Bone density in children: a review of the available techniques and indications.

The recent development of methods for measuring bone mineral content in children has markedly improved our ability to determine changes in bone mass during growth. Currently, the three most generally accepted techniques for measuring the bones of children are dual-energy X-ray absorbtiometry (DXA), quantitative computed tomography (QCT) and quantitative ultrasound (QUS). These techniques vary considerably in their acquisition of data and comparisons between them are difficult and, more often than not, judgment regarding their value has been, at least partially, subjective. DXA is, by far, the most widely used technique for bone measurements. It is low in cost, accessible, easy to use, and provides an accurate and precise quantitation of bone mass in adults. Unfortunately, DXA is unable to account for the large changes in body and skeletal size that occur during growth, limiting its use in longitudinal studies in children. QCT can asses both the volume and the density of bone in the axial and appendicular skeletons, without influence from body or skeletal size, giving it a major advantage over other modalities for bone measurements in children. The cost and inaccessibility of CT scanners, however, has significantly limited its use for bone measurements. Measuring the bones of children by QUS is appealing because ultrasound is low in cost, portable, easy to use and does not emit radiation. In adults, this technique is able to predict fracture risk independent of bone mass determinations in patients with osteoporosis and, therefore, its measurements must be related to certain aspects of bone strength. However, ultrasound values are dependent on so many structural properties not yet fully understood, that it is difficult to use the information meaningfully in children.