[The quality of bone]

The major determinant factors for the mechanical property of bone include the mass, structure and "quality" of bone. The quality indicates the factors which affect the strength of bone other than the mass and the structure. In the spinal cancellous bone, the power-law relationship has been observed and the bone strength increases in proportion to the square (the power number 2) of the bone mass value. The power number represents the quality of cancellous bone, including various factors such as the micro-structure at the tissue level and the physico-chemical bonding at the molecular level. For the long cortical bone, the number or density of the cracks in the tissue represents the quality. The cortical bone strength decreases exponentially with the increase in the number of cracks in the tissue. The increase in the number of cracks with age is observed in the human skeleton. When a crack is made in the bone, the osteoclasts invade the region, refreshing the damaged tissue. Then, the osteoblasts are recruited to appose the new bone tissue, filling the gap made by the crack. Thus, the "quality" of bone is regulated by the bone cell functions as well as by the mass and the structure. Therefore, the regulation of the bone quality is another important target for bone cell biology.