Wear in metal-on-metal total disc arthroplasty.

The wear of a model metal-on-metal ball-and-socket total disc arthroplasty was measured in a simulator. The ball had a radius of 10 mm, and there was a radial clearance between ball and socket of 0.015 mm. The model was subjected to simultaneous flexion-extension, lateral bending, axial rotation (frequency: 1 Hz) and compression (frequency: 2 Hz, maximum load: 2 kN). Throughout the tests, the models were immersed in calf serum diluted to a concentration of 15 g protein per litre, at a controlled temperature of 37 °C. Tests were performed on three models. At regular intervals (0, 0.5, 1, 2, 3, 4 and 5 million cycles), mass and surface roughness were determined; mass measurements were converted into the volume lost as a result of wear. All measurements were repeated six times. Wear occurred in two stages. In the first stage (duration about 1 million cycles), there was a linear wear rate of 2.01 ± 0.04 mm(3) per million cycles; in the second stage, there was a linear wear rate of 0.76 ± 0.02 mm(3) per million cycles. Surface roughness increased linearly in the first million cycles and then continued to increase linearly but more slowly.