We are presenting a retrospective study comparing the results of insitu posterolateral fusiorı and instrumenta-tion with posterolateral fusion (with or vvithout reduction) performed in 18 (13 female and 5 male) patients with spondylolisthesis. The mean age was 46 (range 20-63), average follow-up was 3.5 (range 1.5-8) years. There was grade 1 spondylolisthesis in 2 patients, grade 2 in 9, grade 3 in 5, grade 4 in one and grade 5 spondylolisthesis (spondyloptosis) with para-aortic neurofibroma in one patient. We performed in situ posterolateral fusion in 10 patients and posterolateral fusion with instrumentation in the remaining 8. Decompression was added in 3 of the cases with instrumentation and reduction in 4 of them. Reduction and decompression were combined in the remaining one patient. in addition to reduction, instrumentation and posterolateral fusion, anterior fusion was also performed in the patient with spondyloptosis. VVhile there were no nonunions in the instrumented group, 2 cases with insitu posterolateral fusion failed to ünite. Infection which responded to intravenous antibiotics and debride-ment occurred in two instrumented cases. Radicular type pain was diminished in ali the patients which we achieved union. When reduction was added, in addition to obtaining good decompression, fusion and radicular pain relief, improvement in low back pain due to mechanical imbalance was observed. The two cases who devel-ope pseudoarthrosis were revised with reduction, decompression, instrumentation and posterolateral fusion; union was obtained in both cases. İn conclusion, insitu posterolateral fusion appears to have a higher rate of pseu-doatrhrosis, but in those cases in which union is achieved, there is not any significant difference in radicular pain relief from those vvhich are instrumented. Addition of reduction distinctly improves mechanical pain as well as the radicular pain. İn the instrumented cases however, Ihe infection rate is higher.

Keywords: Spondylolisthesis, in situ fusion, instrumentation