Diagnostic value of estimated volumetric bone mineral density of the lumbar spine in osteoporosis

Abstract

Bone mineral density (BMD) measurements by dual‐energy x‐ray absorptiometry (DXA) are planar measurements. By measuring planar BMD in anteroposterior (AP) and lateral projections, it is possible to estimate the volumetric BMD by assuming that the vertebral body is an ellipsoid cylinder. Correction for the third dimension could improve the diagnostic accuracy of spinal BMD measurement in osteoporosis. The aims of this study were (1) to determine the effect of aging on volumetric BMD compared to planar measurements; and (2) to evaluate the diagnostic accuracy 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 population‐based group. AP and lateral decubitus BMD of the lumbar spine were measured by DXA using a Lunar DPX (precision error for AP measurement, 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 L2–4 were 31, 34, 23, and 23%; expressed as Z scores, the decreases 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 L2–4 were 85, 86, 87, and 87%, respectively. We conclude that there is a greater decrease in volumetric and lateral BMD of vertebra L3 than the AP measurements, both with age and in postmenopausal osteoporosis compared 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 accuracy may be because of the large precision error in lateral decubitus (and hence volumetric) BMD measurements. It is possible that the diagnostic accuracy of estimated volumetric BMD may improve with the use of lateral supine DXA, a measurement with lower precision error.