Thoracic vertebral screw impingement on the aorta in an in vivo bovine model.

ABSTRACT

STUDY DESIGN: A bovine model was used to evaluate the effects of thoracic vertebral screw impingement of the aorta. OBJECTIVES: To evaluate the histologic and biomechanical changes in aortic wall tissue that was severely impinged by abutting instrumentation. SUMMARY OF BACKGROUND DATA Case reports of vascular injury associated with spinal instrumentation generally describe intraoperative injury; some report delayed presentation of large vessel damage. Risks associated with placing instrumentation adjacent to large vessels are largely unknown. METHODS: Six 1-month-old calves underwent left-sided thoracotomies, exposing the anterior thoracic spine and aorta. With the heads removed, screws were inserted in reverse fashion into T6 through T11, leaving the screw tips 1 cm proud and abutting the aorta. After 3, 6, or 12 months (2 calves each), the spines were resected with the adjacent aorta and underwent radiographic, histologic, and biomechanical testing. RESULTS: Computed tomography revealed varying degrees of vessel impingement. Although there were no frank ruptures, 96% of aortic specimens showed histopathologic changes, including 52% with wall thinning; 43% were no longer impinged, yet 60% of these had increased collagen (scar). Biomechanical testing of screw-impinged aortas demonstrated a lower failure stress (1.2 +/- 0.5 N/mm vs. 1.8 +/- 0.4 N/mm, P = 0.016) but no difference in failure strain (42 +/- 9% vs. 32 +/- 10%, P = 0.06) than controls. CONCLUSIONS: Major impingement of vertebral screws on the aorta caused acute and chronic histopathologic and biomechanical changes in the vessel wall. This model represents a severe form of vessel penetration by a screw that confirms such a "worst case" scenario results in marked compromise of the vessel wall integrity. The sequelae of less severe impingement are unknown.