The protrusion on the knee can be caused by the following reasons: first, bursitis of the suprapatellar bursa, when there is chronic inflammation of the synovial membrane of the bursa, resulting in a large amount of effusion and fluid accumulation in the suprapatellar bursa, there will be a protrusion on the knee, and cystic lesions can be palpated and can move during the examination, then the ultrasound and MRI of the knee joint should be examined to diagnose. Second, tumors of the suprapatellar bursa, when there is a tumor in the synovial membrane of the suprapatellar bursa, there can also be pain above the knee, which should be diagnosed by MRI of the knee joint and, if necessary, by arthroscopic resection and pathological examination. Third, tumor of the quadriceps muscle, when there is a tumor of the quadriceps muscle, there can also be a protrusion above the knee, and the swelling can be palpable, then MRI and ultrasound should be examined to diagnose.