MRI Analysis of the Position of the Aorta Relative to the Spine: A Comparison Between Normal Patients and Those with Idiopathic Right Thoracic Scoliosis

Purpose: Despite increasing interest in anterior spinal instrumentation for thoracic scoliosis, there are no studies documenting the relationship of the aorta to the thoracic scoliotic spine. Recent studies suggest that the ends of anterior screws placed during an ASF for right thoracic scoliosis are in proximity to the aorta. The purpose of this study was to analyze the relationship between the aorta to the spine in normal patients and those with idiopathic right thoracic scoliosis.

Methods: We evaluated the relationship of the aorta to the thoracic spine in two groups of patients, those with a normal spine (Group N, 16 patients), and those with adolescent idiopathic right thoracic scoliosis (Group AIS, 28 patients). Radiographs were analyzed to determine the Cobb angle, the apex of the curve and apical rotation (Perdriolle method). The axial MRI images were analyzed to include: the position of the aorta defined by the angle subtended by a reference line in the mid vertebral body and a line through the middle of the aorta (0=aorta directly left-lateral, 90= directly anterior, - 90= spinal canal), the distance from the aorta to the closest point of the vertebral body cortex and distance from the posterior edge of the aorta to the spinal canal. Measurements were obtained from T4 to L3 in each patient.

Results: There were no differences between groups with respect to age or sex. For group AIS, the most common apical vertebra was T8, the average coronal curve measurement was 45.2 (11 to 92) and the average apical rotation was 17.3 (0 to 30). There were no differences between groups with respect to the sagittal or transverse diameters of the vertebral bodies or the aorta diameter. The aorta was positioned more lateral (smaller angle) to the vertebral body at levels T5 through T11in AIS patients compared to the more anterior position in normals (p<0.05). The average angle at these levels was 20.7 in group AIS and 40.9 in Group N (p<0.05). At the apex of the curve in the AIS group, the angle was 15.4 compared to 39.0 at T8 in group N (p<0.05). The distance from the posterior aspect of the aorta to a line drawn parallel to the anterior aspect of the spinal canal was less in group AIS than in group N (11.4 vs 18.7 mm) for levels T5 through T12 (p<0.05). The average distance between the aortic wall and the vertebral body cortex at the apex of the curve was 4.0 mm in group AIS. With increasing Cobb angle and apical vertebral rotation the aorta was positioned more laterally and posteriorly (p<0.05).

Conclusions: In AIS the aorta is positioned more laterally compared to normals in which it is more anterior to the vertebral bodies. This position and proximity to the vertebral cortex poses challenges for safe anterior screw placement in scoliosis. Preoperative MRI may be helpful in defining this relationship when planning surgery.