What is the most significant risk factor for the rupture of an AAA?

The most significant risk factor for the rupture of an abdominal aortic aneurysm (AAA) is indeed the size greater than 5.5 cm. As an AAA enlarges, the wall of the aneurysm becomes thinner and more stretched, which significantly increases the likelihood of rupture. Studies have shown that once the aneurysm reaches this threshold in size, the risk of rupture increases substantially, with estimates indicating a risk of around 25% per year for a 5.5 cm AAA or larger.

While hypertension can contribute to the development and progression of AAAs by exerting additional pressure on the arterial walls and weakening them over time, it does not solely dictate the risk of rupture. Similarly, a family history of AAAs can signify a genetic predisposition, but again, it does not directly correlate with an immediate risk of rupture like size does. Age under 60 is generally considered a lower risk factor for AAA rupture, as the incidence of AAA and associated complications tends to rise with age.

Thus, focusing on the aneurysm's size provides the most reliable indicator for clinical decisions regarding monitoring and intervention, making size greater than 5.5 cm the key risk factor for impending rupture.