The Effects of Body Mass Index and Spine Flexibility on Surgical Outcomes for Patients With Adolescent Idiopathic Scoliosis

Study Design. Retrospective cohort study. Objective. To investigate the relationship between body mass index (BMI), spine flexibility index (FI), and their combined effects on adolescent idiopathic scoliosis (AIS) surgical outcomes. Background. BMI and FI are two factors considered during presurgical planning for AIS correction, but there is sparse research about their relationship. We hypothesize that AIS patients with increased BMI may be associated with decreased FI-a combination which could lead to worsened surgical outcomes. Materials and Methods. AIS patients ages 11 to 19 at surgery, who underwent posterior fusion at a single center from 2011 to 2017, were reviewed. Patients without proper radiographs to assess FI, or a previous spine surgical history, were excluded. FI was categorized as stiff (FI<50) or flexible (FI >= 50), and patients were separated by major curve region. BMI was categorized as underweight (less than fifth percentile), healthy weight (fifth-85th percentile), overweight (85th-95th percentile), or obese (>95th percentile). Regression analysis was conducted to test BMI and FI's effects on intraoperative, immediate postoperative, and two-year postoperative outcomes. Results. A total of 543 patients (82% female), with an average age of 14.9 years, were included. In all, 346 patients had available two-year data. A 10% increase in BMI was associated with a 1.3% decrease in FI for patients with major thoracolumbar/lumbar curves (P=0.01). Obese patients were most likely to have a postoperative complication (P=0.003) or a two-year complication (P=0.04). Revision surgery occurred after 58% of postoperative complications (15/26) and 80% of two-year complications (4/5). FI was negatively associated with initial curve magnitude (P<0.001), operative time (P=0.02), and blood loss (P=0.02). Overweight patients with flexible curves were 10.0 times more likely to sustain a postoperative complication than healthy weight patients with stiff curves (P=0.001). Conclusions. Elevated BMI was associated with decreased FI in patients with major thoracolumbar/lumbar curves. Patients with a high BMI and high FI were associated with the greatest risk of postoperative complication.