Evolution during growth of the mechanical properties of the cortical bone in equine cannon-bones

Two specimens (70.0 x 4.5 x 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 x 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 (Sb = 226 +/- 27 MPa), average Young's modulus (E = 16 +/- 2 GPa) and average yield stress (Sy = 110 +/- 23 MPa) than the cranial quadrant (Sb = 209 +/- 23 MPa, E = 15 +/- 2 GPa, Sy = 99 +/- 18 MPa) while the caudal quadrant gave the lowest values (Sb = 195 +/- 30 MPa, E = 14 +/- 2 GPa, Sy = 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.