There are several cases of joint popping: 1, the front of the joint, the patella rattling: “nail scratching hair sound”, is the patellofemoral joint cartilage damage or degeneration, the joint surface is not smooth, when the activity is produced. 2, the sound of the joint before the lower inside: “popping low”, is the disease of the lower inside of the patella, Plica (synovial crease) syndrome, male prevalence, upstairs pain is dominant. Can be touched in the patella below the “strips” like things, there is pressure pain. 3, the sound of the inner (outer) side of the joint: “play nail” crisp sound. Injury to the medial (lateral) meniscus is more likely. It may cause twisting pain, flat road pain and squatting pain. 4.The sound inside the joint: the sound of “grinding sand grains”. Femoral and tibial articular surface cartilage damage, degeneration, free body caused. Performance of middle-aged and elderly people walking a long way pain, some “O” or “X” shape back. 5, the front and outside of the joint, the patella sounds when moving: “boom” + “sliding dislocation”, is caused by patellar dislocation or subluxation. Mostly children and adolescents. Individuals are elderly people. 6, rattling sound directly in front of the knee: “twisting pronation”, swelling in front of the knee, mostly prepatellar bursitis.