Clinical meaning
An abdominal aortic aneurysm (AAA) is a pathological dilation of the aorta to ≥3.0 cm (normal infrarenal diameter 1.5-2.5 cm) resulting from degeneration of the elastic media layer. The pathophysiology involves a complex interplay of atherosclerosis, chronic inflammation, and matrix metalloproteinase (MMP) overactivity. Atherosclerotic plaques cause intimal injury and chronic transmural inflammation with macrophage and T-lymphocyte infiltration. These inflammatory cells release MMP-2 and MMP-9, which degrade elastin and collagen in the media, weakening the arterial wall. Simultaneously, vascular smooth muscle cells undergo apoptosis, reducing the wall's ability to repair and maintain structural integrity. The aortic wall progressively thins and dilates according to Laplace's law: wall tension = (pressure × radius) / (2 × wall thickness). As the radius increases, wall tension rises disproportionately, creating a positive feedback loop that accelerates expansion. Rupture occurs when wall stress exceeds tensile strength, typically at the posterolateral wall where the aorta is unsupported.