Variation of the position of the aorta relative to a kyphotic thoracic spine: safety margin for pedicle screw placement in the adult Japanese population.

STUDY DESIGN: An analysis of computed tomography images of the spine in adult Japanese patients.

OBJECTIVE: To evaluate the variation of the position of the aorta relative to a spine with a thoracic kyphotic angle and to verify the safety margin for transpedicular screw fixation.

SUMMARY OF BACKGROUND DATA: Recent studies have suggested that the aorta is positioned more laterally and posteriorly in patients with idiopathic scoliosis than in normal patients; however, variation of the position of the aorta in thoracic sagittal alignment has not been performed.

METHODS: T4-T12 vertebrae were studied using computer tomography images in 58 Japanese patients (aged 31-90 years) with normal coronal alignment of the spine. Pedicle diameter, pedicle length, chord length, closest distance from the aorta, and pedicle-aorta angle (PAA) were evaluated using a Mann-Whitney U test and Spearman rank correlation analysis.

RESULTS: Pedicle diameter showed an increase in size from T4 (4.7 mm) to T12 (8.2 mm), pedicle length had an almost constant size of about 20 mm, and chord length increased from T4 (37.7 mm) to T12 (47.2 mm). Closest distance from the aorta decreased from T4 (29.6 mm) to T6 (24.6 mm) and then increased to T12 (49.3 mm). PAA increased from T4 (31.5°) to T6 (38.6°) and then decreased to T12 (5.0°). Sagittal curvature showed a correlation with PAA, with an increasing sagittal thoracic Cobb angle related to decreases in PAA at the T5 (r = -0.31, P = 0.034), T6 (r = -0.37, P = 0.008), and T7 (r = -0.29, P = 0.036) levels.

CONCLUSION: The descending aorta is positioned more medially at the T5, T6, and T7 levels in patients with a kyphotic spine. These spatial relationships should be considered when planning pedicle screw insertion for a kyphotic spine.