DIAGNOSTIC-VALUE OF ESTIMATED VOLUMETRIC BONE-MINERAL DENSITY OF THE LUMBAR SPINE IN OSTEOPOROSIS

Bone mineral density (BMD) measurements by dual-energy x-ray absorptio metry (DXA) are planar measurements. By measuring planar BMD in antero posterior (AP) and lateral projections, it is possible to estimate the volumetric BMD by assuming that the vertebral body is an ellipsoid cy linder. Correction for the third dimension could improve the diagnosti c accuracy of spinal BMD measurement in osteoporosis. The aims of this study were (1) to determine the effect of aging on volumetric BMD com pared to planar measurements; and (2) to evaluate the diagnostic accur acy of volumetric BMD in comparison to planar measurements. We studied 26 postmenopausal women with osteoporotic vertebral fractures and 114 age- and sex-matched controls without vertebral fractures from a popu lation-based group. AP and lateral decubitus BMD of the lumbar spine w ere measured by DXA using a Lunar DPX (precision error for AP measurem ent, 0.8%; for lateral measurement of vertebra L3, 6.4%). Between the ages of 51 and 85 years the decreases in volumetric BMD of L3 (vol L3) , lateral BMD of L3 (lat L3), AP BMD of L3 (AP L3), and AP BMD of L2-4 (AP L2-4) were 31, 28, 17, and 17%, respectively. The decrease in BMD compared to controls in the osteoporotics for vol L3, lat L3, AP L3, and AP L24 were 31, 34, 23, and 23%; expressed as Z scores, the decrea ses were -1.36, -1.33, -1.46, and -1.47 standard deviation units. The areas under ROC analysis curves for vol L3, lat L3, AP L3, and AP L24 were 85, 86, 87, and 87%, respectively. We conclude that there is a gr eater decrease in volumetric and lateral BMD of vertebra L3 than the A P measurements, both with age and in postmenopausal osteoporosis compa red to age-matched controls. However, the diagnostic accuracy was not improved by correcting BMC for vertebral volume. The greater decreases observed with aging may result from the high proportion of cancellous bone in the vertebral body. The lack of improvement in diagnostic acc uracy may be because of the large precision error in lateral decubitus (and hence volumetric) BMD measurements. It is possible that the diag nostic accuracy of estimated volumetric BMD may improve with the use o f lateral supine DXA, a measurement with lower precision error.