Leg cap pain generally refers to knee pain, which may be caused by rheumatoid arthritis, trauma, joint cartilage damage and other reasons. 1. Rheumatoid arthritis: its cause is still unclear, but it may be related to heredity, infection, environment and other reasons. Under the stimulation of inflammation, patients may experience knee pain. 2. Trauma: such as external impact on the knee, knee joint overextension, etc., can cause knee joint injury, meniscus injury, ligament tear, fracture and other symptoms, blood vessels can be ruptured and bleeding, tissue fluid exudation, knee joint swelling and cause pain. 3. Articular cartilage damage: such as overwork, long-term weight bearing, excessive activities, etc. can make the cartilage and articular surface of the knee joint is destroyed, or articular cartilage cumulative damage. Knee pain can also be seen in other causes, it is recommended to go to the hospital in a timely manner, clear the cause of the disease, follow the doctor’s instructions for treatment, to avoid delaying the condition.