Knee pain when straightening the leg can be caused by several reasons: first, osteoarthritis of the knee joint, severe osteoarthritis of the knee joint can have bone spurs in the knee joint, as well as synovial hyperplasia and meniscal damage, which can have obvious pain in the knee when straightening the leg, which should be diagnosed by early examination of knee x-ray and CT. Second, patellofemoral arthritis of the knee joint, patellofemoral arthritis of the knee joint is due to damage to the patella and femoral cartilage, resulting in chronic inflammation of the patellofemoral joint, resulting in pain when the knee is straightened, which should be diagnosed by early examination of x-ray and MRI of the patellofemoral joint. Third, patients with patellar tendinitis or quadriceps tendinitis can also have pain below and above the knee when the leg is straightened, which should be diagnosed by early examination and MRI of the knee joint.