Background Despite the high prevalence of lumbosacral transitional vertebrae (LSTV), little is known about the segmental innervation in this condition.

Methods The authors performed a prospective comparative clinical evaluation and an intraoperative electromyographic (EMG) investigation on patients with six lumbar vertebral bodies (6LVB) and on patients with five lumbar vertebrae (5LVB). First, clinical pain distribution in 80 patients (46 patients with 6LVB, 34 patients with 5LVB) with degenerative lumbar diseases were analysed between patient groups. Intraoperative EMG monitoring of five lower-limb muscles was performed. Compound muscle action potentials were obtained from 100 nerve roots of our 80 patients.

Results The EMG results compared fairly to the clinical findings: 40 CMAPs from 5LVB and 60 CMAPs from 6LVB patients were compared with each other within L3 to S levels. First, there was no difference between groups in the pattern of radicular pain and myotomal innervation at the level L3/4 and L4/5 (p=0.39–1.0). Second, the nerve root stimulated at the L5/6 level compares to the S1 root in 5LVB patients; the only difference was found in a coinnervation of the biceps femoris muscle that is less frequent in 6LVB patients (p=0.02). Third, the nerve root at the L6/S level corresponds to the S1 as well as to the S2 root in 5LVB patients.

Conclusion Intraoperative EMG monitoring of surgically decompressed nerve roots was found to be the ideal means of unequivocal determination of segmental innervation in LSTV patients.