Pain when flexing the knee joint can be caused by the following reasons: first, patellofemoral arthritis, patellofemoral arthritis is due to damage to the cartilage of the patellofemoral joint resulting in chronic inflammation of the patellofemoral joint, there can be pain at the patellofemoral joint when flexing the knee joint, this time to check the X-ray of the patellofemoral joint, MRI to diagnose and early treatment. Second, meniscal injury, when the posterior horn of the meniscus is damaged or torn, there can be pain at the back of the knee joint when the knee joint is flexed, then the MRI of the meniscus should be examined and examined for diagnosis and early treatment. Thirdly, in severe osteoarthritis, patients with osteoarthritis can have pain when the knee joint is flexed and straightened. This is because severe osteoarthritis can cause damage to the cartilage, meniscus and surrounding soft tissues, resulting in pain when the joint is flexed.