CrN etc) or oxides (AI2O3, Z1O2) or borides or diamond like carbon or polycrystalline/nanocrystalline diamond coatings to reduce the metal ion release. These coatings may be applied using physical vapor or chemical vapor deposition processes. As an example, a coated femoral head may articulate against a coated or uncoated liner. Alternatively, a coated liner may articulate against a coated or uncoated femoral head.

[0064] The surface roughness of the femoral head 14 and/or the liner 16 may be specified such that the finished surface allows for maximum lubrication during articulation. Moreover, the diametrical clearance between the femoral head 14 and the liner 16 may be specified such that the finished surface allows for maximum lubrication during articulation.

[0065] Two illustrative and non-limiting embodiments of the present invention will now be described. It should be understood that these do not represent the entire scope of the invention. Modifications that will be understood by those having ordinary skill in the art are also part of the present invention. This includes the use of additional layers of material, as well as different compositional characteristics of each layer.

[0066] In a first illustrative non-limiting embodiment of the present invention, in FIG. 2 there is an orthopaedic implant component 20 (a crosssectional portion of which is shown) comprising a surface layer 24 of a homogenous ceramic material. This surface layer 24 could be, for example, 100% alumina. Alternatively, the surface layer 24 could be a mixture of ceramics, for example, 80 % alumina and 20 % zirconia by volume. However, other possible homogenous ceramic materials, known to those of ordinary skill in the art also can be used. Some examples include mixtures of silicon nitride and silicon carbide, chromium carbide and titanium carbide. In general, several such combinations of ceramic particles can be made. The thickness of this ceramic surface layer 24 is preferably about 25 pm. However, other thicknesses may be used. The thickness of the ceramic layer will depend on the adhesion of the ceramic particles to each other and to a subsurface composite structure 28 at the interface. The thickness should be such that it will not de-laminate the ceramic layer from the interface. Below the homogenous ceramic layer, is the subsurface composite structure 28 is a graded ceramic/metal mixture of metal and ceramic. In the illustrative example using alumina as a ceramic material in this second layer, the alumina would have an initial concentration which decreases monotonically as the depth from the surface increases. Preferably, the initial concentration is 95% to 99% by volume but it may be 100% by volume or some level below 95% by volume and then monotonically decreases. Below this layer, there is a homogenous metal layer 32. In one example, the second layer 28 could have an initial concentration (directly below the first layer of homogenous ceramic) of 95% ceramic and 5 % metal by volume and decrease in ceramic content to a final concentration of 10% ceramic and 90 % metal by volume. The third layer 32, which can be 100% metal by volume then begins. The thickness of the individual layers can range from 5 microns to 500 microns; the ceramic volume fraction can decrease as a step function (abrupt change) or can decrease in a linear fashion. The layers comprising ceramic/metal mixtures may be characterized by an average free distance between ceramic particles that is less than half the average size of said ceramic particles. However, this average free distance between ceramic particles is not required in multilayered compositions.

[0067] A second illustrative non-limiting embodiment 40 of the invention is shown in FIG. 3. This embodiment differs from the first embodiment in that the second graded layer 48 decreases to some steady-state value of ceramic/metal. The second graded layer 48 is below a surface layer 24 of a homogenous ceramic composition. In preferred embodiments, the steady-state level is primarily metallic (for example, 10% ceramic and 90% metal by volume). However, it can be primarily ceramic as well. In this embodiment, there is no discrete third layer and the final steady-state composition forms the substrate of the material. Thus, in FIG. 3 we see a structure 40 having homogeneous ceramic layer 24 and graded ceramic/metal layer 48. At some depth within layer 48, there is reached a steady-state composition of ceramic and metal which then forms the remainder of the substrate. The layers comprising ceramic/metal mixtures may be characterized by an average free distance between ceramic particles that is less than half the average size of said ceramic particles. However, this average free distance between ceramic particles is not required in multilayered compositions.

[0068] In both illustrated embodiments the surface layer 24 and the second layer 28/48 are formed by sintering the particulate components .

