Total body bone mineral measurements in children with Prader-Willi syndrome: the influence of the skull's bone mineral content per area (BMA) and of height.

Background. The aim of this study was to investigate the influence of body height and growth on total body measurements with dual energy X-ray absorptiometry (DEXA) in children. Material and methods. Seventeen children with Prader-Willi syndrome were studied as part of a clinical investigation of the effect of growth hormone (Genotropin) treatment. Bone mineral areal mass (BMA), in g/cm2, was studied with DEXA at 0, 12, 24 and 30 months after the start of the study. The effect of increased bone volume on BMA was studied by making a rough estimate of bone width, which was correlated with BMA. Results. There was a weak correlation between total body BMA and body height (r = 0.58), which increased after exclusion of the head (r = 0.84). The BMA of the head was more than twice as high as that of the rest of the body. In the shortest children more than 50 % of the total bone mineral was contained in the skull, which decreased with height to below 20 % in the tallest children. The correlation between the so-called bone width and BMA (total body, head excluded) was 0.97. Conclusion. The results indicate that (a) the bone mineral content (BMC) of the head and (b) the bone volume and body height have a major influence on BMA measurements with DEXA in children. A theoretical method for evaluating the relative bone density (g/cm3) has also been described.