ObjectiveTo observe the character of spino-pelvic sagittal alignment in patients with high-grade L5 isthmic spondylolisthesis, and to analyze the sagittal alignment alteration after operation. MethodBetween January 2009 and June 2014, 25 patients with high-grade L5 isthmic spondylolisthesis underwent posterior surgery, and the clinical data were retrospectively analyzed as study group. There were 14 males and 11 females with a mean age of 42.5 years (range, 20-65 years). The mean disease duration was 6 months (range, 3-12 months). According to the Meyerding evaluating system, 15 cases were rated as degree III, and 10 cases as degree IV. Eighty healthy adult volunteers were recruited simultaneously as control group. The lumbar sacral angle (LSA), pelvic incidence (PI), pelvic tilt (PT), sacral slope (SS), lumbar lordosis (LL), thoracic kyphosis (TK), and sagittal vertical axis (SVA) were measured on preoperative and last follow-up standing full length lateral X-ray films. And these parameters were compared between study group (preoperative parameters) and control group. Then the patients in study group were divided into 2 subgroups according to Hresko's method:the balanced pelvis subgroup (n=14) and unbalanced pelvis subgroup (n=11) . The sagittal parameters were compared between 2 subgroups, and the alteration of sagittal parameters before surgery and at last follow-up was analyzed in each subgroup respectively. ResultsAll patients in study group were followed up 18 months on average (range, 6-48 months). After surgery, spondylolisthesis was reduced from degree III to degree 0 in 12 cases and to degree I in 3 cases, and from degree IV to degree 0 in 6 cases and to degree I in 4 cases. Bone fusion was obtained in all patients at last follow-up. The preoperative PI, SS, PT, and SVA of study group were significantly greater than those of control group (P<0.05) , while the TK was significantly smaller than that of control group (P<0.05) . In the balanced pelvis subgroup, LSA, LL, and SVA at last follow-up significantly decreased while TK significantly increased when compared with preoperative ones (P<0.05) . In the unbalanced pelvis subgroup, LSA, PT, and SVA at last follow-up significantly decreased while SS, LL, and TK significantly increased when compared with preoperative ones (P<0.05) . The preoperative LSA and PT in the unbalanced pelvis subgroup were significantly greater, while SS, LL, and TK were significantly smaller than those of balanced pelvis subgroup (P<0.05) ; while at last follow-up, significant differentce was found only in LSA between 2 subgroups (P<0.05) . ConclusionsThe LSA should be paid more attention in surgery to assure recovery of the sagittal balance because patients with high-grade L5 isthmic spondylolisthesis have greater PI, abnormal lumbosacral kyphosis, and sagittal imbalance.