Locked Jaw (Trismus)
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What is the Locked Jaw (Trismus)?
Locked jaw (Trismus) is a TMJ disorder in which the jaw is unable to achieve normal range of motion - usually in all directions, but especially with opening or closing the mouth. Since mouth opening is an important movement for everyday function, functional activities such as speaking, chewing, and yawning can be affected. Trismus can therefore be highly debilitating due to the effects on a person’s quality of life. A person suffering from trismus will experience jaw tightening accompanied by varying degrees of pain or discomfort, depending on the cause of trismus or the duration of the episode.
Normal Mouth Opening
The normal range of mouth opening is approximately 40-60mm, or about 2-3 fingers wide¹. A quick and simple test to test for a current or developing state of trismus is the 3 finger test. If mouth opening is wide enough to fit 3 fingers, then trismus is unlikely to be a problem. If not, this may be suggestive of trismus and should be further investigated by a physiotherapist.
Image 1: the 3 finger test.
Open Vs Closed Lock Jaw - What is the difference?
If a person is experiencing difficulty opening their mouth, this is called a closed lock - as the jaw remains in a relatively closed position. Alternatively, if the mouth is stuck open and a person is having difficulty closing their mouth, this is called an open lock.
Closed Locked Jaw
Closed lock jaw can be caused by muscle disorders, joint-related disorders (e.g. disc displacement, arthritis, joint stiffness) or even neck pain or stiffness. For example, muscular spasm of the temporalis, masseter and medial pterygoid (responsible for mouth closure) may result in difficulty opening the mouth and fighting against the muscles. Loss of muscle coordination of the lateral pterygoids (responsible for mouth opening) can lead to difficulty opening the mouth, also due to the superior fibres attaching to the articular disc leading to disc displacement (note: can link highlighted to part 4 - explaining the pathophysiology/how disc issues happen).
During anterior disc displacement the condyle may be unable to return back underneath the disc - the disc therefore stays anterior to the condyle and prevents further mouth opening, leading to a closed lock. Stiffness in the TMJ due to degenerative joint disease (e.g. arthritis) may also contribute to a closed lock.
Open Locked Jaw
Open lock jaw is usually the consequence of an articular disc problem or dislocation of the mandibular condyle. With the mouth in an open position the condyle can become stuck in front of the articular eminence or disc, blocking it from sliding back into the fossa and thus restricting the jaw from closing. A dislocated TMJ stuck in an open lock requires urgent medical attention and will need to be reduced (put back in place) by a qualified medical practitioner. Rehabilitation following joint reduction can be guided by a physiotherapist to regain muscular strength and stability around the TMJ.
Other causes of Locked (Trismus) Jaw?
We know that various muscle or joint-related subcategories of TMD contribute to the development of trismus. Trauma to the TMJ has also been shown to result in trismus to 33% within one week of the traumatic event.³ However, it is also common for trismus to occur as a secondary effect of certain medical conditions and clinical procedures.
Head and neck surgery, or dental procedures such as tooth extraction may cause trismus. This can be due to inflammation involving the jaw muscles or trauma to the TMJ⁴. Drug therapy is also capable of causing trismus as a secondary effect, depending on the drugs used⁵.
Radiotherapy to the head and neck can cause a buildup in toxins or fibrosis (thickening) of TMJ muscles which can reduce range of motion at the jaw and lead to trismus.³
Less common but more severe and potentially life-threatening causes include oral infections or tumours. If suffering from limited jaw opening and trismus is suspected, help from a qualified health professional should be sought in order to assess the cause of trismus.
Further Information
Reference
¹Nelson, S. J., Nowlin, T. P., & Boeselt, B. (1992). Consideration of linear and angular values of maximum mandibular opening. Compendium (Newtown, Pa.), 13(5), 362-363.
³Dhanrajani, P. J., & Jonaidel, O. (2002). Trismus: aetiology, differential diagnosis and treatment. Dental update, 29(2), 88–94. https://doi.org/10.12968/denu.2002.29.2.88
⁴Berge, T. I., & Bøe, O. E. (1994). Predictor evaluation of postoperative morbidity after surgical removal of mandibular third molars. Acta odontologica Scandinavica, 52(3), 162–169. https://doi.org/10.3109/00016359409027591
⁵Cunningham, P. A., & Kendrick, R. W. (1988). Trismus as a result of metoclopramide therapy. Journal of the Irish Dental Association, 34(4), 128–129.