The diamond-shaped posterior surface of the human knee is known as the popliteal fossa. It is through this area that neurovascular structures pass between the upper and lower leg.
The four main structures passing through the popliteal fossa are the common fibular nerve, the tibial nerve, the popliteal vein and the popliteal artery. Of these four, the popliteal artery is the deepest and perhaps most important. This artery is a continuation of the femoral artery, and it supplies blood to the lower regions of the leg.
The two most common causes of the appearance of a mass in the popliteal fossa include a Baker's cyst and an aneurysm of the popliteal artery.