Field

This disclosure relates to a medical implant comprising a structure formed of super hard material, particularly but not exclusively a structure comprising polycrystalline diamond (PCD) and a method of making same.

Background

Traditionally, medical implants such as hip or knee replacement joints are made of metal alloys such as nickel, chrome and cobalt. However, these may cause allergic responses, due to bulk or trace elements. Ceramic materials are another group of materials increasingly being used in, for example, hip joint prostheses due to their wear resistance and biocompatibility properties. However, implants formed of such materials may suffer from sudden failures or fractures due to lack of toughness of the material. As an alternative to ceramic materials, diamond-like carbon (DLC) coatings have also been exploited in biomedical applications, as coatings on conventional prosthetic structures. These coatings are typically extremely hard, have low friction, are bio-inert and may also inhibit leaching of metallic ions into the body. Such coatings may be deposited from carbonaceous precursors, and some offer the means to incorporate other elements such as nitrogen, titanium, or silver. For optimum tribological performance, however, the DLC must be deposited onto highly polished surfaces and DLC coatings may be susceptible to possible delamination of the coating from its substrate during use which could cause serious medical complications. Whilst DLC has proved to be an excellent coating to prolong the life of and reduce complications associated with replacement hip joints and artificial knees, ever increasing drives for improved working life with improved abrasion and impact resistance are desired, as is a method of forming such constructions.

Summary

Viewed from a first aspect there is provided a medical implant comprising a structure formed of super hard material, the structure having porosity greater than 20% by volume and up to around 80% by volume. l Viewed from a second aspect there is provided a body joint prosthesis comprising one or more elements formed of the medical implant defined above.

Viewed from a third aspect there is provided a method of forming a medical implant comprising: forming a skeleton structure of a first material, the skeleton structure having a plurality of voids; at least partially filling some or all of the voids in the skeleton structure with a second material to form a pre-sinter assembly; wherein one or other of the first material or the second material comprises grains of super hard material; and treating the pre-sinter assembly at an ultra-high pressure of around 5 GPa or greater and a temperature to sinter together the grains of super hard material to form a body of polycrystalline super hard material comprising a first region of super hard grains, and an interpenetrating second region; the second region being formed of the other of the first or second material that does not comprise the super hard grains; the super hard grains forming a sintered medical implant structure of super hard material having a porosity greater than 20% by volume and up to around 80% by volume.

Brief Description of the Drawings

Various versions will now be described by way of example and with reference to the accompanying drawings in which:

Figure 1 is a parts exploded view of a conventional hip-joint prosthesis;

Figure 2 is a schematic cross-section through a hip-joint prosthesis according to a first example;

Figure 3 is a schematic cross-section through a hip-joint prosthesis according to a second example;

Figure 4 is a schematic cross-sectional view of the microstructure of conventional PCD material; Figure 5 is a plan view of a pre-sinter composite assembly of a material according to an example;

Figure 6 is a plan view of a representation of a microstructure of an example composite suitable for use in the prosthesis of Figures 1 to 3;

Figure 7 is a perspective view from above and enlarged cross-sectional view through a portion of the microstructure of a material suitable for use in the prosthesis of Figures 2 and 3 according to a further example; and

Figures 8a and 8b are cross-sectional views of a portion of the microstructure of a material according to a further example.

The same references refer to the same general features in all the drawings. Detailed Description

Figure 1 shows a typical conventional hip prosthesis 1 comprising a leg component 2 which is inserted into the patient's femur and comprises a stem portion 10, and a head portion12 attached thereto which is typically in the form of a truncated sphere with a partially extending through-bore 14 into which one end of the stem portion is located. The hip component 1 6 comprises a dome-shaped liner element 18 which covers a portion of the head portion 12 and a shell 20 which is also dome shaped and is located on and over the liner element 18.

Figure 2 is a schematic cross-section through an assembled portion of the prosthesis of Figure 1 showing the shell 20 and liner element 18 located on the head portion 12.

