Prosthetic bearing element and implant incorporating such an element

Abstract: A prosthetic bearing element having a bearing surface which is made from an ultra-high molecular weight polyethylene with a molecular weight greater than 1,000,000 with a multi-axially molecular orientation.

Description: This invention relates to a prosthetic bearing element and to an implant incorporating such an element. Prosthetic bearing elements are employed in many prosthetic implants and can be made from various materials, for example polyethylene or nylon, provided the bearing material is compatible with the human body. Such elements are usually held in a metal housing which carries the load although, in certain circumstances, they may be used by themselves, for example as an acetabular cup. Typical use of such elements is in knee prostheses where they are used on a tibial tray and co-operate with a femoral component. They are also used in patella constructions and act between the femoral and patella components. Yet another use, as referred to above, is as bearing cups in hip implants. A favoured material, at the present time, is polyethylene, particularly ultra high molecular weight polyethylene (UHMWP) with a molecular weight greater than 1,000,000. It has been found however that even this material wears and the present Applicants have therefore carried out research into the possibility of treating the material to improve the wear qualities. It has been found that a multi-axially molecular orientation in the material provides such qualities. United States Patent No. 5,030,402 shows a method for producing an UHMWPE product with enhanced planar mechanical properties. A initial UHMWPE semicrystaline morphology is solid-state deformed under compression in a mould including a mould cavity with an unenclosed perimeter zone or a perimeter zone with a reservoir enabling flow-through for moulded material. In this method the deformation under compression is applied from one direction and the material is pressed flat. The general direction of forming is therefore in only one direction. EP-A-0 310 449 described a process for producing a polyethylene material for use in products required high mechanical strength and high elastic modulus. This process involves the use of a specific class of particulate polyethylenes having an intrinsic viscosity of 5 - 50 dl/g in decalin at 135°C and the steps of compression-moulding, solvent immersing, solid-phase extruding or rolling and finally drawing the material in that order. Although compressing, extruding and drawing are referred to there is no suggestion that the present Applicant's result can be achieved by the process as set forth. According to the present invention therefore a prosthetic bearing element having a bearing surface is made from an ultra-high molecular weight polyethylene with a molecular weight greater than 1,000,000 characterised in that said bearing element is taken from a drawn polyethylene tube or a drawn solid strip which have been subjected to a single solid phase deformation in two directions to cause an orientation of the polymeric chains parallel to a bearing surface of said bearing element in only two perpendicular directions in the same area of the bearing surface, said deformations in two directions having a deformation ratio of 1.3 to 1.9. From tests which have been applied to such material it has been found that the wear rate is lowered and preferably the direction of one of said axial molecular orientations extends parallel with or along the length of the bearing surface. The preferred deformation ratios in each direction are 1.5 to 1.6. The ultra-high molecular weight polyethylene preferably has a molecular weight greater than 4,000,000. The first deformation can be performed in a lengthwise direction of the workpiece and a second deformation performed in a substantially transverse direction thereto. The second deformation ratio can be greater than the first. In a preferred method the workpiece is hollow and is passed over a former of increasing cross-sectional area, the second deformation taking place in a hoop direction. The hollow workpiece can be passed over the former without applying external force. Where the workpiece is solid it can be square or of rectangular cross-section transverse to its length. In another method the workpiece can be drawn through a die and/or over a former. The invention also includes a prosthetic bearing element as set forth above incorporated in a prosthetic implant. The invention can be performed in various ways and some methods will now be described by way of example and with reference to the accompanying drawings in which : Figure 1 is a part cross-sectional front elevation of a tibial tray incorporating the invention; Figure 2 is a part cross-sectional end elevation of the tray shown in Figure 1; Figure 3 is a part cross-sectional front elevation of an alternative construction of an alternative construction of a tibial tray incorporating the invention; Figure 4 is a cross-sectional side elevation of a knee joint prosthesis incorporating the invention; Figure 5 is a part cross-sectional side elevation of a prosthetic hip joint