Bone mineral density (BMD) is considered a strong biomarker for fracture risk. It is not a true density measurement but rather a mathematical construct obtained by dividing the bone mineral content (BMC) by the area of the scanned bone envelope (BA), and it is expressed as g/cm2:
BMC is measured in vivo by absorptiometry, based on the attenuation of energy from a beam of penetrating photons (X-ray) during a scan across the skeletal region of interest. Single-energy absorptiometry (SXA) are used for measuring the BMC of the bones of the arms and legs, but dual energy (DXA) instruments are required for axial (hip and spine) and whole-body measurements that need correction for the overlying soft tissue of variable composition.94 The effective dose of radiation per measurement is low and remains generally within natural background radiation levels.95 The method is precise with coefficients of variation usually less than 1-2% for repeated scans of phantoms and between 2-5% for repeated scans of the same person. In addition long-term reproducibility is good. The accuracy of absorptio-metry is more problematical, being affected by choice of calibrating materials, assumptions built up in the computer algorithms, the depths of tissues in the scan path, the nonuniformity of soft tissues overlying bone, and differences in clothing and bedding. The result of absorptiometry is not limited to osseous tissue per se. Indeed, it represents an integration of absorption over all elements within the bone envelope. Moreover, it cannot differentiate between cancellous and cortical bone, nor exclude abnormalities that interfere with the measurement of bone mineral content (e.g., crush fractures or calcifications).
Absorptiometry can be used both in humans and in laboratory animals. In each case specific computer software is required. Especially whole-body bone mineral content (WBBMC) and whole-body bone area (WBBA) can be measured in anaesthetized rats using DXA to allow calculation of whole body bone mineral density (WBBMD). The coefficient of variation (CV ± sem) is 1.74 ± 0.15, 96 and the method is highly accurate as demonstrated by a correlation coefficient of 0.99 between total body Ca measured by DXA and atomic absorption spectrometry of bone ash, respectively.97
When performing absorptiometry studies in humans seeking to evaluate the impact of an (e.g., dietary) intervention on bone mineral status, 6-12 and 24-36 months are a minimum to evaluate short- and long-term effect, respectively.3 The intervention periods need thus to be much longer than for evaluating parameters of Ca metabolism (e.g., bioavailability). Ideally, it would have the duration of 3-4 complete cycles of bone turnover (i.e., ±4 months in man).54
More modern technologies are being used increasingly for bone mineral assessment, but there are no prospective data yet that link these measurements to fracture incidence, and their value in longitudinal studies need further evaluation. These technologies include quantitative computed tomography and quantitative bone ultra-sound.3
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