Bone Abstracts

Aim: Lean mass is a strong determinant of bone structure, particularly during growth and development. We sought to determine if there are sex differences in bone acquisition in pre-pubertal children and the potential relation with skeletal loading.

Methods: This study included 3602 children (50.7% girls) whose total body BMD and hip scans were measured on the same iDXA devise (GE-Lunar) at 6 years of age. Hip scans underwent hip structural analysis (HSA) of which shaft parameters were analysed. Sex-differences across bone parameters were assessed by multivariate regression models (comparing least-squares means), adjusted for age, height, ethnicity, fat mass and with/without total-body lean mass. We also examined the relationship between bone strength and skeletal loading calculated as an index of mechanosensitivity (ratio of bone strength divided by height (moment arm) to total body lean mass (TB-LM).

Results: TB-BMD (less head) was 0.77% lower in girls than in boys (P=0.01) after adjustment for age, height, fat mass and ethnicity. Additional adjustment for TB-LM resulted in girls having 1.78% higher TB-BMD than boys (P=1.9E−14). Similarly, HSA models showed that girls had significant (all P<0.01) −5.8% thinner cortices (CT), 0.6% greater width (W), −4.7% axial (CSA) and −4.6% bending (SM) strength than boys. After TB-LM correction, girls had −1.4%, CT, 2.2% W, 1.3% CSA and 3.7% SM. The shaft mechanosensitivity index was 2.9% lower in boys than in girls after adjustment for age and ethnicity (P=4.76E−11).

Conclusion: At 6 years of age girls have higher bone mass and strength relative to muscle load than boys. These results are in line with previous studies reporting higher mechanosensitivity in women, attributing these differences to (post) pubertal hormonal changes. Our results show that when considering the relation with lean mass small but significant sex differences in bone density, structure, strength and mechanosensing are already present at pre-pubertal ages.