Long fusions to S1 with or without pelvic fixation can induce relevant acute variations in pelvic incidence: a retrospective cohort study of adult spine deformity surgery

Purpose: The purpose of this study is to verify if any changes occur in pelvic incidence (PI) in adult patients undergoing long fusion to sacrum for spine deformity and to describe the effect of fixation to pelvis on these variations. Methods: We conducted a cross sectional study. Sixty-six adults patients, 87% females (mean ± SD age: 65.1 ± 7.6 years), undergoing fusion from the thoracic spine to the sacrum for adult spine deformity were included. Patients were divided in two different groups: Group A: sacral fixation alone and Group B: sacral fixation plus pelvis fixation. Pre and postoperative standardized full-standing X-rays were analyzed with measurement of: PI, pelvic tilt (PT), lumbar lordosis (LL) and sagittal vertical axis (SVA). Results: A significant effect of time interaction (preoperative to early postoperative; F = 59.93, F = 44.78 and F = 39.87, all p < 0.001) existed for PT, SS and TK in both groups (all, p < 0.001). After adjustment for patients’ age (>65 vs. <65 years), a statistically significant increase of PI was observed in patients >65 years (p = 0.006) in Group A between preoperative and postoperative measurements. All patients in Group B exhibited a decrease in PI from preoperative to postoperative. Conclusions: Older patients undergoing long fusion to the sacrum without pelvic fixation had an increase in PI after surgery. Conversely, pelvic fixation with hips intraoperatively extended has decreased the value of PI from pre- to early postoperative. These changes could be related to degeneration of the sacroiliac joints causing increased rotational mobility and the magnitude of the differences is in the range of clinical relevance.