A Method for Assessing Axial Vertebral Rotation Based on Differential Rod Curvature on the Lateral Radiograph

Abstract

Bench-top and retrospective radiographical analyses to determine apical vertebral rotation based on differential rod curvature on the postoperative lateral radiograph. To develop a clinically relevant methodology for measuring apical vertebral rotation on postoperative lateral radiographs in patients with adolescent idiopathic scoliosis, based on the distance between the spinal rods. Traditional methods of analyzing vertebral rotation on plain radiographs are limited in the postoperative spine with segmental instrumentation. A previous methodology based on pedicle screw tip to rod distances on the posteroanterior radiograph is effective but limited by surgical technique and patient positioning relative to the x-ray beam. The trigonometric relationship between the inter-rod distances on lateral radiographs was defined and validated on a biomechanical model, with apical rotation varying from 0° to 20°. The ability to correct for malposition on the lateral radiograph was tested on 11 postoperative radiographs and correlated against corresponding postoperative computed tomographic scans. The bench-top model had a strong correlation between actual apical rotation and calculated rotation for the full range of image rotations (intraclass correlation coefficient, 0.99). For the 11 clinical cases, comparisons of apical rotation measured on computed tomographic scans were highly correlated to the proposed lateral radiograph calculations (r = 0.84). A technique for measuring apical vertebral rotation based on the inter-rod distance on the lateral and posteroanterior radiographs was developed and validated. This technique is resilient to rotation of the patient within the x-ray machine and can complement measurement of rotation on postoperative posteroanterior radiographs.