Bone Structure

Bone is a complex, highly organized and specialized connective tissue. It is characterized physically by the fact that it is a tissue that is hard, rigid and strong, and microscopically by the presence of relatively few cells and much intercellular substance formed of collagen fibers and stiffening substances [1,2].

Bone consists of 65% mineral, 35% organic matrix, cells, and water. The bone mineral is in the form of small crystals in the shape of needles, plates, and rods located within and between collagen fibers. The mineral is largely impure hydroxyapatite, Ca10 (PO4)6 (OH)2, containing constituents such as carbonate, citrate, magnesium, fluoride, and strontium incorporated into the crystal lattice or absorbed onto the crystal surface.

The rate of turnover of the skeleton approaches 100% per year in the first year of life, declining to about 10% per year in late childhood, and then usually continues at approximately this rate or more slowly throughout life. Much of the turnover of bone during growth results from bone-modeling, but presumably at least some remodeling also occurs. After the completion of skeletal growth, the turnover of bone results primarily fromremodeling. Modeling and remodeling result from coordinated resorption and formation of bone over extensive regions of the tissue, over prolonged periods of

The pathways by which mechanical forces areexpressed in osteoclast and osteoblast activity is currently one of the main unresolvedissues in bone mechanobiology. The current concept is that the bone architecture iscontrolled by a local regulatory mechanism. This idea originates from Roux (1881), whoproposed that bone remodeling is a self-organizing process. Frost captured these conceptsin his ‘mechanostat’ theory (Frost, 1964, 1987). It assumes that local strains regulatebone mass. If strain levels exceed a so-called mechanical ‘set-point’, new bone is formed.If strain levels are below this set-point, bone is removed. It is a qualitative theory, but itforms the theoretical basis for several mathematical and computational theories that weredeveloped to study bone