The knee suddenly can not straighten very painful like stuck, may be the knee joint degenerative disease, cruciate ligament injury, meniscus injury. 1. Knee degenerative disease: knee degenerative disease is a kind of articular cartilage degeneration, destruction, subchondral bone sclerosis, bone cumbersome formation of the disease, the patients of this disease can appear the knee suddenly can not straighten very painful, like stuck like symptoms. Treatment can use ibuprofen and other drugs for pain relief, but also through the knee arthroscopy surgery for treatment. 2. Cruciate ligament injury: Cruciate ligament injury is a kind of disease caused by trauma, violence and other factors that interrupt the continuity of the cruciate ligament, and patients with this disease can also have this symptom, the treatment should reduce the burden on the knee joint, and can be iced, compression bandage, elevation of the affected limb, but also in the arthroscopic ligament reconstruction surgery. 3. Meniscus injury: meniscus injury is a disease in which the integrity and continuity of meniscus is damaged, and patients with this disease may also have this symptom, and the treatment can be carried out by resting, icing, gauze compression bandage, elevating the affected limb and other ways of treatment. For patients with meniscus injury who have joint effusion, the joint cavity can be punctured and extracted, and if the meniscus injury is serious, it can be treated by surgery under arthroscopy. If there is a sudden inability to straighten the knee is very painful, like stuck symptoms, it is recommended to consult a doctor to identify the cause and treatment.