How is the trachea at risk of injury from pedicle screw insertion in proximal thoracic curve of adolescent idiopathic scoliosis patients?

The rotation or translation of vertebrae in adolescent idiopathic scoliosis (AIS) patients could cause the relative migrations of surrounding vital structures and lead to smaller safe zones for pedicle screw insertion. This study aimed to determine the changed relative position of trachea to spine in the proximal thoracic curve (T1-T4) and to analyze the potential risks of tracheal injuries from pedicle screw insertions in AIS patients. Twenty-three patients with complete proximal thoracic curve (CPT group), 25 patients with fractional proximal thoracic curve (FPT group) and 19 normal subjects with a straight spine (normal group) were included. Axial computed tomography images from T1 to T4 level were obtained to evaluate trachea-vertebral distance (TVD, the closest distance between trachea and vertebral body) and trachea-vertebral angle (TVA, defined as 0A degrees when the trachea was located directly laterally to the left and 180A degrees when directly laterally to the right). The extension line of pedicle axis could cross the anterior wall of vertebra and the posterior wall of the trachea at two points when the trachea was located in the trajectory of the screw passage. If the distance between the two points was less than 5 mm, the trachea was considered to be at a potential risk of injury. The percentages of vulnerable trachea were calculated at each level. The TVA in the CPT group was significantly larger than that in the FPT group and in the normal group, while the TVA in the FPT group was significantly larger than that in the normal group at the T2-T4 level. The TVD in the FPT group was significantly smaller than that in the CPT group and in the normal group at each level, while the TVD in the CPT group was significantly smaller than that in the normal group at the T2 and T3 levels. No trachea was found to be at risk from screw insertion on both sides in both the CPT group and the normal group. However, it was at a high risk of injury from anterior cortex penetration during right screw insertion in the FPT group. The percentage of trachea at risk from right screw insertion was 40 % at T1 level, 92 % at T2 level, 100 % at both T3 and T4 levels. This CT-based study demonstrates that the FPT curve has a smaller safe zone with respect to tracheal injury during screw insertion. Spine surgeons should choose the appropriate screw length to avoid anterior wall perforation.