Wear and rim damage of UHMWPE acetabular cups in total hip replacement

Wear and fatigue of polyethylene acetabular cups have been reported to play a role in the failure of total hip replacements. Edge loading of hip replacements can occur where there is sub-optimal component positioning and/or joint laxity. Wear resistance can be improved by crosslinking but the manufacturing process of these materials involves post-irradiation thermal treatments to recombine free radicals and to stabilise the materials. Stabilisation can also be achieved by adding antioxidants. Material degradation due to oxidation and manufacturing process can result in rim cracking and/or fracture due to a reduction in mechanical properties and this has been observed in vivo. A requirement for pre-clinical hip simulator testing under edge loading conditions for all of these materials has therefore been identified.
This thesis describes the development and evaluation of a hip simulator edge loading protocol using accelerated aged conventional UHMWPE acetabular liners as positive controls and commercially available crosslinked UHMWPE acetabular liners as negative controls. The edge loading protocol was then used to evaluate antioxidant stabilised liners in hip simulator tests. Explanted UHMWPE acetabular liners were evaluated for wear and damage mechanisms and compared with the damage observed on the hip simulator tested liners and new methodologies were developed to measure and analyse these explanted liners.
The edge loading protocol produced cracking and subsurface damage in the aged UHMWPE liners but not in the non-aged crosslinked liners. Rim deformation was observed on all liners and the volume change produced was reduced under edge loading conditions for both types of UHMWPE liner. The antioxidant liners performed as well as the commercially available crosslinked liner in hip simulator tests and the rim deformation that was observed on explanted liners was replicated under edge loading conditions in the hip simulator tests.
The edge loading protocol can be used in the future to test a range of UHMWPE materials, including aged materials, and explant analysis using the methodologies developed in this study can be used to inform the design of future simulator tests.