99 Osteoporosis

Osteoporosis is defined as a proportional decrease of both bone mineral and bone matrix, leading to frac­ture after minimal trauma. It differs from osteo­malacia in which there is a normal amount of bone matrix (osteoid) but decreased mineralization.

Osteoporosis is an enormous public health prob­lem, responsible for at least 1.2 million fractures in the United States each year. Fractures of the verte­bral bodies and hip comprise the majority, and the complications of hip fracture are fatal in 12 to 20 percent of cases. Nearly 30 percent require long­term nursing home care. The direct and indirect costs of osteoporosis in the United States are esti­mated at over 8 billion in 1989 dollars annually.

Age-related bone loss or involutional osteoporosis begins about age 40 in both sexes at an initial rate of about 0.5 percent per year. The bone loss increases with age until slowing very late in life. In women, an accelerated postmenopausal loss of bone occurs at a rate of 2 to 3 percent per year for about 10 years. Over their lifetime, women lose about 35 percent of their cortical bone and 50 percent of their trabecular bone, whereas men lose about two-thirds of these amounts (Riggs and Melton 1986).

Etiology

The process of age-related osteoporosis is universal, although certain populations are affected to a greater degree or at an earlier age. Osteoporosis in the elderly is exceedingly common and involves mul­tiple factors of age, sex, endocrine system, genetics, environment, nutrition, and physical activity. Age- related factors include decreased osteoblast func­tion, decreased intestinal calcium absorption, de­creased renal activation of vitamin D, and increased parathyroid hormone secretion combined with de­creased clearance of the hormone by the kidney.

In women, decreased estrogen production in meno­pause accounts for 10 to 20 percent loss of total bone mass. Although men do not undergo the equivalent of menopause, gonadal function declines in many elderly men and contributes to osteoporosis. Multi­ple pregnancies and extended breastfeeding in pre­menopausal females may also produce negative cal­cium balance and subsequent bone loss. Underlying medical conditions or medications can contribute to osteoporosis, particularly hyperthryoidism, hemi­plegia or paralysis, alcoholism, use of glucocorticoid steroids, and smoking.

Population and familial studies implicate both ge­netic and environmental factors in causing or inhib­iting osteoporosis. Blacks, for example, tend to have greater initial bone mass or bone density than do Caucasian populations. Even with similar rates of bone loss, their osteoporosis begins at a later age than that of their white counterparts. Black females living in Michigan had similar relative vertebral density by age to blacks living in Puerto Rico. Both black groups had significantly less osteoporosis by age group than did Caucasian females living in Michigan. Within the Caucasian group, women of British Isles’ ancestry had a higher incidence of osteoporosis than did whites of other national ori­gins (Smith and Rizek 1966).

Nutrition, particularly calcium intake, plays a role in the inhibition of osteoporosis. Long-term tri­als of calcium supplementation, however, have had mixed results in postmenopausal women. Such sup­plementation may be more effective if initiated in women years before onset of menopause. A high- protein diet increases urinary excretion of calcium and may therefore induce a negative calcium bal­ance. The decreased renal activation of vitamin D with age may also be a contributing factor in popula­tions without vitamin D supplementation or with the elderly confined indoors.

Physical activity decreases the rate of bone loss in the elderly. The skeletal stresses from weight bear­ing and muscle contraction stimulate osteoblast func­tion, and muscle mass and bone mass are directly related.

Epidemiology, Distribution, and Incidence Osteoporosis was defined as a clinical entity by Fuller Albright and colleagues in 1941. Since that time a large number of epidemiological studies have been performed to evaluate the varying rates of osteoporosis and osteoporosis-related fractures in dif­ferent populations. The process of age-related and postmenopausal bone loss occurs in all populations. It appears to start earlier for women in Japan and India as compared to women in Britain and the United States. It occurs later in Finnish women, and age-related rates of bone loss of women between 35 and 64 years of age are lower in Jamaica and the four African countries studied than in the other coun­tries examined (Nordin 1966).

Relating the differing rates to calcium intake cor­relates well with the populations in Japan, India, and Finland but not Jamaica or the African coun­tries. Other factors such as genetically determined initial bone density or increased physical activity may also be involved. The rate of hip fracture varies according to the age distribution of osteoporosis in all countries. This distribution involves a much broader age group in India secondary to the preva­lence of rickets and osteomalacia.

Black females in both Puerto Rico and Michigan have less osteoporosis than do white females in Michigan. However, both groups demonstrate a very high prevalence with advanced age: 60 percent of those over 65 years in Puerto Rico and 80 percent over 65 years in Michigan (Smith and Rizek 1966). The incidence rates of hip fracture are higher in white populations in all geographic areas including Israel and Singapore, compared to the indigenous populations. A very low incidence rate is also noted among the Maoris of New Zealand and Bantu of South Africa compared to white populations in both areas. The age-adjusted incidence rate of hip frac­ture for females and males per 100,000 is 5.3 and 5.6 among the Bantu compared to 101.6 and 50.5 among blacks in the United States (Cummings et al. 1985).

In the United States, over 90 percent of all hip fractures occur in individuals over age 70 years. By age 90, a third of all women will have sustained a hip fracture. Vertebral fractures are present in 25 percent of women over age 70.

History

Age-related bone loss has also been examined in prehistoric skeletal populations. Archaic Indians dat­ing back to 2500 B.C. and subsisting as hunter­gatherers showed differential rates between females and males as well as overall rates of osteoporosis quite similar to those of a modern hospital popula­tion. A more recent Hopewell Indian population showed greater age-related bone loss, probably re­sulting from genetic or nutritional factors (Perzigian 1973). A study of three ancient Nubian skeletal popu­lations found that osteoporosis occurred earlier in life among these women as compared to modern Western samples, perhaps secondary to inadequate calcium intake or extended lactation (Dewey et al. 1969).

R. Ted Steinbock

Bibliography

Albright, F., et al. 1941. Post-menopausal osteoporosis. Journal of the American Medical Association 116: 2465-8.

Chalmers, J., and K. C. Ho. 1970. Geographical variations in senile osteoporosis.

Cummings, S. R., et al. 1985. Epidemiology of osteoporosis and osteoporotic fractures. Epidemiology Review 7: 178-208.

Dewey, J. R., et al. 1969. Femoral cortical involution in three Nubian archaeological populations. Human Bi­ology 41: 13-28.

Ericksen, M. F. 1976. Cortical bone loss with age in three native American populations. American Journal of Physical Anthropology 45: 443-52.

Gam, S. M. 1970. The earlier gain and the later loss of cortical bone. Springfield, Ill.

Nordin, B. E. C. 1966. International patterns of osteo­porosis. Clinical Orthopedics 45: 17—30.

Perzigian, A. J. 1973. Osteoporotic bone loss in two prehis­toric Indian populations. American Journal of Physi­cal Anthropology 39: 87-96.

Resnick, N. M., and S. L. Greenspan. 1989. Senile osteo­porosis reconsidered. Journal of the American Medical Association 261: 1025-9.

Riggs, B. L., and L. J. Melton. 1986. Involutional osteoporosis. New England Journal of Medicine 314: 1676-86.

Smith, R. W., and J. Rizek. 1966. Epidemiologic studies of osteoporosis in women of Puerto Rico and southeast­ern Michigan with special reference to age, race, na­tional origin and to other related or associated find­ings. Clinicial Orthopedics 45: 31—48.