Aneurysms of the thoracic and abdominal aorta represent a degenerative process of the aorta that is often attributed to atherosclerosis. Aneurysms are defined as a focal dilatation with at least a 50% increase over normal arterial diameter, usually associated with a degradation of the aortic media, or other structural defect in the aortic wall.
Medical research has suggested that these lesions are prone to occur in areas subjected to significant redirection of blood flow during diastole; however, the exact cause of such aneurysms is not known. A familial tendency to symptomatic aneurysms has been suggested. Degenerative aneurysms account for more than 90% of all infrarenal aneurysms of the abdominal aorta. Other potential causes include infection, cystic medial necrosis, arteritis, trauma, collagen vascular disorders, and anastomotic disruption.
Abdominal aortic aneurysms most commonly begin in the infrarenal aorta, and extend down to the iliac bifurcation. Aneurysms of the thoracic aorta are most commonly located in the descending thoracic aorta, beginning just distal to the origin of the left subclavian artery.
Aortic aneurysms generally affect elderly Caucasian men. Aortic aneurysms are less commonly reported among persona of African American, Asian, and Hispanic heritage. Abdominal aortic aneurysms are five times more common in men than in women. In men, the aneurysm process appears to begin at approximately age fifty years and reaches peak incidence at approximately age eighty years. Women appear to have a more delayed onset in which the aneurysm process appears to begin at approximately age 60 years. Smoking has been associated as a potential risk factor for the development of aortic aneurysms. Other risk factors include previous aneurysm repair or peripheral aneurysm (such as femoral or popliteal), coronary artery disease, and hypertension.
Although the reported findings from autopsy series vary widely, the incidence of aortic aneurysms probably exceeds 3-4% in individuals older than 65 years. Death from aneurysmal rupture remains one of the 15 leading causes of death in the United States. In addition, the overall prevalence of aortic aneurysms has increased significantly in the last 30 years. This is partly due to an increase in diagnosis based on the widespread use of imaging techniques. However, the prevalence of fatal and nonfatal rupture has also increased, suggesting a true increase in incidence. An aging population probably plays a significant role.
The surgical management of aortic aneurysms dates back to the early twentieth century, and has involved a variety of methods, including ligation, intraluminal wiring, cellophane wrapping, homografts, and grafts using nylon and polytetrafluoroethylene [PTFE] fabrics.
Prior to the development of endoaneurysmorrhaphy in 1962, postoperative surgical mortality rates were high (>25%). Endovascular repair techniques have reduced the operative mortality to 1.8-5%.
Existing techniques for endovascular treatment of aneurysms involve placement of a tubular graft with seals to normal aortic walls above and below the aneurysm to create a tubular bridge to carry flow across the aneurysm without allowing flow to fill the aneurismal sac. Using these techniques, grafts may be placed using percutaneous access to the femoral arteries, and delivery/implantation using vascular catheters and fluoroscopic visualization. The deficiencies associated with existing endograft technology relate to leakage at the graft/aortic interface and/or post-implantation migration of the endograft. Small post-implantation leaks may be repaired with the placement of one or more extension cuffs above the endograft proximally, or below the implant distally to attempt to obtain a better seal with the vessel. The required use of such cuffs may add significantly to the overall cost and morbidity of the procedure. Major failures with endograft repair generally require emergent open surgery to avert catastrophic rupture of the aneurysm. Also, current endovascular systems require accurate size matching of endograft implants, leaving a very small margin for error.
In order for a patient to be a candidate for existing endograft methods and technologies, a proximal neck of at least 15 mm. of normal aorta must exist between the origin of the most inferior renal artery and the origin of the aneurysm in the case of abdominal aneurysms or the left subclavian artery for thoracic aortic aneurysms in order to permit an adequate seal. Similarly, at least 15 mm. of normal vessel must exist distal to the distal extent of the aneurysm for an adequate seal to be achieved.
Migration of existing endografts has also been a significant clinical problem, potentially causing leakage and re-vascularization of aneurysms and/or compromising necessary vascular supplies to arteries such as the carotid, subclavian, renal, or internal iliac vessels. This problem has been partially addressed by some existing endograft designs, in which barbs or hooks have been incorporated to help retain the endograft at its intended site. However, these existing endograft designs are not removable and repositionable once they are deployed. Thus, once such an endograft has been placed, open surgery is necessary if there is failure due to leakage or undesired occlusion of other vascular structures.
Because of the limitations imposed by existing vascular endograft devices and endovascular techniques, approximately eighty percent of abdominal and thoracic aneurysms repaired in the U.S. are still managed though open vascular surgery, instead of the lower morbidity of the endovascular approach.