Negative exploration for lumbar disc prolapse--findings and possible causes

32 cases of negative exploration in a joint follow-up series of 3,032 lumbar disc operations are described. Data from 15 hospitals were analyzed by means of questionnaires and computer-aided analysis. Detailed information is given about pre as well as intraoperative findings. The causes of negative lumbar disc exploration can be divided into two groups. The most important causes in the first group were osteochondrotic compression, distended epidural veins, and root tension. The results are less favourable in these patients than in the overall series but, compared to the other complicated courses, the results were markedly better. A review of the literature is given, the possible causes for negative explorations are discussed, and a directive is established for the surgical procedure in cases where an expected prolapse is not found at operation.