The possible causes of sudden pain on the outside of the knee that dares not to bend include trauma, meniscus injury, synovitis, etc. 1. Trauma: it may be due to trauma caused by external impact, knee pain, accompanied by activity limitation, if there is no fracture, there may be local soft tissue injuries, such as ligament injuries, etc., there will be a sudden pain in the lateral side of the knee do not dare to bend the phenomenon. 2. Meniscus injury: it may be in half squat or squatting position for a long time, or due to degenerative damage of knee meniscus, which may lead to meniscus injury, resulting in knee pain, swelling, impaired mobility and other symptoms, resulting in sudden pain on the outside of the knee, not daring to bend the phenomenon. 3. synovitis: when the synovial membrane is subjected to various stimuli, such as trauma, infection, osteophytes, joint degeneration, rheumatism, etc., resulting in damage to the inflammatory reaction caused by synovitis, so that the lateral part of the knee sudden pain is afraid to bend. Sudden pain in the lateral part of the knee is not afraid to bend may also have other reasons, it is recommended to go to the regular hospital in time to improve the relevant examination, clear cause, under the guidance of the doctor to carry out targeted treatment or treatment.