Postoperative Distal Coronal Decompensation After Fusion to L3 for Adolescent Idiopathic Scoliosis Is Affected by Sagittal Pelvic Parameters

STUDY DESIGN: Retrospective study. OBJECTIVE: To identify on early postoperative radiographs the risk factors for late distal decompensation in adolescent idiopathic scoliosis (AIS) patients undergoing posterior fusion surgery to L3. SUMMARY OF BACKGROUND DATA: Sparing distal fusion levels in AIS surgery is considered beneficial for postoperative mobility and outcomes; nonetheless, late distal decompensation is of concern. L3 is often advocated as lower instrumented vertebra in posterior fusion, but progressive angulation of the L3/4 disc is commonly observed. METHODS: A retrospective analysis was conducted on 78 AIS patients who underwent posterior fusion to L3 from 2007 to 2014. Patients' demographic data, early and 2-year postoperative standing radiographs by biplanar imaging system were investigated. Late decompensation was defined as progressive increase of L3-4 disc wedging angle at 2-year follow-up. Coronal, sagittal, and rotational radiographic parameters were compared between those with and without decompensation. SRS-30 scores were reviewed. RESULTS: Mean age was 14.5-year, and fusion levels averaged 12.0 (range: 6-15); 43 out of 78 patients (55%) experienced progressive L3-4 disc wedging, with 6 showing wedging >5°. L3 translation from the central sacral vertical line (13.9 vs. 11.1 mm, P = 0.13) and increased pelvic tilt (13.3° vs. 8.6°, P = 0.06) on the early postoperative radiograph were associated with increased L3-4 disc wedging. Multivariate analysis revealed that larger pelvic tilt was a significant risk factor for decompensation (odds ratio = 1.1 per 1°, 95% confidence interval: 1.0-1.1, P = 0.04). SRS-30 scores did not differ significantly between the two groups (4.0 vs. 4.1, P = 0.44). CONCLUSIONS: Pelvic retroversion and increased translation of L3 from the central sacral line on the early postoperative radiograph were associated with late L3-4 disc wedging in AIS fusions to L3. Careful surgical planning and correction of sagittal alignment are imperative to ensure the long-term outcomes. LEVEL OF EVIDENCE: 4.