Introduction: Reduction of ultra high molecular weight polyethylene (UHMWPE) wear in total knee replacement (TKR) bearings may delay the onset of osteolysis and subsequent loosening of components. This study used finite element (FE) modelling and in vitro simulator testing to investigate the effect of wear path geometry on UHMWPE surface wear.

Methods: The wear of PFC Sigma fixed bearing TKRs (DePuy) was investigated using a six-station force/ displacement controlled knee simulator (frequency 1 Hz) using previously developed methods [1]. High, intermediate and low kinematic inputs were simulated for up to five million cycles (Table 1) with identical flexion-extension and axial loading for all components. This kinematic data was also applied to a FE model of the PFC Sigma TKR and run using PAM-CRASH-SAFE software. The anterior-posterior (AP), medial-lateral (ML) and inferior-superior data were recorded and the resulting wear paths generated by selecting nodes from the contacting surface of the polyethylene relative to the femoral.

Results and Discussion: The mean wear rates with 95% confidence limits on the simulator when subjected to high, intermediate and low kinematics were 22.75 ± 5.95, 9.85 ± 3.7 and 5.2 ± 3.77 mm3 per million cycles, respectively. All FE models exhibited looped wear paths. An example wear path for the first 60% of the gait cycle for a lateral node is displayed in Figure I. The high kinematics model generated the greatest ML displacement and similar AP displacement to the intermediate kinematics model. The low kinematics model showed least ML and AP displacements. The AP displacements for medial wear paths differed little when subjected to the different kinematics. A looped wear path on the surface of UHMWPE results in greater cross shear transverse to the principal direction of motion, which is parallel to AP displacement in TKR and is the axis along which strain hardening occurs. This study revealed that increased AP displacement and tibial rotation kinematics generate more looped wear paths, increase ML and AP displacements on the surface of fixed bearing TKR and result in greater cross shear which ultimately increases UHMWPE surface wear.