Multilevel spinal growth modulation with an anterolateral flexible tether in an immature bovine model.

STUDY DESIGN: A bovine model was used to evaluate the effects of a multilevel anterolateral flexible tether in a growing spine. OBJECTIVE: To evaluate the radiographic changes in a growing spine with a multilevel anterolateral tether. SUMMARY OF BACKGROUND DATA: Spinal growth modulation has long been considered as a conceptually attractive and elegant possible alternative to arthrodesis in the treatment of idiopathic scoliosis. Although some experimental studies have described spinal growth modulation, few have described a purely mechanical tether. Clinical studies of spinal epiphysiodesis have described inconsistent curve stabilization and/or correction. METHODS: A total of 33 one-month-old male calves underwent a single thoracotomy and placement of vertebral screws at T6-T9. In 11 animals, one screw per level was connected by a 3/16 in. stainless steel cable (single tether). In 11 animals, two screws per level were connected by two cables (double tether). In the remaining 11 animals, single screws in each level were left unconnected (control). After 6 months, the spines were harvested and underwent radiographic analysis. RESULTS: In the control group, there was little change in the coronal and sagittal measurements during the survival period. In the single tether group, there was variable instrumentation fixation and inconsistent creation of coronal deformity, which ranged from 0 degrees to 31 degrees. The double-tether group had more consistent creation of deformity, ranging from 23 degrees to 57 degrees. CONCLUSIONS: Given adequate bony fixation, a flexible lateral spinal tether can affect growth modulation. This technique of growth modulation may serve as a future fusionless method of correction in a growing patient with scoliosis.

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

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.