There are more reasons for the pain when bending the knee, and it is fine after standing upright, and there are common diseases such as osteoarthropathy, meniscus injury, lumbar disc herniation, and hip and femoral head lesion. 1. Osteoarthropathy: Osteoarthropathy, due to the degeneration of the articular cartilage on the joint surface of the patella and femur cartilage in front of the knee, leads to pain when the knee is bent due to the increased pressure on the patellofemoral joint, and when the pressure is reduced after straightening, the pain will be reduced or disappeared. 2. Meniscus injury: If there is a meniscus tear, when the knee is bent, the torn meniscus will be squeezed and pain will occur, but when it is straightened, the anterior meniscus is intact, and there will be no pain. 3. Herniated lumbar disc: If you suffer from herniated lumbar disc, such patients will appear forced position, incorrect walking posture will lead to muscle balance is broken and cause lower limb discomfort symptoms, including leg pain when bending the knee, straightening will relieve. 4. Hip and femoral head lesions: Hip and femoral head lesions can also cause corresponding symptoms in the knee, including inner and outer pain in the knee and impaired knee movement. Patients with knee pain when bending, straightening will be relieved, it is recommended to go to the hospital in a timely manner, in order to clarify the causes of the situation, reasonable treatment, in order to avoid delaying the condition.