The principal causal components of an osteoporotic fracture are a fall and weakened bone strength. While bone quality measures have been frequently studied, the ability of simple measures of impaired balance to predict fracture risk has received less attention. Computer-assisted telephone interviews were conducted between 1998 and 2000 among 24,598 Swedish twins aged 55 years or older. Impaired balance at the time of interview was reported by 2,890 (12%) of the twins. Twin pairs who were discordant with regard to impaired balance were selected for analysis and followed for fractures through 2005. In a pairwise analysis, the odds ratio for hip fracture was 3.13 (95% confidence interval (CI): 1.62, 6.05) among twins with impaired balance as compared with their co-twins with normal balance. When previously recognized clinical risk factors for osteoporotic fracture were considered in the model, the odds ratio for hip fracture with impaired balance was 3.88 (95% CI: 1.40, 10.72). Approximately 40% of all hip fractures were attributable to impaired balance. The odds ratios for any fracture and any osteoporotic fracture for twins with impaired balance were 2.00 (95% CI: 1.29, 3.11) and 2.39 (95% CI: 1.49, 3.82), respectively. These results imply that self-reported impaired balance is a novel and readily assessed risk factor for future fractures in the elderly.

Bone mineral density (BMD), bone size and bone turnover are independent determinants of fractures in elderly. Earlier twin studies of these phenotypes have revealed high heritability for BMD and bone area, and more moderate heritability for bone turnover markers. No previous Scandinavian study has evaluated the genetic and environmental contribution to the variance of these phenotypes, despite the fact that Scandinavian countries have the highest incidence of osteoporotic fractures worldwide. Participants were selected from the Swedish Twin Registry. All intact like-sexed twin pairs born in 1965 or earlier and living in the county of Uppsala were invited to participate. A total of 102 twin pairs (45 monozygotic and 57 dizygotic) accepted the invitation to participate. All twins underwent measurement of BMD and bone area using dual-energy X-ray absorptiometry. Hip geometry was also calculated. Markers for bone formation (osteocalcin) and bone resorption (CrossLaps) were measured in serum. We observed a substantial heritability for BMD at the lumbar spine (0.85; 95 % CI 0.54-0.90), the femoral neck (0.75; 95 % CI 0.62-0.83), and the proximal femur (0.84; 95 % CI 0.74-0.90). The values for bone area were approximately similar to those for BMD. Bone turnover markers had a slightly lower genetic influence with a value of 0.69 (0.53-0.80) for osteocalcin and 0.58 (95 % CI 0.33-0.75) for CrossLaps. As a comparison, the heritabilities of body height and weight were 0.95 and 0.82, respectively. The high heritability on bone phenotypes among Swedish middle-aged and older men and women should encourage further work on the identification of specific genetic pathways. Continuing research in this area could reveal the mechanisms behind the strong genetic susceptibility of bone-related phenotypes.

These results imply that a self-reported impaired balance, an independent risk factor for osteoporotic fractures, has a modestly heritable etiology in older subjects. Our observation can partly explain the previously observed modest heritability for osteoporotic fractures even though there is a high heritability for bone mineral density.

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