[Measurement of femoral bone density using dual photon absorptiometry. Values in osteoporosis].

X-ray absorptiometry allows measuring the density of bone in the upper end of the femur in 3 mains areas: the neck, Ward's triangle and the greater trochanter. Failin prospective data, it is not possible at present to know whether one of these areas has better performances than the others for the assessment of fracture risks. The interpretation of the measurement is based on the idea that bone demineralization is the main risk factor of osteoporotic fracture. It is carried out on the basis of reference normal values, which must be determined in the French population. More sophisticated predictive models of fracture risks are being developed. In young adults, the reproducibility of the measurements is lower than 2% for the neck and the trochanter, and around 2.5 to 3% for Ward's triangle. The density of femoral bone is not well correlated to that of the other bony sites (os calcis, ulna, vertebrae: r = 0.3 to 0.7). On the other hand, the correlation between the right and left femur is good, as well as that of the three femoral measurement areas between them. A fracture of the upper end of the femur is often associated with a decrease in femoral bone density (-12 to -15%, according to the site of measurement), while the measurement in the ulna or the spine is normal or just slightly lowered. Femoral bone rarefaction is also noted in vertebral osteoporosis. Dual photon absorptiometry allows, in most cases, an accurate and reproducible measurement of the femoral bone density. This measurement is indicated to assess the risk of osteoporosis on menopause and in elderly subjects and/or those with lumbar osteoarthritis.