Left knee pain when bending can be caused by arthritis, meniscus injury, and surrounding muscle lesions, and can be treated with general therapy, physical therapy, medication, and surgery. 1. Knee arthritis: Knee arthritis can cause pain when bending the knee when going up and down stairs, squatting on the toilet and other activities. It can be treated with analgesic drugs such as ibuprofen as prescribed by the doctor, and sodium vitrate can be injected into the joint cavity to lubricate and nourish the articular cartilage, and knee replacement surgery is feasible for serious conditions. 2. Meniscus injury: Meniscus injury caused by trauma or sports injury can cause pain when bending the knee. The knee joint should be strictly braked, avoiding activities, and the meniscus injury should be clarified by the doctor, and if necessary, the meniscus can be removed or repaired and other surgical treatments. 3. Peripheral muscle lesions: due to trauma or long-term weight-bearing activities of the knee joint can cause injury or lesions of the muscles around the knee joint, resulting in pain when the knee bends muscle activities. Oral analgesic and anti-inflammatory drugs such as Celecoxib can be taken as prescribed by the doctor, and local massage, physiotherapy, hot compresses and other treatments in regular hospitals can help to accelerate blood circulation and alleviate local muscle fatigue. When the knee bending pain should be timely consultation, under the guidance of specialists to actively carry out appropriate treatment, will help to reduce the pain and discomfort symptoms as soon as possible.