Knee bending pain is usually caused by trauma, synovitis, osteoarthritis and other reasons. 1. Trauma: due to trauma caused meniscus injury, ligament injury around the knee joint, etc., there will be obvious pain when the knee bends, resulting in limited activities, and exertion can aggravate knee bending pain. 2. Knee synovitis: it is abnormal proliferation or inflammation of synovial membrane, when the knee bends, the pain site is not fixed because the surface of synovial membrane becomes not smooth and rough. It manifests as swelling of the knee joint, limitation of movement, and pain. 3. Osteoarthritis: Osteoarthritis seen in the knee joint, commonly seen in middle-aged and old people, due to degeneration and proliferation of articular cartilage, inducing aseptic inflammation of the knee joint and bony joint pain when bending the knee. There are many causes of knee bending pain, it is recommended that patients seek timely medical treatment, improve the relevant examination, targeted treatment.