Design and development of a pre-clinical test for cemented femoral hip replacements.

Femoral hip implants loosen. The rate at which they loosen is determined by many factors, but dominated by implant design. Loosening has been correlated with the migration measured at 2 years post-operatively. This thesis presents a pre-clinical testing protocol which measures prosthesis migration, migration rates, and inducible displacement (i.e. amplitude of motion) in six degrees-of-freedom, for 2 million cycles. Using this protocol, the performance of two prostheses known to loosen at different rates clinically are assessed: the Müller Curved prosthesis, which has a high revision rate, and the Lubinus SPII prosthesis, which has a low revision rate.

The implants tested migrated at a relatively fast rate for the first 0.2 million cycles then migrated at a lower rate thereafter. The Müller Curved prostheses subsided significantly more than the Lubinus SPII prosthesis at p = 0.05 level of significant, after 2 million loading cycles, and at p = 0.04 after 1 million loading cycles. Qualitatively, this mirrors the differences expected clinically. A three-fold increase in the steady state migration rate of the Müller prosthesis, evaluated at 1 million cycles, compared to the Lubinus prosthesis was quantified. Those prosthesis with a tendency to migrate by a comparatively large amount in any direction were found to have a high inducible displacement in that direction.

This experimental test has been proven to differentiate between what may be termed a 'poor' prosthesis design that is prone to early loosening, and a 'good' design that is not. The testing protocol is therefore proposed as a suitable method for assessing between prosthesis performance in vitro before clinical trials are begun.