How Reliable Are Routine Radiological Imaging Methods in Diagnosing Lumbosacral Transitional Vertebrae and Identifying the Pathological Disc Level?

Lumbosacral transitional vertebrae (LSTV) include morphological variants in which L5 is partially or completely fused with the sacrum (sacralization), or in which S1 assumes additional lumbar characteristics (lumbarization). The most commonly used classification is the Castellvi system, which categorizes these variations from Type I to Type IV. Based on data from the past decade, the prevalence of LSTV ranges between 7% and 36%. LSTV may restrict motion at the transitional segment and consequently lead to disc degeneration in adjacent levels. One study reported that the Pfirrmann grade of the disc above the transitional vertebra was significantly higher in individuals with LSTV. The most reliable method for diagnosing LSTV is counting vertebrae caudally from the C2 vertebra on whole-spine imaging. Since the presence of LSTV can result in incorrect vertebral level identification during surgical or interventional procedures, recognizing these variations is essential.

Transitional segments located in the thoracolumbar junction that display both thoracic and lumbar characteristics are defined as thoracolumbar transitional vertebrae (TLTV). TLTV commonly involves atypical rib formation at the last thoracic or first lumbar vertebrae. The presence of a 13th rib-also termed a lumbar rib-is another variant that complicates vertebral level identification. In radiography, the last vertebra with a true rib is generally accepted as T12, and lumbar vertebrae are counted accordingly. However, in some individuals, the transverse process of the first lumbar vertebra develops a rib-like morphology. This structure is referred to as a lumbar rib and may create ambiguity during lumbar level identification on radiographs. Although numerous studies have examined the prevalence and clinical implications of LSTV, data on TLTV remain insufficient.

Various imaging modalities and anatomical markers have been used for detecting transitional vertebrae. The most reliable method for identifying both LSTV and TLTV is whole-spine MRI or CT imaging that allows vertebral counting from C2 caudally. This approach directly reveals segmental anomalies and numbering variations. Although anatomical markers such as the aortic bifurcation, inferior vena cava bifurcation, right renal artery, and conus medullaris have been proposed for determining lumbar vertebral levels, their anatomical variability limits their reliability, particularly when used in isolation. Moreover, studies have shown that the position of these markers shifts depending on the presence of transitional vertebrae: markers tend to be positioned more caudally in sacralized vertebrae and more cranially in lumbarized vertebrae.

In this cross-sectional study, patients presenting with low back and leg pain to the Algology Division of the Department of Physical Medicine and Rehabilitation at Marmara University, who are diagnosed with lumbar radicular pain based on clinical evaluation and MRI, will be assessed. Demographic data, including age, sex, height, weight, and body mass index, will be recorded.

In the first stage of evaluation, the level of the disc herniation responsible for lumbar radicular pain will be determined on routine lumbar MRI sequences using anatomical markers described in the literature (right renal artery, conus medullaris, aortic and inferior vena cava bifurcations). Thoracolumbar and lumbosacral two-view radiographs will also be evaluated to identify transitional vertebrae and to note the presence of lumbar ribs, rudimentary thoracic ribs, or other costal anomalies. For each anatomical marker, the disc level suggested by that marker will be recorded separately, and the level indicated by the majority of markers will also be noted. If a transitional vertebra is identified on radiographs, the disc level determined on MRI will be revised accordingly.

In the second stage, an independent, blinded investigator will determine the presence of TLTV and LSTV, as well as the pathological disc level, using a sagittal whole-spine MRI localizer that includes the entire spinal axis from C2 to the sacrum. In the final analysis, the agreement between the two assessment methods will be examined. The study will determine the accuracy of level identification based on axial and sagittal lumbar MRI sequences and thoracolumbar/lumbosacral radiographs. Additionally, the detectability of lumbar ribs or rib agenesis based solely on radiographic findings will be evaluated. Finally, the true anatomical level corresponding to each marker will be identified using the whole-spine localizer and reported accordingly.