BACKGROUND CONTEXT: In situ fusion is the gold standard method of treatment of spondylolisthesis. There is no study in the literature evaluating the effect of sagittal contour realignment on clinical outcomes in comparison with the addition of anterior slippage reduction. PURPOSE: The correction of sagittal plane vs. reduction with instrumentation in the patients with low or high dysplastic spondylolisthesis. STUDY DESIGN/SETTING: A prospective randomized study in patients treated with the same surgical team at the same center. PATIENT SAMPLE: 40 patients, 20 with low and 20 with high dysplastic spondylolisthesis (mean age: 33.1±10.6; average follow-up: 37.9±11.9 Mo.). OUTCOME MEASURES: The extent of displacement, lumbosacral angle values, lumbar sagittal contours, correction rates, JOA scores, SRS-22 questionnaire were evaluated preoperatively, postoperatively, and at the final visit. Fusion rates, complications and quality of fusion were recorded. METHODS: 4 groups of patients were generated. Only posterolateral fusion, neural decompression, and sagittal plane correction with posterior instrumentation using 3rd generation instrumentation system transpedicular screws was accomplished in 20 patients (low dysplastic: 10 patients, high dysplastic: 10 patients). Additional reduction of anterior slippage was done in the remaining 20 patients (low dysplastic: 10 patients, high dysplastic: 10 patients). RESULTS: No statistically significant difference was found between low vs. high dysplastic patients and between patients with sagittal contour realignment vs. patients with additional anterior slippage reduction (p>0.05). The correction rates for displacement were statistically similar at the final visit. Postoperative and final JOA and SRS scores were similar between in situ fusion and reduction groups (p>0.05). A solid fusion mass of 77.5 % was achieved in both groups. CONCLUSIONS: A high percentage of fusion was achieved with posterolateral in situ fusion with or without reduction; and an additional reduction procedure did not have a statistically detectable impact on clinical outcomes. Successful fusion and neural decompression were the most important parameters that have an impact on clinical outcomes in patients with developmental spondylolisthesis, irrespective of the extent of preoperative displacement and the type of listhesis
