The relationship between measures of spinal deformity and measures of thoracic trunk rotation

Adolescent idiopathic scoliosis (AIS) is associated with both asymmetry of the torso (rib hump) and vertebral body rotation (VBR). Current surgical techniques aim to reduce the VBR and rib hump. However, it is not clear how the vertebral rotation and thoracic asymmetry are linked.

A retrospective cohort study was performed in which all adolescent patients with a diagnosis of AIS (Lenke curve type one to four only), a minimum 2-year follow up and a complete data set of radiographs, Integrated Shape Imaging System 2 (ISIS2) surface topography and axial imaging within a 6-week period were included. The Cobb angle was obtained from the radiograph, the maximum VBR was measured from the axial imaging using the Aaro and Dahlborn technique and the largest maximum skin angle (MSA) was taken from the ISIS2 topography. MSA is the ISIS2 parameter and is similar in nature to a scoliometer.

From the surface topography database of AIS, 51 met the inclusion criteria. There were 6 males and 45 females with a mean age of 14.6 years (SD 1.4, range, 11.2 to 17.7). The mean Cobb angle was 54.4° (SD 13.8°, range, 29° to 92°). Mean MSA was 11.7° (SD 4.0°, range, 4° to 23°). Mean VBR was 14.3° (SD 4.3°, range, 8° to 24°). Through linear regression techniques, the relationships between Cobb angle, MSA and VBR were examined. The R² between Cobb angle and MSA was 9%, between Cobb angle and VBR was 23% and between MSA and VBR was 16%. A multiple regression analysis did not improve these results.

Whilst AIS features both VBR and torso asymmetry, they are poorly related to each other. This may help to explain why surgical de-rotation of the spine does not fully address the rib hump as other factors, yet to be defined, must be involved.