Volumetric bone mineral density in normal subjects, aged 5-27 years

Concerns have been raised regarding the validity of using areal bone mineral density (aBMD) as a substitute for the true volumetric bone mineral density (vBMD) in the pediatric population. We studied 209 normal subjects (109 males), aged 5-27 yr, to examine the influence of age, gender and growth on vBMD. The femoral neck, midthird of the femoral shaft, and the four lumbar vertebral bodies (L1-L4) were studied. Using data on bone width and height obtained by dual energy x-ray absorptiometry, bone volume was calculated with the assumption that all three sites are cylinders. In contrast to aBMD, vBMD of the femoral neck bore no relationship to age or weight in both sexes, but was significantly related to height in females (r2 = 0.07; P = 0.01). Similarly, vBMD of the femoral shaft (vFBMD) did not change with age or height in either sex. In females, a significant inverse relationship was seen between vFBMD and weight (r2 = 0.14; P = 0.001). Male subjects had higher vFBMD than females (mean +/- SD, 0.73 +/- 0.11 vs. 0.70 +/- 0.12; P = 0.047), but no sex difference was seen in vBMD of the femoral neck. Conversely, vBMD of L1-L4 remained age and growth dependent, although the strength of the relationship was weaker than that for aBMD (data not shown). In conclusion, the vBMD of the femoral neck and shaft is independent of age and is less dependent on growth variables in children and young adults than is aBMD. These observations offer a different perspective from our previous concepts of aBMD.