WO2009115790A2

WO2009115790A2 - Replacement bone joints

Info

Publication number: WO2009115790A2
Application number: PCT/GB2009/000715
Authority: WO
Inventors: Peter White
Original assignee: Peter White
Priority date: 2008-03-19
Filing date: 2009-03-18
Publication date: 2009-09-24
Also published as: WO2009115790A3; GB0805122D0

Abstract

A replacement bone joint has joint members, such as a femoral prosthesis (15) and an acetabular prosthesis (20), which define a ball and socket joint between a head (25) and a cup (45). The cup has a lining (60) and the head (25) is provided with an adhered coating. The coating and/or the lining (60) are formed from a non-cross-linked thermoplastics material such as Nylon. In particular, the coating may be Nylon 11.

Description

REPLACEMENT BONE JOINTS

This invention relates to replacement bone joints and is particularly although not exclusively concerned with replacement hip joints.

Replacement hip joints have a femoral component such as tapered titanium implant which is fixed within the femur bone, a ball at the upper end of the femoral component, and an acetabular component fitted within the acetabulum socket of the pelvis. The acetabular component may comprise a lined shell and the femoral ball fits movably within this.

The contacting surfaces of the ball and the acetabular component should be large and strong enough to be capable of withstanding the impact and movement stresses which occur in use. At the same time the surfaces of the ball and socket should be such as to permit easy articulation movement, i.e. such as to have a low coefficient of friction, whilst being hard-wearing. Further, it is generally preferred that a small radial clearance exist between the ball and socket of the bone joint to enhance the lubrication of the joint by the synovial fluid.

Metal-on-metal contact surfaces are known, typically stainless steel and cobalt chrome. They are strong and hard-wearing. However there is growing concern over the effects of metal wear debris and the possibility of dispersed metal ions being taken up by the immune system both locally and distant with the potential to cause swelling of the lymph nodes and genetoxicity. Allergy is another possible side effect, for instance, nickel and chromium ions can act as allergens in some individuals.

Ceramic-on-ceramic joint surfaces are also known, made from aluminium oxide. These can be very hard-wearing. However, there can be practical size limitations, particularly due to the inherent brittleness of the material, although newer formulations are less brittle. In addition, the effect of aluminium oxide particles resulting from wear on the joint surfaces is as yet unknown.

Ceramic-on-metal joints may also be used. However, it has been found that ceramic linings, where thin wall thicknesses are employed, are liable to fracture as the ceramic is generally too brittle to offer a robust contact surface.

Metal-on-polyethylene and ceramic-on-polyethylene joint surfaces are also known and are widely used. These give excellent articulation. The use of plastics material advantageously gives an impact cushioning or damping effect. Early systems utilised non-cross-linked polyethylene but were found to be too soft with poor wear characteristics. Current systems utilise high molecular weight, highly cross-linked polyethylenes which are better than lower molecular weight and non-cross-linked grades. However, even the cross-linked systems currently in common use remain insufficiently hard-wearing. In particular, cross-linked polyethylene systems are prone to fragment with prolonged use producing debris in the joint, and this is a known cause of osteolysis and a major source of revision operations. The sterilisation and cross-linking processes are carried out by irradiating the polyethylene with gamma radiation in an inert atmosphere. Low dosages of radiation are required for sterilisation but much larger multiple doses are utilised to obtain the degree of hardness required. The short and long-term durability can be compromised both by a failure to fully cross-link the system and also in a system where the dosage of radiation has been excessive. Degradation of the polymer, embrirtlement and environmental stress cracking can also occur during the moulding/extrusion process used to produce the joint component.

A further problem arising from the use of artificial bone joints is that the synovial fluid is less efficient as a lubricant in an artificial bone joint, such as a hip joint. Ceramic-on-ceramic joints appear to have the best fluid film lubrication whilst other combinations show mixed results. It has been found that the hydrophobic nature of polyethylene appears to neutralise the effectiveness of the synovial fluid as a lubricant.

Accordingly, an object of the present invention is to provide strong, hard-wearing replacement bone joints and components for bone joint replacement that have some or all of the following properties. Specifically, the replacement bone joints are intended to give excellent articulation, maintain their properties during the life time of the product, have good tolerance to processing, sterilization, provide improved cushioning and damping characteristics when compared to metals and ceramics, be inert, biocompatible, non toxic, have anti microbial properties, and surface hydrophilicity to promote improved synovial lubrication.

According to the invention therefore, there is provided a replacement bone joint having structural joint members with respective contact surfaces, said surfaces being movable relative to each other wherein at least one surface is formed separately to the respective structural member from a hard-wearing plastics material, characterised in that the plastics material is a non-cross-linked thermoplastic material.

