Does lateral vertebral translation correspond to Cobb angle and relate in the same way to axial vertebral rotation and rib hump index? A radiographic analysis on idiopathic scoliosis.

The deformity in idiopathic scoliosis (IS) is three dimensional in nature and effective correction involves all three planes. Even though the vertebral translation (VT) is an accepted element in the deformity along with vertebral rotation(VR) as reported by Asher and Cook (Spine (Phila Pa 1976) 20(12):1386-1391, 1995), Kotwicki et al. (Study Health Technol Inf 123:164-168, 2006) and Kotwicki and Napiontek (Pediatr Orthop 28(2):225-229, 2008), rib hump (rib hump index (RI)) and Cobb angle as reported by Aaro and Dahlborn (Spine (Phila Pa 1976) 6(6):567-572, 1981), it was assumed that VT was represented by adequately by Cobb angle and it was not analysed individually. We hypothesized that the Cobb angle and the VT measured in axial plane on CT scan and may not represent the same measurement and factors like coronal plane vertebral tilt,VR and vertebral deformation might affect them in different ways. Hence, VT should be considered as a separate variable and its relationship with VR, RI and Cobb angle should be investigated. Since the newer implants depend on curve translation and derotation for correction studying the role of VT and the relationships is important. VT, VR and RI were measured in CT scans of 75 patients with IS and correlated with Cobb angle. Regression analysis was used to identify the influence of the variables on each other. All the variables significantly correlated with one another but the correlation of Cobb and VT is not perfectly linear and it cannot be used to represent VT. VT influences RI much more than Cobb angle or VR. VT, therefore, merits further study treating it as an independent variable.