The knee is a complex hinge joint that manages the body’s weight while allowing for extensive movement like walking, running, and bending. While attention often focuses on the patella and front ligaments, the posterior portion of the knee is equally intricate. This back region is a critical anatomical passageway, making it prone to specific issues when injury or disease occurs. Understanding this structure is the first step toward recognizing potential health concerns.
The Popliteal Fossa
The depression often described as the “under part of your knee” is formally known as the Popliteal Fossa. This shallow, diamond-shaped hollow is situated directly behind the knee joint and is easily felt as a soft concavity when the knee is slightly bent.
This space is defined by the arrangement of large muscles in the thigh and calf. The superior boundaries are formed by the hamstring tendons: the semimembranosus and semitendinosus medially, and the biceps femoris laterally. The inferior borders are created by the two heads of the gastrocnemius muscle. The floor of the fossa is formed by the posterior surface of the femur and the capsule of the knee joint.
Why This Area is So Important
The Popliteal Fossa functions as a protected, yet confined, thoroughfare for the major structures connecting the upper leg to the lower leg and foot. It is a transitional zone where essential blood vessels and nerves travel. This concentration of structures makes the fossa vital to the function and health of the lower extremity.
The deepest structure within this space is the Popliteal Artery, a continuation of the femoral artery that supplies oxygenated blood to the entire lower leg. Just superficial to the artery is the Popliteal Vein, which is responsible for draining blood back up toward the heart. Both of these vessels are highly vulnerable because they are compressed against the bone of the femur when the knee is fully bent.
More superficially, the major nerves of the lower leg—the Tibial Nerve and the Common Fibular (Peroneal) Nerve—pass through the fossa. These nerves originate from the large sciatic nerve and control both the movement of the muscles and the sensation in the lower leg and foot. Because all these structures are packed tightly within the diamond-shaped space, any swelling or abnormal mass in the fossa can immediately compress them, leading to significant symptoms.
Common Problems Behind the Knee
The most frequent issue causing swelling and discomfort here is a Baker’s Cyst, also known as a Popliteal Cyst. This fluid-filled sac forms when excess synovial fluid leaks from the knee joint and collects in the gastrocnemius-semimembranosus bursa. A Baker’s cyst is often secondary to underlying knee problems, such as arthritis or a meniscus tear, which cause the knee to produce too much fluid. Symptoms typically include a feeling of fullness, tightness, or a noticeable lump behind the knee, especially when the leg is straightened.
While usually benign, a large or ruptured cyst can cause severe complications due to the tight confines of the fossa. If the cyst expands, it can press on adjacent nerves, leading to symptoms of nerve compression. Tibial Nerve compression causes pain, tingling, or weakness in the calf muscles, while Common Fibular Nerve compression can result in numbness or foot drop.
Less common but more serious issues involve the Popliteal Artery and Vein. The artery can be affected by aneurysms or, rarely, by Popliteal Artery Entrapment Syndrome (PAES), where surrounding muscles squeeze the vessel. Vein compression is more frequent than arterial issues and can cause pain and swelling in the lower leg. Evaluation is necessary to rule out a deep vein thrombosis (DVT), a serious blood clot. Any persistent swelling, pain, or changes in sensation behind the knee warrant a professional medical examination to prevent potential long-term damage to the neurovascular structures.