IMPACT OF ANOMALOUS VERTEBRAL SEGMENTATION ON MEASUREMENTS OF BONE-MINERAL DENSITY

In anteroposterior (AP) bone mineral density (BMD) measurements of the lumbar spine (LS), the presence of ribs is used to identify vertebra T12. Similarly, in lateral LS-BMD measurements, the position of the il iac crest is used to identify the lumbar vertebrae. The aim of this st udy was to determine the impact of variations in spinal segmentation a nd iliac crest position on BMD measurements. In 375 women (ages 50-85 years) radiographs were taken of the thoracic and lumbar spine, as wel l as AP measurement of LS-BMD, by dual-energy x-ray absorptiometry (DX A). In 121 subjects lateral decubitus LS-BMD was also measured. Anomal ous spinal segmentation was found in 16.5%, and L1 would have been inc orrectly identified on the AP-DXA image in 13%. The change in BMC and BMD between adjacent vertebrae was greater in the upper than the lower lumbar spine. Misidentification of L1 for T12 resulted in underestima tion of the bone mineral content in grams (BMC) of L1 by a mean of 11. 5 +/- 14.4% (SD; range -33.5 to 33.5%). For the usual region of intere st, L2-4, the BMC (g) was underestimated by 8.4 +/- 8.7% (range -1.5 t o 29.2%), with the BMD (g/cm2) underestimated by 3.6 +/- 4.8% (range - 5.4 to 11.6%). The position of the iliac crest on the lateral decubitu s DXA scans would have led to misidentification of either L2 or L4 for L3 in 15 cases (12%). This resulted in the BMD of L3 being underestim ated by 2.7 +/- 19.4% (range -242.4 to 34.6%). We conclude that (1) an atomic variations in spinal segmentation are common; (2) the impact of mididentification of vertebra L1 on the AP scan is considerable if on ly one lumbar vertebra is studied but small if a larger area is analyz ed and expressed as g/cm2; and (3) misidentification of L3 in the late ral projection can result in a large error in the BMD measurement of L 3, although the mean error in terms of a population is small. Thus the impact of misidentification of lumbar vertebrae does not result in im portant overall changes when studying groups of subjects but on an ind ividual basis could lead to incorrect diagnosis of osteopenia.