The postoperative lumbar spine. A radiological investigation of the lumbar spine after discectomy using MR imaging and CT.

Problems and aims: The clinical outcome of repeat lumbar discectomy is not as satisfactory as that of the primary surgical procedure. One reason is the difficulty of assessing post-operative radiological investigations and particularly the significance of the various radiological changes. A second problem is differentiating between the two forms of post-operative discitis, a rare but important complication after lumbar discectomy. The aims of this study were: to improve understanding of the significance of certain post-operative radiological changes; and to evaluate differences in the MR features of septic and aseptic post-operative discitis.

Material and methods: A total of 192 patients (209 disc levels) took part in this study; all had had prior lumbar discectomy. Nineteen of these patients were regarded as asymptomatic and they served as a control group in the evaluation of certain post-operative changes in the symptomatic patients. Twelve patients with post-operative discitis were examined in order to compare the MR features of septic and aseptic discitis. MR imaging was performed first without and then with contrast enhancement in all patients. Contrast-enhanced MR imaging was also compared with contrast-enhanced CT.

Results: Owing to its superiority in distinguishing the nerve roots at the surgical site, MR imaging was found to be a more valuable diagnostic method than CT. Disc herniations were found in 16% of the disc levels in asymptomatic patients and in 38% of the disc levels in the symptomatic patients. Significantly more disc herniations were found in patients who had only a short duration of recurrent symptoms (maximum 3 months) before MR investigation than in the asymptomatic patients. Nerve-root displacement due to disc herniation was also significantly more frequent in patients with the short symptom duration than in patients with a longer symptom duration. True intradural nerve-root enhancement was found in 7% of symptomatic patients, and focal enhancement in the root sleeve was found in 26% of them; there was good correlation to clinical symptoms and other pathological findings. Thickened nerve roots were found with equal frequency in asymptomatic and symptomatic patients. Epidural scar tissue diminished with time, showing no significant difference between asymptomatic and symptomatic patients. Out of 6 patients with septic post-operative discitis, 3 showed extensive MR changes; the remaining 3 showed moderate changes which were similar to those in another 6 patients who had aseptic discitis.

Discussion and conclusion: MR is the imaging method of choice in the evaluation of patients with recurrent clinical symptoms after disc surgery. Disc herniations may be found in asymptomatic patients; it is therefore important to assess the plausibility of the assumption that the finding of a herniated disc correlates well with the actual clinical symptoms. Nerve-root displacement and nerve-root enhancement caused by recurrent disc herniation may strengthen the indication for repeat discectomy. On the other hand, the finding of a thickened nerve root seems to be of no diagnostic value. The MR features in post-operative discitis develop only gradually and the differentiation between septic and aseptic forms of discitis is thus difficult at the early stage.