Bone mineral accrual during childhood and adolescence : a critical analysis of size-correction techniques
Davison, Kenneth Shawn
The true pattern of bone mineral density changes during childhood and adolescence is unclear owing to a lack of longitudinal investigations, a lack of adequate control for maturational differences, and size-mediated errors in the measurement of areal bone mineral density (aBMD). This investigation incorporated mixed-longitudinal (distance data) and longitudinal (velocity data) designs to describe pediatric changes in bone density at the total body (TB), femoral neck (FN) and lumbar spine (LS). Maturational differences were controlled by aligning participants on their age at peak height velocity (PHV). Changes in areal BMD (aBMD) were compared with densities obtained from two methods of size correction: bone mineral apparent density (BMAD) based on geometric assumptions, and statistically corrected BMD (sBMD), which utilizes linear regression. Correlations between bone projected area (PA) and density were strongest with aBMD, intermediate with BMAD, and generally insignificant with sBMD, supporting sBMD's size independence. With the distance data aBMD increased over the entire growth period at all sites. Contrastingly, TB BMAD declined initially and then stabilized after PHV, and FN and LS BMAD were generally stable until PHV, increasing afterward. Similarly, TB and LS sBMD decreased until PHV, increasing afterward, and FN sBMD was stable until PHV, increasing thereafter. aBMD velocity was positive at all sites and all ages. In contrast, TB and FN BMAD had a negative velocity until after PHV, and LS BMAD velocity was generally stable until near PHV, with a positive velocity afterwards. sBMD velocity was negative at the TB and LS until PHV and there was a stabilization (males) or decrease in velocity (females) in FN sBMD until PHV. Velocity curves for PA and bone mineral content displayed a consistent dissociation at all sites with bones increasing in area first and later consolidating when rapid growth ceased or slowed. The point of minimal density suggested from the corrected data coincided with PHV and is supported by epidemiological data that reports the highest rates of fracture in adolescents during this time. These results highlight the size-dependence of aBMD and cautions against its use in the pediatric population. Physiologically, sBMD appeared the more appropriate size-correction technique.