Knee pain when the leg is straight can be caused by several reasons: First, injury to the meniscus. When there is a tear in the anterior horn of the meniscus, there can be pain when the anterior horn of the leg is squeezed by the femur and tibia when the leg is straightened, at which point an MRI of the meniscus is examined for diagnosis and early treatment. If there are symptoms of popping and interlocking, surgery is considered. Second, synovial crease syndrome, when the synovial crease is stuck between the patella and the femur during extension, the knee joint pain can occur when the knee is straightened, then the MRI of the knee joint should be examined to exclude other diseases for diagnosis. Third, diseases of the tibial tuberosity of the knee joint, when the tibial tuberosity of the knee joint is injured or inflammation, can have the symptoms of knee pain when straightening, commonly in adolescents, after a trauma or a lot of exercise and the symptoms of knee pain when straightening, then we should check the x-ray of the tibial tuberosity, CT to diagnose.