Objectives The aorta inhibits paravertebral ossification in diffuse idiopathic skeletal hyperostosis. We investigated if syndesmophytes in ankylosing spondylitis (AS) occurred less often at the vertebral rim near the aorta.

Methods We performed thoracolumbar CT scans in 60 subjects in this cross-sectional study. The mid-thoracic spine was also scanned in 22 subjects. We divided the rim of each intervertebral disc space (IDS) into 72 angular sectors, each of 5°. We computed syndesmophyte height in each sector, and the distance from the sector to the aorta. We evaluated if syndesmophyte size or frequency in a sector was associated with its distance from the aorta.

Results In the 180° region of the vertebral rim centered on the sector closest to the aorta, syndesmophyte height and/or frequency varied with the distance of the sector to the aorta, with the lowest frequency and smallest mean syndesmophyte height at the sector along the rim nearest the aorta. Additionally, syndesmophytes were less common in subjects and at IDSs where the aorta was anatomically closer to the vertebra. No syndesmophytes were present in the sector closest to the aorta in subjects whose aorta-vertebral distance was less than 2 mm, but syndesmophytes were progressively more common among subjects whose aortas lay further from the rim.

Conclusions Syndesmophytes occurred less commonly and were smaller at the thoracolumbar vertebral rim near the aorta. These findings suggest that mechanical factors extrinsic to the spine and not solely vertebral inflammation, influence syndesmophyte development in AS.