In the absence of trauma, the knee joint suddenly becomes painful, consider the possibility that the following factors exist: First, intra-articular free bodies, commonly found in middle-aged and elderly patients with osteoarthritis, where cartilage is damaged, degenerated, exfoliated, and falls out behind the joint to form joint rats. Free bodies do not cause jamming and interlocking of the joint when they are fixed and immobile, but some small free bodies move with the activities of the human knee joint. When jammed between the joints, it can cause sudden popping and interlocking, and even the patient hits a weak leg. Second, it often occurs in meniscal injuries, also seen in some middle-aged and elderly patients with degeneration of the meniscus, or congenital discoid meniscus, where there are longitudinal or transverse tears of the meniscus. When the position is suddenly changed and the torn meniscus is stuck between the femoral condyles causing compression and interlocking, the patient will also feel sudden pain. Third, patients with patellofemoral dysplasia and patellar subluxation may also experience sudden pain in the knee joint due to sudden dislocation of the patella caused by a small external force. In such cases, patients should visit the Department of Bone and Joint Surgery or the Department of Sports Medicine as much as possible, and ask a professional physician to combine imaging and physical examination to make a clear diagnosis.