Three-dimensional simulations of the scoliosis derotation maneuver with Cotrel-Dubousset instrumentation

The derotation maneuver using Cotrel-Dubousset instrumentation (CDI) is intended to correct the counterdirectional transverse plane rotations of the spine and of the vertebrae in thoracic scoliosis. This was simulated in a finite element model of an idealized thoracic scoliosis with an initial 65 degrees scoliosis angle and 0 degree kyphosis angle. After 90 degrees of rod rotation the apical vertebra derotated 50 degrees towards the sagittal plane but the apical vertebra axial rotation worsened by 8 degrees. The scoliosis angle corrected to 29 degrees and a 54 degrees kyphosis was created. If the initial rod curvature was reduced by 9 degrees, the model predicted only small changes in spinal curvature resulting from the forces required to connect the vertebrae to the hooks. Decreased kyphosis and scoliosis curvatures but increased vertebra axial rotation were produced by the derotation maneuver. The increase in apical vertebra axial rotation was reversed by modifying the representation of the motion segments by repositioning their effective axes 30 mm posteriorly.