RESUMEN
BACKGROUND: For patients who have back pain or radiating pain, lumbar intervention should be performed at the correct lumbar segment that triggers pain. It is quite common for pain physicians to identify lumbar segments based upon the 12th pair of ribs to do an interventional procedure. CASE: We experienced two cases of rib number abnormality (absent 12th rib pair) that made the injection ineffective. In both cases, we had misidentified the lumbar segmentation due to rib abnormality. Although the procedure was performed properly, the clinical symptoms of the patient were not well correlated with the dermatome, and the diagnosis was delayed. CONCLUSIONS: These cases suggest that rib counting is necessary for more accurate lumbar segmentation. If the expected effect has not appeared after intervention, rib numbers should be checked.