RISK-EVALUATION OF MEDIASTINAL OR RETROPERITONEAL PROTRUSION OF PEDICLE SCREWS IN THORACOLUMBAR SPINE

Purpose of the study Within an anatomical and a clinical study, the au thors employed computerized tomographic scans to evaluate the risks of anterior surrounding tissues injuries during screw insertion. Materia l and methods CT scans of 20 patients suffering from cardiac disease w ere reviewed retrospectively. Scans through the thoracic and lumbar sp ine were obtained using 6 mm slice thickness. These examinations were performed with intravenous contrast medium. Measurements of vessel dia meters and distance of the soft tissues situated directly anterior to the spine were done. A retrospective study of 61 pedicle screws implan ted for spine fractures evaluated the penetration of the anterior vert ebral cordex with X rays and CT scans. Results Computerized tomographi c scans of the thoracic and lumbar spine of the 20 patients in the con trol group confirmed proximity of the posterior mediastinal structures to the anterior vertebral cortex. Many structures of the posterior me diastinum were within five millimeters of-the anterior vertebral corte x and thus were at risk: aorta, azygos vein, vena cava, parietal pleur a and lungs. The theoritical risk of unrecognized screw penetrations e valuated on geometric shape of the anterior vertebral body is as high as 21 per cent when screw position is only seen with an antero posteri or and a lateral X Ray. In the other group, computerized tomographic s cans showed that 30 per cent of the implanted screws were outside the bundaries of the anterior thoracic spine. Discussion and conclusion Tw o orthogonal incidences do not enable determination of whether the ext remity of the screw is slightly outside the anterior cortex of the ver tebral body. However the geometric shape of the anterior vertebral bod y enables peroperative definition of a safety zone on two orthogonal i ncidences. Even if a breach of a few millimeters of the anterior corti cal bundaries of the vertebral body may not initially damage the adjac ent soft-tissue structures, chronic irritation may result in late dama ges of these structures. The use of metallic markers and the respect o f a safe vertebral zone on X Rays could guide the choice of the approp riate screw length.