Physical characteristics of bone as a structural material. The collagen matrix resists tension and binds together the rigid crystalline component which resists compression and shear. The small size of the hydroxyapatite crystals contribites to the strength of the bone because fractures propogate more easily through a uniform rigid substance such as glass, than through a composite substance such as bone or fiberglass. In a composite substance, failure may be confined to one crystal and not propagate. It has long been known that bones are formed to best resist the stresses acting on them.
Bones can lengthen their lever arms or become thicker even after growth in length has ceased.
The mechanism of this remodeling is due to the response of bone cells to piezoelectric fields produced when bone is stressed as a result of its two phase construction. Bones as structural units. They are frequently hollow at the centers of their shafts where the main stress is a bending stress. A hollow tube is stronger, for its weight, than a solid tube of the same weight. Cross sections are shaped to best resist the strains encountered.