The biomechanical effect of changes in cancellous bone density on synthetic femur behaviour

Biomechanical researchers increasingly use commercially available and experimentally validated synthetic femurs to mimic human femurs. However, the choice of cancellous bone density for these artificial femurs appears to be done arbitrarily. The aim of the work reported in this paper was to examine the effect of synthetic cancellous bone density on the mechanical behaviour of synthetic femurs. Thirty left, large, fourth-generation composite femurs were mounted onto an Instron material testing system. The femurs were divided evenly into five groups each containing six femurs, each group representing a different synthetic cancellous bone density: 0.08, 0.16, 0.24, 0.32, and 0.48 g/cm3. Femurs were tested non-destructively to obtain axial, lateral, and torsional stiffness, followed by destructive tests to measure axial failure load, displacement, and energy. Experimental results yielded the following ranges and the coefficient of determination for a linear regression (R2) with cancellous bone density: axial stiffness (range 2116.5-2530.6N/mm; R2 = 0.94), lateral stiffness (range 204.3-227.8N/mm; R2 = 0.08), torsional stiffness (range 259.9-281.5N/mm; R2 = 0.91), failure load (range 5527.6-11 109.3 N; R2 = 0.92), failure displacement (range 2.97-6.49 mm; R2 = 0.85), and failure energy (range 8.79-42.81 J; R2 = 0.91). These synthetic femurs showed no density effect on lateral stiffness and only a moderate influence on axial and torsional stiffness; however, there was a strong density effect on axial failure load, displacement, and energy. Because these synthetic femurs have previously been experimentally validated against human femurs, these trends may be generalized to the clinical situation. This is the first study in the literature to perform such an assessment.