In a prosthetic joint such as a hip joint as shown in Figures 1 and 2, the surfaces between the head portion 12 and liner element 18 suffer significant wear from the rubbing effect during limb movements such as walking. Wear debris may have significant detrimental implications on a patient's health. For example, if such parts were formed of metal, metal debris in form of free metal ions may be absorbed by the body and lead to severe long-term complications. In addition, accelerated wear of the head 12 and liner 18 surfaces shortens the life span of the implant, meaning the patient would potentially go through several surgical operations in his/her life time. If ceramic parts are used they may not have the necessary fracture resistance and may suffer for sudden fractures.

To address and potentially ameliorate such issues, it is proposed that any one or more of the shell 20, liner element 18 and head portion 12 may be formed of a material according to the examples described below, or that, as shown in Figure 3, any one or more of these components may have all or part of a surface coated with such a material or a layer of such a material bonded thereto. As shown in Figure 3, the outer surface of the head portion 12 may be coated with a material according to an example, the coating forming a layer 22 over the head portion 12 and bonded thereto, the head portion 12 may itself be formed of a conventional material. Similarly, the surface 24 of the liner element 18 that abuts or contacts the head portion in use may be formed of or coated with a material according to an example, or have a layer of such a material bonded thereto. In this case, instead of the head portion 12 and liner element 18 being made of an example material, a functionally graded structure may be employed with the example composite materials only forming a layer adjacent to the mating surfaces as shown in Figure 3. This may also be used to facilitate applications of coatings on the mating surfaces if required.

Some examples propose using a material with 3D continuously interpenetrating networks of PCD and metal or ceramic or a combination thereof. Such a composite may offer the high abrasion resistance of diamond and combine it with the fracture resistance of a metal or an engineered ceramic. Diamond also has a lower coefficient of friction and is biostable and biocompatible, which add to the benefits of using such composite material in these applications. The method of making PCD-ceramic and PCD-metal 3D interpenetrating material composites is discussed below.

As used herein, a "super hard material" is a material having a Vickers hardness of at least about 28 GPa. Diamond and cubic boron nitride (cBN) material are examples of super hard materials.

As used herein, a "super hard construction" means a construction comprising a body of polycrystalline super hard material. In such a construction, a substrate may be attached thereto or alternatively the body of polycrystalline material may be freestanding and unbacked.

As used herein, polycrystalline diamond (PCD) is a type of polycrystalline super hard (PCS) material comprising a mass of diamond grains, a substantial portion of which are directly inter-bonded with each other and in which the content of diamond is at least about 80 volume percent of the material. As used herein, "interstices" or "interstitial regions" are regions between the diamond grains of PCD material. In some examples of PCD material, interstices between the diamond grains may be at least partly filled with a binder material comprising a catalyst for diamond. In some examples of PCD material, interstices or interstitial regions may be substantially or partially filled with a material other than diamond, or they may be substantially empty. PCD material may comprise at least a region from which catalyst material has been removed from the interstices, leaving interstitial voids between the diamond grains.

A "catalyst material" for a super hard material is capable of promoting the growth or sintering of the super hard material.

The term "substrate" as used herein means any substrate over which the super hard material layer is formed. For example, a "substrate" as used herein may be a transition layer formed over another substrate.

As used herein, the term "integrally formed" regions or parts are produced contiguous with each other and are not separated by a different kind of material.

The micro-structure of a conventional PCD material is shown in Figure 4. During formation of a conventional polycrystalline diamond construction, the diamond grains are directly interbonded to adjacent grains and the interstices 32 between the grains 34 of super hard material such as diamond grains in the case of PCD, may be at least partly filled with a non-super hard phase material. This non-super hard phase material, also known as a filler material, may comprise residual catalyst/binder material, for example cobalt, nickel or iron. The typical average grain size of the diamond grains 34 is larger than 1 micron and the grain boundaries between adjacent grains is therefore typically between micron-sized diamond grains, as shown in Figure 4. Polycrystalline diamond (PCD) is an example of a super hard material (also called a super abrasive material or ultra hard material) comprising a mass of substantially inter- grown diamond grains, forming a skeletal mass defining interstices between the diamond grains. PCD material typically comprises at least about 80 volume % of diamond and is conventionally made by subjecting an aggregated mass of diamond grains to an ultra-high pressure of greater than about 5 GPa, and temperature of at least about 1 ,200°C, for example. A material wholly or partly filling the interstices may be referred to as filler or binder material.

