Quantification of third body damage to the tibial counterface in mobile bearing knees

Abstract

Fourteen pairs of explanted low contact stress (LCS) tibial interface components: six rotating platform (RP), six meniscal (MN) and two anterior-posterior (AP) glide designs, have been analysed with particular attention paid to the condition of the tibial counterfaces. The average surface roughness, Ra, for the tibial trays ranged from 0.01 to 0.087 μm, significantly greater than the unworn control measurement of 0.008 μm. The scratch geometry analysis showed that the scratch peaks were found to be consistently of a lower aspect ratio than the scratch valleys and under 1 μm in height (average asperity height - Rp = 0.52 μm, aspect ratio Δ p = 0.01, average asperity depth Rv = 1.10 μm, Δ v = 0.05). The largest scratches were 3-4 μm in both Rp and Rv In vitro tests have shown that ultra-high molecular weight polyethylene (UHMWPE) wear increases in the presence of counterface scratches perpendicular to the direction of motion. In these explants, the unidirectional motion produced scratches parallel to the direction of sliding which is predicted to produce a smaller increase in UHMWPE wear. Other designs in mobile bearing knees have less constrained motion at the tibial counterface and this has been shown to accelerate wear; it may also lead to a further increase in wear in the presence of third body scratches. It may be possible in future knee designs to reduce this type of wear damage by introducing alternative materials or coatings which are more resistant to scratching and surface roughening.