A series of experiments was performed to evaluate the mechanical properties of the equine cannon (third metacarpal) bone from Thoroughbred racehorses. Monotonic and fatigue tests were conducted under four-point-bend loading in a 37°C saline bath. In total, 144 dorsal, medial and lateral mid-diaphyseal specimens from forty-two horses were tested. Testing included monotonic (deflection-control) and high strain range (10,000 microstrain) fatigue tests (load-control and deflection-control modes), and fatigue tests at various initial strain ranges (4000, 5000, 7000, 9000, 10,000 microstrain);​ and a post-test histologic evaluation of high strain range fatigue specimens. All fatigue tests were performed at 2 Hz., and most were conducted under load-control for a specified initial strain range. Damage accumulation was assessed by reduction in specimen modulus.

Initial monotonic and high strain range fatigue tests revealed significant regional variations in the mechanical properties. Lateral specimens were strongest, stiffest and exhibited the shortest fatigue lives. Dorsal specimens were weakest, least stiff and had the longest fatigue lives. Further testing showed that regional variations in life are consistent at various strain ranges, and deflection controlled tests produced expectedly higher fatigue lives than load controlled tests.

Modulus behavior over specimen fatigue life indicated that regional variations also exist in damage accumulation mechanisms of cannon bone tissue. The modulus degradation curves over specimen life were combined and averaged to produce damage surface plots for each anatomic region. When compared to fatigue data from the human femur, these data showed similar lives in lateral specimens, and substantially longer lives in dorsal and medial specimens.

Results from the microstructural evaluation suggested that specimens with more (secondary) osteons, higher osteonal density and greater cement line length exhibited lower modulus values and longer fatigue lives. And for the lateral region only, specimens with larger average osteon diameter had longer fatigue lives and lower moduli. Regional differences in fatigue damage accumulation mechanisms were also indicated by variations in the ratio of mechanical damage to cement line length.

From our experiments on the equine cannon bone, we’ve gained a better understanding of its variations in mechanical and histologic properties, and established some important relationships between these measurements.