Clinical significance of heterogeneity of vertebral mineral density

Important variations in Z-score per vertebra, which is a common expression of bone mineral density (BMD), are sometimes observed. The present study evaluates the clinical significance of this heterogeneity. Normal and osteoporotic subjects were defined by using strict criteria. For every scan, the minimal Z-score (the vertebra with the lowest Z-score) and the delta Z (highest Z-score--lowest Z-score) was calculated. Of the investigated subjects, 30% presented a delta Z > or = 1. No significant correlation could be found between delta Z and age, BMD, height and weight. There was no difference in delta Z between scans of good, average or poor quality. Osteoporotic subjects had significantly lowered BMD values, whether evaluated through Z-scores for the L2-L4 site (P < 0.001; t = 3.71) or by minimal Z-score (P < 0.001; t = 3.97). Reproducibility calculated for the L2-L4 site on phantoms as well as on patients was excellent (C.V. < 1%). When reproducibility was calculated on each vertebra in vitro or in vivo, an increase in variability was observed. These data show that marked heterogeneity in BMD per vertebra is not infrequent. In some subjects low BMD may be measured at certain vertebrae but not at the total site. Our data suggest that in those cases the lowest BMD should be considered. In follow-up studies however, the BMD should be calculated on the L2-L4 segment, since a loss of precision is observed when only one vertebra is measured.