incorporating the invention; Figure 6 is a graph showing the relative density of drawn materials (B) which can be used in the bearing elements relative to drawn isotropic source material (I); Figure 7 is a graph showing the relative crystallinity of the material (B) which can be used in the bearing elements relative to isotropic source material (I); Figure 8 is a diagrammatic representation of one method used to provide the material for the bearing element; Figure 9 is a diagrammatic perspective view of a material produced by the method shown in Figure 3; Figure 10 is a graph showing typical engineering stress strain curves; Figure 11 is a graph showing strain energy to failure and elastic modulus of material made by the method shown in Figure 3 in comparison with isotropic materials; Figure 12 is a graph showing stress strain curves for materials made by the method described relative to isotropic materials; Figure 13 is a graph showing wear test results; Figure 14 is another graph showing wear test results; Figure 15 is a diagrammatic view of a pin used for testing; Figure 16 is a diagrammatic view of a plate used for testing; Figure 17 is a diagrammatic perspective view of another method of making material for the bearing elements; and Figure 18 is a diagrammatic side elevation of a third method of making material for the bearing elements. Figures 1 and 2 show a typical tibial tray for use in a prosthetic implant. The tray comprises a base portion 1 the outer circumference 2 of which is provided with an upstanding lip 3. A spigot 4 extends downwardly from the base 1 to provide attachment to the bone in which it is inserted. The base 1 carries a bearing element 5 which has spaced apart indentations to accept the condyles of the femoral implant (not shown). Tibial trays of this kind are well known in themselves and the construction will not therefore be described further. According to the present invention the bearing element 5 is made from ultra high molecular weight polyethylene (UHMWP) with a molecular weight greater than 1,000,000 and with a multi axially molecular orientation. Figure 3 shows an alternative form of tibial tray and the same reference numerals are used to indicate similar parts but in this construction the base 1 is provided with two separate bearing elements 7 and 8 which are located in recesses 9 and 10. Once again the bearing elements, each of which has a bearing surface 11, are made from an UHMWP with a molecular weight greater than 1,000,000 with a multi axially molecular orientation. The direction of one of said axial molecular orientations extends parallel with or along the length of the bearing surface, thus, it could extend in a substantially horizontal direction and along the length of the indentation 6 or bearing surfaces 11 along a substantially medio lateral plane. A second molecular orientation may extend at right angles thereto, again, substantially horizontal, or it could extend vertically through the bearing element. Again, a combination of various directions can be used. Figure 4 shows another knee prosthesis but in this arrangement a tray 15 is provided on the tibia on which is carried a pair of sliding meniscal bearing elements 16. The upper surface of the element 16 is engaged by a femoral implant 17 on which is also carried a patella implant 18 having a metal backing which engages the patella remnant 19. A patella bearing element 20 is arranged between the metal backing and the femoral implant 17. As will be seen the bearing elements 16 each have a proximal bearing surface 21 which engages the bearing surface 22 on the femoral implant 19 and further bearing surfaces 23 which engage bearing surfaces 24 on the tray 15. The patella element 20 has a bearing surface 25 which engages the bearing surface 22 on the femoral implant 14 and a further bearing surface 26 which engages the metallic backing. As described with regard to Figures 1, 2 and 3 the bearing elements 16 and 20 are made from an UHMWP with a molecular weight greater than 1,000,000 with a multi axially molecular orientation. The orientations can be as described with regard to Figures 1, 2 and 3. Figure 5 shows a hip prosthesis which comprises a femoral insert 30 provided with the usual ball 31. The ball seats in a hip cup 32 which comprises a metallic backing 33 designed for location by any convenient means, for example cement or mechanical means to the pelvis and which carries a bearing element in the form of a liner 34. The liner 34 is made from an UHMWP with a molecular weight greater than 1,000,000 and the multi axially molecular orientation in the liner 34 and can again be in any way desired and as referred to with regard to Figures 1, 2 and 3. It has been found that a preferred molecular weight is greater than 4,000,000. There are various ways of treating UHMWP to provide the biaxial orientation and some will now be described with reference to Figures 6 to 13. The method is focused on biaxial orientation of UHMWP to enhance its properties in at least two directions, and establish conditions where the strain energy to failure of the modified material is maintained and that proof of yield stress of the material