CT-based anatomical evaluation of pre-vertebral structures with respect to vertebral body using a clock-face analogy

Study Design. Retrospective Chart and CT Scan Review Objective. To define the relationship of the pre-vertebral structures for each level to assist in easier intraoperative visualization. Summary of Background Data. Vascular and visceral injuries from pedicle screws are well-known. This study will define the relationship of the pre-vertebral structures for each level to assist in avoiding potential complications. Methods. Pre- and post-operative CT scans were reviewed to define the pre-vertebral structures in relation to a clock-face. On reformatted axial slices, a clock-face was superimposed so that the left transverse process (TP) represented 8 o'clock and the right TP represented 4 o'clock. The positions of the TP on the clock-face did not change with rotation of the vertebra. Results. 108 patients had pre-operative CT scans. 78 had postoperative CT scans. Median age was 15 years, median Cobb angle was 50°, fused were 12, with 21 fixation points. 6324 axial CT slices were reformatted and analyzed. The trachea was located at 12 o'clock at T1, 1 o'clock at T2-T4, and between 12 and 1 o'clock at T5. The esophagus starts as a midline structure at 12 o'clock from T1-T2, moves to 11 o'clock from T3-T6, and further to 10 o'clock from T7-T9. The aorta starts at 10 o'clock at T5-T6, moves left at T7-T8 to 9 o'clock, and returns to 10 o'clock from T9-T11. It appears at 11'clock at T12, and at 12 o'clock from L1-L4. In about a third of cases, it is at 1 o'clock from L1 to L4, where it bifurcates. Conclusions. This CT-based anatomical study provides a simple reference frame to help surgeons visualize the vital structures at each level. This three-dimensional visualization is facilitated by fixing the position of TP on the clock-face. Knowledge of this anatomical relationship can help avoid direct injury, and is easier to recall intra-operatively.