Comparison of the aorta impingement risks between thoracolumbar/lumbar curves with different convexities in adolescent idiopathic scoliosis: a computed tomography study.

PURPOSE: To compare the positions of the aorta relative to vertebral bodies and the potential risk of the aorta impingement for pedicle screw (PS) placement between right-sided and left-sided thoracolumbar/lumbar curves of adolescent idiopathic scoliosis (AIS).
METHODS: Thirty-nine AIS patients with a main thoracolumbar or lumbar curve were recruited. The Lenke's classification was type 5C in all patients. According to the convexity of the thoracolumbar or lumbar curves, the patients were divided into either group R or Group L. The patients in Group R had a main right-sided thoracolumbar/lumbar curve, and the patients in Group L had a main left-sided thoracolumbar/lumbar curve. Axial CT images from T12 to L4 at the midvertebral body level were obtained to evaluate Aorta-vertebra angle (α), Vertebral rotation angle (β), Lefty safety distance (LSD), and Right safety distance (RSD). The risks of the aorta impingement from T12 to L4 were calculated and then compared between the two groups.
RESULTS: The α increased from T12 through L4 in Group R, increased from T12 through L1, and then decreased from L1 through L4 in Group L. The β decreased from T12 through L4 in both groups. The LSD constantly increased from T12 through L4 in Group R, increased from T12 through L3, and then decreased from L3 through L4 in Group L. The RSD increased from T12 through L3 and then decreased from L3 through L4 in both groups. With the increment of the lengths of the simulated screws, the aorta impingement risks were constantly elevated at all levels in both groups. The aorta was at a high risk of impingement from left PS regardless of the diameters of the simulated screws in Group R (80-100 % at T12 and 53.3-100 % at L1). In Group L, the aorta was completely safe when using 35 mm (0 at all levels) PS and at high risks of the aorta impingement on the right side from 45 mm PSs (31.8-72.7 %). In all, the risks of the aorta impingement were mainly from left PS in Group R and from right PS in Group L, and the risk of the aorta impingement from PS placement was generally higher in right thoracolumbar or lumbar curves when compared with that of the left.
CONCLUSIONS: The present study illustrated different changed positions of the aorta relative to vertebrae between thoracolumbar/lumbar curves with different convexities. In right-sided curve, the risks of the aorta impingement were mainly from left PS while in left-sided curves, from right PS. The aorta was more proximal to entry points in right-sided lumbar curve when compared with left-sided curve; thus placing PS carries more risks in right-sided thoracolumbar/lumbar curve. Surgeons should be more cautious when placing PSs on the concave sides of T12 and L1 vertebrae of right-sided thoracolumbar/lumbar curves.