Wear characteristic and biocompatibility of some polymer composite acetabular cups
Introduction
In the early days of hip surgery metal-on-metal prostheses were used. Both femoral implants and acetabular cups made of stainless steel, Co–Cr–Mo or its alloys were considered suitable until Sir John Charnley proposed metal to plastic pairing for total hip joint prosthesis in early 1960s [1], [2]. Initially polytetrafluoroethylene (PTFE) was used as a socket material to achieve low friction but this was later abandoned due to its high wear rate. Ultra-high molecular weight polyethylene (UHMWPE) has been successfully used as acetabular cup material for its superior mechanical toughness, wear resistance and biocompatibility over the most other polymer candidates. A large bulk of work has been carried out on the development of suitable materials for acetabular cups, their mechanical and wear characteristics, effect of multidirectional motion on wear, effect of irradiation on the material performance and such other aspects [3], [4], [5], [6], [7], [8]. Recently, however there are concerns regarding the adverse biological tissue responses resulting in osteolysis by the wear debris from UHMWPE [9], [10], [11], [12], [13], [14]. This initiated work on the methods of reducing wear rate of UHMWPE, such as, the use of crosslinked form of UHMWPE. However, based on some recent experimental work with retrieved components of highly crosslinked UHMWPE the claim that these materials have ‘no detectable wear’ was questioned [15]. It was felt that all orthopaedic bearings are expected to wear after implantation and a more realistic approach would be to characterise the wear rate and plan the subsequent protocol accordingly. The problem of osteolysis by the wear debris from UHMWPE also regenerated interest on metal-to-metal [16], [17] and ceramic-on-ceramic prosthesis [18]. It has been shown clinically that metal-on-metal prostheses may survive for over 20 years in some cases [19]. However, the low wear level in these hard-on-hard bearings is due to the formation of thick film lubrication, which in its turn depends on manufacturing, and design parameters such as roughness, radial clearance and others [20], [21]. There are also concerns over the long-term systematic problems due to metal ion release and increase in cobalt and chromium concentrations in blood and urine [22].
In view of these observations the work on the improvement of UHMWPE was not abandoned. Some work on filled UHMWPE and PEEK, carried out in search of alternative prosthesis materials, indicated that non-abrasive fillers in UHMWPE can result in a considerable improvement in wear resistance due to strong chemical bonding of the filler material with the metal surface [23]. Carbon fibre reinforced PEEK also offers a superior wear characteristics over unfilled UHMWPE when rubbed against either metal or ceramics in ball and socket configuration [24], [25]. Work on the development of UHMWPE filled with MoS2 or short carbon fibre with fibre fraction volume of around 10% showed encouraging wear results [26], but these materials are yet to be tested in body environment for more realistic assessment of their suitability as socket materials. There are also some evidence of improvement in both wear resistance and biocompatibility of filled UHMWEPE and HDPE [27]. Very little attention has, however, been paid towards the development of polymer composites suitable for hip or knee prostheses. The present work is an attempt to develop acetabular components using polymer composites and to test their mechanical strength and in vitro wear characteristics in a standard tribo-tester and also in a newly developed walk simulator. Biocompatibility tests of these materials were also carried out in order to judge the overall suitability of the composites as socket materials.
Section snippets
Material characterisation
Since both carbon and kevlar fibres are known to be biocompatible it was felt that HDPE reinforced with carbon and kevlar fibres may be a starting point of the present proposal. Composites of HDPE reinforced with 10, 15 and 20 wt.% of grade 49 kevlar fibre (designated as KFRP10, KFRP15 and KFRP20) and 20 wt.% of gradeT-300 carbon fibre (designated as CFRP20) were prepared by a technique of uniform mixing and moulding. These materials along with the unfilled HDPE and UHMWPE were characterised
Wear tests
Wear tests were carried out on a pin-on-disc apparatus and also on a newly developed walk simulator.
Biocompatibility tests
Biocompatibility refers essentially to the compatibility of biomaterials with the biological systems. Since it is rarely possible to find a fully biocompatible material, it is necessary to identify the materials, which are physiologically tolerable. To this extent, the results of in vivo and in vitro tests are the only guiding criteria for the choice of materials. There are a number of standard methods for testing the biocompatibility of materials; some are more suitable than others in specific
Conclusions
Polymer composites of HDPE reinforced with different percentages of kevlar and carbon fibre have been developed and using these materials new acetabular cups were prepared by a simple compression moulding technique. The mechanical and tribological characteristics of these materials were studied in standard universal testing machine and pin-on-disc machine. A walk simulator, developed in-house was also used for testing the tribological performance of the acetabular cups and the results are
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Present address: IGIT, Sarang, Talcher, Orissa, India.