The possible causes of pain in the posterior lateral fossa tendon flexion of the knee include arthritis, posterior cruciate ligament injury of the knee, posterior horn injury of the meniscus, and popliteal cyst. 1. Arthritis: when degenerative changes occur in the knee joint, osteoarthritis of the knee joint is prone to occur, or the patient himself suffers from rheumatoid arthritis and so on. Long-term inflammatory stimulation may cause osteomalacia of the knee joint, and when the knee is bent, the proliferated bone will stimulate the joint capsule inside the popliteal fossa, resulting in the symptom of painful bending of the posterior side of the knee tendon. 2. Posterior cruciate ligament injury of the knee joint: If the stopping point of the posterior cruciate ligament of the knee joint is hit by external force, it is easy to occur damage and rupture of the cruciate ligament, or even avulsion fracture, which leads to extrusion of the damaged area when the knee bends, and the symptom appears. 3. Posterior meniscus corner injury: If the knee joint is traumatized, wear and tear of the posterior meniscus corner may occur, and when the knee bends, it will squeeze the damaged posterior meniscus corner, and the phenomenon will also occur. 4. Popliteal cysts: the popliteal fossa is connected to the knee joint cavity, and the synovial fluid in the knee joint can penetrate to the back of the knee, forming popliteal cysts. With the increase in the size of the cyst, it can produce local compression. As a result the symptom appears when the knee is bent. There may also be other reasons for the pain in the posterior side of the knee fossa tendon bending, it is recommended to go to the regular hospital in time, improve the relevant examination, clarify the cause of the disease, and then give targeted treatment or treatment under the guidance of the doctor.