A protruding bone below the knee is abnormal and may be caused by tibial tuberosity epiphysitis, synovitis of the knee joint, osteochondroma and other conditions. 1. Tibial tuberosity epiphysitis: it is usually caused by acute or repeated chronic injury of tibial tuberosity epiphysis under the pull of patellar tendon. It usually occurs in adolescence, and patients often have pain and swelling in the tibial tuberosity, accompanied by redness and warmth, which will be aggravated by active knee extension, passive knee flexion, squatting and so on. 2. Knee synovitis: the pathogenesis is not clear, and it is considered to be related to knee injury, degenerative changes in the knee joint, obesity, long-term physical exertion and other factors. Patients often appear knee swelling, pain, limited activities and other symptoms, some patients can also be manifested in the leg below the knee bone protruding. Some patients may also show the protrusion of the bone below the knee, and the patients may also be accompanied by elevated skin temperature and atrophy of quadriceps muscle. 3. Osteochondroma: the cause of the disease is still unclear, osteochondroma is a kind of autosomal dominant hereditary disease, the patients mostly have family history, and will be accompanied by mutation and loss of function of oncogenes EXT1 and EXT2. Patients often have multiple bony masses, which may be accompanied by deformity, pain, fracture and limb dysfunction. It is recommended that when patients have the phenomenon of protruding bone below the knee, they should find out the cause of the disease in time and follow the doctor’s instructions to treat it actively.