Trismus commonly referred to as “lock jaw”, is a medical condition in which the normal motion of the mandible (jaw) is reduced as a result of sustained, tetanic spasm of the masticatory muscles mediated by the trigeminal nerve. Hence interfering with the patient’s eating, normal speech, swallowing, oral hygiene and in some cases increased risk of aspiration. Cases of Trismus can be of temporal (typically resolves after two weeks) or of permanent development, with most falling into the former. 
The normal range of mouth opening or maximal intercisal opening (MIO) varies from one individual to the other, with males having a larger mouth opening compared to females. It is usually between 40-60mm, some authors however insist on 35mm lower margin. This range often corresponds to two-three finger breadths when inserted sideways. Lateral movement is 8-12mm.
Relevant Anatomy/Pathological Process
For mouth opening to occur, a coordinated function of an intact sensory and motor neural activity and also a normal muscular and temporomandibular joint apparatus is required. They are two primary groups of muscles that determine mandibular (jaw) motion these are temporalis, masseter and medial pterygoid responsible for mandibular elevation (mouth closure). While the lateral pterygoid inserts into the articular disc as well as the neck of the condyle. Therefore, is primarily responsible for depression (mouth opening) of the mandible and for coordination of the disk condyle relationship during function. Opening is assisted by the mylohyoid, anterior belly of digastric, geniohyoid and infrahyoid muscles and possibly the posterior belly of digastric. All the muscles of mastication have motor and afferent sensory supply from mandibular division of the trigeminal nerve except infrahyoid muscles being supplied by branches of the ansacervicalis. The muscles of closure are approximately 10 times more powerful than the opening muscles and are made up slow twitch fibres. This fact is helpful in exercising planning for trismus patients. 
The masticatory muscles act in antagonism, as neurogenic stimulation of one group causes reflex neural inhibition of the other. In trismus, while the inciting insult may be unilateral, the reflex activated is bilateral.
The prevalence of trismus ranges widely, partly because no clear criteria have been established by the various authors on the subject. Some regard normal jaw opening as greater than 30 to 40 mm. Trismus has also been defined as mouth opening less than 40 mm; others have defined it as an opening to 15 to 30 mm, or even less than 20mm. Additionally, other authors have graded trismus according to visual assessment of mouth opening (light/moderate/severe or grades 1 to 3, again corresponding to mouth opening). Its incidence varies greatly, anywhere from 5% to 38% and dependent on the inciting etiology. It is said to increase in irradiated patients, head and neck cancer diagnosis, congenital micrognathia syndrome and rare in common conditions such as pharyngitis.
Trismus has a number of potential causes which are single and ranges from simple and non- progressive to those that are complex and potentially life threatening. These causes are congenital disorders, infections, trauma, iatrogenic, neoplasia, radiotherapy, temporomandibular disorders, drugs, psychogenic and miscellaneous causes. 
- Congenital Disorders: Certain congenital disorders such as hypertrophy of the coronoid process causing interference of the coronoid against the anteromedial margin of the zygomatic arch have been associated with trismus. Other congenital conditions include Pierre-Robin syndrome and Trismus-pseudo-camptodactyly syndrome. 
- Infections: A classical symptom of masticatory space infections is Trismus. Infections causing Trismus could be odontogenic or non odontogenic in nature. Odontogenic infections include pulpal infections, periodontal infections and pericoronal infections. While examples of non-odontogenic infection include tonsillitis, tetanus, meningitis, parotid abscess and brain abscess. 
- Trauma: These include fracture or dislocation of zygomatic arch, hemarthrosis/hematoma, temporomandibular joint (TMJ) contusion, intra articular bone islands/foreign bodies, displaced meniscus, direct injury to muscles of mastication. 
- Iatrogenic: 3rd molar extraction related inflammation, inaccurate nerve block injection procedure, puncturing medial pterygoid muscle or vessels, radiotherapy for head and neck cancer, can all lead to trismus.
- Neoplasia: Trismus is a common complication of oncology. Especially all malignant tumors involving the mandible, muscles of mastication and associated structures can cause limitation of mandibular movement. Also, primary tumors or neoplastic diseases occurring in many parts of the body could metastasize to the epipharyngeal region, parotid gland, mandible or temporomandibular joint whose clinical sign may be trismus.
- Radiotherapy: A known significant effect of radiotherapy around the head and neck region is trismus. This often occurs where there is involvement of the medial pterygoid muscles during the treatment. Generally when the muscles of mastication are within field of radiation, there is fibrosis which may lead to trismus. This outcome has been attributed to the ischaemia caused by endarteritis obliterans.
- Temporomandibular Disorders: Disorders involving the temporomandibular joint (TMJ) can lead to trismus. These disorders are divided into intra articular or extra articular. Intra articular causes include fibrous ankylosis, anchored disc phenomenom, bilateral anterior disc displacement without reduction, arthritis and unilateral condylar hypoplasia. The extra articular covers all myofascial related causes. 
- Drugs: Trismus has been shown to be a secondary effect of some drugs. These drugs include succinylcholine, phenothiazines and tricyclic antidepressants being the most common. Others having same effect include metaclopramide and phenothiazines. Strychnine poisoning is also a possible cause of trismus. 
- Psychogenic: Hysteria is a cause of trismus. However diagnosis of this condition must first exclude other causes followed by psychiatric assessment. Electromyography is a further useful test in diagnosing hysterical trismus.  
- Miscellaneous: Trismus has also been described in association with multiple sclerosis, pseudobulbar palsy, lupus erythematoses, scleroderma, acquired deformity e.g. burns, and neck flexion deformity.  
