Sagittal Imbalance Deterioration After S2-Alar-Iliac Fixation in Adult Spinal Deformity: What Role Does Dynamic Hip Joint Coverage Play?

Objective: To determine the role of dynamic hip joint coverage in maintaining postoperative sagittal balance in adult spinal deformity (ASD) patients following S2-alar-iliac (S2AI) fixation. Methods: A total of 224 ASD patients who underwent S2AI fixation were enrolled. Patients were stratified into 2 groups based on pre-to-post (from preoperative to postoperative) changes in femoral head coverage (ΔFHC): change group (group C) and noncoverage change group (group NC). Group C was further subdivided according to FHC recovery during follow-up into rebound (group C-R) and nonrebound (group C-NR) groups. Clinical outcomes and radiographic parameters of hip and spinopelvic alignment were assessed preoperatively, at the initial postoperative standing, and at the 2-year follow-up. Results: Compared to group C, patients in group NC demonstrated a higher incidence of sagittal imbalance-related mechanical complications at 2-year follow-up, with a greater tendency for sagittal imbalance progression (p=0.013), a larger post-to-follow-up change in sagittal vertical axis (ΔSVA) (p=0.029), and a higher incidence of proximal junctional kyphosis (PJK) (p=0.031). Although there was no significant difference in PJK incidence between group C-NR and group C-R (p=0.845), group C-NR showed a greater tendency for postoperative sagittal imbalance aggravation (p=0.025), with a significantly larger ΔSVA during follow-up (p=0.002). The optimal cutoff values for predicting postoperative sagittal imbalance aggravation were 3.5% for pre-to-post ΔFHC (area under the curve [AUC]=0.694) and 1.8% for post-to-follow-up ΔFHC (AUC=0.713). Conclusion: Dynamic postoperative changes in hip joint coverage, characterized by the FHC, are associated with postoperative sagittal balance maintenance. Patients with limited pre-to-post and post-to-follow-up changes in the FHC demonstrate compromised hip joint compensatory capacity, thereby increasing the risk of postoperative sagittal imbalance-related mechanical complications.