Lateral Spine Densitometry in Obese Women

The lateral (LAT) spine scan has been suggested as a more sensitive measure than posterior-anterior (PA) scanning for assessing age-related bone loss in normal-weight postmenopausal women. The measurement error of PA and LAT bone mineral density (BMD) using dual energy X-ray absorptiometry (DXA) has also been shown to rise with incremental increases in fat and from large variance in fat thickness, respectively. The purpose of this cross-sectional study was to determine specific affects of obesity on paired PA and LAT lumbar (L₂–L₄) BMD and Z score (BMD of patient versus age-matched reference database) correlation in 30 obese postmenopausal women (mean BMI ± SD = 33.3 ± 4.06). The mean PA and LAT BMD ± SD were 0.946 ± 0.123 and 0.749 ± 0.134, respectively. The mean PA and LAT Z scores were −0.17 ± 1.15 and 0.80 ± 1.7. The correlation between PA and LAT BMD was significantly lower (r = 0.55; P < 0.05) than previously reported, and PA and LAT Z score correlation was (r = 0.57; P= 0.0016). After adjusting for body mass index (BMI), percent body fat, fat mass, and truncal fat by DXA, waist:hip ratio (WHR) and visceral and subcutaneous abdominal fat by computerized axial tomography (CT), PA and LAT Z score correlation increased to r = 0.62; P= 0.0065. In our subjects, the mean LAT Z score was 4.6 times higher than the mean AP Z, contrary to previous observations in normal-weight postmenopausal women. Our findings may be due to increased soft tissue composition and fat inhomogeneity in the LAT scanning field resulting in increased X-ray attenuation in obesity. Received: 22 July 1997 / Accepted: 26 January 1998