Abdominal aortic aneurysm (also known as AAA, pronounced "triple-a") is a localized dilatation of the abdominal aorta exceeding the normal diameter by more than 50 percent. It is caused by degeneration of the aortic wall, but the exact etiology remains unknown. Some 90 percent of abdominal aortic aneurysms occur infrarenally (below the kidneys), but they can also occur pararenally (at the level of the kidneys) or suprarenally (above the kidneys). Such aneurysms can extend to include one or both of the iliac arteries in the pelvis.
Abdominal aortic aneurysms occur most commonly in individuals between 65 and 75 years old and are more common among men and smokers. They tend to cause no symptoms, although occasionally they cause pain in the abdomen and back (due to pressure on surrounding tissues) or in the legs (due to disturbed blood flow). The major complication of abdominal aortic aneurysms is rupture, which can be life-threatening as large amounts of blood spill into the abdominal cavity, and can lead to death within minutes.
Symptomatic and large aneurysms (i.e., those greater than 5.5cm in diameter) are considered for repair by one of several surgical methods. There is moderate evidence to support screening in individuals with risk factors for abdominal aortic aneurysms.
There have been many calls for alternative approaches to rupture-risk assessment over the past number of years, with many believing that a biomechanics-based approach may be more suitable than the current diameter approach. Numerical modelling is a valuable tool to researchers allowing approximate wall stresses to be calculated, thus revealing the rupture potential of a particular aneurysm.
Experimental models are required to validate these numerical results, and provide a further insight into the biomechanical behaviour of the AAA. ''In vivo'', AAAs exhibit a varying range of material strengths from localised weak hypoxic regions to much stronger regions and areas of calcifications. Experimental models can now be manufactured using a novel technique involving the injection-moulding lost-wax manufacturing process to create patient-specific anatomically-correct AAA replicas.
Work has also focused on developing more realistic material analogues to those ''in vivo'', and recently a novel range of silicone-rubbers was created allowing the varying material properties of the AAA to be more accurately represented. These rubber models can also be used in a variety of experimental testing from stress analysis using the photoelastic method to deterimining whether the locations of rupture experimentally correlate with those predicted numerically.
With the recent advancements in AAA research, coupled with the increasing collaboration between clinicians and engineers, the future research into AAA rupture-prediction and treatment appears to be in a strong position to combat what is currently ranked as the 13th leading cause of death in the US and the 10th leading cause of death in men over the age of 55 years.
A recent animal study published in the journal ''Nature Medicine'' showed that removing a single protein prevents early damage in blood vessels from triggering a later-stage, frequently lethal complication of atherosclerosis. By eliminating the gene for a signaling protein called cyclophilin A (CypA) from a strain of mice, researchers were able to provide complete protection against abdominal aortic aneurysm (AAA).
New endovascular devices are being developed that are able to treat more complex and tortuous anatomies.
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