The objective of this study was to determine the influence of articular surface motion and contact area on wear of Ultra High Molecular Weight Polyethylene (UHMWPE) in orthopaedic bearing systems. A particular focus was on the clinical, mechanical, and material factors associated with surface degradation in total knee replacements (TKR).

Five studies were undertaken to investigate specific aspects of the problem, both under controlled conditions in vitro and through clinical studies. In the first study, a standardized method was developed for the quantitative assessment of surface degradation in retrieved TKR. The technique also provided a qualitative assessment of the patterns of wear associated with specific clinical features such as lower limb alignment.

The second study examined the influence of UHMWPE microstructure on surface degradation features from retrieved knee implants and in vitro wear testing using a customized nine-channel reciprocating tester. Two forms of pitting were observed:​ one associated with surface abrasion, and one related to a fatigue process initiated at interparticle boundaries at the surface.

A third study examined the articular surface motion of a commercial mobile bearing TKR design. The relative motion between femoral and tibial components was highly sensitive to the placement of the femoral component with respect to ligamentous structures. The results were consistent with clinically observed failure modes of this TKR design.

The results of the first three studies suggested that the kinematic condition of relative surface motion was a significant influence on UHMWPE degradation in orthopaedic hearings. This hypothesis was tested in the fourth study in which UHMWPE-Cobalt Chrome wear couples were evaluated under three conditions:​ (1) Sliding, in which the contact point on the polymer remained stationary;​ (2) Gliding, in which the contact point on the polymer moved, and (3) Rolling, in which the contact point on the polymer moved at the same speed as the contact point on the metal. The results indicated that wear was an order of magnitude greater when the contact point in the polymer moved.

A final study was undertaken to determine whether results similar to those observed in the in vitro studies could be observed clinically. Wear in conventional fixed-bearing TKR and mobile bearing TKR were compared in a study of 89 retrievals. Fixed-bearing designs, which incorporate rolling and sliding, showed a predominance of fatigue wear mechanisms. In contrast, mobile bearing designs which have two independent sliding interfaces showed a predominance of adhesive mechanisms.

This work indicates that while wear in UHMWPE in total joint replacements is a multifactorial problem, this wear is strongly influenced by the design of the articular surfaces and that an optimum surface shape may exist for a given set of biomechanical conditions.