Background:The treatment of high-grade developmental spondylolisthesis (HGDS) is still challenging and controversial.In this study,we investigated the efficacy of the posterior reduction and monosegmental fusion assisted by intraoperative three-dimensional (3D) navigation system in managing the HGDS.Methods:Thirteen consecutive HGDS patients were treated with posterior decompression,reduction and monosegmental fusion of L5/S1,assisted by intraoperative 3D navigation system.The clinical and radiographic outcomes were evaluated,with a minimum follow-up of 2 years.The differences between the pre-and post-operative measures were statistically analyzed using a two-tailed,paired t-test.Results:At most recent follow-up,12 patients were pain-free.Only 1 patient had moderate pain.There were no permanent neurological complications or pseudarthrosis.The magnetic resonance imaging showed that there was no obvious disc degeneration in the adjacent segment.All radiographic parameters were improved.Mean slippage improved from 63.2％ before surgery to 12.2％ after surgery and 11.0％ at latest follow-up.Lumbar lordosis changed from preoperative 34.9 ± 13.3° to postoperative 50.4 ± 9.9°,and 49.3 ± 7.8° at last follow-up.L5 incidence improved from 71.0 ± 11.3° to 54.0 ± 11.9° and did not change significantly at the last follow-up 53.1 ± 15.4°.While pelvic incidence remained unchanged,sacral slip significantly decreased from preoperative 32.7 ± 12.5° to postoperative 42.6 ± 9.8°and remained constant to the last follow-up 44.4 ± 6.9°.Pelvic tilt significantly decreased from 38.4 ± 12.5° to 30.9 ± 8.1 ° and remained unchanged at the last follow-up 28.1 ± 11.2°.Conclusions:Posterior reduction and monosegmental fusion ofL5/S 1 assisted by intraoperative 3D navigation are an effective technique for managing high-grade dysplastic spondylolisthesis.A complete reduction of local deformity and excellent correction of overall sagittal balance can be achieved.