Towards the standardization of in vitro load transfer investigations of hip prostheses

Many types of hip prosthesis exist on the market, and no standard exists for testing their functionality. There is a lack of agreement in the literature about the relevance of several design parameters on the load transfer between the femoral stem and bone. This can be explained considering the lack of investigations on the methodology of load transfer measurements. A testing protocol for such purpose must fulfil three requirements: it must be physiological, it must be repeatable and reproducible and it must be sensitive to different prostheses. A methodological investigation is presented that was necessary to design a repeatable and reproducible protocol. The loading and constraints were derived from physiological data. The set-up and protocol were enhanced by means of (a) selection of an appropriate test specimen, (b) definition of reproducible reference systems, (c) sensitivity analyses on the sources of experimental error and (d) modification of the loading system to minimize the sources of error. The variability of results was as low as 0.8 per cent for repetitions on the same specimen, and 11 per cent between femurs. The capability of the protocol of detecting differences between stems was tested on different prostheses, including a prototype device. Significant differences between the load transfer patterns for the different designs were detected. The prototype device yielded satisfactory results.