Bone Composition

Bone is composed primarily of calcium and phosphorus, in the form of hydroxyapatite crystals deposited in a collagen matrix (23). Adults have two types of bone: cortical and trabecular. Cortical bone provides rigidity and is the major component of the long bones of the skeleton. Cortical bone accounts for approximately 80 to 85% of the skeletal mass. Examples of cortical bone are the dense long bones of the hands, arms, and legs and bones of the feet. Trabecular bone is spongy in appearance and provides strength and elasticity. Trabecular bone makes up only about 15 to 20% of the skeletal mass. Examples of trabecular bone are the skull, the sternum, ribs, hip and shoulder girdle, and spinal column.

It would appear from the distribution of cortical and trabecular bone, that the cortical skeleton would be the most important area of bone turnover. However, bone turnover is a surface phenomenon, and trabecular bone has the greatest surface area. Therefore, 80% of bone turnover occurs in trabecular bone with only 20% occurring in cortical bone (4,5,19). This accounts for the greater fracture rate of trabecular bone versus cortical bone. A postmenopausal woman may lose 35% of her cortical bone and approximately 50% of her trabecular bone. A man of the same age, conversely, will lose about 12% of his cortical bone and approximately 17% of his trabecular bone (19).

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