[0069] Although the depicted embodiments illustrate the use of the ceramic metal composite in a hip implant, the ceramic metal composite could equally be implemented in reconstructive knee components and particularly in femoral knee components that articulate against polyethylene or other types of polymers. The invention described herein is applicable to orthopaedic implants generally, and although the preferred embodiments are hip implants and knee implants, it is also applicable in, for example, shoulder, vertebral, and other orthopaedic implants familiar to those of skill in the art.

[0070] As various modifications could be made to the exemplary embodiments, as described above with reference to the corresponding illustrations, without departing from the scope of the invention, it is intended that all matter contained in the foregoing description and shown in the accompanying drawings shall be interpreted as illustrative rather than limiting. Thus, the breadth and scope of the present invention should not be limited by any of the abovedescribed exemplary embodiments, but should be defined only in accordance with the following claims appended hereto and their equivalents.

[0071] The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

[0072] Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

Claims (23)

1. CLAIMS What is claimed is:

   1. A method of making an orthopaedic implant component, comprising: mixing powders of ceramic components and metal components to desired ratios to form a first pourable powder; mixing powders of ceramic components and metal components to desired ratios to form a second pourable powder, the second pourable powder and the first pourable powder having different ratios of ceramic components and metal components; pouring said first pourable powder onto said second pourable powder to create a first layered powder; and, sintering said first layered powder to form a graded composition that is a transition region between an exterior layer of material suitable for bearing articulation and an interior region of the orthopaedic articulation implant component, wherein the concentration of ceramic decreases from the exterior layer to the interior region.
2. 2. The method of claim 1, wherein at least one mixing step comprises mixing using a ball milling process.
3. 3. The method of claim 1 or claim 2, wherein said metal components in said first pourable powder and said second pourable powder have a Young’s modulus in the range of about 60 GPa to about 150 GPa.
4. 4. The method of any one of claims 1 to 3, further comprising the step of altering the size and volume fraction of ceramic particles in a ceramic/metal mixture such that the average free distance between ceramic particles is less than the size of said ceramic particles.
5. 5. The method of claim 4, wherein said step of altering the size and volume fraction of ceramic particles in a ceramic/metal mixture such that the average free distance between ceramic particles is less than the size of said ceramic particles comprises altering the size and volume fraction such that the average free distance between ceramic particles is less than half of the size of said ceramic particles.
6. 6. The method of any one of claims 1 to 5, further comprising mixing powders to form one or more additional mixtures and pouring said one or more additional mixtures on said first layered mixture; wherein sintering said first layered mixture comprises sintering a combination of said first layered mixture and said one or more additional mixtures.
7. 7. A method of making an orthopaedic articulation implant component, comprising: mixing powders of ceramic components and metal components to desired ratios to form a first pourable powder; mixing powders of ceramic components and metal components to desired ratios to form a second pourable powder; pouring said first pourable powder onto a layer of ceramic powder and pouring said second pourable powder onto said first pourable powder to create a first layered powder; and, sintering said first layered powder to form a graded composition, the graded composition having a ceramic material suitable for bearing articulation at an exterior layer of the graded composition, wherein the concentration of ceramic decreases from the exterior layer of the graded composition to an interior of the graded composition.
8. 8. The method of claim 7, further comprising diffusing one or more of boron, nitrogen, oxygen, and carbon into the exterior layer.
9. 9. The method of claim 7, further comprising coating the exterior layer with a carbide, nitride, oxide, boride, or crystalline coating by physical vapour deposition or chemical vapour deposition.
10. 10. A method of making an orthopaedic articulation implant component, comprising: mixing powders of ceramic components and metal components to desired ratios to form a first powder; mixing powders of ceramic components and metal components to desired ratios to form a second powder, the second powder mixture and the first powder mixture having different ratios of ceramic components and metal components; pouring said first pourable powder onto said second pourable powder to create a first layered powder; and, sintering said first layered powder to form a graded composition, the graded composition having an exterior layer of material suitable for bearing articulation, wherein the sintered second pourable powder forms the exterior layer, and wherein the concentration of ceramic decreases from the exterior layer to an interior of the graded composition.
