Pain when curling the knee may be caused by chondromalacia patella, meniscus injury, synovitis and other factors. 1. Chondromalacia patella: a disease caused by chronic injury to the cartilaginous surface of the patella, often leading to infrapatellar pain, accompanied by knee pain and weakness, and the pain will be more pronounced when the knee is curled up. 2. Meniscus injury: most of them are caused by trauma or sports, which often cause obvious pain in the knee joint area, especially when curling up the knee, the activity of the joint will aggravate the pain, and may be accompanied by joint popping. 3. Synovitis: Infection, trauma and other reasons can cause aseptic inflammation of the knee joint area, resulting in synovitis, swelling of the joint area, pain, and symptoms of limited movement. The pain when the knee is curled up may also be caused by other reasons, the patient should go to the hospital in time to find out the cause and timely treatment.