Paul A. Clark & John S. Kuo
BONE IS THE main weight—bearing tissue of the body and functions to withstand mechanical forces several times the weight of the body. Despite the numerous shapes and sizes of the bones in the body, bone tissue is structurally and microscopically similar throughout. Bone tissue exists in two general forms: Cortical bone composes the shell of many of the long bones and has low porosity and high density and strength. Cancellous (or tra—becular) bone is found at the ends of long bones or in low—weight bearing areas (such as the lower jaw) and is composed of microscopic interconnecting bony trabecula, giving macroscopically high porosity and low density. Although bone tissue is generally thought of as a rigid structure, it contains both elastic and stiff components. The bone extracellular matrix is approximately 65 percent inorganic, mainly calcium and phosphate in the form of hydroxyapatite, and 35 percent organic matrix (osteoid), mainly collagen, ...