The axial rotation (rotation about a vertical axis) of the vertebrae, of the ribs, and of the back surface are components of the deformity recognized clinically as the "rib hump" in thoracic scoliosis. Relationships of these rotations to the lateral deviation and lateral curvature of the spine were studied in 40 patients with idiopathic scoliosis. Stereoradiographs of the spine and rib cage were used to measure three components of axial rotation: rotation of the vertebrae, of the rib cage, and of the plane of maximum curvature of the spine. Stereotopographs of the back surface were digitized to measure the axial rotation of the back surface.
In individual patients, there were high correlations of all components of axial rotation at each spinal level with the corresponding vertebral lateral deviation from the spinal axis. By regression analyses of the maximum values of each rotation in each curve, the rotation of the apex vertebra was found to be generally of lesser magnitude than the rotation of the plane of maximum curvature of the spine and in an opposite sense in kyphotic curves. The rib cage rotation was generally of lesser magnitude than the vertebra rotation, and the back surface rotation was less than both of these skeletal rotations.
Vertebra rotation correlated most closely with lateral deviation of the spine. Simple segmental coupling of axial rotation and lateral bending could not be responsible for this axial rotation.