Fusion mass shift: role in postoperative distal curve adding-on in adolescent idiopathic scoliosis

Abstract

INTRODUCTION: The ideal fusion mass in adolescent idiopathic scoliosis (AIS) is parallel endplates of the upper and lower instrumented vertebra without any shift. Distal adding-on is an important complication to consider as it is undesirable with increased risk of revision surgery. However, its relationship with fusion mass shift (FMS) is unknown. Hence, this study investigates this relationship and identifies a cut-off value of FMS that will lead to distal adding-on. METHODS: Consecutive patients since 2006 who underwent selective thoracic fusion for Lenke type 1 AIS were recruited prospectively and assessed preoperatively and at 2 years postoperatively. Residual fusion mass Cobb’s angle and shift, and any distal adding-on were measured. Receiver operating characteristic (ROC) analysis was used to determine the cut-off point of FMS for adding-on. RESULTS: Of the 69 cases recruited, 11 cases had FMS and 6 of those cases had distal adding-on at final follow-up. Although global spinal balance did not differ significantly between cases with or without FMS, the occurrence of adding-on phenomenon was significantly higher in the non-ideal FMS group (odds ratio=8.6; 95% confidence interval, 1.9-39.2). Based on the ROC analysis, a FMS of >18 mm was observed as the cut-off point for distal adding-on phenomenon (area under the curve, 0.70; 95% confidence interval, 0.52-0.89; sensitivity, 0.64; specificity, 0.73). CONCLUSION: Our study illustrates the significant utility of the fulcrum bending radiograph in determining fusion levels that can avoid FMS, thereby underlining its importance in designing personalised surgical strategies for scoliosis patients. Preoperatively, determining fusion levels that can avoid residual FMS is imperative.