Trismus, or lockjaw, refers to reduced opening of the jaws caused by spasm of the muscles of mastication, or may generally refer to all causes of limited mouth opening. It is a common problem with a variety of causes, and may interfere with eating, speech, oral hygiene, and could alter facial appearance. There is an increased risk of aspiration. Temporary trismus is much more common than permanent trismus, and may be distressing and painful, and limit or prevent medical examination or treatments requiring access to the oral cavity.
Classically, the definition of trismus is an inability to open the mouth due to muscular spasm, but more generally it refers to limited mouth opening of any cause. Another definition of trismus is simply a limitation of movement. Historically and commonly, the term lock jaw was sometimes used as a synonym for both trismus and tetanus. Definitions from popular medical dictionaries vary, e.g.:
Normal mouth opening ranges from 35 to 45 mm. Males usually have slightly greater mouth opening than females. (40-60mm)=(avg-35mm). The Normal Lateral movement is (8-12mm). Some have distinguished mild trismus as 20–30 mm interincisal opening, moderate as 10–20 mm and severe as less than 10 mm.