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 q uadrants at mid-diaphysis of the third metacarpus and metatarsus of Fr ench saddle horses (12 males and seven females) aged from I day to 4 y ears. The mechanical properties (bending strength, Young's modulus, yi eld stress and ultimate specific deflection) were determined by a 4-po int bending test, loading at a rate of 166 x 10(-6) ms(-1). During gro wth, the mechanical properties of the cortical bone were not significa ntly 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 h ad greater average bending strength (S-b = 226 +/- 27 MPa), average Yo ung'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 a nd the yield stress were Positively correlated with age (p < 0.01) and the total body weight (p < 0.001) of the horses. Conversely, the ulti mate specific deflection decreased slightly during the same period. Th e mechanical properties of the cortex were also related (p < 0.05) to the mineral content (ash or calcium). The Young's modulus was particul arly correlated to calcium content (p < 0.0001). It is also linearly r elated to the bending strength (r = 0.8), and its in vivo determinatio n by tile ultrasound method should provide an easy and non-invasive in vestigation means of the mechanical properties of the cortical bone in equine cannon-bones.