Sudden pain in the knee generally has the following causes. First, tendonitis. Young people who exercise a lot can develop tendonitis of the quadriceps tendon or chronic inflammation of the tendons of the goose feet on the inside of the knee joint, resulting in knee pain. Second, synovitis, there is synovial tissue in the knee, when there is acute inflammation of the synovial membrane, there will be sudden pain in the knee, often accompanied by swelling of the knee, fluid accumulation, and limited movement of the knee joint, an MRI is needed to further confirm the diagnosis. Third, arthritis, arthritis often occurs more often in the spring and autumn, when the knee often has a stiff feeling, especially in the morning when the stiffness is more obvious, and can be accompanied by pain and stiffness after sitting and standing up more obvious, after the activity can be relieved, can be further confirmed by X-ray examination.