Comparison of Site-Specific Bone Mass Indices in South African Children of Different Ethnic Groups

Abstract

This study reports on ethnic differences in bone mass before and after adjusting for differences in body size and bone area (BA). Lumbar spine (LSBMC), proximal femur (PFBMC) and femoral neck (FNBMC) bone mineral contents were measured in black (‘black’; n = 263) and white (‘white’; n = 73) children from Johannesburg and children of mixed ancestral origin (‘mixed’; n = 64) from Cape Town, South Africa. Geometric estimates and the power coefficient from the regression analyses of BMC on BA were calculated. After adjusting for age, weight, and height, LSBMC in girls and FNBMC in girls and boys were greatest in mixed, followed by black and then white, groups. Mixed boys and girls also had greater PFBMC than their black and white peers, but only in the boys was PFBMC greater in the black than the white groups. When including BA in the adjustment, differences remained at the FN in boys and girls, and the LS and PF in girls, but disappeared at the PF in mixed and black boys. The difference in LSBMC between mixed and black boys became significant after adjustment for age, weight, height, and BA. Geometric estimates at the femoral neck were greater in the mixed group. Power coefficients were greater in the white group, suggesting differences in shape or bone distribution. In conclusion, this study suggests that, in addition to differences in BMC, differences in bone strength and geometry are present which might confer advantages to the bone of mixed-ancestry children.

Appendix

The method for estimating geometry in the femoral neck (FN; T. J. Beck, Sc.D.) is as follows. Structural geometry can be calculated directly from DXA mass image data, but in cases where the image data are not available it is possible to estimate the geometry from conventional outputs, although the method is meaningful only in DXA regions that traverse the long axis of a long bone (FN). It should be realized that the methods are necessarily crude and assume that the bone within the region is a uniform right circular cylinder. This is not a very good assumption in children and persons with smaller bones since cross sections of the FN change greatly in shape along the length of the neck. How much cross sections change in a fixed region length varies with the size of the bone.

FN width (W) is estimated by dividing the neck region area by its fixed length in Hologic scanners, i.e., 1.5 cm. The cross-sectional area (CSA) is computed as

$$ {\text{CSA}} = {\frac{{\text{BMD}}*W}{{\rho_{\rm m} }}} $$

where ρ_m is the effective density of bone mineral in fully mineralized bone tissue (~1.05 g/cm³).

The section modulus (Z) is computed as

$$ Z = {\frac{{\text{CSMI}}}{W/2}} $$

The cross-sectional moment of inertia (CSMI) is computed as

$$ {\text{CSMI}} = {\frac{\pi }{4}}\left( {\left( {{\frac{W}{2}}} \right)^{4} - p\left( {{\frac{{\text{ED}}}{2}}} \right)^{4} } \right) $$

where p is the trabecular porosity and ED is the estimated endosteal diameter.

$$ {\text{ED}} = 2\sqrt {\left( {{\frac{W}{2}}} \right)^{2} - f_{\rm c} {\frac{{\text{CSA}}}{\pi }}} $$

$$ p = 1 - \left[ {{\frac{{(1 - f_{\rm c}){\text{CSA}}}}{{\pi \left( {{\frac{{\text{ED}}}{2}}} \right)^{2} }}}} \right] $$

where f _c is the assumed proportion of cortical mass in the FN (0.6).

An estimate of mean cortical thickness is given by

$$ t = (W - {\text{ED}})/2 $$

Buckling ratio can then be estimated as

$$ {\text{BR}} = {\frac{W/2}{t}} $$