Pre-operative prone radiographs can reliably determine spinal curve flexibility in adolescent idiopathic scoliosis (AIS)

Purpose: The purpose of this study was to evaluate the correlation between non-effort prone and bending radiographs in determining curve flexibility in adolescent idiopathic scoliosis (AIS). Methods: A retrospective review of AIS patients who underwent pre-operative full spine radiographic imaging from 2006 to 2019 was performed. The Cobb angle (CA) of proximal thoracic (PT), main thoracic (MT) and thoracolumbar/lumbar (TL/L) curves were measured and correlated on standing, prone and bending radiographs. Standing, bending, and prone measurements were correlated using Spearman’s analysis, and intra-rater reliability was evaluated using intraclass correlation analysis. Results: A total of 381 patients (74% female) with a mean age of 15.1 ± 2.5 years were identified. A strong correlation existed between the prone and bending CA for the PT (r_s = 0.797, p < 0.01) and MT (r_s = 0.779, p < 0.01) curve and a moderate correlation existed between the prone and bending TL/L curve (r_s = 0.641, p < 0.01). For a non-structural PT curve, a prone CA < 25° correctly identified a bending CA < 25° 96.7% of the time (p < 0.005). For a non-structural MT curve, a prone CA < 35° correctly identified a bending CA < 25° 90.2% of the time (p < 0.005). For a non-structural TL/L curve, a prone CA < 35° correctly identified a bending CA < 25° 95% of the time (p < 0.005). Conclusion: Prone radiographs demonstrated a moderate to strong correlation with bending radiographs and may be used as a proxy for determining spinal flexibility, especially when bending films are deemed unreliable. Level of evidence: III.