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What is Trismus and explain in detail?
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".

Differential diagnosis
Traditionally causes of trismus are divided into intra-articular (factors within the temporomandibular joint [TMJ]) and extra-articular (factors outside the joint, see table).

Joint problems
Ankylosis
True bony ankylosis: can result from trauma to chin, infections and from prolonged immobilization following condylar fracture
Treatment – several surgical procedures are used to treat bony ankylosis, e.g.: Gap arthroplasty using interpositional materials between the cut segments.
Fibrous ankylosis: usually results due to trauma and infection
Treatment – trismus appliances in conjunction with physical therapy.

Arthritis synovitis
Meniscus pathology
Extra-articular causes
Infection
Odontogenic- Pulpal
Periodontal
Pericoronal
Non-odontogenic- Peritonsillar abscess
Tetanus
Meningitis
Brain abscess
Parotid abscess
The hallmark of a masticatory space infection is trismus or infection in anterior compartment of lateral pharyngeal space results in trismus. If these infections are unchecked, can spread to various facial spaces of the head and neck and lead to serious complications such as cervical cellulitis or mediastinitis.
Treatment: Elimination of etiologic agent along with antibiotic coverage
Trismus or lock jaw due to masseter muscle spasm, can be a primary presenting symptom in tetanus, Caused by Clostridium tetani, where tetanospasmin (toxin) is responsible for muscle spasms.
Prevention: primary immunization (DPT)

Dental treatment
Dental trismus is characterized by a difficulty in opening the jaw. It is a temporary condition with a duration usually not longer than two weeks. Dental trismus results from some sort of insult to the muscles of mastication, such as opening the jaw for a period of time or having a needle pass through a muscle. Typical dental anesthesia to the lower jaw often involves the needle passing into or through a muscle. In these cases it is usually the medial pterygoid or the buccinator muscles.
Oral surgery procedures, as in the extraction of lower molar teeth, may cause trismus as a result either of inflammation to the muscles of mastication or direct trauma to the TMJ.
Barbing of needles at the time of injection followed by tissue damage on withdrawal of the barbed needle causes post-injection persistent paresthesia, trismus and paresis.
Treatment: in acute phase:
Heat therapy
Analgesics
A soft diet
Muscle relaxants (if necessary)
Note: When acute phase is over the patient should be advised to initiate physiotherapy for opening and closing mouth.

Trauma
Fractures, particularly those of the mandible and fractures of zygomatic arch and zygomatic arch complex, accidental incorporation of foreign bodies due to external traumatic injury.
Treatment: fracture reduction, removal of foreign bodies with antibiotic coverage

TMJ disorders
Extra-capsular disorders – Myofascial pain dysfunction syndrome
Intra-capsular problems – Disc displacement, arthritis, fibrosis,  etc.
Acute closed locked conditions – displaced meniscus

Tumors and oral care
Rarely, trismus is a symptom of nasopharyngeal or infratemporal tumors/ fibrosis of temporalis tendon, when patient has limited mouth opening, always premalignant conditions like oral submucous fibrosis (OSMF) should also be considered in differential diagnosis.

Drug therapy
Succinyl choline, phenothiazines and tricyclic antidepressants causes trismus as a secondary effect