Pain in the knee, when the leg is straightened, can be caused by the following reasons: First, patellofemoral arthritis of the knee joint, patellofemoral arthritis of the knee joint is due to damage to the cartilage between the patella and the femur of the knee joint, which causes pain when the patella and the femur rub against each other when straightened, and this is diagnosed and treated by examining X-rays and MRI of the patellofemoral joint. Second, injury to the meniscus of the knee joint, when the anterior horn of the meniscus of the knee joint is damaged or torn, the anterior horn of the meniscus can be compressed when the leg is straightened, resulting in pain in the knee joint, then the MRI of the meniscus should be examined for diagnosis and early treatment. Third, synovial crepitus syndrome, patients with synovial crepitus syndrome can have pain in the knee joint when the leg is straightened because the synovial crepitus is caught between the patella and the femur, which should be diagnosed by early physical examination and examination to exclude other diseases.