Complications of Chemotherapy:
Oral mucosal cells have high growth rate and are susceptible to the toxic effects of chemotherapy, which lead to stomatitis. Congenital and developmental causes
Hypertrophy of coronoid process causes interference of coronoid against the anteromedial margin of the zygomatic arch. Treatment: Coronoidectomy
Trismus-pseudo-camtodactyly syndrome is a rare combination of hand, foot and mouth abnormalities and trismus. Miscellaneous disorders
Hysteric patients: Through the mechanisms of conversion, the emotional conflict are converted into a physical symptom. E.g. : trismus
Scleroderma: A condition marked by edema and induration of the skin involving facial region can cause trismus

Common causes
Pericoronitis (inflammation of soft tissue around impacted third molar) is the most common cause of trismus. 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. Peritonsillar abscess, a complication of tonsillitis which usually presents with sore throat, dysphagia, fever, and change in voice. Temporomandibular joint dysfunction (TMD). 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.
Trismus, commonly called lockjaw as associated with tetanus, is a condition of limited jaw mobility. It may be caused by spasm of the muscles of mastication or a variety of other causes. Temporary trismus occurs much more frequently than permanent trismus. It is known to interfere with eating, speaking, and maintaining proper oral hygiene. This interference, specifically with an inability to swallow properly, results in an increased risk of aspiration. In some instances, trismus presents with altered facial appearance. The condition may be distressing and painful. Examination and treatments requiring access to the oral cavity can be limited, or in some cases impossible, due to the nature of the condition itself. Definition
Trismus is defined as painful restriction in opening the mouth due to a muscle spasm, however it can also refer to limited mouth opening of any cause. Another definition of trismus is simply a limitation of movement. Historically and commonly, the term lockjaw was sometimes used as a synonym for both trismus and tetanus.Normal mouth-opening ranges from 35 to 45 mm. Males usually have slightly greater mouth opening than females. (40–60 mm, average of 50 mm). The normal lateral movement is 8–12 mm, and normal protrusive movement is approximately 10 mm. Some have distinguished mild trismus as 20–30 mm interincisal opening, moderate as 10–20 mm and severe as less than 10 mm.Trismus is derived from the Greek word trigmos/trismos meaning "a scream; a grinding, rasping or gnashing".