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

Fourteen pairs of explanted low contact stress (LCS) tibial interface compo nents: six rotating platform (RP), six meniscal (MN) and two anterior-poste rior (AP) glide designs, have been analysed with particular attention paid to the condition of the tibial counterfaces. The average surface roughness, R-a, for the tibial trays ranged from 0.01 to 0.087 mum, significantly gre ater than the unworn control measurement of 0.008 mum. 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 mum in height (aver age asperity height R-p = 0.52 mum, aspect ratio Deltap = 0.01, average asp erity depth R-v = 1.10 mum. Deltav = 0.05). The largest scratches were 3-4 mum in both R-p and R-v. In vitro tests have shown that ultra-high molecular weight polyethylene (UH MWPE) wear increases in the presence of counterface scratches perpendicular to the direction of motion. In these explants, the unidirectional motion p roduced scratches parallel to the direction of sliding which is predicted t o produce a smaller increase in UHMWPE wear. Other designs in mobile bearin g 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 w ear in the presence of third body scratches. It may be possible in future k nee designs to reduce this type of wear damage by introducing alternative m aterials or coatings which are more resistant to scratching and surface rou ghening.