Design and test of novel tribometer: Investigation into wear performance of materials used in total hip replacements

Abstract

A six-station, programmable load and motion wear machine was designed to further the tribological investigation into wear, subsurface plasticity, and debris generation of ultra high molecular weight polyethylene (UHMWPE). This paper describes the pin-on-plate device and validation protocol in accordance with ASTM F732-00 (2006). Unlike conventional pin-on-plate wear testing machines which commonly statically load a UHMWPE pin against a metallic/ceramic rotating disc or sliding plate, this new design incorporates programmable dynamic loading and motion profiles. There is a requirement for this type of machine in order to investigate the directional dependence of wear and debris generation of UHMWPE more accurately, where ‘simplified’ tribometers (adequate for constant load/constant velocity and constant load/sinusoidal velocity work) are not capable or suitable. To validate the machine, three orthopaedic polymers of clinical relevance: polytetrafluroethylene, polyacetal and UHMWPE were subjected to linear reciprocating and novel elliptical motion paths under a Paul type load profile. All three polymers showed higher wear factors under elliptical motion, by up to two orders of magnitude, agreeing well with explanted values. The UHMWPE elliptical wear factor was comparable to that reported for clinical, where K_elliptical=1·56×10^–6 mm³ Nm^–1. Wear rankings agreed with the ASTM standard: polytetrafluroethylene>>polyacetal≥UHMWPE. However, only under elliptical motion were the wear rate ratios comparable in magnitude to published clinical data.