The opposite side of the knee is called the popliteal fossa, which is an important tissue of the knee joint and houses many important nerves and blood vessels on its medial side, including popliteal vessels such as the popliteal artery and popliteal vein, as well as the tibial and common peroneal nerves and fat and lymph nodes. The popliteal fossa is rhombic in shape and its superior border is the biceps femoris, the superior medial border is the semimembranosus and semitendinosus, while the inferior lateral and inferior medial borders are the lateral and medial heads of the gastrocnemius, and its base is the joint capsule of the knee joint. When a tendon sheath cyst grows in the popliteal fossa, it can compress the nerves and blood vessels in the popliteal fossa, resulting in pain, numbness, and swelling of the lower extremity.