Cobb angle estimation without X-ray using angle of trunk rotation and characteristics of unpredictable cases

Investigate whether Cobb angle can be predicted by angle of trunk rotation (ATR) measurements with a scoliometer and identify the characteristics of hard-to-predict cases.

We retrospectively analyzed 176 patients who were examined for AIS at our department between 2011 and 2021. The cohort included 160 girls and 16 boys aged 11 years (83 patients) or 12 years (93 patients). ATR was determined with an OSI-Scoliometer (Orthopedic Systems, Inc., Union City, CA, USA). Risser grade, Cobb angle, coronal balance, and clavicular angle were all measured using standing full-length frontal radiographs. Coronal balance was measured as the distance of the C7 plumb line (C7PL) to the central sacral vertebral line. We defined the underestimated group as having a predicted Cobb angle > 1 standard deviation (SD) lower than the actual Cobb angle.

ATR and Cobb angle were strongly and positively correlated in the cohort (r = 0.883, P < 0.001). To investigate ATR values for use in treatment decision-making, the determined cut-off value of ATR for > 25° Cobb angle was 7.5° (area under the receiver operating characteristic curve [AUROC]: 0.895, sensitivity: 0.810, specificity: 0.838) and for > 45° Cobb angle was 10.5° (AUROC: 0.941, sensitivity: 1.000, specificity: 0.781). We observed 23 patients in the underestimation group. Actual Cobb angle was significantly higher in the underestimation group, which also included significantly more patients with larger C7PL deviation and with right shoulder up. Logistic regression analysis revealed that the larger the Cobb angle at the time of ATR measurement, the higher the risk of underestimating the Cobb angle in our prediction formula. Specifically, the odds ratio suggests that a 10° increase in the Cobb angle results in a 2.3-fold increase in this risk.

ATR and Cobb angle were strongly correlated, with ATR cut-off values of 7.5° and 10.5° to predict Cobb angle > 25° and > 45°, respectively. Although the accurate prediction of Cobb angle appears difficult, especially for larger curves, this method may be useful in determining the need for bracing or surgical treatment without radiography.