DISTRIBUTION OF BONE-MINERAL DENSITY IN THE LUMBAR SPINE IN HEALTH AND OSTEOPOROSIS

The significance of variability in bone mineral density (BMD) between lumbar vertebrae L1 to L4 in the same individual was investigated by dual-energy X-ray absorptiometry in 1000 normal women aged 40-60 years (average 65 years) with vertebral osteoporosis. The mean BMD increased from L1 to L4 in normal women from 0.841 g/cm2 to 1.017 g/cm2, and in osteoporotics from 0.562 g/cm2 to 0.709 g/cm2. Z scores for osteoporotic women (Z = osteoporotic BMD - age-normal BMD/normal SD) were significantly lower for individual vertebrae compared with L1-4 and at L4 compared with L1, L2 and L3 (p < 0.001). The mean difference between Z scores for the highest and lowest vertebrae in an individual was 0.70 for normals (SD = 0.40) and 0.64 for osteoporotics (SD = 0.36). The mean Z score difference between the L1-4 Z score and the lowest individual vertebral Z score was 0.36 for normals (SD = 0.23) and 0.06 for osteoporotics (SD = 0.31). However, receiver operating analysis (ROC) curves showed that the lowest Z score for any individual vertebra did not provide improved discrimination between normals and osteoporotics when compared with the L1-4 Z score. The area under the ROC curve for L1-4 was significantly greater than for individual vertebrae (p < 0.05) and that for L4 was significantly smaller than for L1, L2 or L3 (p < 0.001). In conclusion, L1-4 BMD gives greater diagnostic sensitivity for osteoporosis than individual vertebrae, and L1, L2 and L3 are better than L4. Although there is considerable individual variation in Z scores for different vertebrae in the same individual, the lowest vertebral Z score does not offer improved diagnostic sensitivity over the L1-4 Z score.