Pain when bending the knee is often caused by the following causes, according to the different causes and different treatment: First, severe osteoarthritis of the knee, which can have pain when bending the knee, early X-ray and CT to diagnose, after diagnosis to take oral drugs to nourish the cartilage, oral anti-inflammatory and pain medication, but also to the joint cavity injection of sodium glass acid, oral Chinese medicine to treatment. Second, patellofemoral osteoarthritis, patellofemoral osteoarthritis is due to damage to the cartilage of the patellofemoral joint and leads to pain when bending the knee joint, which should be diagnosed by taking knee x-ray and MRI at an early stage, and after the diagnosis is confirmed, oral cartilage-nutritive drugs and intra-articular injection of sodium vitrate should be taken for treatment. Third, meniscus injury, meniscus injury can be painful when flexing the knee joint, can be accompanied by popping and interlocking, then the MRI of the knee joint should be checked, and after the diagnosis is confirmed, intra-articular injection of sodium vitreous acid is required.