Choosing the site to estimate bone mineral density with DXA method

Abstract

Osteoporosis is defined as a systemic metabolic skeletal disease in which bone mass loss and micro-architectural deterioration of bone tissue occurs, leading to a reduction of bone strength and increased risk of fractures. This disease can be classified as primary or secondary due to a variety of causes, and has been shown to represent a major public health problem. Diagnosis of osteoporosis focuses on the assessment of bone mineral density (BMD) using dual-energy X-ray absorptiometry (DXA). It is considered to be the “gold standard” method of diagnosis and assesses bone mineral content (grams of hydroxyapatite) per area (cm) at prespecified sites of the axial and appendicular skeleton. As a result, the technique provides a two-dimensional image that is affected by the size of the bones, and does not provide a true (“volumetric”, mg/cm) density, since the relation between area and volume is non-linear. DXA is preferably performed on skeletal sites such as the lumbar spine, proximal femur, and distal forearms, where fracture risk is the highest. The measures provided by DXA are bone mineral content (BMC; in gr), bone area (in cm) and areal BMD (in gr/ cm). To diagnose osteoporosis, the results of areal BMD measurements obtained with DXA, should be reported as the difference in standard deviations (SD’s) with the “peak” bone mass, ie the mean mass of young individuals, thus producing a T-score. For females, three general diagnostic categories have been proposed by WHO, for assessments done with DEXA: normal (T-score -1 or above), low bone massosteopenia (T-score between -1 and -2.5) and osteoporosis (T-score -2.5 or below). BMD represents approximately 60-70% of bone strength of isolated bones in vitro and is used as a substitute measure of bone strength in fracture risk prediction.