Zirconia (ZrO2) on Polyethylene (PE) has been used for18 years. However, a majority of clinical results expressed caution, citing surprisingly high wear and osteolysis. The most recent clinical study (Walters 2004) reports 20% higher wear with ZrO2/PE compared to CoCr/PE The PE wear may be the result of increased surface roughness of the ball, due to zirconia transformation from tetragonal to monoclinic phase. Impingement of metal cup backing on zirconia may result in high stress that drives the transformation. Our objective was to simulate the conditions most likely to transform the zirconia ball surface.

In phase one, the rim of a titanium cup was loaded against a zirconia ball with a static load range of 0.01kN to 10kN to simulate impingement-dislocation stress. The ball was cleaned with acid, ultrasonic cleaning, and then observed under SEM. Ball sections were made for XRD study. In phase two, the zirconia balls were pressure stressed in a hip simulator (static load range of 1kN to 4kN) for 600 cycles. Wear studies follow.

In phase one, the rim of a titanium cup was loaded against a zirconia ball with a static load range of 0.01kN to 10kN to simulate impingement-dislocation stress. The ball was cleaned with acid, ultrasonic cleaning, and then observed under SEM. Ball sections were made for XRD study. In phase two, the zirconia balls were pressure stressed in a hip simulator (static load range of 1kN to 4kN) for 600 cycles. Wear studies follow.

Pressure alone does not appear to transform zirconia in the articular surface. The phase two simulator studies include combinations of mechanical stress, environmental aging (autoclave) and lubricants (air, water, serum). The role of the heavy metal transfer may actually protect the zirconia surface from transformation phenomena.