Clinical sign depends mainly on what the cause of the trismus is. However they show some common characteristics. These include:
- Restricted mouth opening, preventing the 2-3 fingers positioned side by side from fitting into the intercisal space as seen in normal subjects. Inability to perform lateral mandible movements often indicates trismus due to bony TMJ ankylosis
- Pain during forced mouth opening. Palpation of the masticatory muscles affected in the acute phase also elicits pain.
- Deviation of the mandible towards the affected side as a result of muscles not functioning properly because of spasms.
- Sensation of muscle tightness, cramping, or stiffness
- Diffuse facial swelling and fever when associated with infections.
- Speech impairments often referred to as “hot potato voice”.
- Impaired oral intake, mastication and nutrition leading to weight loss. Weight loss may also be associated with neoplastic cause.
- Poor oral hygiene
- Difficulty breathing
The diagnosis of trismus is clinical.
- History: First, a thorough history is conducted to ascertain the cause and duration of the trismus.
- Neck mobility is screened to rule out neck muscles shortening, especially the flexors.
- Palpate masticatory muscle for tenderness. Also palpate joint by inserting the index finger in the patient’s ear and ask the patient to open the mouth. This is to determine if there is available motion at the TMJ.
- Imaging adjuncts may be useful to determine its etiology and determine the articular involvement of the TMJ. Computed tomography may be useful to identify traumatic etiologies including hematomas or facial and mandibular fractures when suspected. Magnetic resonance imaging may also be helpful in identifying space-occupying lesions or abnormalities in the pharyngeal or oral structures.
- Mandibular Function Impairment Questionnaire (MFIQ).
The management of trismus is often dependent on the factor causing it. If trismus results due to fibrosis of tissue or immature scar formation, physiotherapy and appliances can be of help. If trismus results due to TMJ ankylosis or intra articular pathologies, causing dense fibrous tissue formation, it may require surgical management. 
- Conservative Medical Management for Acute Trismus
1. Heat - Placement of moist hot towels on the affected area for 10–20min/h.
2. Analgesic therapy - Aspirin is the most common. When discomfort is extensive, narcotic analgesic may be indicated.
3. Soft diet.
4. Muscle relaxants - In extensive masticatory muscle spasm, benzodiazepines 2.5–5mg 3 times a day may be indicated.
- Surgical Management
Surgery is often indicated, when the cause of trismus is as a result of intra articular pathologies involving the temperomandibular joint. Bony interferences from styloid or coronoid processes, the presence of a foreign body may require surgical intervention. If trismus is caused due to dense fibrotic band formation in the submucosa, lysis of these bands is done using laser. Myotomy of the masseteric muscle helps in certain cases.  
- Physical Therapy Management
When the cause of the trismus is of extra articular origin, it is recommended that physiotherapy should commence after cessation of the acute phase.
1. Reduction of oedema
2. Soften and cause stretch of scar tissue
3. Increase the range of joint movement
4. Increase the strength of muscles of mastication.
Heat: A heat emitting modality such as ultrasound is commonly used as an adjunct to stretching exercises involving the muscles of mastication. This is to bring about an increase in collagen tissue extensibility, decreased stiffness of joint, and relieve pain and muscle spasm. Heat has also been known to increase flow, thereby washing away exudates and reducing oedema of the muscles of mastication. 
Exercise: Active and passive stretching/strengthening exercises to the muscles of mastication have been advocated by various authors in the treatment of trismus. As they work to stretch scar tissue, relax the muscle that are in spasm and increase muscle strengthen, bringing about increased range of motion of the TMJ.   
Sugarless Chewing Gum: This is another means of providing lateral movement of the TMJ. 
Trismus Devices: In conjunction with physiotherapy, they are devices designed for mandible motion rehabilitation. The devices are divided into externally and internally activated. Externally activated devices cause forcible stretching of the elevator muscles by depressing the mandible. While the internally activated device stretches the affected elevator muscles and other tissue that limits mandibular opening.  
- Externally Activated Devices
- Inflatable Bite Opener
- Dynamic Bite Opener
- Threaded Tapered Screw
- Shell Shaped Mouth Opener
- Screw-Type Mouth Gag
- Tongue Blades
- Therabite Jaw Motion Rehabilitation System.
- Internally Activated Devices
- Tongue Blades
- Plastic Tapered Cylinder.
Post-Operative Physical Therapy Management
Post-operative physiotherapy is highly recommended to maintain the mouth opening obtained intra-Op, prevent scar contracture and trismus recurrence.   Allevi F, et al  conducted a comparative study of two cases of post-traumatic pseudoankylosis of the jaw treated with bilateral coronoidectomy and postoperative physiotherapy. After a one year follow-up Case A showed no relapse, while Case B relapsed and this was attributed to non-compliance to prescribed exercises.
A consensus about the timing (how many times per day and how long) of physiotherapy has not been reached by the different authors. Some authors propose that post operative physiotherapy should commence few days post surgery as this will reduce the chances of scar tissue formation. However, severe pain remains a common deterrent to patient’s compliance to immediate post operative physiotherapy. It has been advocated that patients be kept under strong analgesic cover.
The facial nerve may be affected following surgery especially when a periauricular incision is made. Commonly the assault results in neuropraxia, this can be managed with facial exercises and electrical stimulation.
Temporal form of trismus which is more common has been shown to be self- limiting and transient, typically resolving within 2 weeks. In more complex cases of trsimus like TMJ ankylosis, the critical factor of successful treatment is early detection, correct surgery approach, implementation of an intensive physiotherapy program, and a good post-operative conduct.
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