Bone mineral lies mainly outside collagen fibrils: predictions of a composite model for osteonal bone.

We propose that the elastic properties of osteonal bone can be modeled accurately as a simple fiber-reinforced composite, provided that accurate properties for the mineral and collagen phases of the ultrastructure are available. Off-axis stiffness coefficients were measured in anterior quadrant of canine femora at 10 degrees increments from longitudinal to transverse direction using an acoustic microscope. The resolution of these measurements was about 60 microns or less than the radius of one osteon. The bone specimens were subsequently demineralized and the off-axis measurements were repeated to determine the elasticity of bone collagen. Bone collagen fibrils were not principally aligned along the long axis of the bone, but demonstrated an alignment that was 30 degrees from the long axis. A simple composite model was developed based on the experimental data. The model that best fit experimental data assumed that (1) bone collagen was aligned 30 degrees from the long axis of the bone, (2) 75% of mineral crystals reside outside of collagen fibrils, and (3) mineral crystals outside of collagen fibrils have their c-axis in the longitudinal direction.