Microscratching Characterization of the Wear-in of UHMW Polyethylene in Prostheses

A new measurement technique of prosthesis wear by microscratching has been demonstrated. The technique has been applied in a study of the backside wear of a UHMW polyethylene tibial insert of a rotating platform knee prosthesis. Four disc-shaped UHMW polyethylene plugs, prepared with 5-μm deep microscratches, were carefully recessed into the backside of the tibial insert. It was demonstrated that the scratches are not affected by creep under static load. A realistic in vitro wear simulation experiment was performed over 0.8 × 10⁶ flexion cycles. SEM and AFM show that following the experiment the initial microscratches are effectively absent in all four locations with only residual depressions observed. This implies that typically at least 5 μm of polyethylene material was worn over the first 0.8 × 10⁶ cycles by processes other than creep. Evidence from AFM and SEM indicates the in-fill and reintegration of polyethylene wear particles into residual scratch depressions. This supports a two-phase model of the wear process that has been independently confirmed by radioisotope tracing. For an initial wear-in phase, the model implies large, but rapidly decreasing, wear, resulting from abrasive wear and a competition between the loss and the reintegration of wear particles. The in vitro wear-in of the backside may typically produce a wear debris volume of 8.5 mm³. In addition to wear-in, the model assumes a much lower, constant long-term wear rate. The model has been used to correct published backside wear rates for the effects of the wear-in phase. A best estimate of 0.7 mm³/10⁶ cycles has been determined for the long-term in vitro backside wear rate of a tibial insert in a rotating platform design.