The possible causes of sudden knee pain and fear of bending include meniscus injury, patellofemoral arthritis, osteoarthritis, synovitis and so on. 1. Meniscus injury: it may be due to degenerative damage of meniscus of knee joint or in half squatting or squatting position for a long period of time, which leads to meniscus injury, resulting in knee joint pain, swelling, impaired mobility and other symptoms, which triggers sudden pain in the knee, not daring to bend the situation. 2. Patellofemoral arthritis: patellofemoral arthritis is one of the causes of knee pain. Inflammation of the joint surface between the patella and the femur due to degenerative changes of the cartilage and osteophytes on the joint surface may also cause the phenomenon. 3. Osteoarthritis: Osteoarthritis is caused by age, heredity, obesity, metabolic abnormalities, accumulated microtrauma, and overloading of joints, etc. It may also occur. 4. Synovitis: Synovitis is an inflammatory reaction caused when the synovial membrane is subjected to various stimuli such as infection, trauma, osteophytes, joint degeneration, rheumatism, etc., resulting in injury. Sudden pain in the knee dare not bend may also have other reasons, it is recommended to go to the regular hospital in time, improve the examination to clarify the cause of the disease, under the guidance of the doctor to give targeted treatment or treatment.