Can postoperative changes in pelvic incidence occur after adult spine deformity surgery? When do they occur, and what factors influence them? A systematic review with pooled analysis

Objective: Pelvic incidence (PI) is traditionally considered a constant anatomic parameter in adult spinal deformity (ASD) surgery. However, emerging evidence suggests that PI may change postoperatively, potentially influencing sagittal balance and mechanical complications. This study aims to systematically review the literature on postoperative PI variations in ASD patients, identifying potential radiologic and surgical predictors. Methods: Papers describing postoperative PI changes in ASD patients were included in the analysis. The rate of PI changes, in different subgroups of patients was considered for outcome measure. Meta-analyses were performed to determine the prevalence and factors influencing PI changes. Results: Thirteen studies (1055 patients) met the inclusion criteria. All included studies assessed spinopelvic parameters using standing full-spine X-rays, except for one that used full-body standing X-rays, demonstrating moderate\high reproducibility in PI measurement (ICC: 0.71–0.96). While the mean absolute PI value remained stable postoperatively, 46.3% of patients experienced significant changes (> 5°). Factors associated with PI variations included fusion area, pelvic fixation type, preoperative sagittal imbalance, and extreme PI values. Sacropelvic fixation reduced PI instability, whereas prolonged anterior imbalance and compensatory pelvic retroversion increased the likelihood of PI shifts. Long-term follow-ups indicated that PI changes could not recover or increase in patients without sacropelvic fixation and in patients with higher preoperative PI values if they experience a decrease in PI postoperatively. Conclusion: The current literature challenges the traditional belief that PI is a static parameter in ASD surgery, particularly in severely imbalanced patients or those with extreme PI values. Acknowledging that PI may change under certain preoperative conditions could help optimize postoperative sagittal realignment in selected groups of ASD patients.