We aimed to develop a better understanding and method of rating the success or failure of low back surgery by studying 185 patients prospectively. Identical pre-operative and postoperative assessment by an independent observer included pain, disability, physical impairment, psychological distress and illness behaviour. Outcome was assessed by the patient, by the observer and by return to work. There was 96% follow-up at two years. Correlation co-efficients varied considerably between the various measures of outcome, both patient and observer appearing to base their assessment mainly on postoperative status rather than on any change produced by surgery. The observer was influenced most by postoperative pain, disability and physical impairment. Patients were influenced most by residual physical impairment, type of surgery and proportional change in disability. Return to work was moderately influenced by postoperative disability and to a larger extent by social and work-related factors. We developed a simple formula to judge overall success or failure which accurately reproduced the combined assessment of patient and observer. If surgical audit is to be meaningful it must be based on an improved understanding of how the outcome of surgery should be assessed.

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.