Abstract 9

STRAIN IN THE PROXIMAL FEMUR: THE EFFECT OF EXETER STEM IMPLANTATION AND POSITION

C. Bell, J. Ciampolini, P. Weinrauch, M. Pearcy, R.Crawford.

Queensland University of Technology, School of Engineering Systems

This investigation has been designed to determine the strain patterns in the proximal femur for an intact non-implanted native femur (three specimens) and to compare this to the strain of the proximal implanted femur for both neutral (five specimens) and varus (four specimens) stem geometries.

Sawbones femurs were implanted with Exeter stems. Implant position with respect to the femur was controlled using specially designed positioning devices. Implanted specimens were instrumented with strain rosettes positioned along the lateral and medial aspect of the femur with the third rosette positioned at the medial calcar. A static anatomical loading vector of 650N was applied to the head of the implant for 7½ minutes. Data was gathered during the application of the load and for a further 7½ minutes following the removal of the load.

There was a significant variation in longitudinal strain at the medial calcar with a strain of -1800µå observed on the native femur specimens and -430µå and -360µå measured on the neutral (P=0.04) and varus (P=0.02) stem position specimens respectively. However, there was no significant difference between the strains of the neutral and varus stem position specimens in the longitudinal direction at the calcar. The magnitudes of the circumferential strains reduce for both the neutral and varus stem positions when compared to the tensile native femur strain. The similarity of the strain at the medial calcar for the neutral and varus specimens may explain why varus malposition of the Exeter stem has not been associated with adverse clinical outcomes.

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