is increased. The first method demonstrates die drawing of GUR412 (MW 4.5 million) UHMWP in both uniaxial and biaxial forms. The drawn materials had similar density and crystallinity as shown in Figures 6 and 7 to the isotropic source material. The molecular orientation of the drawn material was confirmed by X-ray diffraction. The material is intended for use as polymer components in artificial joints as set forth above which undergo complex loading regimes, stress field and wear patterns, it is considered preferable to enhance the properties of the UHMWP in two directions, by using biaxial drawing methods. Figure 8 shows a method used for biaxial orientation of a thick walled tube 41, by drawing a material 42 over a mandrel 43 which is of increasing diameter over its length in the direction of draw indicated by reference numeral 44. Such a method of drawing polyethylene material to produce solid phase deformation is described in GB 2 225 551, but the effects produced by the present method were unexpected considering the information set out, for example in the prior art document referred to above. This method produces orientation in both the longitudinal direction indicated by arrow 45 and hoop direction indicated by arrow 46, as shown in Figures 8 and 9, with a higher draw deformation ration and orientation near the internal surface of the tube 47 than the outside 48 in the hoop direction 46. Material is produced such that the draw ratios close to the internal surface 47 is similar in magnitude in both the hoop 46 and longitudinal direction 45. Figure 10 shows typical engineering stress strain curves taken in tension in the longitudinal direction for the isotropic or standard material (A) and biaxially orientated material (B) with a draw ratio of 1.5 in each direction. This shows that the drawn material had a reduction in the strain to failure, approximately similar strain energy to failure and a higher proof stress and working stress range compared to the isotropic material. Increasing the draw ratio further adversely affected both the strain and strain energy to failure, producing a much more brittle material as shown in the curve for the highly drawn uniaxial material with a ratio of 4 to 1 as shown in Figure 10. Biaxially drawn material with a draw ratio of approximately 1.5 in each direction was considered to produce the most appropriate change in properties with a statistically significant increase in proof stress and working stress range while maintaining a similar strain energy to failure and elastic modulus to the isotropic material as shown in Figure 11. A detailed examination of the stress strain curve for the two materials at low strains shows clearly the improved characteristics of the biaxially orientated material as shown in Figure 12. Seven one metre long lengths of thick walled tube were produced from three different 75 mm thick slabs of GUR412 UHMWP. The final dimensions of the drawn tube produced a wall thickness of between 10 and 15 mm with an outside diameter of 63 mm. The drawn material was found to have some dimensional instability when machined components were taken from the drawn tube, due to the release of inbuilt strains. Two types of wear tests were carried out, a polymer pin on metal reciprocating plate with reciprocating motion, and a spherical ended metal pin on polymer plate under reciprocating motion. The first test configuration maybe considered more appropriate for hip joint applications, while the second test may be considered more suited to knee joint applications. The polymer pins and plates were taken with their wear surfaces close to the internal diameter of the drawn tube, where the draw ratio was approximately 1.5 in each direction such a pin 49 and a plate 50 are shown in Figure 9. Each test consisted of a direct comparison between the biaxial material and the isotropic control material. Tests were carried out for sliding distances greater than 250 km (greater than 10 years equivalent), with more than 15 measurements in each test to allow statistical analysis. Tests were run in bovine serum as a lubricant, and the wear rate was expressed as a normalised wear factor K when, Wear factor = Wear Volume Load and Sliding Distance The results of five sets of polymer pin 49 on plate tests are shown in Figure 13. The specific conditions for each of the tests are set out below. POLYMER PIN ON PLATE WEAR TESTS 1 200N LOAD 2 160N LOAD 3 160N LOAD 4 80N LOAD 5 80N LOAD (ROUGH COUNTERFACE) In each of the tests the biaxial material has a lower wear rate than the isotropic control material. The differences were statistically significant at the 20 % level in Tests 2 and 3. Overall, the wear factor for the biaxial material was reduced by 22% compared to the isotropic material. The results for the polymer plate 50 on a pin test are shown in Figure 14. The isotropic material showed a higher wear factor than the biaxial material and this was statistically significant at the 5% level. The biaxial drawing of the material was predicted to produce a 25 % reduction in the wear rate of this test. Figures 15 and 16 are intended to illustrate what is thought to be the lines of molecular orientation