The mechanical properties of equine third metacarpals as affected by age

Twenty-five pairs of equine third metacarpals (McIII) were collected from horses of varying breeds and ranging in age from 1 d to 27 yr. Standard three-point bending tests were performed on the McIII to obtain comparative data on breaking load (BL), breaking strength (BS), and elasticity (E). Broken bones were reassembled and 2-cm sections cut from the bone 1 cm below the nutrient foramen and at the site of the break. Bone mineral content (BMC) and geometrical properties of these sections were determined. Maximum BMC (17.3 g/2 cm) was reached at 6.0 +/- 1.8 yr of age. Horses in this study achieved 76% of maximum BMC by 1 yr of age. Maximum BL (1,888 kg) was reached at 4.6 +/- 1.8 yr of age. The coefficient of determination (r2) between BMC and BL was .92 (P < .001). Breaking strength peaked at 2,272 kg/cm2 at 6.3 +/- 1.2 yr of age. The correlation between BS and BMC was .84 (P < .0001). At 1 yr of age BS was 1,919 kg/cm2, or 85% of maximum BS. Elasticity increased linearly with log 10 age (r2 = .91). The r2 of BMC and E was .79 (P < .001). Area moment of inertia (I) of the cortex of the bone was best approximated by the area of an ellipse (r2 = .91; P < .01). Results suggest that bone strength in horses peaks after most race horses have completed their careers.