With this arrangement the required properties can be inherent in the polymer without having to resort to potentially detrimental modifications brought about by high levels of radiation and/or processing.

A durable, long lasting joint which has good impact resistance, cushioning and damping properties, and low friction can be achieved together with the additional benefits of surface hydrophilicity, increased surface hardness, microbial resistance and improved tensile strength.

In particular, the use of a non-cross-linked thermoplastics material removes dependence upon the degree of cross-linking as a control of the physical properties of the material. For instance, the surface hardness of the material will typically be at a level which minimises the internal shearing which is common in polyethylene joints.

As a result, the amount of debris formed in the joint can be significantly reduced.

In some embodiments the replacement bone joint will comprise two contact surfaces formed from the hard-wearing non-cross-linked thermoplastic material.

The non-cross-linked thermoplastic surface may be a lining or coating. Preferably it is a coating adhered to the respective joint member. The joint member may be treated to increase the effective surface area of the substrate and promote secure firm adhesion of the non-cross-linked thermoplastic material of the coating or lining to the member. Such treatment may include the provision of holes, perforations, or convolutions in the surface of the member. In addition, the surface of the member may be etched, grit-blasted or chemically treated. In one embodiment a recess e.g. in the form of a notch or groove or hole is formed in the respective member and non-cross-linked thermoplastic material of the coating or lining penetrates the recess so that the coating or lining is keyed in position relative to the respective member. The invention can be applied to a range of replacement bone joints including hip joints; joints for knees, elbows, fingers, or toes; bone plates for use as replacements of articulating surfaces such as the knee; bone plates and for use in splinting fractures; and bone plates used for bone bridging as in major resections of the mandible in the case of trauma or cancer surgery. In preferred embodiments the replacement bone joint will be a hip joint.

Preferably the non-cross-linked thermoplastic material is Nylon 11. Nylon 11 is often made from linseed oil. Other polyamides (Nylon 11, 12, 66, 6) may be also be used, as may other non-cross-linked thermoplastic materials including acrylonitrile butadiene styrene, ethylene vinyl alcohol, polyacetal, polyacrylates, polyamide-imide, polybutylene terephthalate, polycarbonate, polyester, polytetrafluoroethylene, polyetheretherketone, polyimide, polyphthalamide, and polysulfone.

The non-cross-linked thermoplastic materials may be used either alone or in combination. Where used in combination, one of the materials used will often be a polyamide so that the resulting material benefits from the high tensile strength found in polyamides. For this purpose the term "in combination" applies to the combination of polymers by intimate mixing to form a single surface, or the combination of polymers as separate contact surfaces where more than one surface is formed from a non-cross-linked thermoplastic material. Body-compatible additives may be incorporated within the polymer matrix of the non-cross-linked thermoplastic material. Examples of body-compatible additives include lubricants, reinforcing products, and anti-bacterial products.

Lubricants for use in the invention include silicone, hydrogels, molybdenum sulphide, polyvinyl pyrollidone, glycerine, mineral oil grease, and/or graphite. Lubricants selected from silicone, hydrogel, molybdenum sulphide and graphite lubricants are commonly used.

Reinforcing products may be selected from glass, stainless steel, and/or carbon fibre. Preferably however the thermoplastic material contains no, or substantially no, fibrous reinforcing material, and preferably contains no, or substantially no, reinforcing material of any kind.

At least one anti-bacterial substance, such as anti-bacterial products that allow the slow release of silver ions either onto the surface or the zone above the surface of the non-cross-linked thermoplastic material so making it hostile to bacteria such as e- coli, staphylococcus, aureus pneumophilia, salmonella typhimurium, Iegionella &

MRSA may also be used.

Surface treatment of the non-cross-linked thermoplastic material, such as ultra thin surface treatments, can be employed to increase hydrophilicity where necessary, for instance, if the surface of the non-cross-linked thermoplastic material is not already inherently hydrophilic to the extent required to promote production of the synovial fluid. Other surface treatments can be used for any other suitable purpose e.g. to increase the hardness or wear resistance, for instance, inorganic coatings such as tungsten disulphide, molybdenum disulphide, boron nitride, and diamond like carbon may be used. As used hereinafter the term non-cross-linked thermoplastic material is to be interpreted as including non-cross-linked thermoplastic material in combination with other materials and substances or modified as mentioned above.

