Tribometer with programmable motion and load to investigate the influence of molecular structure on wear of orthopaedic polyethylene

Abstract

Total hip arthroplasty commonly involves a hard metallic/ceramic femoral ball component articulating against an acetabular ultra-high molecular weight polyethylene (UHMWPE) counter-bearing. A novel six-station, wear tribometer, featuring programmable load and motion, was designed to further the investigation into wear, sub-surface plasticity and debris generation of UHMWPE. This thesis describes the pin-on-plate device, its validation and subsequent use to assess unirradiated (-PE) and gamma-irradiated highly crosslinked (+PE) UHMWPE wear behaviour. With the emphasis on dynamic loading and a closer gait matched open wear path, the tribometer improves on the clinical relevance of pin-on-plate testing. 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. For the first time in orthopaedic pin-on-plate studies, tests were conducted using an advanced dynamic load synchronised to a more physiologically accurate elliptical motion path. To validate the machine, three orthopaedic polymers of clinical relevance; Polytetrafluroethylene, Polyacetal, and UHMWPE were subjected to linear-reciprocating (LR) and novel elliptical motion paths under a Paul-type load profile. All three polymers showed higher wear factors under elliptical motion, by up to 2 orders of magnitude, agreeing well with explanted values. The UHMWPE elliptical wear factor was comparable to that reported for clinical, where kelliptical = 1.56 x 10-6 mm3/Nm. In the crosslinked study, the mean steady state wear of -PE and +PE groups under linear reciprocating motion was not significantly different. However, under elliptical motion, crosslinking reduced UHMWPE wear by up to 92% when compared to the unirradiated group. In –PE pins worn under LR motion and in +PE pins subjected to both motion paths a sub-surface damage zone with reduced crystallinity and increased strain was measured using Raman spectroscopy. This was attributed to large strain accumulation in the slower wearing surfaces providing a mechanism for de-crystallisation. The discovery of such a near-surface layer is in good agreement with critical strain wear models. In disagreement, however, we found the sliding induced layer to extend to greater depths than previous reported.