Assessment of cortical bone elasticity and strength: Mechanical testing and ultrasound provide complementary data

Cortical bone is a compact tissue with anisotropic macroscopic mechanical properties determined by a microstructure and the quality of a mineralised collagen matrix. Anisotropic elastic properties and strength are usually measured on different groups of sample which can hardly be pooled; as a consequence little is known on the relationships between strength and elasticity in the different anatomical directions. A method is presented to measure on a same cortical bone sample: (1) Young's modulus and strength (σmax) in the longitudinal direction; (2) stiffness (C11) in the transverse direction. Longitudinal and transverse direction are taken along and perpendicular to the diaphysis axis, respectively. Ultrasonic techniques yield Young's modulus (Ea) and C11; three-point bending tests yield Young's modulus (E) and σmax. The relationships between strength, elasticity and density and their anatomical distributions were investigated for 36 human femur samples. (i) A marginal negative correlation was obtained for Ea and C11 (R = −0.21; p = 0.08); (ii) σmax was significantly correlated to E and Ea (R ∼ 0.5; p < 0.005) but not to C11 (p > 0.2); (iii) density was not correlated with E and moderately with strength (R = 0.38; p < 0.3). Small density variability (±30 kg m−3) may partly explain the results. The techniques presented are suited to a systematic characterization of bone samples.