PCD is typically formed in the presence of a sintering aid such as cobalt, which promotes the inter-growth of diamond grains. Suitable sintering aids for PCD are also commonly referred to as a solvent-catalyst material for diamond, owing to their function of dissolving, to some extent, the diamond and catalysing its re-precipitation. A solvent-catalyst for diamond is understood be a material that is capable of promoting the growth of diamond or the direct diamond-to-diamond inter-growth between diamond grains at a pressure and temperature condition at which diamond is thermodynamically stable. Consequently the interstices within the sintered PCD product may be wholly or partially filled with residual solvent-catalyst material. Materials that do not promote substantial coherent intergrowth between the diamond grains may themselves form strong bonds with diamond grains, but are not suitable solvent - catalysts for PCD sintering.

A first example of a composite material for use either to form one or more components of a prosthesis or other implant for biomedical applications or for use as a coating or layer bonded to such components is described with reference to Figures 5 to 8b. The examples of such a composite material comprise a three dimensionally (3D) continuous interpenetrating network of sintered inter-bonded polycrystalline super hard material 40 such as PCD, and one or more phases 42 comprising materials such as a ceramic, a metal alloy, hardmetals and/or polymers. In some examples, all respective phases, except the catalyst used for sintering the super hard material, are continuous in three dimensions. The super hard material 40 forms at least 10% of the composite by volume and up to around 80 % of the composite by volume. The other phases 42 fill the remaining volume fraction. The secondary phases (such as any one or more of a ceramic, a metal alloy, a hardmetal and a polymer) may also be chemically removed after the composite is manufactured to form a porous super hard structure, with porosity greater than 20% by volume and up to around 80% by volume. Such a porous super hard structure may be particularly suitable for use as a scaffold or skeleton structure for tissue regeneration as the tissue material may grow through the pores in the porous structure and in the example of a hip prosthesis,, may find particular application as the material for forming the shell 20.

The construction and formation of examples of material as shown in Figures 5 to 8b are discussed in more detail below with reference to the following examples, which are not intended to be limiting.

Example 1 :

Commercially available alumina foam 42 as shown in Figure 5 was infiltrated with a plurality of diamond particles having an average particle size of around 15 microns. In one method, diamond particles were added on top of the foam inside a niobium cup and the assembly was subjected to mechanical vibration in order to force the diamond particles to fill the pores inside the foam. In another method, a slurry of diamond particles was formed and poured on top of the foam in order to fill the pores. The assembly was then allowed to settle and dry. A tungsten carbide substrate with 13wt% cobalt was placed on top of the diamond infiltrated alumina foam inside a niobium cup to form a pre-composite assembly. The pre-composite is then sintered at a pressure above 5GPa and temperature of about 1400°C in the presence of cobalt infiltrated from the WC-Co substrate. This forms an intergrown (interbonded) PCD skeleton 40 with a three dimensionally continuously interpenetrating structure of alumina 42. In some applications, depending on the end use, the WC substrate may be removed from the sintered structure by conventional methods such as EDM, or laser cutting techniques.

