Cellular Mechanisms of Aneurysm Occlusion after Treatment with a Flow Diverter

Purpose: To characterize the progression of healing across aneurysm necks following treatment with a flow diverter in a rabbit aneurysm model. Materials and Methods: With institutional animal care and use committee approval, saccular aneurysms were created in 20 rabbits and treated with flow diverters. On days 1, 3, and 7 and weeks 4 and 8 after implantation, the aneurysm and the device-implanted vessel were harvested. En face staining of the gross specimen was performed for endothelial cells, endothelial progenitor cells, smooth muscle cells, and inflammatory cells. Results: The parent artery segments covered by the flow diverters were completely devoid of endothelial cells at 1 and 3 days but had completely reendothelialized by 7 days. At all time points, the struts along the patent portions of the aneurysm necks harbored scattered tissue islands composed exclusively of inflammatory cells. At 4 and 8 weeks, all samples contiguous with the tissue along the parent arteries had translucent tissue present along the occluded segments of the aneurysm neck. The vast majority of endothelial cells were contiguous with the parent artery and had smooth muscle cells underlying them. Endothelial progenitor cells were not observed along the neck of any aneurysm. Aneurysm closure was noted only when complete or nearly complete endothelialization over the device struts was present. Conclusion: The initial event following flow diversion treatment is adherence of clusters of inflammatory cells across the aneurysm neck. Endothelialization is relatively delayed and derived exclusively from cells in the adjacent parent artery.