Analysis of Screw Placement Relative to the Aorta and Spinal Canal Following Anterior Instrumentation for Thoracic Idiopathic Scoliosis

Study Design. Axial computed tomographic (CT) evaluation of the position of anterior vertebral body screws placed thoracoscopically in patients with adolescent idiopathic scoliosis (AIS). Objective. To evaluate the position of the anterior vertebral body screws relative to the spinal canal and the thoracic aorta following anterior spinal fusion and instrumentation for AIS. Summary of Background Data. Thoracoscopic anterior instrumentation and fusion is gaining more widespread use in the treatment of idiopathic scoliosis. However, the accuracy in the positioning of instrumentation has not been previously studied for this technically difficult surgery. Methods. CT examinations were performed following thoracoscopic anterior spinal fusion and instrumentation in 14 patients with right thoracic AIS. The vertebral body width (transverse dimension) and depth (anterior-posterior dimension) was measured for each thoracic vertebra. At each instrumented level, the position of the screw was analyzed to determine its proximity to the spinal canal and the aorta. The distance from the anterior cortex of the spinal canal to the posterior edge of the screw was measured. The position of each screw relative to the aorta was determined: D, the screw tip was distant to the aorta; A, the screw tip was adjacent to the aorta; C, the screw tip was felt to be against the aorta and creating some contour deformity on the outer wall of the aorta. Results. All 14 patients were female and had a single right thoracic curve. The average age of the patients was 13.3 years (range 12.4-15.1 years). The average preoperative coronal Cobb measurement was 55.9° (bending 26.4°) with correction to 8.9° at 2 years after surgery. The average number of levels fused was 6.6 (range 5-8) and a total of 106 screws were used (average 7.6/patient). The width of the vertebral bodies increased from T4 (24.0 mm) to T12 (33.6 mm), increasing an average of 1.2 mm per level, while the depth increased from 17.7 mm at T4 to 25.5 mm at T12. The average distance from the posterior aspect of the screw to the spinal canal was 5.3 mm (range -1.2 to 11. 4 mm). There were no neurologic deficits in any patient. When analyzing the position of the screw tip relative to the aorta, 78 (73.6%) screws were distant from the aorta, 15 (14.2%) were adjacent to the aorta, and there were 13 (12.3%) screws that were thought to create a contour deformity of the aorta. There were no vascular complications at 2 years after surgery. Conclusions. Thoracoscopic instrumentation and fusion Is technically demanding and relies on adequate visualization for accurate screw placement. The vertebral body width and depth are consistent between patients, with the vertebral body width increasing approximately 1.2 mm when progressing down the thoracic spine. Safe screw placement was achieved with adequate distance from the spinal canal; however, close screw proximity to the aorta was seen. The aorta was positioned on the left lateral aspect of the vertebral body in these patients, making anterior screw placement challenging in right thoracic AIS.