Redefining minor curve structural criteria: a novel supine radiograph assessment for the Lenke classification of Adolescent Idiopathic Scoliosis (AIS).

PURPOSE: Current methods for assessing spinal flexibility in adolescent idiopathic scoliosis (AIS) typically rely on multiple side-bending radiographs to determine the structural nature of minor curves. However, these methods are subject to inherent variability due to differences in radiographic technique and patient effort, limiting their reproducibility. To address this limitation, we aimed to evaluate the effectiveness of a single supine full-spine anteroposterior (AP) radiograph as an alternative to side-bending radiographs for the identification of minor structural curves in AIS.

METHODS

Preoperative full spine standing, bending and supine AP radiographs of 123 surgical patients with AIS were evaluated. Coronal Cobb angles and sagittal parameters were measured classified according to the Lenke classification. Patients were grouped by the location of the major curve. Group one included Lenke Types 1 to 4. Group two included Lenke Types 5 and 6. Supine Percent Correction (SPC) was calculated for each curve. Sensitivity and specificity for our proposed supine structural criteria, Pearson's correlation coefficient between the supine and side-bending films, and area under the receiver operator characteristic (ROC) curve was calculated.

RESULTS

Both groups demonstrated a statistically significant strong positive correlation between supine and side-bending radiographs across all curve regions. Receiver operating characteristic (ROC) analysis identified a residual Supine Cobb angle ≥ 30° and Supine Percent Correction (SPC) ≤ 30% on supine radiographs as the optimal threshold for distinguishing structural curves, yielding the highest combined sensitivity and specificity. Using these thresholds, our proposed Supine Structural Criteria achieved a sensitivity of 89%, specificity of 93%, positive predictive value (PPV) of 94%, and negative predictive value (NPV) of 88%. Area under the curve (AUC) analysis revealed very good predictive accuracy (AUC > 0.8) for proximal thoracic (PT) and thoracolumbar/lumbar (TL/L) curves in Group 1, and excellent predictive performance (AUC > 0.9) for main thoracic (MT) curves in Group 2.s

CONCLUSION

A threshold of ≥ 30° residual supine Cobb angle in conjunction with ≤ 30% Supine Percent Correction on a preoperative supine anteroposterior radiograph demonstrated to be a reliable and reproducible method for identifying structural minor curves in adolescent idiopathic scoliosis.

LEVEL OF EVIDENCE: Level III.