Structures at Risk Following Anterior Instrumented Spinal Fusion for Thoracic Adolescent Idiopathic Scoliosis

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Abstract
With the increasing popularity of anterior instrumented spinal fusion for adolescent idiopathic scoliosis, there has also been a rising concern over the proximity of the descending aorta to the screw tips and the possibility of vessel wall erosion over time. This computed tomography (CT) study attempts to define the relative position of the thoracic aorta and other vital structures to the spine (preoperatively) and to the projected instrumentation (postoperatively) by level and curve magnitude in deformity patients. Twenty consecutive patients (17 female, 3 male) with an average age of 14.5 years (range 12.4-18.5 years) and a right main thoracic/Lenke 1 curve (average 55.2°, range 50-66°) underwent preoperative and postoperative CT scanning as part of their planned open anterior spinal fusion with instrumentation. All images were analyzed for proximity (distance from the midvertebral body) and position to (as defined relative to the center of the vertebral body in the axial plane) the spine preoperatively and the projecting screw tip postoperatively. As a control, 10 age-matched nondeformity thoracic CT scans were analyzed to assess the relative position of the thoracic aorta to the vertebral bodies by level. Preoperative and postoperative plain radiographs were also analyzed for curve magnitude, correction, and fusion levels to assess the possible effect of these variables on the various thoracic structures. The postoperative curve magnitude averaged 26.9° (range 17-40°; 51% correction) using 151 screws (7.5 screws/patient) and an average follow-up of 4.1 years (range 3.2-7.0 years). Screw-to-spinal-canal distance averaged 5.3 mm (range 3.5-8.2 mm) at an average of +4.5° (range −11° to +15°) from the coronal axis. Screw tip extrusion (distance beyond the far cortex) averaged 2.8 mm (0-5 mm). The trachea, azygous vein, esophagus, and lungs/pleura were not found to be at risk from screw penetration. The postoperative screw-tip-to-descending-aorta distance varied by vertebra level, with the periapical and distal screws being positioned closer to the aorta (1.6-2.4 mm) (P < 0.05). Additionally, 23 of 151(15%) screws were thought to be adjacent (≤2 mm) to the aorta. This includes 4 of 60 (7%) of the proximal screws but 6 of 40 (15%) of the periapical screws and 13 of 51 (26%) of the distal screws (P < 0.05). There were no screws compressing the aorta and no perioperative or postoperative complications. The course of the thoracic aorta may vary in individuals; however, in deformity patients, it generally moves from a relatively anterolateral position proximally to a posteromedial position at the apex. Distally, it moves more anteriorly. Consequently, the aorta moves closer to the screw tips both at the apex and distally, whereas the distal screws are more frequently juxtaposed to the descending aorta (P < 0.05).

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