Postoperative Deterioration of Sagittal Balance in Adult Spinal Deformities: Influence of Preoperative Alignment and Pelvic Incidence.

Introduction: This study aimed to identify the factors associated with the postoperative deterioration of sagittal balance after surgery for adult spinal deformity (ASD), focusing on preoperative alignment and pelvic incidence (PI). Methods: We retrospectively reviewed the medical records of 87 patients who underwent corrective surgery for ASD (2017-2020). Sagittal balance was assessed using the sagittal balance classification (SBC). The patients were classified as balanced (maintained SBC grade 1 or 2) or imbalanced (deteriorated to grade 3). Radiographic parameters, clinical outcomes (Japanese Orthopedics Association scores and mechanical complications), and bone mineral density were analyzed. Results: In 15 patients (17.2%), the sagittal balance deteriorated to grade 3 (imbalanced group). Factors significantly associated with postoperative deterioration of sagittal balance included preoperative grade 3 SBC (73.3% vs. 23.6%, p<0.001), steroid use (26.7% vs. 2.8%, p<0.01), pelvic fusion (80% vs. 51.4%, p=0.049), higher preoperative PI, sacral slope, and sagittal vertical axis. At 2 years, the imbalanced group showed a significantly greater corrective loss of the lumbar spine. Receiver operating curve analysis identified preoperative PI ≥52.1° as predictive of postoperative imbalance (sensitivity 86.7%, specificity 66.7%). The clinical outcomes were similar, but the reoperation rates were higher in the imbalanced group (20.0% vs. 2.8%, p=0.03). Conclusions: High preoperative PI, severe sagittal imbalance, steroid use, and pelvic fusion were predictive of postoperative sagittal balance deterioration, underscoring the need for personalized preoperative planning.