Effects of Applied Stress on Preferential Alignment of Biological Apatite in Rabbit Forelimb Bones

Effects of stress distribution and its change in vivo on density and orientation of biological apatite (BAp) were investigated in rabbit forelimb bones. Bone mineral density (BMD) and BAp alignment were analyzed by a peripheral quantitative computed tomography (pQCT) and a micro-beam X-ray diffraction system, respectively. The intact original rabbit forelimb bone consisting of an ulna and a radius exhibited a one-dimensional preferential alignment of the c-axis of BAp along the longitudinal direction, but the degree of orientation depended strongly on the distance from the neutral point on the bone cross section. This suggests that the BAp alignment is sensitively controlled by the bending stress in addition to the axial stress along the long bone. In the rabbit model with a 10mm defect on the ulna, the BAp alignment as well as BMD in the ulna sensitively changed depending on the loading condition during the bone healing in the defect. The BAp alignment decreased and increased during unloading and reloading, respectively. Therefore, it is concluded that not only BMD but also BAp alignment changes depending on the applied stress on the basis of the functional adaptation in bones.