Not good...see below. An aneurysm is an dilitation of a blood vessel. This is the result of weakening in the wall of the artery. The weakened wall can rupture, and bleeding into the tissues around the knee occurs, causing tremendous pain and swelling. The blood outside the artery can then impinge on flow to the lower leg and foot, causing ischemia. In all, very bad, and one of the reasons to fix aneurysms electively.
Bleeding. The wall of the aneurysm has "split" and blood escapes. The speed of the bleeding depends on the location of the aneurysm and the surrounding structures which may support it. It is unusual for a popliteal aneurysm to rupture. It usually clots or causes clot and debris from inside the aneurysm to go downstream blocking the arteries. Both of these can lead to amputation.
A serious problem. Popliteal aneurysms very rarely rupture, but instead they more commonly thrombose. When this occurs small pieces of clot shower downstream into all of the small artery branches below the knee. This is a vascular surgery emergency. Over half of patients who suffer thrombosis of a popliteal artery aneurysm will require amputation, below or often above the knee. Read more...