The surface of the joint member to which the non-cross-linked thermoplastics material of the lining or coating is applied may be formed from any suitable substrate material known in the art. Substrate materials may include metals, such as stainless steel, cobalt, chrome, titanium and its alloys; ceramics, such as aluminium oxide; composite materials reinforced with fibres such as carbon or stainless steel; and combinations of these substrate materials. Use of the non-cross-linked thermoplastic material as a coating over another substrate can result in the properties of the combination being greater than the individual parts e.g. a Nylon 11 coated cobalt chrome femoral head can retain all the strength of the metal substrate and impart the beneficial properties of the coating but negate the undesirable properties of the metal such as nickel allergies and release of potentially toxic metal ions. In other embodiments the substrate material may also be formed from the same said non-cross-linked thermoplastic material as that which is used to form the aforesaid coating or lining. In such embodiments the substrate material may be composed primarily or entirely or substantially entirely of the non-cross-linked thermoplastic material. The coating or lining may be of a typical but not exclusive mean depth or thickness in the range 0.025mm - 11.00mm, particularly dependent on the coating or lining technique used. For a coating the range may be 0.025mm - 0.50mm. With electrostatic coating a range of 0.025mm - 0.1mm may be used. With fluid bed coating the range may be 0.25mm - 0.50mm. In the case of a lining the thickness may be 3mm - 11.00mm. Where two surfaces are formed from the non-cross-linked thermoplastic material, and both of these surfaces are formed either by coating or lining, the mean depth and depth range of each coating or lining may be selected independently. Such depth selection and tolerance would consider, for instance, the radial gap required between the femoral head and the acetabulum cup, the nature of the substrate to be coated or lined, the physical abrasion to which the coating or lining will be subjected and the tensile strength of the non-cross-linked thermoplastic material of each surface.

In some applications it may be advantageous to incorporate into bone replacement joint at least one substance which inhibits, controls, or promotes the growth of tissue or bone cells over the surface of the joint. For instance, where the joint is a replacement hip joint, it may be desirable to inhibit or control the growth of tissue cells but to promote the growth of bone cells thereby providing a secure adhesion between the replacement hip joint and the femur. However, where the joint is a bone bridging plate, it may be desirable to promote tissue re-growth across the treated area. Such substance or substances may be incorporated in or applied to the surface of the non-cross-linked thermoplastic material.

Growth inhibitory substances include polytetrafluoroethylene, polyethylene glycol and biomaterial matrices which impart growth inhibitory properties to the coating. Growth promoting substances include tissue sealants, such as fibrin glue; growth factors, such as platelet-derived growth factor (PDGF), insulin-binding growth factor- 1 (IGF-I), insulin-binding growth factor-2 (IGF-2), epidermal growth factor (EGF), transforming growth factor-α (TGF-α), transforming growth factor-jS (TGF- jS), platelet factor 4 (PF-4), osteogenin and other bone growth factors, collagen growth factors, heparin binding growth factor- 1 (HBGF-I), heparin binding growth factor-2 (HBGF-2) and biologically active derivatives of any of said growth factors; and potentiating compounds, such as heparin.

The invention will now be described, by way of example only, by reference to the accompanying drawings, of which: Figure 1 shows a schematic cross-sectional view of an artificial hip joint including a femoral prosthesis and an acetabulum prostheses. Figure 2 shows a side elevational view of the femoral prosthesis of Figure 1;

Figure 3 shows a rear elevational view of the femoral prosthesis of Figure 2; Figure 4 shows a perspective view of the acetabulum prosthesis of Figure 1 from above.

As shown in Figure 1 , one embodiment of the invention comprises a bone replacement hip joint 10 composed of a femoral component (a femoral prosthesis 15) and an acetabular component (an acetabulum prosthesis 20).

The femoral prosthesis 15 is further illustrated in Figures 2 and 3, and comprises a femur head 25, having a neck 30 and shoulder 35 with a tapering stem 40 extending therefrom. As shown in figure 3, the head 25 may be a separate component to the remainder of the femoral prosthesis 15 being connected by way of a spigot 65 forming part of the femoral stem 40. The head 25 which is made of stainless steel has been machined 250 microns undersize. It is then pre-treated by degreasing, aqueous grit-blasted to give a (30-60) micron peak to trough profile primed with a water based primer prior to being preheated for 30 mins at 220 degree Centigrade and coated with a USP Class Vl grade non-cross-linked Nylon 11, using the fluid bed process to give a finished coating thickness of 350 microns. The Nylon 1 1 coating is allowed to normalise prior to being machined and polished so as to produce a coating thickness of 250 micron; giving the correct radial gap when fitted into the cup 45 of the acetabulum prosthesis 20. The Nylon 11 coating is machined and polished according to ISO 7206-2 1996 whereby the spherical articulating surface of the Nylon 11 femoral head has a finish, when measured in accordance with the principles given in ISO 468, with an Ra value not greater than 0.05micron, using a cut off value of 0.08mm. When examined by normal corrected vision, the articulating surface shall be free from embedded particles and from scratches and score marks other than those arising from the finishing process. The remainder of the femoral prosthesis 15, specifically the neck 30, the shoulder 35 and the stem 40, is made of titanium. The stem 40 is of a length such that it will extend a substantial distance into the medulary canal of a femur to be treated and adhere thereto using friction. If necessary, the femoral prosthesis 15 may be cemented into place. Pins (not shown) may also be used to retain the femoral prosthesis 15 in position until bone in-growth and adhesion to the prosthesis 15 has occurred. There may be a notch beneath the head 25 into which the Nylon coating flows to key the coating in position.

