The knee’s complex mechanics are often simplified to the patella and the visible joint. However, the area directly behind the knee is a significant anatomical region. This posterior aspect is a shallow, diamond-shaped space that serves as a thoroughfare for the major vessels and nerves of the lower leg. Understanding this area provides insight into why pain, swelling, or numbness can occur below the thigh.
The Popliteal Fossa: Naming and Boundaries
The anatomical term for the under part of the knee is the Popliteal Fossa, which translates to the “hollow of the knee.” This depression is shaped like a rhombus or diamond when the knee is slightly bent, and it acts as a protected space for delicate structures passing from the thigh to the calf. The physical boundaries of this space are defined by the powerful hamstring and calf muscles that converge around the knee joint.
The upper boundaries are formed by the hamstring muscles: the semimembranosus and semitendinosus muscles on the inner side and the biceps femoris muscle on the outer side. These tendons create the superior angles of the diamond shape. The lower boundaries are formed by the two prominent heads of the gastrocnemius muscle (the calf muscle).
The floor of the popliteal fossa is comprised of the popliteal surface of the femur bone and the capsule of the knee joint, which provides a rigid base. The roof is formed by the skin and a strong layer of deep tissue called the popliteal fascia. This muscular and fascial arrangement helps protect the contents within, especially during movement.
Critical Structures Contained Within the Fossa
The popliteal fossa is an important passage for the neurovascular bundle that supplies the leg and foot. The most deeply positioned structure is the Popliteal Artery, a continuation of the femoral artery from the thigh. This artery is the primary supplier of oxygenated blood to the lower leg.
Lying immediately superficial to the artery is the Popliteal Vein, which carries deoxygenated blood back up toward the heart. The vein and artery are closely associated, running parallel through the center of the fossa. The most superficial major structures are the nerves, including the Tibial Nerve and the Common Fibular Nerve (also called the Common Peroneal Nerve).
Both of these nerves split off from the large Sciatic Nerve in the thigh before entering the fossa. The Tibial Nerve descends vertically through the center of the fossa with the artery and vein, controlling the muscles of the calf and foot. The Common Fibular Nerve angles toward the outer side of the knee, wrapping around the head of the fibula to supply the muscles on the front and side of the lower leg.
Common Causes of Pain and Swelling in the Posterior Knee
Pain and swelling in the popliteal fossa often arise when the confined space is compromised, placing pressure on the delicate nerves and vessels. One of the most common causes of swelling is a Baker’s Cyst, also known as a popliteal cyst. This is a fluid-filled sac that forms when excess joint-lubricating fluid, or synovial fluid, is pushed out of the knee joint and collects in the bursa at the back of the knee.
Baker’s cysts are often secondary problems, caused by underlying knee conditions like osteoarthritis, rheumatoid arthritis, or a meniscal tear. The fluid accumulation creates a bulge and can cause stiffness or pain, especially when the knee is fully bent or straightened. If a cyst ruptures, the fluid leaks into the calf, causing sharp pain, swelling, and redness that can mimic the symptoms of a blood clot.
More serious causes of discomfort relate to the major vessels within the fossa. A Popliteal Artery Aneurysm is an abnormal ballooning of the artery wall that can cause a pulsing feeling or swelling behind the knee. The danger is that blood clots can form inside it, potentially blocking blood flow to the lower leg and causing a lack of circulation.
Another serious vascular issue is Deep Vein Thrombosis (DVT), a blood clot in the popliteal vein, causing swelling and pain in the calf and behind the knee. The nerves in the fossa are also susceptible to compression. Swelling from any cause, including a large cyst or muscle tightness, can compress the tibial or common fibular nerves, leading to altered sensation, tingling, or weakness in the lower leg and foot.
Simple muscle strain can also refer pain to this area, particularly strains of the hamstring tendons or the gastrocnemius muscle heads. Because the symptoms of simple strains, cysts, and dangerous vascular events can overlap, persistent pain or swelling in the back of the knee should always be evaluated by a healthcare professional.