Impact of anomalous vertebral segmentation on measurements of bone mineral density

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Abstract
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 iliac crest is used to identify the lumbar vertebrae. The aim of this study was to determine the impact of variations in spinal segmentation and iliac crest position on BMD measurements. In 375 women (ages 50–85 years) radiographs were taken of the thoracic and lumbar spine, as well as AP measurement of LS‐BMD, by dual‐energy x‐ray absorptiometry (DXA). In 121 subjects lateral decubitus LS‐BMD was also measured. Anomalous spinal segmentation was found in 16.5%, and L1 would have been incorrectly 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 underestimation 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 interest, L2–4, the BMC (g) was underestimated by 8.4 ± 8.7% (range ‐1.5 to 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 decubitus 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 underestimated by 2.7 ± 19.4% (range ‐ 242.4 to 34.6%). We conclude that (1) anatomic variations in spinal segmentation are common; (2) the impact of mididentification of vertebra L1 on the AP scan is considerable if only one lumbar vertebra is studied but small if a larger area is analyzed and expressed as g/cm2; and (3) misidentification of L3 in the lateral projection can result in a large error in the BMD measurement of L3, although the mean error in terms of a population is small. Thus the impact of misidentification of lumbar vertebrae does not result in important overall changes when studying groups of subjects but on an individual basis could lead to incorrect diagnosis of osteopenia.
Funding Information
  • Arthritis and Rheumatism Council (R44)