Pelvic Incidence Changes in Patients with Adult Spinal Deformity Following S2AI Fixation: Is the Hip Joint Involved?

Study design: Retrospective study.

Objective: To investigate whether the hip joints are involved in PI changes and explore its implications on pre-to-post PI changes.

Summary of background data: Pelvic incidence (PI) is a key parameter in formulating surgical strategies for adult spinal deformity (ASD) patients requiring spinopelvic fixation. Emerging evidence of PI variability has challenged traditional formulations based on PI being a fixed parameter. Serving as a hinge between spine and lower extremity, the hip joints are involved in the compensation for spinal sagittal imbalance, yet its role in PI changes is poorly understood.

Methods: A total of 155 consecutive patients were enrolled, with 121 patients following S2 alar-iliac fixation (S2AI group) and 34 following iliac screw fixation (IS group). Patients with S2AI fixation were further categorized into C and NC groups. The preoperative supine CT scout-view images were retrieved from S2AI group to compare the differences in hip parameter changes during positional changes. Finally, a correlation analysis was performed between hip parameters changes and ∆PI in S2AI group, with further validation conducted using fluoroscopic imaging of a 3D-printed pelvic model.

Results: Patients in both S2AI and IS groups showed a significant decrease in PI postoperatively, with subsequent rebound during follow-up. Compared to group NC, patients in group C showed significantly greater pre-to-post changes in hip parameters. In S2AI group, pre-to-post ∆PI showed a significant negative correlation with preoperative position-related ∆femoral head coverage (FHC) and ∆lateral center-edge (LCE) angle, and a positive correlation with ∆extrusion index (EI). Based on the ROC curve, the optimal threshold of preoperative position-related ∆FHC, ∆LCE angle, ∆EI were 6.65%(AUC=0.844), 5.40°(AUC=0.664), and 4.55%(AUC=0.792), respectively.

Conclusion: The pre-to-post PI changes in ASD patients following S2AI fixation are significantly associated with the hip joint coverage changes. These findings suggest that hip joints should be considered in formulating surgical corrective strategies based on PI in patients with ASD.