Restricted jaw movement: lockjaw, trismus

The maximum jaw opening, lateral thrust and advancement of the lower jaw can be partially or completely restricted in the case of diseases of the temporomandibular joints and the masticatory muscles.

Trismus

If the opening of the jaw is restricted, this is called trismus. Usually the cause is a frontal position of the joint disc (the discus). The anterior position of the discus is usually asymptomatic except for an audible crack, because the discus pops open when the joint head of the lower jaw, the condyle, is opened. In some cases, however, the disc does not jump back onto the condyle, but remains displaced. This can lead to a mandibular wedge. The trismus can occur slowly over months, but also acutely through an accident or a blow to the jaw. Then there is usually strong pain in the affected temporomandibular joint at the same time and typically the opening of the jaw is restricted to a distance of about two fingers.
A trismus can also be triggered by shortened and painful chewing muscles. Another possible cause is a reflective protective tension of the muscles when a structure in the oro-facial region is painful.

Lockjaw

A lockjaw can also be caused by a displaced discus. More often, excessive mobility of the temporomandibular joint, hypermobility, is probably the cause. In the case of hypermobility, the condyle at the end of the articular path can slide over a small bony elevation, the eminentia articularis. Normally the condyle can also slide back, but occasionally the pull of the chewing muscles prevents it from sliding back and the jaw is locked in maximum jaw opening – hence lockjaw.