The acetabulum prosthesis 20 is further illustrated in Figure 4 and comprises a hemispherical cup 45 into which the head 25 of the femoral prosthesis 15 is placed and an angular flange forming a lip 50 around the cup 45. In this embodiment the acetabulum prosthesis 20 is composed of machined titanium lined at the contact surface 55 with femoral prosthesis 15, with moulded non-cross-linked Nylon 66. The lining 60 may have a thickness in the range 5-1 lmm. The acetabulum prosthesis 20 can be retained in the pelvis using medical grade cement (not shown). Where necessary the acetabulum prosthesis 20 may be temporarily retained in place using pins (not shown), until such time as bone in-growth has sufficiently adhered the acetabulum prosthesis 20 to the pelvis.

Figure 1 shows the placement and interaction of the femoral 15 and acetabulum 20 prostheses, the stem 40 of the femoral prosthesis 15 being inserted into the medulary canal of a femur with the prosthetic shoulder 35 being supported on the shoulder thereof. The acetabulum prosthesis 20 is placed in a cup shaped depression formed by scraping out the damaged socket in the pelvis. Except where otherwise explicitly indicated, all numbers in this description are to be understood as modified by the word "about." All amounts are by weight of the final composition, unless otherwise specified.

The invention resides as well in sub-combinations of the components and features described. While the drawings and written description may disclose features and components in connection with a specific embodiment, the features and components described may be used, alone or in combination, with any embodiment.

The appearance of these features is a matter of design preference from a large array of options. These features may be selected or changed to create any desired functional combination of strengths of the bone replacement joint.

Thus, a novel bone replacement joint has been shown and described. Many changes, substitutions and uses of equivalents may of course be made without departing from the scope of the invention. The invention therefore, should not be limited, except by the claims and their equivalents. AU aspects of the invention are preferably applied to joints having surfaces which are movable relative to each other by a sliding movement without direct connection therebetween. However other modes of relative movement and interconnection are also possible.

Claims

1. A replacement bone joint having structural joint members with respective contact surfaces, said surfaces being movable relative to each other wherein at least one surface is formed separately to the respective structural member from a hard-wearing plastics material, characterised in that the plastics material is a non-cross-linked thermoplastic material.

2. A bone joint according to claim 1 characterised in that the non-cross-linked thermoplastic material is a coating adhered to the respective joint member.

3. A bone joint according to claim 1 or 2 characterised in that the non-cross-linked thermoplastic material penetrates a recess in the respective joint member so as to be keyed in position relative thereto.

4. A bone joint according to any one of claims 1 to 3 characterised in that the non-cross-linked thermoplastics material is a polyamide.

5. A bone joint according to claim 4 characterised in that the polyamide is Nylon 11.

6. A bone joint according to any one of claims 1 to 5 characterised in that the non-cross-linked thermoplastics material contains at least substantially no fibrous reinforcement material.

7. A bone joint according to any one of claims 1 to 5 characterised in that the non-cross-linked thermoplastics material contains at least substantially no reinforcing material.

8. A bone joint according to any one of claims 1 to 7 characterised in that the non-cross-linked thermoplastics material has a thickness in the range 0.025mm - 0.05mm in the case of a coating and 3mm - 1 lmm in the case of a lining.

9. A bone joint according to any one of claims 1 to 8 characterised in that the non-cross-linked thermoplastics material contains at least one anti-bacterial substance.

10. A bone joint according to any one of claims 1 to 9 characterised in that the non-cross-linked thermoplastics material is surface treated to increase hydrophilicity and/or wear resistance and/or hardness.

11. A bone joint according to any one of claims 1 to 10 wherein the non-cross-linked thermoplastics material is applied to the surface of the respective joint member characterised in that said surface is formed from a substrate material selected from metals, ceramics, fibre-reinforced composite materials, and combinations thereof.