Gravity-induced torque and intravertebral rotation in idiopathic scoliosis

Biomechanical analysis.

To investigate the relationship between gravity-induced torques acting on the scoliotic spine and rotation within the vertebrae.

Vertebral rotation is an important aspect of spinal deformity in idiopathic scoliosis, associated with ribcage asymmetry. Although both lateral curvature and rotation seem to increase together in progressive scoliosis, the mechanisms driving vertebral rotation are not clearly established and it is not known whether lateral curvature precedes rotation, or vice versa.

Three-dimensional spinal curvature was measured for a small group of idiopathic scoliosis patients using standing radiographs, and equations of static equilibrium were used to calculate gravity-induced torque profiles along the length of each spine because of head, neck, and torso weight. Vertebral rotations were then measured for the same patients using Aaro and Dahlborn's technique with reformatted computed tomography images. The gravity-induced torque curves were compared with rotation measurements to see whether gravity-induced torque is a likely contributor to intravertebral rotation in scoliosis.

Gravity-induced torques as high as 7.5 Nm act on the spines of idiopathic scoliosis patients because of body weight in the standing position, and maximum intravertebral rotations (for a single vertebra) are approximately 4 degrees. There is a statistically significant relationship between gravity-induced torque and intravertebral rotation in the scoliotic spine.

Gravity-induced torque is a likely cause of intravertebral rotation in progressive idiopathic scoliosis. Because the spine must be curved in 3-dimensions (out of plane) to produce such torques, vertebral rotation would be expected to occur subsequent to an initial lateral deviation, suggesting that lateral curvature precedes vertebral rotation in progressive idiopathic scoliosis.