Intraoperative 3D reconstruction of the scoliotic spine from radiographs

We have developed a new per-operative three dimensional (3D) reconstruction technique to evaluate the 3D correction of a scoliotic spine induced by su rgery using Cotrel-Dubousset instrumentation. A small object with 15 embedd ed markers was used to calibrate the radiographic system. During surgery, t he calibration object was sterilized and fixed to the patient just before t he acquisition of two pairs of posterior-anterior and sagittal radiographs; one pair before the rotation maneuver of the rod and one pair after the ma neuver. The markers were digitized on each radiograph and their relative 3D positions were measured to establish the relation between the 3D positions of the anatomical structures and their 2D positions on the radiographs. Th is relation was used to calculate the 3D position of six anatomical landmar ks per vertebra (the centers of the superior and inferior vertebral body en dplates and the proximal and distal bodies of both pedicles) from the ident ification of these landmarks on each radiograph. We made a 3D representatio n of the thoracic and lumbar spine of three patients with idiopathic scolio sis undergoing corrective surgery by the posterior approach. Clinical indic es (Cobb angle, axial rotation and the plane of maximum curvature) computed from the 3D reconstruction of the spine obtained before and after the rota tion maneuver of the rod were compared to evaluate the 3D correction perfor med during the surgery. The new proposed approach allows the surgeon to eva luate the per-operative shape of the spine. This approach is simpler, faste r and less risky for the patient than the previous method which employed an electromagnetic digitizer to measure the 3D coordinates of anatomical land marks located on the posterior part of the spine. Furthermore, the 3D repre sentation of the spine visualized from different points of view gives the s urgeon an accurate evaluation of the 3D correction during the surgical proc edure.