Development of an Objective Method for Evaluating Waistline Asymmetry in Adolescent Idiopathic Scoliosis

Purpose: Quantitative evaluation of appearance deformation is important for patients with adolescent idiopathic scoliosis (AIS), in whom the accompanying appearance deformation, such as waistline asymmetry, can affect psychology. This study was conducted to develop an objective method to evaluate waistline asymmetry in patients with AIS and to investigate the factors affecting waistline asymmetry before and after surgery. Methods: This study included 35 patients who underwent posterior spinal fusion for idiopathic scoliosis at our institute between March 2020 and October 2021. The left-right difference in the waist area was measured for each patient from X-ray images, and the value corrected for the entire waist area was defined as the waistline asymmetry index (WAI), which increased in conjunction with the difference between the left and right waist areas. We further investigated the factors that correlated with WAI. Results: The WAI significantly decreased after surgery in most cases; however, in cases where the vertebra distal to the lower instrumented vertebra was located to the left of the central sacral line preoperatively, the WAI worsened postoperatively. The WAI was positively correlated with lateral shifts of the thorax and apical vertebrae. The Lenke classification had no significant effect on the WAI before and after surgery. Conclusion: We successfully developed a method to measure waistline asymmetry before and after surgery. The application of this method showed that the deviation of the lower instrumented vertebrae from the central sacral line influenced the exacerbation of waistline asymmetry after surgery.