Effect of Hip Flexion on the Measurement of Spinal Bone Mineral Density in the Norland Eclipse XR

It is recommended that the hip joints be flexed to 90° during dual-energy X-ray absorptiometry scanning of the lumbar spine in the anteroposterior projection; however, some patients are unable to maintain this position because of the presence of degenerative changes in lower limb joints. This study examines the effect of a lesser degree of hip flexion on the lumbar spine bone mineral density (BMD) measurement and its interpretation. Fifty women were scanned on the Norland Eclipse XR, initially in the standard position with the hips flexed to 90° and then in the adjusted position after allowing for some degree of hip extension to keep them comfortable (hip flexion of 60°-70°). Higher bone mineral content (BMC), surface area, and BMD values were seen in the standard position compared to the adjusted position, but none of the differences was statistically significant. There were strong correlations for BMC, surface area, and BMD measured in the two positions. In the standard position, 26 women were found to have osteoporosis and 18 had osteopenia. In the adjusted position, osteoporosis was noted in 27 women, and 18 had osteopenia. Four women showed a reduction, whereas 12 women showed an increase in BMD in excess of the least significant change at the 95% confidence level, defined as 2.77 times the precision error (0.008 g/cm2 × 2.77 = 0.120 g/cm2). Our study demonstrates that a lesser degree of hip flexion in women who find it difficult to maintain the recommended 90° hip flexion during the lumbar spine BMD measurement would not affect the patient classification based on T-scores recommended by the World Health Organization; however, variation in hip flexion can be a major confounding factor when interpreting a change in BMD over time.