Normal mouth opening in an adult ranges from 35–55 mm inter-incisally. Trismus (lockjaw) (from the Greek Trimos = ‘grating’, ‘grinding) is the inability to open the mouth normally usually due to muscle spasm. Trismus can have consequences including impaired mastication, difficulty in speaking, in achieving adequate oral hygiene and in access for oral care. If left untreated, degenerative processes in the masticatory muscles, with disuse atrophy, may ensue.
In some people, such as persons who have received radiation to the head and neck, trismus is often seen in conjunction with difficulty in swallowing. In trismus caused by radiation treatment, hyposalivation and mucositis are also common associated challenges. Occasionally in temporomandibular joint trauma or infection, and rarely in pain-dysfunction syndrome, the joint may become fibrotic, or even ankylosed.
Limited opening of the jaw is usually due to extra-articular disease with masticatory muscle spasm secondary to stress, trauma or local infection (e.g. pericoronitis around a partially erupted mandibular third molar). Occasionally trismus is caused by joint (intra-articular and intra-capsular) disease, or conditions affecting the adjacent soft (peri-capsular) tissues such as scarring, infiltrating neoplasms or oral submucous fibrosis.
Trismus is usually caused by inflammation and masticatory muscle spasm, or inflexible scarring or other tissues (Boxes 23.1 and 23.2). Life-threatening causes include tetanus, malignant neoplasms and fascial space infections. In stroke patients, trismus may appear as a sequel to the CNS dysfunction. Some psychotomimetics (e.g. amphetamines and ecstasy (methylenedioxymethamphetamine; MDMA)) can cause masticatory muscle spasm which forcibly causes bruxism and difficulty in mouth opening, which can be reduced by sucking on a pacifier or lollipop. In some mental states such as hysteria, it may appear that the patient has trismus.