Wear Simulation of Total Disc Arthroplasties: Sensitivity to Device Design and Test Parameters

The challenges of measuring in vivo total disc replacement (TDR) kinematics are well recognized, meaning that it is difficult to establish appropriate input conditions for wear simulation. Therefore it is desirable to ascertain the sensitivity of implant wear in vitro to perturbations of the kinematics and other testing parameters. It has previously been demonstrated in other metal-on-polyethylene joint replacements that cross-shear strongly influences wear rate. This study investigates this phenomenon by altering the phasing of the inputs by making the lag in the flexion-extension and lateral bend displacements zero. Further, the effect of an additional anterior-posterior shear, which has been reported in vivo, was investigated for two different TDR designs using an extra load or displacement input in addition to those prescribed by the standard ISO 18192-1. Altering the standard ISO 18192-1 waveform phasing significantly reduced the mean wear rate of the constrained polyethylene disc. The addition of an anterior-posterior input showed no significant change in the rate of wear for the constrained TDR but was increased for the unconstrained device. These data demonstrate the strong dependency of the wear in these types of joints to the input conditions as well as the devices design parameters. Hence, these factors should be given prime consideration when designing both the device itself and the assessment regime in which the construct is to be tested.