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Reprinted from ref. 12 with permission from the American Society for Bone and Mineral Research.

Reprinted from ref. 12 with permission from the American Society for Bone and Mineral Research.

Knowledge of the frequency of anomalous vertebral segmentation, the characteristic shapes created by the posterior lumbar elements on a PA lumbar spine study and the expected incremental change in BMC and BMD can be used to label the vertebrae correctly. If the vertebrae are mislabeled, comparisons to the normative databases will be misleading. The expected effect of mislabeling T12 as L1 is a lowering of the BMC or BMD at L1, which would then compare less favorably to the reference values for L1. The BMC and BMD averages for L1-4 or L2-4 would also be lowered. The degree to which BMC is lowered by mislabeling is substantially greater than BMD as shown in Table 2-4 (12). The assumption that the lowest set of ribs is found at the level of T12 is often used as the basis for labeling the lumbar vertebrae. As can be seen in Table 2-3, this assumption would result in the vertebrae being labeled incorrectly in 13.3% of the population. As a consequence, all of the criteria noted here should be employed in determining the correct labeling of the lumbar vertebrae. This should obviate the need for plain films for the sole purpose of labeling the vertebrae in the vast majority of instances. Figure 2-7 is a PA spine study in which the labeling of the lumbar vertebra was not straightforward. The characteristic shapes of the vertebrae are easily seen, but no ribs appear to be projecting from what should be T12. Note the block H shape of the vertebra labeled L4 and the visible transverse processes on the vertebra labeled L3. Statistically, it is likely that there are five lumbar vertebrae here with the lowest set of ribs on T11. The appearance of L3 and L4 would also support this labeling. Plains films, acquired for the purpose of diagnosing spine fracture, confirmed that the labeling shown in Fig. 2-7 is correct.

Fig. 2-7. A DXA PA spine study acquired on the Lunar DPX. The vertebra labeled L4 has a classic block H or X shape. No ribs are seen however, protruding from the vertebra that should be T12. It is far more likely that this represents five lumbar vertebrae with the lowest ribs on T11 than six lumbar vertebrae with the lowest ribs on T12. Also note that the BMD at L1 is higher than at L2, which is unusual. A lateral lumbar spine X ray of this patient, shown in Fig. 2-9, confirmed a fracture at L1.

Fig. 2-7. A DXA PA spine study acquired on the Lunar DPX. The vertebra labeled L4 has a classic block H or X shape. No ribs are seen however, protruding from the vertebra that should be T12. It is far more likely that this represents five lumbar vertebrae with the lowest ribs on T11 than six lumbar vertebrae with the lowest ribs on T12. Also note that the BMD at L1 is higher than at L2, which is unusual. A lateral lumbar spine X ray of this patient, shown in Fig. 2-9, confirmed a fracture at L1.

PA and AP lumbar spine bone density measurements are extremely useful in predicting fracture risk and following the effects of a variety of disease processes and therapeutic interventions. Unfortunately, the lumbar spine is also the site most commonly affected by structural changes or artifacts that may affect either the accuracy or precision of the measurement or both.

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