The fossa behind the knee usually refers to the popliteal fossa. The most common causes of popliteal pain are skin infections, trauma, and popliteal cysts. 1. Skin infection: if the patient’s popliteal fossa has a break in the skin, the broken skin is infected will be prone to inflammatory reaction, and then local pain and redness and swelling. 2. Trauma: If the patient’s popliteal fossa is hit by an external impact, it can lead to soft tissue damage in the popliteal fossa, which can lead to localized swelling and pain. 3. Popliteal cysts: popliteal cysts can be caused by over-exercise or prolonged sitting and standing, etc. A lump may appear in the popliteal fossa, accompanied by pain, and in severe cases, it may cause limited knee joint activities. The causes of pain in the popliteal fossa behind the knee are many, and may be related to meniscus injury, osteoarthritis, etc. If the symptoms continue to be unrelieved, it is recommended to go to the hospital in time to find out the cause of the disease, and then give targeted treatment.