Quantitative ultrasound at the hand phalanges in monozygotic twins: a preliminary report.

The aim of this study was to assess the contribution of genetic and environmental factors on ultrasound (US) values within hand phalanges in monozygotic (MZ) twins. A total of 83 pairs of MZ twins (54 pairs of females, mean age 14.9 +/- 8.3 years, SD, and 29 pairs of males, mean age 14.1 +/- 7.1 years) and a control group consisting of 21 pairs of dizygotic (DZ) twins (14 pairs of females, mean age 13.9 +/- 6.9 years and 7 pairs of males, mean age 13.0 +/- 5.6 years) with no reasons for potential influence on bone metabolism were studied. Skeletal status was assessed using DBM Sonic 1200 (IGEA, Carpi, Italy) which measures at the hand phalanges amplitude-dependent speed of sound (Ad-SoS, m/s). The intrapair correlation coefficients of Ad-SoS values were in MZ pairs 0.96 for males, 0.98 for females and, in DZ pairs, 0.92 and 0.93, respectively (p < 0.0001). The intrapair correlation coefficients for Ad-SoS were significantly stronger in MZ pairs in comparison to DZ pairs (Fisher's test, p < 0.05). To study the hypothesis that the intrapair differences in Ad-SoS are connected with environmental factors, a simple linear regression analyses of absolute differences in Ad-SoS values with age were performed. In MZ pairs, increasing age was connected with significantly greater intrapair differences in Ad-SoS (for males r = 0.37, p < 0.05 and, for females, r = 0.32, p < 0.05). The results of multivariate linear analysis by using intrapair differences in Ad-SoS in MZ pairs as a dependent variable and age, weight and height as independent variables, showed that intrapair difference in Ad-SoS was influenced only by age, and not by weight, height, or intrapair differences in weight or height, for both males and females. The study provides the support for a significant contribution of heredity to skeletal status in MZ twins, although significantly greater intrapair differences in Ad-SoS with age suggest also the importance of environmental factors.