Popliteal aneurysms occur in the leg arteries behind the knee.

They differ from AAA in one major aspect. Instead of rupturing, the popliteal aneurysm blocks off with thrombus - a thrombosed popliteal aneurysm. Although this can happen without any ill effects, usually the leg goes cold and numb and if a bypass isn't carried out within a few hours, the leg dies and an amputation is necessary.

Therefore the idea of treating a popliteal aneurysm is to bypass it BEFORE it blocks off.

A bypass operation is a long operation where the vein is taken from the leg, turned around, and used to bypass the popliteal aneurysm. Sometimes the vein isn't suitable and so a plastic graft is used instead.