The anisotropic Young's modulus of equine secondary osteones and interstitial bone determined by nanoindentation

The equine radius is a useful subject for examining the adaptation of bone histology to loading because in life the anterior cortex is loaded almost e ntirely in tension, the posterior cortex in compression. The histology of t he two cortices is correspondingly different, the osteones and the intersti tial lamellae in the posterior cortex having a more transversely oriented f ibre arrangement than those in the anterior cortex. Presumably as a result of this histological difference, the posterior cortex is stronger in compre ssion than the anterior cortex; the anterior cortex is stronger in tension than the posterior cortex. We here use nanoindentation to examine how the Y oung's modulus of elasticity of secondary osteones and interstitial lamella e in the anterior and posterior cortices varied as a function of angle. The anterior osteones were stiffer than the posterior osteones when tested in the direction parallel to the bone's long axis, but became progressively relatively less stiff as the angle increased; at 90 degrees, they were les s stiff than the posterior osteones. Although the interstitial lamellae wer e stiffer than their neighbouring osteones, the same relationship between a nterior and posterior interstitial lamellae as a function of angle was foun d as for the osteones. The anisotropy of these Young's moduli determined by nanoindentation shows a close relationship with what was to be expected fr om the histological findings.