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What Is Osteoporosis

Osteoporosis is a common condition affecting as many as 15-20 million individuals in the United States. About 1.3 million fractures attributable to osteoporosis occur annually in people age 45 and older. Among those who live to be age 90, 32 percent of women and 17 percent of men will suffer a hip fracture, most due to osteoporosis. The cost of osteoporosis in the United States has been estimated at $3.8 billion annually.

Primary osteoporosis is an age-related disorder characterized by decreased bone mass and by increased susceptibility to fractures in the absence of other recognizable causes of bone loss.

Bone is composed of collagen-rich organic matrix impregnated with mineral--largely calcium and phosphate. Two major forms of bone exist. Compact cortical bone forms the external envelopes of the skeleton; trabecular or medullary bone forms plates that traverse the internal cavities of the skeleton. The proportions of cortical and trabecular bone vary at different sites. Vertebral bodies contain predominantly trabecular bone, while the proximal femur contains predominantly cortical bone. The responses of the two forms of bone to metabolic influences and their susceptibility to fracture differ.

Bone undergoes continuous remodeling (turnover) throughout life. Osteoclasts resorb bone in microscopic cavities; osteoblasts then reform the bone surfaces, filling the cavities. Normally, bone resorption and formation are linked closely in space, time, and degree. Mechanical and electrical forces, hormones, and local regulatory factors influence remodeling.

Peak bone mass is achieved at about 35 years of age for cortical bone and earlier for trabecular bone. Sex, race, nutrition, exercise, and overall health influence peak mass. Bone mass is approximately 30 percent higher in men than in women and approximately 10 percent higher in blacks than in whites. In each group, bone mass varies among individuals.

After reaching its peak, bone mass declines throughout life due to an imbalance in remodeling. Bones lose both mineral and organic matrix but retain their basic organization. In women, bone mass decreases rapidly for 3 to 7 years after menopause. Bone loss also is enhanced in a variety of diseases.

Women have more fractures than men, and whites have more fractures than blacks. Three factors determine the likelihood of fractures: first, the magnitude, direction, and duration of the applied force; second, the dissipation of that force by muscle contraction and soft tissue absorption; and finally, bone strength. Injuries are more frequent and energy dissipation diminishes with advancing age. Reduction in bone mass is the most important reason for the increased frequency of bone fractures in postmenopausal women and in the elderly.