Elastic anisotropy and collagen orientation of osteonal bone are dependent on the mechanical strain distribution

Abstract

There is evidence that the collagen microarchitecture of bone is influenced by mechanical stresses or strains. We hypothesized that peak functional strains correlate with the elastic anisotropy and collagen orientation of bone tissue and that the bone anisotropy might be changed by altering the strain patterns in canine radii for 12 months. We tested these hypotheses in studies using nine adult foxhounds. The baseline group (n = 3) had three rosette strain gauges placed around the midshaft of the radius, and strain distributions were measured during walking. The osteotomy group (n = 3) had 2 cm of the ulna surgically removed, and the sham group (n = 3) received a sham osteotomy. The osteotomy and sham groups were allowed free movement in cages with runs for 12 months, after which strain distributions were measured on the radii during walking. Bone-tissue anisotropy and collagen architecture were measured in radii from which the in vivo longitudinal strain patterns had been measured. The collagen birefringence patterns were measured with use of a circularly polarized light technique, and the elastic anisotropy of the bone, mineral, and collagen matrix was evaluated with a novel acoustic microscopy technique. Peak longitudinal strains in the radius correlated with the normalized longitudinal structure index (a polarized light measure of collagen birefringence) and the tissue anisotropy ratio. The average anisotropy ratio was 1.28+/-0.01 in the posterior (compressive) cortex and 1.43+/-0.01 in the anterior (tensile) cortex (these values are significantly different at p < 0.0001). The ulnar osteotomy changed the strain pattern on the radius, causing increased tensile strains in the medial cortex by more than 5-fold that were associated with a significant increase in the anisotropy ratio in the bone tissue. The longitudinal structure index was strongly correlated (r = 0.62, p < 0.005) with the anisotropy ratio of demineralized bone but was not correlated with that of deproteinized bone; this indicates that it reflects collagen fibril orientation in the bone matrix. These results indicate that mechanical strains affect both collagen and mineral microarchitecture in bone tissue, i.e., tensile strains are associated with increased tissue anisotropy and compressive strains, with decreased anisotropy.