Scoliosis correction by Cotrel-Dubousset instrumentation. The effect of derotation and three dimensional correction.

In 30 patients undergoing Cotrel-Dubousset instrumentation, computerized tomography was performed before and after surgery. The RAsag-angle was measured for each vertebral level. Coronal and sagittal translation were evaluated in the same computerized tomographic scans by means of a new method. Based on the hooks used, various sections of the spine were defined. In these sections relative rotation as well as relative sagittal and frontal translation were calculated by an algorithm comparing the most severely rotated or translated vertebra with the mean value of both end vertebrae. Significant correction of rotation did not occur in any of these sections. A significant increase of kyphosis, however, could be demonstrated in all thoracic sections. Above the second intermediate hook combined correction of all three parameters was achieved. Below this hook correction occurred almost independently in the frontal and sagittal planes.