Example 2:

A PCD-titanium 3D continuously interpenetrating was formed by initially forming a titanium metal skeleton using 3D laser sintering of titanium powder. The skeleton was introduced into a niobium cup and diamond powder added to fill the pores in the titanium skeleton. In this case a bimodal diamond powder was used comprising around 15wt% of diamond particles having an average grain size of around 2 microns and 85wt% of diamond particles having an average grain size of around 22 microns. A tungsten carbide substrate with 13wt% cobalt was then added and the assembly was subjected to mechanical vibration to ensure the loose powders fill the empty spaces in the titanium skeleton. The pre-composite was then sintered at a pressure above 5GPa and temperature of about 1400°C in the presence of cobalt infiltrated from the WC-Co substrate to form an interbonded PCD skeleton with a three dimensionally continuously interpenetrating structure of titanium.

Example 3:

In the above described examples 1 and 2, the process of making a PCD-Ceramic and PCD-Metal 3D continuously interpenetrating composite starts with forming a porous ceramic or metal and then infiltrating it with diamond. However, one may also start by forming a porous diamond green body and then infiltrating it with ceramic, metal, polymer or a combination thereof. Diamond green bodies may be prepared via conventional freeze casting techniques or other manufacturing methods such as injection moulding, 3D inkjet or laser printing and robocasting. The porous green body is then infiltrated with a second phase such as a ceramic, a metal, a polymer or a combination thereof and the product sintered as described above with respect to examples 1 and 2. An alternative source of catalyst material for the PCD other than a WC substrate may be used in one or more examples such as admixing the catalyst such as Co into the diamond grains prior to sintering.

Example 4:

A 3D interpenetrating network of PCD and metal of polymer or any leachable material is prepared via any of the examples 1 to 3 described above. The secondary phase(s) 42, namely any one or more of a metal, a polymer or any other leachable material are then removed via conventional chemical leaching techniques or any other suitable method to form a porous intergrown PCD structure. This porosity is not less than 20% by volume of the PCD body and may be up to around 80% by volume of the PCD body.

Whilst not wishing to be bound by any theory, it is submitted that PCD-Ceramic-Metal composites with three dimensionally (3D) continuous interpenetrating networks may exploit the superior tribological and mechanical properties, and biocompatibility of intergrown polycrystalline diamond. By imbedding a PCD skeleton inside the metal alloys of any one or more of, for example, nickel, chrome, alumina, cobalt and titanium, the wear resistance may be significantly improved whilst at the same time the likelihood of allergic reactions caused by elements of these metals may be reduced. In the case of PCD-ceramic composite with interpenetrating networks, the wear resistance and biocompatibility may be further improved, together with the fracture toughness. Diamond also has a very low coefficient of friction. Thus diamond is an ideal material for biomedical applications, in particular as a medical implant manufactured to replace a missing biological structure, support a damaged biological structure, or enhance an existing biological structure, such as hip and knee prostheses, due to the superior tribological and mechanical properties of diamond as well as good corrosion resistance, biocompatibility, and biostability.

Further examples of the second phase material 42 which may be used include but are not limited to TieAUV which has been proven to be biocompatible and has good biostability, and other metal alloys with similar properties, such as cobalt-chrome.

In other examples, the PCD-Metal composite structure may be formed with a thin CVD diamond layer adhered to the free surface which may assist in inhibiting any residual catalyst material elements used in the sintering of the PCD being absorbed by the body.

While various versions have been described with reference to a number of examples, those skilled in the art will understand that various changes may be made and equivalents may be substituted for elements thereof and that these examples are not intended to limit the particular examples or versions disclosed. For example, in some embodiments of the method, the PCD material may be sintered for a period in the range from about 1 minute to about 30 minutes, about 2 minutes to about 15 minutes, or from about 2 minutes to about 10 minutes.

In some examples of the method, the sintering temperature may be in the range from about 1 ,200 degrees centigrade to about 2,300 degrees centigrade, about 1 ,400 degrees centigrade to about 2,000 degrees centigrade, about 1 ,450 degrees centigrade to about 1 ,700 degrees centigrade, or about 1 ,450 degrees centigrade to about 1 ,650 degrees centigrade. Also, whilst it is conventional to sinter PCD using a catalyst such as cobalt, a range of catalysing materials comprising metals and/or non- metals may be used.