Abstract 28

INFLUENCE OF TESTING CONDITIONS ON SURFACE SHEAR STRAIN IN LONG BONE TORSIONAL TESTS

Pelletier, M H; Okamoto, K; Walsh, W R

Orthopaedic Research Laboratories, University of New South Wales, Prince of Wales Hospital, Sydney, NSW, Australia

This study was designed to examine the cortical shear strain distributions during torsional testing (2.5Nm) under load or displacement control and internal or external rotation in rabbit femurs. A 1.7mm thick photoelastic coating was bonded to the surface of seven adult rabbit femora. Specimens were mounted in a servo-hydraulic testing machine such that the anatomical axis of the femur was collinear with the testing machine's rotational axis. A zero load and displacement state was achieved at the beginning of each test. Four loading protocol were applied utilizing axial displacement or axial force control feedback mode in internal or external rotation. Photoelastic measurements were taken at three points along the anterior and posterior diaphyses, the measurements were normalized by the maximum shear strain in each bone and grouped (Anterior, Posterior). The results were then analyzed by ANOVA. Displacement controlled tests produced tensile loads of 51.7N for internal and 34.8N for external rotation. Shear strain was statistically higher during external versus internal rotation posteriorly and statistically lower anteriorly. The anterior surface of the femur also showed an increase in the surface strains using displacement control versus load control. Although not statistically significant all displacement control strains were higher than their load control counterparts. The differences noted are likely due to the geometry of the anterior and posterior femur, these differences suggest that direction of rotation should be matched by side, i.e. internal or external rather than simply positive or negative torque, for all torsional testing to control for potential confounding variables.

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