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Study Design: A retrospective study. Purpose: To examine the effectiveness of using an electrodiagnostic technique as a new approach in the clinical diagnosis of extraforaminal stenosis at L5–S1. Overview of Literature: We introduced a new effective approach to the diagnosis of extraforaminal stenosis at the lumbosacral junction using the existing electrophysiological evaluation technique. Methods: A consecutive series of 124 patients with fifth lumbar radiculopathy were enrolled, comprising a group of 74 patients with spinal canal stenosis and a second group of 50 patients with extraforaminal stenosis at L5–S1. The technique involved inserting a pair of needle electrodes into the foraminal exit zone of the fifth lumbar spinal nerves, which were used to provide electrical stimula tion. The compound muscle action potentials from each of the tibialis anterior muscles were recorded. Results: The distal motor latency (DML) of the potentials ranged from 11.2 to 24.6 milliseconds in patients with extraforaminal stenosis. In contrast, the DML in patients with spinal canal stenosis ranged from 10.0 to 17.2 milliseconds. After comparing the DML of each of the 2 groups and at the same time comparing the differences in DML between the affected and unaffected side of each patient, we concluded there were statistically significant differences ( p