The area informally known as the “knee pit” is a significant anatomical junction at the back of the leg. This shallow, protected space acts as a passageway for structures traveling between the thigh and the lower leg. Because it contains a concentration of nerves, blood vessels, and soft tissues, it is important for circulation and nerve function in the limb. Understanding its anatomy is necessary due to its susceptibility to various clinical issues.
The Popliteal Fossa: The Technical Name
The technical term for the knee pit is the popliteal fossa. The word “fossa” is derived from Latin, meaning a shallow depression or hollow, which aptly describes the space. While sometimes referred to by common names like the hough or the poplit, the popliteal fossa remains the universal medical designation. The term popliteal refers specifically to the area at the back of the knee.
This region is structurally similar to the cubital fossa, the analogous depression found at the front of the elbow. The popliteal fossa serves as a conduit for structures that pass from the upper to the lower leg. It is a diamond-shaped space that becomes more noticeable when the knee is bent.
Defining the Boundaries and Structure
The popliteal fossa is defined by muscular boundaries that create its diamond shape. The upper, or superior, border is formed by the tendons of the hamstring muscles. Specifically, the semimembranosus and semitendinosus muscles form the superomedial boundary, while the biceps femoris muscle forms the superolateral boundary.
Below the knee joint, the lower, or inferior, borders are created by the calf muscles. The medial and lateral heads of the gastrocnemius muscle form the inferomedial and inferolateral boundaries. The plantaris muscle also contributes to the inferolateral border. This muscular arrangement creates the space for the contents within.
The structure has a roof and a floor that enclose the space. The roof, the most superficial layer, consists of the skin, superficial fascia, and deep popliteal fascia. The floor, the deepest layer, is formed by the popliteal surface of the femur and the capsule of the knee joint.
Major Structures Housed Within
The popliteal fossa functions as a neurovascular highway for the lower limb. Several major structures pass through this space, allowing for blood flow and nerve function below the knee. The deepest structure is the popliteal artery, a continuation of the femoral artery. This artery supplies oxygenated blood to the leg and foot.
The popliteal vein runs alongside the artery, returning deoxygenated blood. It lies relatively superficial to the artery. The major nerves passing through are the tibial nerve and the common fibular nerve, both branches of the large sciatic nerve. These nerves are the most superficial neurovascular structures in the fossa.
The position of these structures is clinically significant due to the confined space. While the artery is deepest and most secure, the vein and nerves are more susceptible to compression or injury. The fossa also contains popliteal lymph nodes, which can become enlarged during infection.
Common Clinical Conditions
Due to the concentration of structures within a confined space, the popliteal fossa is prone to specific clinical conditions. One frequent issue is a Baker’s cyst, also known as a popliteal cyst. This fluid-filled sac forms when excess synovial fluid from the knee joint leaks into the back of the fossa.
Baker’s cysts are commonly associated with underlying joint problems like osteoarthritis or meniscal tears, which cause an overproduction of joint fluid. When the cyst enlarges, it can cause tightness, discomfort, or a feeling of fullness behind the knee. In some cases, a ruptured cyst can mimic the symptoms of a deep vein thrombosis, causing calf pain and swelling.
A less common but more serious condition is a popliteal artery aneurysm, which is an abnormal enlargement of the artery. Because the artery is deep and against the bone, an aneurysm can expand and compress the adjacent popliteal vein or tibial nerve. This compression can lead to symptoms like venous obstruction or nerve entrapment.