Localizing the L5 Vertebra Using Nerve Morphology on MRI: An Accurate and Reliable Technique

Accurate and reliable spine numbering is important for the diagnosis of pathology and preprocedure planning. This can be challenging in patients with vertebral numeric variation (VNV) or lumbosacral transitional vertebrae (LSTV), particularly when full spine imaging is unavailable. VNV refers to the variation of the total number of presacral vertebrae (PSV). Approximately 89% of the population have 24 PSV (5 lumbar-type vertebrae), 8% have 25 PSV (6 lumbar-type vertebrae), and 3% have 23 PSV (4 lumbar-type vertebrae).1 LSTV are congenital spinal anomalies in which an elongated transverse process of the last lumbar vertebra fuses with the “first” sacral segment to varying degrees.2 The morphologic variation of LSTV can range from partial/complete L5 sacralization to partial/complete S1 lumbarization.3,4 The prevalence of LSTV in the population varies throughout the literature because of differences in definition and diagnostic modalities.1,4⇓-6 LSTV can also vary with sex, with lumbarization of S1 seen more commonly in women and sacralization found to be more common in men.3 A person can have VNV without LSTV, or conversely, one can have LSTV without VNV.1 Approximately 5% of subjects have been found to have both.1

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Multiple anatomic landmarks have been used to determine the lumbar vertebral level in cases without full spine imaging. A leading method of localizing the iliolumbar ligament, most frequently arising from L5, has been found less accurate in the setting of LSTV and VNV.7⇓⇓⇓-11 Other landmarks, including the level of the conus, right renal artery, superior mesenteric artery, aortic bifurcation, and iliac crest height, are also less accurate.9,12⇓-14 Choosing the appropriate level for surgical or interventional procedures is essential and relies on accurately and reliably numbering the spine in patients with “normal” anatomy as well as those with variant or transitional anatomy.4,15 This is especially important in patients with LSTV and/or VNV undergoing surgical planning, as up to 32% of neurosurgeons have reported an event of wrong-level spinal surgery occurring at least once in their careers.16 LSTV can also create challenges for approach in interventional pain procedures and can increase the risk of iatrogenic vascular injury.17

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Multiple imaging modalities have been used to evaluate LSTV and VNV, with MR imaging found to be most reliable.18 Anteroposterior radiographs have demonstrated high intermodality agreement with MR imaging.19 Studies show that one can accurately number the vertebrae by counting down from C2 to the sacrum on sagittal MR imaging by using a cross-referencing tool.1,8,19,20 Although most counting methods have focused on the ossified structures, 1 postmortem study numbered the vertebrae by dorsal spinal nerve morphology and found up to 95% probability that the lower spinal nerves correspond to their respective spinal segment.21 We hypothesized that nerve morphology on lumbar spine MR imaging would aid in L5 vertebra localization, particularly when full spine imaging was not available. We aimed 1) to determine whether MR imaging morphologic features of the lumbar nerves could be used to distinguish the lower lumbar levels and 2) to apply these characteristics in localizing the L5 vertebra.