The functional muscle-bone unit in obese children - altered bone structure leads to normal strength strain index

Obese children have a twofold increased risk of fracture of the forearm compared to non-obese children.

To investigate bone strength and bone structure of the forearm, and the relationship between muscle and bone in obese children.

The study-group consisted of 84 (40 female) overweight children (mean (SD)) age 11.8 (3.2) years, BMI 29.0 (5.1) kg/m(2)). Bone geometry and strength were measured at the proximal radius of the non-dominant forearm (65% measurement site) by means of pQCT (XCT 2000). Bone mineral density and lean mass of the total body was determined by means of DXA (Lunar, DPXL/PED). Results were compared to reference values by calculating age (SDS(CA)) and height-age (SDS(HA)) dependent standard deviation scores (SDS).

Cortical density, -1.11 (1.74) SDS(HA), -0.45 (1.52) SDS(CA); cortical thickness, -1.46 (1.33) SDS(HA), -1.01 (1.46) SDS(CA); cortical area, -0.42 (1.31) SDS(HA), 0.26 (1.58) SDS(CA); total bone area +2.21 (1.47) SDS(HA), 2.91 (1.80) SDS(CA), marrow area +3.12 (2.29) SDS(HA), 3.37 (2.38) SDS(CA); strength strain index +0.10 (1.10) SDS(HA), 0.95 (1.57) SDS(CA). These changes in bone structure were independent from pubertal stage. Measurements revealed correlations between muscle area and SSI (R(2)=0.67, p<0.001), and muscle mass and bone mineral content (DXA; R (2)=0.81, p<0.001).

Low cortical density, normal cortical area and increased total bone area led to a normal strength strain index adjusted both for height and for age. We assume that this normal bone strength is not appropriate for the higher kinetic energy of impact in case of a fall in overweight children.