In vitro lateral versus medial wear of a knee prosthesis

A knee wear test was conducted in which the wear of the medial and lateral tibial components was measured separately to determine the medial–lateral (M–L) wear distribution and gain insight into the operating wear mechanisms. This was achieved by pairing and wear testing as a total knee, six medial and six lateral low constraint unicondylar ultrahigh molecular weight polyethylene (UHMWPE) prosthetic components in a displacement-controlled wear simulator. The medial and lateral components were found to have substantially different cumulative gravimetric wear rates, 6.58±0.50 and 2.93±0.29 mg per million cycles, respectively. This corresponds to 124±25% more wear on the medial component, despite similar loading on both components. This differential medial–lateral wear appears to be connected with the well-established sensitivity of the UHMWPE wear rate to multidirectional shear effects. A wear model was thus developed in which only the transverse sliding component contributes significantly to the wear of UHMWPE. The model was shown to be in good agreement with this observed differential medial–lateral wear. This transverse wear effect from the model accounts for 63% of the increased medial component wear. Taking into account the lengths of the wear tracks, this number increased to 88%, which is in good agreement with the observed value of 124±25% (standard error), ±65% (95% CI). Further work is required to refine the calculations of the tibio–femoral sliding motion paths and the transverse polyethylene wear model.