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

The research explores the differing stiffness properties of bone structures in the anterior and posterior cortices of the equine radius, specifically studying the modulus of elasticity in secondary osteones and interstitial lamellae. The study found a correlation between the bone’s stiffness and the angle of examination, with the anterior elements being stiffer longitudinally but less so laterally.

Subject and Methods

• The study uses the equine radius, a bone in the foreleg of a horse, as a subject for examination due to its distinct loading characteristics in life where the anterior cortex is mainly loaded in tension and the posterior cortex in compression.
• Such unique loading conditions lead to histological differences in the two cortices, with the fiber arrangement in the osteones and interstitial lamellae of the posterior cortex being more transverse than those in the anterior cortex.
• The test of the bone’s stiffness was conducted via a technique known as nanoindentation, which measures the mechanical properties of small volumes of materials.

Findings

• The study found that the anterior osteones were stiffer when tested in the direction parallel to the bone’s long axis. However, as the angle increased, they progressively become less stiff. At 90 degrees, the anterior osteones were less stiff than the posterior osteones.
• The interstitial lamellae, though stiffer than the osteones in their vicinity, demonstrated the same pattern of stiffness variation with the angle of testing.
• Both types of bone structures in the anterior cortex showed greater strength under tension, while those in the posterior cortex were stronger under compression, corresponding with the in vivo loading conditions of the equine radius.

Conclusion

• The study concludes that the anisotropy (directional dependence) of the Young’s moduli (a measure of stiffness) determined by nanoindentation shows a close relationship with what was expected from the histological findings. The alignment and arrangement of fibers in the bone play an important role in its material properties.
• The anterior osteones and interstitial lamellae were discovered to be stiffer in the direction of the bone’s long axis, reflecting their functional adaptation in resisting tension in real life loading conditions.
• The research findings contribute to the broader understanding of bone biomechanics and can be applied to develop medical or veterinary treatments and interventions that cater to the detailed mechanical behavior of bone at the microscopic level.