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ORTHOPAEDIC BIOPOLYMER WEAR SCREENING RIG VALIDATED AGAINST CLINICAL DATA

Centre for Biomedical Engineering, University of Durham, Durham, DH1 3LE, UK

Introduction: The wear of orthopaedic biopolymers is recognised as a major factor in the failure of total joint replacements. Clinical wear data exists for acetabular cups manufactured from three biopolymers: ultra high molecular weight polyethylene (UHMWPE); poly tetra fluoro ethylene (PTFE); and polyacetal. The aim of this paper was to wear test these biopolymers and compare the results with clinical data.

Method and Materials: The biopolymers were tested using a modified, four-station, pin-on-plate wear rig [1]. In the tests, two of the four stations applied reciprocating motion and two applied multi-directional motion. Biopolymer pins articulated against stainless steel plates under a load of 40N. The lubricant consisted of 25% bovine serum and 75% distilled water. A standardised cleaning and weighing protocol was followed, and the biopolymer wear factors were calculated by dividing the volume lost by the product of the load and the sliding distance.

Discussion and Conclusions: Failed and retrieved UHMWPE acetabular cups have been reported as having a clinical wear factor of 2.1 x 10^–6mm³/Nm [2]. However, UHMWPE cups which have been functioning well until removal at post-mortem have been said to show 45 to 69% less wear than revised UHMWPE cups [3]. Combining these values suggests clinical wear factors for functional UHMWPE in the range of 0.95 to 1.45 x 10^–6mm³/Nm. This range fits well with the value of 1.1 x 10^–6mm³/Nm shown in table 1 for UHMWPE under multi-directional motion. A clinical wear factor of 37 x10^–6mm³/Nm has been calculated for PTFE acetabular cups [4]. When compared with the mean wear factor for PTFE pins under multi-directional motion obtained from the pin-on-plate rig, the match is remarkable. For polyacetal cups a mean volumetric wear of 136mm³/ year has been reported [5] and it has been calculated that explanted hip prostheses averaged 1.54 million cycles/year [2]. In polyacetal acetabular cups of 37mm diameter, an average sliding distance of 25mm/cycle can be calculated [6] and it has been said that an equivalent static load of 1000N applies [7]. Taking these four values permits a clinical wear factor for polyacetal cups of 3.5 x 10^–6mm³/Nm to be calculated. This number compares well with the value of 3.8 x 10^–6mm³/Nm seen for the polyacetal test pins under multi-directional motion. In summary, all three biopolymers subject to multi-directional motion exhibited clinically relevant values of wear.

Correspondence should be addressed to Dr Carlos Wigderowitz, Honorary Secretary of BORS, Division of Surgery & Oncology, Section of Orthopaedic & Trauma Surgery, Ninewells Hospital & Medical School Tort Centre, Dundee, DD1 9SY.