The May issue of Vascular Disease Management highlights aortic aneurysmal diseases. A thought-provoking study by Dr. Filardi demonstrates the potential of biomechanics, specifically computational fluid dynamics, for understanding rupture risk of aneurysms over the conventional use of diameter measurements. Additionally, in interviews with Drs. Fillinger and Makaroun, newer endografts for treatment of complex aortic aneurysmal disease are discussed.
In 1991, Dr. Juan Parodi presented what was at that time a controversial but successful use of an endoprosthesis for treatment of abdominal aortic aneurysms. Custom-made Dacron grafts were inserted transfemorally and fixed with balloon-expandable stents. Since the seminal work of Dr. Parodi, endoprosthesis technology for aortic disease has made and continues to make tremendous strides. The reduced perioperative mortality vs open repair and improving long-term outcomes with newer iterations of endoprostheses has allowed endovascular aneurysm repair (EVAR) to flourish. At some institutions, management of abdominal aortic aneurysms with EVAR has surpassed that of open repair.
Anatomical factors remain the cornerstone for successful EVAR, whether in the abdominal and/or thoracic location. Device manufacturers are challenged with the task of developing endografts that minimize or eliminate endoleaks, improve conformability to match arterial anatomy, provide enhanced graft stabilization, and preserve flow to important branch vessels. Newer generation stent grafts, low-profile delivery systems, and advances in percutaneous arteriotomy closure will allow the adoption of endovascular therapy for the most complex and previously untreatable patients.
The tremendous advancements made in EVAR are mitigating the risk for the highly lethal consequence of aortic rupture. Presently, our decision about who to treat is based on the presence or absence of symptoms, aneurysm size, and aneurysm growth rate. For abdominal aortic aneurysms, we typically reserve treatment for asymptomatic patients with an aortic diameter >5.5 cm or with rapid rates of expansion (>0.5 cm in 6 months or >1 cm per year). Cumulative evidence from prospective, randomized trials, including the United Kingdom Small Aneurysm Trial, indicates that larger abdominal aneurysms (>5.5 cm) expand at a greater rate than smaller aneurysms and consequently have higher risk for rupture. Additionally, early open or endovascular repair of smaller, asymptomatic aneurysms 4 cm to 5.5 cm does not appear to afford a mortality advantage over continued surveillance. Nevertheless, aortic rupture in patients with smaller aneurysms undergoing surveillance has been reported to occur in 2% to 5% of cases and post-mortem studies have shown that 10% to 24% of ruptured aneurysms have diameters less than 5.5 cm.
This leads us to the importance of identifying novel methods for assessing patients at risk for aortic rupture. Aortic rupture occurs at regions where the stress of the wall (or force per unit area) created by several variables exceeds its strength. Traditionally, the Law of Laplace, where simplistically the stress in the abdominal aortic aneurysm (AAA) wall is proportional to its diameter, has been the theoretical basis for aortic rupture potential. The applicability of Laplace Law fails because AAAs are not simple cylinders and have complex geometries. Additionally, other components of aneurysms, such as intraluminal thrombus and atherosclerotic plaque, may influence wall stress. Several investigational methods have been employed to more accurately predict rupture risk in patients with AAAs. Ultrasound, computed tomography, and magnetic resonance imaging have been used to measure multiple dynamic properties of the aortic wall beyond simple diameter measurements (e.g., distensibility and compliance). Positron Emission Tomography offers the potential of radiotracers to image functional activity at sites of aortic instability. Finally, patient-specific biomechanical profiling has shown significant promise. Biomechanical assessments require detailed information of the 3-dimensional geometry of an AAA and hemodynamic forces acting on the aneurysm wall. Computational methods for calculating AAA wall stress can then be employed to assess potential risk of rupture.
Ultimately, the Achilles’ heel for predicting rupture risk on a clinically relevant scale is based on the identification of techniques that are both reproducible and cost effective. At present, neither criterion is adequately satisfied for clinically relevant use. However, investigators in the field of identifying aortic rupture risk are applauded for their ongoing, and potentially life-saving, efforts.