Purpose <p>Discrepancies between clinical and imaging findings complicate accurate diagnoses of spinal levels in lumbar disorders. Extreme variations in the 12th rib length may be associated with cranial or caudal shifts of the lumbosacral plexus; however, the anatomical basis remains speculative because of limited in vivo visualization. This study assessed whether the conus medullaris position and L4 morphology—particularly the branch to the lumbosacral trunk (LST)—can be visualized using magnetic resonance imaging (MRI) and their correlations with the 12th rib length and symptom deviation.</p> Methods <p>This study included 127 patients who underwent lumbar disc herniation surgery and 25 cadaveric specimens. MRI and radiography were used to measure the conus medullaris level, LST branch visibility, and 12th rib length. Cadaveric dissections evaluated the LST branch thickness and its association with the rib length and plexus position.</p> Results <p>Patients with symptoms of caudal shift had shorter 12th ribs, a more cranially located conus medullaris, and more frequent visibility of the LST branch using MRI (all <i>p</i> &lt; 0.001). LST branches visible with MRI were linked to shorter ribs and higher conus levels. The conus position positively correlated with the rib length (<i>r</i> = 0.36). A cadaveric analysis revealed a strong inverse correlation between the rib length and LST branch thickness (<i>r</i> = –0.86).</p> Conclusion <p>The 12th rib length, conus medullaris level, and L4 morphology reflect cranial–caudal shifts in neural structures and symptoms. These anatomical markers may be clinically useful predictors of level-specific symptoms and support more accurate surgical planning.</p> Graphical abstract <p></p>