Determinants of bone size and strength in 13-year-old South African children: the influence of ethnicity, sex and pubertal maturation.

We have previously shown ethnic differences in bone mass between pre-pubertal black and white children using DXA. To investigate these ethnic differences further, using pQCT, and to determine the influence of sex and pubertal development, we measured appendicular bone variables in 13-year-old children using pQCT. We collected pQCT data on a cohort of 471 black and white children at age 13years. Black boys and girls were shorter and had less lean mass than their white peers, and black boys were lighter than white boys at an earlier stage of pubertal development. Metaphyseal (4%) radial trabecular density was greater in the black girls than their white peers (239.5±49.5 vs. 222.7±34.2 mg/cm(3); p<0.05). Bone strength index was not different between the ethnic groups. All metaphyseal measures were 3-41% greater in boys than girls, after adjusting for height where appropriate. Diaphyseal (38%) tibial values, including total area, endosteal diameter, tibial diameter, periosteal circumference and polar strength-strain index were 4-22% greater in the black than white children and in boys than in girls. Cortical density was greater in black than white boys (1079.0±39.4 vs. 1058.7±34.5 mg/mm(3); p<0.001) and greater in the girls than boys (black: 1129.3±33.7 vs. 1079.0±39.4 mg/mm(3); p<0.001; white: 1126.8±28.3 vs. 1058.7±34.5mg/mm(3); p<0.001). Cortical thickness was less in the black groups. Lower leg muscle cross-sectional area (MCSA) was higher in white than black children, and forearm MCSA was higher in white than black boys. There was no difference in fat cross-sectional area between the ethnic groups. In conclusion, ethnic and sex differences in both metaphyseal and diaphyseal bone parameters exist during puberty, which are not accounted for by differences in body size or skeletal maturity. South African black children have wider diaphyseal regions of appendicular bones with greater measures of bone strength.