Heritable and environmental factors in the causation of clinical vertebral fractures

Vertebral fractures are common osteoporotic fractures, and their incidence is greater in Scandinavia than in other European regions. Vertebral fractures are strongly associated with low bone mineral density, which has a predominant genetic etiology. The heritability of radiological vertebral deformities has been estimated in one recent family study. The objective of our study was to determine the genetic liability to clinical vertebral fractures and to what extent individual-specific environmental factors can explain the variance of these fractures. Participants were ascertained from the Swedish Twin Registry. Twin pairs born 1896-1944 formed the study base, a total of 33,432 subjects. Vertebral fractures after the age of 50 years were identified in the National Patient Register (n = 1,037) or by self-report (n = 35). The age-adjusted heritability for all vertebral fractures was 0.17 (95 % CI 0.00-0.40). Restricting the fracture cases to low-energy causes of injury, the heritability was 0.24 (95 % CI 0.00-0.47). Individual-specific environmental influences were found to explain one-third of the variance in vertebral fracture occurrence before the age of 70 years (0.33, 95 % CI 0.16-0.56), whereas they explained most of the variance among those 80 years of age or older (0.83, 95 % CI 0.61-1.00). We conclude that the occurrence of clinical vertebral fractures is largely explained by environmental influences and not by genetic factors. Individual-specific environmental influences such as lifestyle become more important with increasing age, and it is of importance to identify those environmental factors that cause more fracture cases in Scandinavia than in other European settings.