What Is the Actual 3D Representation of the Rib Vertebra Angle Difference (Mehta Angle)?

STUDY DESIGN.Cross-sectional study. OBJECTIVE.To establish the relevance of the conventional two-dimensional (2D) rib vertebra angle difference (RVAD) and the relationship with the complex three-dimensional (3D) apical morphology in scoliosis. SUMMARY OF BACKGROUND DATA.The RVAD, also known as Mehta angle, describes apical rib asymmetry on conventional radiographs and was introduced as a prognostic factor for curve severity in early onset scoliosis, and later applied to other types of scoliosis as well. METHODS.An existing idiopathic scoliosis database of high-resolution computed tomography scans used in previous work, acquired for spinal navigation, was used. Eighty-eight patients (Cobb angle 46°–109°) were included. Cobb angle and 2D RVAD, as described by Mehta, were measured on the conventional radiographs and coronal digitally reconstructed radiographs (DRR) of the prone computed tomography scans. A previously validated, semiautomatic image processing technique was used to acquire complete 3D spinal reconstructions for the measurement of the 3D RVAD in a reconstructed true coronal plane, axial rotation, and sagittal morphology. RESULTS.The 2D RVAD on the x-ray was on average 25.3° ± 11.0° and 25.6° ± 12.8° on the DRR (P = 0.990), but in the true 3D coronal view of the apex, hardly any asymmetry remained (3D RVAD3.1° ± 12.5°; 2D RVAD on x-ray and DRR vs. 3D RVADP