Rotations of a helix as a model for correction of the scoliotic spine

A prospective study using intraoperative stereophotogrammetry to analyze helical motion of the spine during the correction of scoliosis.

To determine whether derotation systems rotate the scoliotic helix.

Scoliosis is a complex three-dimensional deformity that is difficult to visualize on standard radiographs. The use of stereophotogrammetry has allowed study of the deformity in three dimensions during surgical correction.

Thirty-five patients with right thoracic adolescent idiopathic scoliosis were studied using a stereophotogrammetry technique during surgical correction. Changes in vertebral unique rotations and spinal plane of maximum deformity were measured during three sequential stages of the surgery.

The mean preoperative and postoperative Cobb angles were 58 degrees and 19 degrees, respectively. Most rotation occurred at the top and bottom vertebrae in the curve, averaging 10 degrees each but in opposite directions. The apical vertebra rotated the least in the structural curve, with an average rotation of 5 degrees. Much of the rotation occurred during the derotation maneuver with additional rotation occurring during the final distraction. The plane of maximum deformity changed from a mean of 50 degrees before instrumentation to 19 degrees at the end of the procedure.

Multiple rotations of the scoliotic curve occur, and it can be shown when maximum rotations occur during surgery. Posterior derotational systems unwind or rotate the scoliotic helix and reposition the resultant sine wave toward the sagittal plane as described by the change in the plane of maximum deformity.