Design and Development of a Pre-Clinical Test for Cemented Femoral Hip Replacements

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Abstract

Current state of the art testing methods do not allow implant performance to be assessed relative to known failure modes, prior to clinical use. Even clinically, it can take up to 5 years before implant performance can be readily quantified. This situation is not desirable for either the surgeons who have to chose from amongst the wide array of implants on the market, or for the engineers who are effectively selling a device of un-proven performance.

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 Muller 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 Muller Curved prostheses subsided significantly more than the Lubinus SPII prosthesis at p = 0.05 level o f significance, 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 Muller prosthesis, evaluated at 1 million cycles, compared to the Lubinus prosthesis was quantified. Those prostheses with a tendency to migrate by a comparatively large amount in any direcfion 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 trails are begun.

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Cited By (2)

Year	Entry
2001	Murphy BP. Aspects of the Fatigue Behaviour of Acrylic Bone Cement [PhD thesis]. Trinity College Dublin; May 2001.
2004	Britton JR. Failure of Cemented Hip Implants Under Complex Loading: Experimental and Numerical Analysis [PhD thesis]. Trinity College Dublin; March 2004.