Evolution during growth of the mechanical properties of the cortical bone in equine cannonbones

Abstract

Two specimens (70.0 × 4.5 × 1.8 mm) (proximal and distal) of cortical bone were taken from each of the cranial, caudal, lateral and medial quadrants at mid-diaphysis of the third metacarpus and metatarsus of French saddle horses (12 males and seven females) aged from 1 day to 4 years. The mechanical properties (bending strength, Young's modulus, yield stress and ultimate specific deflection) were determined by a 4-point bending test, loading at a rate of 166 × 10 −6 ms −1. During growth, the mechanical properties of the cortical bone were not significantly different ( p > 0.05) between metacarpus and metatarsus, but they were slightly higher in the proximal than in the distal diaphysis. The variations in these properties were significant ( p < 0.0001) between quadrants. From birth to adult age, the lateral and medial quadrants had greater average bending strength ( S b = 226 ± 27 MPa), average Young's modulus ( E = 16 ± 2 GPa) and average yield stress ( S y = 110 ± 23 MPa) than the cranial quadrant ( S b = 209 ± 23 MPa, E = 15 ± 2 GPa, S y = 99 ± 18 MPa) while the caudal quadrant gave the lowest values ( S b = 195 ± 30 MPa, E = 14 ± 2 GPa, S y = 92 ± 21 MPa). During the growing period, the bending strength, the Young's modulus and the yield stress were positively correlated with age ( p < 0.01) and the total body weight ( p < 0.001) of the horses. Conversely, the ultimate specific deflection decreased slightly during the same period. The mechanical properties of the cortex were also related ( p < 0.005) to the mineral content (ash or calcium). The Young's modulus was particularly correlated to calcium content ( p < 0.0001). It is also linearly related to the bending strength ( r = 0.8), and its in vivo determination by the ultrasound method should provide an easy and non-invasive investigation means of the mechanical properties of the cortical bone in equine cannon-bones.