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

Abstract

STUDY DESIGN: Cross-sectional study. OBJECTIVE: To establish the relevance of the conventional two-dimensional (2D) RVAD and the relationship with the complex three-dimensional (3D) apical morphology in scoliosis. SUMMARY OF BACKGROUND DATA: The rib vertebra angle difference (RVAD, also known as Mehta's 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 CT 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 CT scans. A previously validated, semi-automatic image processing technique was used to acquire complete 3D spinal reconstructions for 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 RVAD: 3.1 ± 12.5°; 2D RVAD on X-ray and DRR versus 3D RVAD: P < 0.001). 2D apical rib asymmetry in the anatomical coronal plane did not correlate with the same RVAD measurements in the 3D reconstructed coronal plane of the rotated apex (r = 0.155; P = 0.149). A larger 2D RVAD was found to correlate linearly with increased axial rotation (r = 0.542; P < 0.001) and apical lordosis (r = 0.522; P < 0.001). CONCLUSIONS: The 2D RVAD represents a projection-based composite radiographic index reflecting the severity of the complex 3D apical morphology including axial rotation and apical lordosis. It indicates a difference in severity of the apical deformation. LEVEL OF EVIDENCE: 4.