Reliability of the L3 Transverse Process as a Landmark for Lumbar Spine Enumeration Across Different Vertebral...

Background/Objectives: Vertebral numbering on limited lumbosacral imaging is difficult when the thoracic or lumbar count varies. The L3 transverse process has been proposed as a landmark because it is often the longest, but this has not been tested across segmentation patterns. This study assessed whether L3 as the level bearing the longest transverse process is independent of thoracic and lumbar vertebral count on whole-spine CT. Methods: Of 499 trauma CT examinations with confirmed numbering, all 49 non-standard thoracolumbar formulas were included, together with 17 C7T12L5 spines that remained after exclusion of transitional and numerical anomalies at every level (n = 66). Bilateral lumbar transverse-process lengths were measured on multiplanar reconstructions. The longest level was identified by morphological and presacral numbering. Associations with thoracic patterns (C7T11, n = 10; C7T12, n = 43; C7T13, n = 13) were tested with the Fisher-Freeman-Halton exact test and Monte Carlo simulation. Nine primary analyses were adjusted with the Holm procedure. Inter-observer agreement was assessed with an intraclass correlation coefficient. Results: In this enriched cohort, L3 was the longest level in 45/66 cases (68.2%) based on the single longest transverse process and 49/66 (74.2%) based on bilateral mean length. These percentages are not population prevalences. Within the C7T12 group, the corresponding proportions were 76.7% and 83.7%, respectively. By bilateral mean length, L3 was longest in 30.0% of C7T11, 83.7% of C7T12 and 76.9% of C7T13 cases (exact p = 0.003, Holm-adjusted p = 0.014). The single-process association did not remain significant after Holm correction (adjusted p = 0.058). The 22nd presacral vertebra showed a comparable dependence on thoracic pattern (Holm-adjusted p = 0.041 and 0.001 for the single-process and bilateral-mean analyses, respectively). Inter-observer ICC was 0.88 for individual transverse-process lengths and 0.93 for bilateral means. Conclusions: Neither L3 nor the 22nd presacral vertebra is a segmentation-independent marker of lumbar identity. Full-spine counting remains necessary when precise numbering is required.