TWO ORTHOPAEDIC BIOPOLYMERS WEAR TESTED AGAINST EACH OTHER: AN ALL-POLYMER KNEE PROSTHESIS REVISITED

Abstract

Introduction The clinical use of an all-polymer knee which articulated a polyacetal femoral component against an ultra high molecular weight polyethylene (UHMWPE) tibial component has been reported [1]. A ‘polyacetal group’ of 63 total knee replacements were followed for at least ten years and no instances of femoral component fracture or failure due to wear occurred [1]. Such results are remarkable for an all-polymer pros-thesis in such a heavily loaded joint as the knee. Recently a wear screening device has been described which reproduced in vitro the clinical wear rates reported for three biopolymers which have been employed as the acetabular cup material in hip prostheses [2]. Given this validated rig, the objective of the work reported here was to undertake wear tests of polyacetal against UHMWPE.

Materials and Methods The polyacetal and UHMWPE couples were tested using a modified, four-station, pin-on-plate wear test rig [2]. The modification entailed the addition of rotational motion to the test pins, in addition to the standard reciprocating motion, to give multi-directional motion. In the wear tests, two stations had reciprocation-only and two applied multi-directional motion. Investigating the influence of both types of motion permitted a fuller tribological analysis to be undertaken. Control pins and control plates were included to account for any weight change due to lubricant uptake. A load of 40N was employed and reciprocating and rotating speeds of 1Hz were chosen. The lubricant consisted of 25% bovine calf serum and 75% distilled water, which was heated to 37°C during testing. A standardised cleaning and weighing protocol was followed, and the pins and plates were weighed on a balance sensitive to 0.1mg.

Results and Discussion After an average of 1.4 million cycles of sliding, the mean wear factors were: UHMWPE pins rubbing against polyacetal plates, 1.5 x 10-6mm3/ Nm under reciprocation, and 4.1 x 10-6mm3/Nm under multi-directional motion. For polyacetal pins rubbing against UHMWPE plates they were 0.7 x 10-6mm3/ Nm under reciprocation, and 2.8 x 10-6mm3/Nm under multi-directional motion. As can be seen, the wear factors depended on both the orientation of the material, whether it was a pin or a plate, and the motion it was subjected to. The increase in weight of the polyacetal control components due to lubricant uptake was many times that of the UHMWPE components. For example the UHMWPE control plate showed an increase of 0.2mg compared with 33.4mg for the polyacetal control plate. Using the same wear screening rig, the wear factors for UHMWPE articulating against stainless steel were measured to be 0.1 x10-6mm3/Nm under reciprocating motion and 1.1 x10-6mm3/Nm under multi-directional motion [2]. Though greater than this latter value, the all-polymer wear factors were not excessively high and were less under reciprocation-only. How much multi-directional motion, or cross-shear, it is appropriate to apply to a wear simulation of an artificial knee joint is worth further investigation, as it may be much less than in the hip joint.