Trismus, also called lockjaw, is reduced opening of the jaws (limited jaw range of motion). It may be caused by spasm of the muscles of mastication or a variety of other causes.

Common causes
  1. Pericoronitis (inflammation of soft tissue around impacted third molar) is the most common cause of trismus.
  2. Inflammation of muscles of mastication. It is a frequent sequel to surgical removal of mandibular third molars (lower wisdom teeth). The condition is usually resolved on its own in 10–14 days, during which time eating and oral hygiene are compromised. The application of heat (e.g. heat bag extraorally, and warm salt water intraorally) may help, reducing the severity and duration of the condition.
  3. Peritonsillar abscess, a complication of tonsillitis which usually presents with sore throat, dysphagia, fever, and change in voice.
  4. Temporomandibular joint dysfunction (TMD).
  5. Trismus is often mistaken as a common temporary side effect of many stimulants of the sympathetic nervous system. Users of amphetamines as well as many other pharmacological agents commonly report bruxism as a side-effect; however, it is sometimes mis-referred to as trismus. Users' jaws do not lock, but rather the muscles become tight and the jaw clenched. It is still perfectly possible to open the mouth
  6. submucous fibrosis
  7. Fracture of the zygomatic arch.