Effect of Spinal Segment Variants on Numbering Vertebral Levels at Lumbar MR Imaging

PURPOSE: To verify iliolumbar ligament (ILL) location, to evaluate magnetic resonance (MR) imaging morphologic features for detecting lumbosacral transitional vertebrae (TVs) (LSTVs), and to determine whether transitional situations are associated with anomalous vertebral numbering. MATERIALS AND METHODS: Investigational review board approval was obtained for this HIPAA-compliant retrospective study. A review of 147 subjects was performed by using spine radiography as the reference standard to determine total and segmental vertebral count and transitional anatomy. Thoracolumbar TVs (TLTVs) and LSTVs were identified. The lumbosacral intervertebral disk angle (LSIVDA), defined as the angle between the endplates, was measured, S1-2 disk morphology was rated according to the classification by O'Driscoll et al, and the ILL level was determined from MR images. Statistical analysis was performed by using χ(2) tests for dichotomous and ordinal variables and the t test for continuous variables. RESULTS: An anomalous total number of vertebrae were present in 12 (8.2%) of 147 subjects. The ILL was identified in 126 (85.7%) of 147 subjects and was present at L5 in 122 (96.8%) subjects; the remaining four (3.2%) subjects had an anomalous total number of vertebrae. A complete S1-2 intervertebral disk was associated with LSTVs (P = .004); however, LSIVDA was not (P = .2). TLTVs were present in six (4.1%) and LSTVs were present in 22 (15.0%) of 147 subjects. Both were present in four (2.7%) subjects. The presence of a TLTV was associated with a higher incidence of a concomitant LSTV and vice versa (P < .001; odds ratio OR, 13.7; 95% confidence interval CI: 2.7, 68.4]). A TLTV was not associated with an anomalous total number of vertebrae (P = .46), but an LSTV was (P < .001; OR, 7.4; 95% CI: 2.2, 24.8). CONCLUSION: The ILL denotes the lowest lumbar vertebra, which does not always represent L5. A well-formed, complete S1-2 intervertebral disk is associated with LSTVs, but alteration in LSIVDA is not. LSTVs are associated with anomalous vertebral numbering.