Role of Iliolumbar Ligament in Identification of L5 Vertebra on MRI in Lumbosacral Transitional Anomalies

Labeling a transitional vertebra (lumbarization or sacralization) as "LSTV" helps in simplifying the reporting process but leads to mis-numbering and must be avoided, as reliable identification of vertebral levels is essential for surgical interventions and image guided interventions. Many parameters have been described to identify the vertebral level; however, there are contradictory results in literature regarding their usefulness. Objective: To determine the role of iliolumbar ligament (ILL) on MRI as an identifier of the L5 vertebra in cases of LSTV anomalies. Methods: 200 patients with confirmed L5 level (identified on MRI screening of whole spine) were included in the study. These were further divided into group I (159 patients), II (31 patients) and III (10 patients) consisting of normal spine, spine with sacralization and with lumbarization respectively. The level(s) of origin of the ILL was documented in all of them and analysis was done. Results: ILL was found bilaterally in all patients. All patients with normal lumbosacral junction had ILL arising from L5 bilaterally. Among sacralization group, 23 patients had ILL arising from L4, 2 patients had ILL arising from L5 and 6 patients had ILL arising from L4 on sacralized side and from L5 contralaterally. Among Lumbarization group 2 patients had ILL arising from S1, 5 patients had ILL arising from L5 and 3 patients had ILL arising from S1 on side of lumbarization and from L5 contralaterally. Conclusion: Our study demonstrated that ILL is not a reliable identifier of the L5 vertebra in the setting of LSTV anomalies. It rather identifies the lowest lumbar-type vertebral segment only.