Variation of the lumbosacral myotomes with bony segmental anomalies.

Clinical localisation of a disc prolapse required dependable knowledge of the muscles supplied by the lumbosacral nerve roots. Localisation is most difficult in the 10 per cent of patients who have lumbosacral bony segmental anomalies. The lumbosacral plexus has been dissected in 11 cadavers with such anomalies and electrical stimulation studies carried out in 15 patients similarly afflicted. It is suggested that whatever the anomaly the "last fully mobile level" should be identified as the lowest level with a fully formed disc space, bilateral facet joints and two free transverse processes which do not articulate with the sacrum or pelvis. In three out of four patients with bony segmental anomalies the fifth lumbar root emerges at the last fully mobile level.