Structural and material changes in osteoporosis: Their impact on the mechanical environment of the osteocyte.

Abstract

The mechanism responsible for controlling the quantity and distribution of bone material is unknown, although many investigators believe that the osteocytes play a role. Osteocytes are located in an ideal location to sense the mechanical environment in the bone, and have been shown to respond to deformation and shear from fluid flow. Osteoporosis, a disease in which the quantity of bone is pathologically low, may be due to an error in the control of bone remodeling. The global hypothesis of this series of studies was that changes in the material properties of the bone at various hierarchical levels or the geometry of the osteocyte lacunae are different in osteoporotic and normal individuals, resulting in a change in the strain of the osteocyte, and a reduction in the strength of the bone. Bone properties were examined at various hierarchical levels, from the trabecular bone continuum to the tissue level, in six age and gender groups, seeking changes in the bone that parallel the risk of osteoporotic fracture. No clear conclusions were drawn, although several features were identified for future studies. Voxel-based finite element models verified the effect of the osteocyte lacuna geometry on the strain in and around the osteocyte. As a result, a separate study measured the three-dimensional geometry of lacunae using confocal microscopy images of trabecular bone from the proximal femur of fractured and normal women, but found no difference. The average mineral content of trabecular bone tissue in osteoporotic and normal individuals near a common fracture site in the proximal femur was found to be statistically similar. However, the osteoporotic individuals had a greater variability in mineral content, suggesting an increase in remodeling in some fractured individuals. Poroelasticity and fluid-solid homogenization were used to estimate the mechanical environment of the osteocyte, incorporating the effects of the bone fluid. The relative properties of the cell and the region between the cell and matrix were found to be particularly important.