The importance of genetic factors in an elderly individual's propensity to bone fractures depends on the individual's age and the type of fracture, according to a study in the September 12 issue of Archives of Internal Medicine, one of the JAMA/Archives journals.

Bone fractures resulting from osteoporosis have a profound impact on quality of life, with only one third of patients regaining their pre-fracture level of function and a substantial risk of death following fracture, according to background information in the article. The authors suggest that twin studies provide one of the most natural study populations for evaluating genetic risk (the relative contribution of genes versus environment). If heritable factors contribute to fractures, monozygotic twins (who have all the same genes, commonly called identical twins) are more likely to have similar rates of fracture than dizygotic twins (who share about half the same genes, commonly called fraternal twins).

Karl Michaelsson, M.D., Ph.D., of the Uppsala University Hospital, Uppsala, Sweden, and colleagues used the Swedish Twin Registry, the Swedish Inpatient Registry and telephone interviews to evaluate the genetic liability to fracture of the elderly. From the registry of Swedish twins born between 1,896 and 1,944 (3,724 identical twins, 6,314 fraternal same-sex twins and 5,736 fraternal different-sex twins), the researchers were able to identify 6,021 twins with any fracture, with a higher proportion among women (23 percent) than men (14 percent). More than half the cases (3,599) were classified as osteoporotic fractures. The most important osteoporotic fracture, hip fracture, was recorded for 1,055 twins.

Genetic variation in liability to fracture differed considerably by type of fracture and age, the authors report. Less than 20 percent of the overall age-adjusted fracture variance was explained by genetic variation. Heritability was considerably greater for first hip fracture before the age of 69 years and between 69 and 79 years than for hip fractures after 79 years of age.

"We conclude that the genetic influence on susceptibility to fractures is dependent on type of fracture and age at fracture event," the authors write. "The heritability of osteoporotic fractures is stronger than has been previously estimated, especially for early-occurring osteoporotic fractures. A search for genes and gene-environmental interactions that affect early osteoporotic fracture risk is likely to be fruitful, but fracture-prevention efforts at older ages should be focused on lifestyle habits."

This study was supported by grants from the Swedish Research Council, Stockholm; the National Institute on Aging, Bethesda, Md.; and Uppsala University Hospital, Uppsala, Sweden.