A bivariate genetic analysis among 217 older female twin pairs showed that, although the structural strength of tibia and radius are mainly regulated by same genetic and environmental factors, the tibia is more affected by environment.

INTRODUCTION:

The habitual loading environment of the bone may modulate the relative contribution of genetic and environmental factors to bone structure. The purpose of this study was to estimate the contribution of the common and site-specific genetic and environmental factors to interindividual variation in compressive structural strength of the weight-bearing tibia and non-weight-bearing radius.

MATERIALS AND METHODS:

pQCT scans were obtained from both members of 103 monozygotic (MZ) and 114 dizygotic (DZ) 63- to 76-yr-old female twin pairs to estimate the compressive strength of the distal tibia and distal radius. Quantitative genetic models were used to decompose the phenotypic variance into additive genetic, shared environmental, and individual environmental effects at each bone site and to study whether these bone sites share genetic or environmental effects.

RESULTS:

The MZ and DZ twins did not differ in mean age, height, weight, or bone structural strength. The age-adjusted Cholesky model showed that additive genetic factors accounted for 83% (95% CI, 77-88%) of the variance in radial strength and 61% (95% CI, 52-69%) of the variance in tibial strength, and these were fully correlated. A shared environmental factor accounted for 15% (95% CI, 10-20%) of tibial strength. An individual environmental factor accounted for 17% (95% CI, 12-23%) of the variance in radial strength and 10% (95% CI, 5-17%) of the variance in tibial strength. The relative contribution of an individual environmental factor specific to tibial strength was 14% (95% CI, 11-18%).

CONCLUSIONS:

The results suggest that, in older women, the majority of the individual differences in the compressive structural strength of the forearm and leg are regulated by genetic and environmental factors that are common to both bone sites. However, the relative importance of environmental factors was greater for the weight-bearing tibia than for the non-weight-bearing radius. Thus, the heritability of bone strength seems to vary between skeletal sites according to differences in the typical loading environment.