The abdominal aorta is a major blood vessel in the body that carries blood from the heart to the major organs in the chest and abdomen. An abdominal aortic aneurysm (AAA) is a balloon-like bulge (dilation) of the aorta that is greater than 3 cm in diameter. If an AAA ruptures (bursts), this is often fatal. Hence, AAAs that are larger than 5.5 cm are usually treated surgically in order to try to prevent such a rupture. Traditionally, AAAs are treated using an open surgical repair (OSR) technique, in which the abdomen is cut open (referred to as open surgery) and the dilated aorta is repaired using fabric graft material. However, over the past 20 years, a newer, 'key hole' technique has been used, in which the AAA is repaired without the need for open surgery - a thin tube is passed via the blood vessels in the groin to the site of the AAA. Once in the correct position, a sheath is introduced that acts to reline the dilated aorta, acting as an artifical blood vessel through which blood can continue to flow, bypassing the aneurysm. Hence, the risk of further expansion or rupture of the AAA is reduced, This technique is referred to as endovascular aneurysm repair (EVAR). As EVAR is a less invasive technique than OSR, in that there is no need for open surgery, it may have advantages over OSR. In addition, some individuals with other medical illnesses, for whom open surgery may be considered a high-risk procedure and who are not fit for OSR, can be offered EVAR instead.