BONE QUALITY - A DETERMINANT FOR CERTAIN RISK-FACTORS FOR BONE FRAGILITY

Low bone quantity alone is insufficient cause for fragility fractures. This paper examines the role of bone quality in the fracture risk ass ociated with age, sex, and race. Aspects of bone quality to be conside red are bone architecture, matrix, mineralization, and fatigue damage. The trabecular network becomes progressively disconnected and weaker with age. Death of old osteocytes leads to hypermineralization and bri ttleness of bone. The stability of bone collagen declines with age, an d unremodeled bone accumulates fatigue damage. The lower bone fragilit y rates in males than in females may be due to a combination of the la rger male skeleton, greater cortical bone density after age 60 years, and greater bone turnover which would replace fatigue damaged bone. Fr agility fracture rates in American and African blacks are lower than i n whites, bone mineral density (BMD) is greater in American but not in African blacks (except for hip BMD), and American blacks have lower a nd African blacks higher bone turnover compared to whites. In South Af rican blacks, trabeculae are thicker and better connected and trabecul ar bone undergoes less destructive age changes than in whites. To reco ncile the disparate findings in American and African blacks we suggest that both groups have a genetic tendency to greater BMD than whites; American blacks realize this potential and African blacks do not, poss ibly because of calcium deficiency. Consequent high turnover removes f atigue damage and so improves bone quality. Weight-bearing activity in African blacks may be responsible for good hip bone density and thick trabeculae. American and African blacks have lower fragility fracture rates than whites for different reasons: American blacks because of h igher BMD, and African blacks because of higher bone turnover, better hip BMD, and sturdier trabeculae.