a single sound is heard and as "crepitation" or "crepitus" when there are multiple, grating, rough sounds. Most joint sounds are due to internal derangement of the joint, which is instability or abnormal position of the articular disc. Clicking often accompanies either jaw opening or closing, and usually occurs towards the end of the movement. The noise indicates that the articular disc has suddenly moved to and from a temporarily displaced position (disk displacement with reduction) to allow completion of a phase of movement of the mandible. If the disc displaces and does not reduce (move back into position) this may be associated with locking. Clicking alone is not diagnostic of TMD since it is present in high proportion of the general population, mostly in people who have no pain. Crepitus often indicates arthritic changes in the joint, and may occur at any time during mandibular movement, especially lateral movements. Perforation of the disc may also cause crepitus. Due to the proximity of the TMJ to the ear canal, joint noises are perceived to be much louder to the individual than to others. Often people with TMD are surprised that what sounds to them like very loud noises cannot be heard at all by others next to them. However, it is occasionally possible for loud joint noises to be easily heard by others in some cases and this can be a source of embarrassment e.g. when eating in company. Pain Pain symptoms in TMD can be thought of as originating from the joint (arthralgia), or from the muscles (myofascial), or both. There is a poor correlation between TMD pain severity and evidence of tissue pathology. Generally, degenerative joint changes are associated with greater pain. Myofascial pain Pain originating from the muscles of mastication as a result of abnormal muscular function or hyperactivity. The muscular pain is frequently, but not always, associated with daytime clenching or nocturnal bruxism. Limitation of mandibular movement The jaw deviates to the affected side during opening, and restricted mouth opening usually signifies that both TMJs are involved, but severe trismus rarely occurs. If the greatest reduction in movement occurs upon waking then this may indicate that there is concomitant sleep bruxism. In other cases the limitation in movement gets worse throughout the day. The jaw may lock entirely. Limitation of mandibular movement itself may lead to further problems involving the TMJs and the muscles of mastication.