Measurement of Spinal Bone Mineral Density on a Hologic Discovery DXA Scanner With and Without Leg Elevation

Abstract 

Although it is generally recommended that patients keep their hips flexed by 90° during the measurement of spinal bone mineral density (BMD), there is no uniform agreement among the manufacturers of dual-energy X-ray absorptiometry (DXA) scanners regarding the positioning of legs while scanning the spine. We measured spinal BMD in 54 postmenopausal women, from L1 to L4 in posterior-anterior projection, using a Hologic Discovery scanner, first with their legs elevated as recommended by the manufacturer and then with their legs flat on the scanning table. Differences of bone mineral content (BMC), area of the region of interest (ROI), BMD, and T-score of the total spine between the 2 scans were compared. The mean (SD) age of the women was 54.3yr (15yr). Between the 2 scans, BMC, area of the ROI, BMD, and T-scores showed high correlations (r=0.98, 0.94, 0.99, and 0.99, respectively). BMC and the area of the ROI changed significantly between the 2 scans, but the changes of BMD and T-scores were not significant. The percentage changes of BMC and the area of the ROI were similar (2.6% and 2.4%, respectively), whereas T-scores showed no change and change of BMD was only 0.6%. The absolute difference in BMD between the 2 scans was only 0.005 (p=0.09). When spinal BMD was measured with their legs elevated, 31 women were found to have osteoporosis and further 13 were found to have osteopenia. When spinal BMD was measured with their legs flat, 32 women were found to have osteoporosis and further 12 were found to have osteopenia. In conclusion, no clinically or statistically significant difference in the total spinal BMD was found when the BMD in a group of women was measured on a Hologic Discovery DXA scanner with their legs positioned flat.

Key Words: Bone mineral density, leg positioning, precision error