Fractional curve following adult idiopathic scoliosis correction: impact of curve magnitude on postoperative outcomes

OBJECTIVE The goal of this study was to assess the impact of fractional curve (FC) severity on curve progression and postoperative outcomes in patients undergoing adult idiopathic scoliosis (AdIS) correction. METHODS Patients with AdIS who had preoperative coronal plane deformity and who had undergone thoracolumbar fusion with a lowermost instrumented vertebra (LIV) between L1 and L4 were included. Patients were stratified by 6-week postoperative FC severity (small FC, ≤ 40th percentile, large FC, ≥ 60th percentile of the entire cohort; calculated as the Cobb angle between LIV and S1) and age groups. Preoperative to 2-year postoperative changes in FC were evaluated using Student t-tests. Demographics, spinopelvic alignment, patient-reported outcome measures (PROMs), and complications were compared using chi-square tests for categorical variables and Student t-tests for quantitative variables. Multivariate regression analyses, accounting for age, sex, frailty, and 6-week postoperative LIV, were also performed when feasible to assess the impact of FC on 2-year postoperative outcomes. RESULTS In total, 86 patients, with 34 in the group with small FCs and 34 in the group with large FCs, were examined (18 were in the group with medium FC). The mean age (36.4 years for those with small FCs vs 36.0 years for those with large FCs, p > 0.05) was similar. Preoperatively, spinopelvic parameters and PROMs were comparable (p > 0.05). Two years postoperatively, higher postoperative FC was associated with larger thoracolumbar deformity (i.e., higher thoracolumbar/lumbar/lumbosacral Cobb angles) and lower perceived lumbar stiffness (p < 0.05); however, other PROMs and complications, including revisions, were comparable (p > 0.05). Bidirectional change in postoperative FC was associated with a lower C7 pelvic angle and lower C7 plumb line (R² = −0.03, 95% CI −0.05 to 0.00, p = 0.050). Across all patients, the mean FC improved from baseline to 6 weeks postoperatively (from 18.1° to 6.5°, p < 0.001) but changed minimally from 6 weeks to 2 years postoperatively (from 6.5° to 6.5°, p = 0.942). After stratification, the cohort with small FCs exhibited a relative increase (from 1.6° to 3.5°, p < 0.001), whereas the cohort with large FCs noted a nonsignificant change (from 11.9° to 9.8°, p = 0.121) in FC over time. CONCLUSIONS Following surgery for AdIS, larger residual lumbosacral FCs were not correlated with adverse events or poor outcomes at 2 years postoperatively. FCs may improve or worsen over time to drive improvement in global coronal balance surgery, but are not associated with adverse outcomes or reoperation during the first 2 years after surgery.