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The aorta is the main trunk of the arterial system that carries oxygenated blood from the heart to all points in the body. The aorta arises from the left ventricle of the heart and ends at the lumbar area where it divides to form the right and left common iliac arteries that run down each leg. The abdominal aorta is the portion that is located in the abdominal area between the renal arteries and the iliac bifurcation (split) and it travels down the frontal surface of the vertebral column (spine).

Arteries consist of three layers:

• Tunica intima - the inner layer
• Tunica media - the middle layer
• Tunica adventitia - the outer layer

An aneurysm is an abnormal widening of an artery with an increase of greater than 1.5 times the normal diameter. It usually involves a weakness in the tunica media resulting in stretching of the tunica adventitia and/or the tunica intima. As blood is pumped through the artery, the weakened wall is stretched further often creating an egg-shaped ballooning. An aneurysm may have deposits of cholesterol, calcium, and even small blood clots. Aneurysms typically grow over time, usually at an average of 1/8 - 1/4 inch per year. Additional expansion can be caused by blood pressure within the aneurysm.

An aneurysm can occur in any blood vessel in the body, but it most commonly occurs in the aorta. A common site for an aortic aneurysm is the part of the vessel that is immediately below the renal arteries (which supply the kidneys with blood) but above the iliac arteries (which supply the legs with blood). This is called an abdominal aortic aneurysm (AAA).

The aorta undergoes growth and increases in diameter during childhood, adolescence, and between the ages of 25 and 75 when the diameter increases by approximately 25%. Diameter of the normal aorta is larger in men than in women. The average size of a normal aorta ranges from approximately 1.7 cm. to 2.4 cm. depending on gender, body habitus, and age.

The International Society for Cardiovascular Surgery/Society for Vascular Surgery defines abdominal aortic aneurysm as a focal dilation (widening) of the abdominal aorta where the diameter is at least 50% larger than the expected normal diameter for that individual. The standard measurement accepted by many clinicians and clinical trials for a focal dilation to be considered an abdominal aortic aneurysm is a diameter greater than 3 cm.

The incidence and prevalence of abdominal aortic aneurysm increases with age, making this condition a significant issue as the population ages. It is most common in elderly men and occurs in 5% to 7% of people over the age of 65 in the United States. The ratio of men to women who develop an abdominal aortic aneurysm is 3:1. After age 65, the prevalence of 3 cm. aneurysms in men increases by approximately 6% per decade. It is estimated that the rate of clinically relevant aneurysm (4 cm. or more) increases approximately 2% to 4% per decade. Children can develop abdominal aortic aneurysms as a result of trauma or certain medical conditions but this is very rare.