Pain behind the knee is usually a condition of the popliteal fossa (the depression on the back side of the knee), which may be caused by trauma, popliteal cyst, popliteal vein thrombosis, osteoarthritis, meniscus injury and other factors. 1. Trauma: If the popliteal fossa is injured by a collision, it can cause pain behind the knee. In addition, if the patient performs high-frequency knee flexion and extension exercises, it may cause muscle or ligament strain in the popliteal fossa, which can also cause pain. 2. Popliteal cysts: If the patient has cysts in the popliteal fossa due to inflammation and other factors, it will compress the surrounding blood vessels and other tissues, resulting in poor blood circulation, which leads to pain. 3. Popliteal vein thrombosis: If a thrombus forms in the popliteal vein and prevents the blood from returning, it can cause the popliteal vein to become swollen and painful, which can cause pain behind the knee. 4. Osteoarthritis: If the patient’s osteoarthritis invades the surrounding tissues, it can cause pain behind the knee. 5. Meniscus injury: Meniscus posterior horn injury will also pull the surrounding tissue, causing pain behind the knee. There are many other factors that can cause pain behind the knee, patients should go to the hospital in time for examination to clarify the cause of the disease and receive appropriate treatment.