The fossa behind the knee of the leg refers to the popliteal fossa. Pain in the popliteal fossa is considered to be popliteal cyst, trauma, infection, meniscus injury, osteoarthritis and so on. 1. Popliteal cyst: mostly caused by intra-articular related lesions, under normal circumstances popliteal cysts do not have symptoms of pain, but when the popliteal cyst is large in size, the pressure inside the cyst is large, and the outside of the cyst has pressure on the surrounding tissues, the patient may also experience pain. 2. Trauma: there is a clear history of trauma to the popliteal fossa, which can be caused by strains and contusions of the ligaments in the popliteal fossa, and in addition to pain, there is also swelling. 3. Infection: the skin of the affected area is often broken, improper treatment can lead to bacterial invasion, the affected area appears red, swollen, hot and painful and other symptoms of infection, and may even form abscesses. 4. Meniscus injury: when the posterior horn of the meniscus is damaged, it stimulates the ligaments and joint capsule of the posterior side of the joint, resulting in pain in the popliteal fossa. 5. Osteoarthritis: Bone growths in the joints can cause pain in the popliteal fossa if they irritate the ligaments and other tissues on the back side of the knee. Popliteal pain may also be distal femur and proximal tibia bone tumor and other diseases, if the clinical symptoms have not been relieved, it is recommended to go to the hospital for examination to clarify the cause.