11. 11. The method of claim 10, wherein said metal components in said first pourable powder and said second pourable powder have a Young's modulus in the range of about 60 GPa to about 150 GPa.
12. 12. The method of claim 10 or 11, further comprising the step of altering the size and volume fraction of ceramic particles in a ceramic/metal mixture such that the average free distance between ceramic particles is less than the size of said ceramic particles.
13. 13. The method of claim 12, wherein said step of altering the size and volume fraction of ceramic particles in a ceramic/metal mixture such that the average free distance between ceramic particles is less than the size of said ceramic particles comprises altering the size and volume fraction such that the average free distance between ceramic particles is less than half of the size of said ceramic particles.
14. 14. A method of making an orthopaedic articulation implant component, comprising: mixing powders of ceramic components and metal components to desired ratios to form a first pourable powder; mixing powders of ceramic components and metal components to desired ratios to form a second pourable powder, the second pourable powder and the first pourable powder having different ratios of ceramic components and metal components; laying said first pourable powder on said second pourable powder to create a first layered powder, wherein laying said first pourable powder on said second pourable powder to create the first layered powder comprises pouring said first pourable powder and said second pourable powder into a mould; and, sintering said first layered powder to form a graded composition that is a transition region between an exterior layer of material suitable for bearing articulation and an interior region of the orthopaedic articulation implant component, wherein the concentration of ceramic decreases from the exterior layer to the interior region.
15. 15. The method of claim 14, wherein at least one mixing step comprises mixing using a ball milling process.
16. 16. A method of making an orthopaedic articulation implant component, comprising: mixing powders of ceramic components and metal components to desired ratios to form a first pourable powder; mixing powders of ceramic components and metal components to desired ratios to form a second pourable powder; laying said first pourable powder on a layer of ceramic powder and laying said second pourable powder on said first pourable powder to create a first layered powder, wherein laying said first pourable powder on said second pourable powder to create the first layered powder comprises pouring said first pourable powder and said second pourable powder into a mould; and, sintering said first layered powder to form a graded composition, the graded composition having a ceramic material suitable for bearing articulation at an exterior layer of the graded composition, wherein the concentration of ceramic decreases from the exterior layer of the graded composition to an interior of the graded composition.
17. 17. The method of claim 16, wherein said metal components in said first pourable powder and said second pourable powder have a Young's modulus in the range of about 60 GPa to about 150 GPa.
18. 18. A method of making an orthopaedic articulation implant component, comprising: mixing powders of ceramic components and metal components to desired ratios to form a first powder; mixing powders of ceramic components and metal components to desired ratios to form a second powder, the second powder mixture and the first powder mixture having different ratios of ceramic components and metal components; laying said first pourable powder on said second pourable powder to create a first layered powder, wherein laying said first pourable powder on said second pourable powder to create the first layered powder comprises pouring said first pourable powder and said second pourable powder into a mould; and, sintering said first layered powder to form a graded composition, the graded composition having an exterior layer of material suitable for bearing articulation, wherein the sintered second pourable powder forms the exterior layer, and wherein the concentration of ceramic decreases from the exterior layer to an interior of the graded composition.
19. 19. The method of claim 18, further comprising the step of altering the size and volume fraction of ceramic particles in a ceramic/metal mixture such that the average free distance between ceramic particles is less than the size of said ceramic particles.
20. 20. The method of claim 19, wherein said step of altering the size and volume fraction of ceramic particles in a ceramic/metal mixture such that the average free distance between ceramic particles is less than the size of said ceramic particles comprises altering the size and volume fraction such that the average free distance between ceramic particles is less than half of the size of said ceramic particles.
21. 21. The method of any one of the preceding claims, comprising forming at least a portion of the orthopaedic articulation implant component by selectively melting a region of the orthopaedic articulation implant component and alloying the melted region with ceramic particles.
22. 22. The method of any one of the preceding claims, wherein the sintering comprises electric field assisted sintering.
23. 23. The method of any one of claims 1 to 21, wherein the sintering comprises spark plasma sintering.