on the test pieces. Thus, Figures 15 shows the work surface 49a on the end of the pin 49. The general direction of the molecular orientation is shown to be in two directions by arrows A and B. The point from which the pin has been taken is shown in Figure 9 and it will be seen that the lines of orientation are substantially at right angles to each other across the surface. In Figure 16 the lines of orientation are again shown in two directions C and D on one face 51a of the plate 50. It will be seen that the lines of orientation are at right angles on this face. On the face 51b however, although the lines of orientation C and D are again at right angles the lines D exhibit their ends towards the face and on the face 51c the lines of orientation D extend across the face but the lines C again exhibit their ends. It will be appreciated that the above Figures only show assumed lines of orientation and are meant to be interpreted in general terms. From the experimental results set forth above it is assumed that the best wear qualities are obtained when the lines are at right angles across the face, as shown in Figure 15, although this assumption may be only the result of the two different types of experiments applied. As will be seen from the above the mechanical properties of ultra high molecular weight polyethylene are been enhanced by biaxial drawing and a significant increase in proof stress and working stress range has been achieved, while maintaining the strain energy to failure. Wear tests have shown that this can produce a 22 to 25 % reduction in the wear rate of the biaxially drawn materials compared to the isotropic UHMWP. An alternative method of causing solid phase deformation in at least two directions to ultra high molecular weight polyethylene (UHMWP) can be applied by a slot type drawing method. As shown in Figure 17 a rectangular or square strip of pre-machined UHMWP 52 is drawn through a slot 53 of a slot-type die 54 having an entry opening 55 and a discharge opening 56. The direction of draw is indicated by arrow 57 and the transverse width of the slot 53 is equivalent to the transverse width of the strip 52 at entry. With this arrangement there is solid phase deformation in the draw direction 57 and at an angle thereto, indicated by arrow 58 in the die slot, thus causing solid phase deformation in two directions to cause the preferred multi-axial orientation. The deformation ratios, that is the draw ratio and in effect the compression ratio in the slot, are both between 1.3 to 1.9. Figure 18 shows a method similar to that described and shown in Figure 8 and the same reference numerals are used to indicate similar parts but in this method the tube of material also passes through a control die 60.

Claims: A prosthetic bearing element (5, 7) having a bearing surface (6, 11) which is made from an ultra-high molecular weight polyethylene with a molecular weight greater than 1,000,000 characterised in that said bearing element (5, 7) is taken from a drawn polyethylene tube (41) or a drawn solid strip (52) which have been subjected to a single solid phase deformation in two directions (44, 46; 57, 58) to cause an orientation of the polymeric chains parallel to a bearing surface (6, 11) of said bearing element in only two perpendicular directions in the same area of the bearing surface (6, 11), said deformations in two directions having a deformation ratio of 1.3 to 1.9. A prosthetic bearing element as claimed in claim 1 characterised in that said deformation ratios in each direction are 1.5 to 1.6. A prosthetic bearing element as claimed in claim 1 or claim 2 characterised in that the ultra-high molecular weight polyethylene has a molecular weight greater than 4,000,000. A prosthetic bearing element as claimed in claim 1, claim 2 or claim 3 characterised in that a first of said two solid phase deformations includes a deformation performed in a lengthwise direction of the tube and a second deformation is performed in a substantially transverse direction thereto. A prosthetic bearing element as claimed in any of the preceding claims characterised in that the second deformation ratio is greater than the first. A prosthetic bearing element as claimed in claim 5 characterised in that the tube (41) is hollow and passed over a former of increasing cross-sectional area, the second deformation taking place in a hoop direction (46). A prosthetic bearing element as claimed in claim 6 characterised in that the tube (41) is passed over the former without applying external force. A prosthetic bearing element as claimed in claim 1 characterised in that the strip (52) is of square or rectangular cross-section transverse to its length. A prosthetic bearing element as claimed in any one of preceding claims 1 to 8 characterised in that the tube (41) or strip (52) is drawn through a die and/or over a former. A bearing element as claimed in any one of preceding claims 1 to 9 incorporated in a prosthetic implant.

Inventors: DOYLE CHRISTINA, DOYLE, CHRISTINA, DR.
Applicants: HOWMEDICA INTERNAT S DE R L, HOWMEDICA INTERNATIONAL S. DE R.L.
Application year: 1996
Main classification: A61F2
IPC codes: A61F2, A61L27, A61L33, B29C55