Restriction of mouth opening is a common symptom of temporomandibular joint disorder. The sudden onset of mouth opening restriction is often a sign of irreducible joint disc displacement. Such patients mainly present with significant mouth opening restriction, mouth opening deviation, facial pain, and even face shape changes. Most of the patients have a history of joint popping of varying duration, and when the joint does not pop when opening the mouth, mouth jamming, failure to open to the maximum, and painful mouth opening occur. The displaced joint disc restricts and blocks the normal magnitude of forward sliding of the condyle, and mouth opening is limited. If you want to open your mouth more, the condyles have to push the disc further, causing a lot of mechanical stress on the ligaments behind the disc and causing pain. This is the reason why many people suddenly experience restricted opening with significant pain. In addition, inflammatory diseases of the joint can also cause restricted mouth opening and pain, and when this occurs, the patient must seek treatment from a TMJ specialist as soon as possible.