There are many causes of knee flexion and pain, the most common of which are external injury, bacterial infection, osteoarthritis, bone tumors, etc., as follows: 1. External injury: The knee joint is subjected to great external force, resulting in soft tissue injury, joint capsule injury, ligament injury, meniscal injury, fracture, dislocation, etc., which can lead to significant swelling and pain in the knee joint, and cannot be bent. 2. 3.Osteoarthritis: If a patient has severe osteoarthritis of the knee joint, it will lead to limited flexion and extension of the knee joint and very severe pain. 4.Bone tumors: Common osteosarcoma, chondrosarcoma, osteofibrosarcoma, synovial sarcoma and metastatic bone tumors will also lead to the existence of the knee joint There is significant pain and inability to bend.