Dental Hypotheses

: 2017  |  Volume : 8  |  Issue : 2  |  Page : 46--47

Potential causative role of involuntary mouth opening in temporomandibular disorders

Yuki Kojima1, Seitaro Suzuki2, Naoki Sugihara2,  
1 Department of Physiology, Tokyo Dental College, Chiyoda-ku, Tokyo, Japan
2 Department of Epidemiology and Public Health, Tokyo Dental College, Chiyoda-ku, Tokyo, Japan

Correspondence Address:
Yuki Kojima
Department of Physiology, Tokyo Dental College, Chiyoda-ku, Tokyo


Introduction: Bruxism has long been suspected as a cause of temporomandibular disorders (TMDs). However, the validity of conventional guidelines and treatment has been discussed since sometime. The association between bruxism and TMDs has recently been questioned, as self-assessments of bruxism are somewhat unreliable, and bruxism appears to have no adverse effects based on histological and physiological findings. The hypothesis: We hypothesize that involuntary mouth opening may play a role in the development of TMDs. Evaluation of the Hypothesis: Recently, some clinical and basic reports have indicated that involuntary mouth opening might be associated with TMDs.

How to cite this article:
Kojima Y, Suzuki S, Sugihara N. Potential causative role of involuntary mouth opening in temporomandibular disorders.Dent Hypotheses 2017;8:46-47

How to cite this URL:
Kojima Y, Suzuki S, Sugihara N. Potential causative role of involuntary mouth opening in temporomandibular disorders. Dent Hypotheses [serial online] 2017 [cited 2021 Sep 20 ];8:46-47
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Bruxism, in which the teeth are gnashed or clenched, has been proposed as a cause of temporomandibular disorders (TMDs), as the condition places high stress on the temporomandibular joint, similar to malocclusion.[1],[2] In the past, if patients without malocclusion reported feeling fatigue and pain around the temporomandibular joint upon waking, the cause of TMD was typically diagnosed as bruxism. Although the causal association between bruxism and TMDs remained unknown for quite some time, dentists have begun to recognize the importance of occlusal splints for buffering against bruxism in self-diagnosed patients.[3],[4]

 The Hypothesis

Some clinical reports have indicated that obstructive sleep apnea syndrome (OSAS) is associated with TMDs. One prospective cohort study of adults aged 18–44 years reported that OSAS preceded the initial onset of TMDs, based on an affirmative response to “difficulty in opening the mouth.”[5] In a large-scale self-assessment survey including 5,820 respondents, researchers observed that individuals who were at risk for OSAS also experienced symptoms of TMDs.[6] Additional studies have reported a tendency towards mouth breathing in patients with OSAS.[7],[8] Based on these clinical observations, we hypothesized that involuntary mouth opening might cause TMDs.

 Evaluation of the Hypothesis

TMDs, including myofascial pain, were first described in the mid-nineteenth century,[9] while an association between TMDs and occlusion has been discussed since the 1920s.[10],[11] In 1934, treatment of improper prostheses such as dentures and crowns resulted in TMD improvement in 11 cases.[12] At this time, many patients with abnormalities of the temporomandibular joint exhibited infraocclusion, and symptoms of TMD were relieved following restoration of proper bite alignment, suggesting that TMDs were associated with malocclusion.

The “Diagnostic Criteria for Temporomandibular Disorders” (DC/TMD) include guidelines based on evidence obtained from several studies and advanced diagnostic imaging.[13] Published in 2014, the DC/TMD suggested new criteria for the classification of TMDs, including myofascial pain. This report discussed the validity of the conventional guidelines and treatment, and brought the association between bruxism and TMDs into question. However, some researchers have recently reported findings that do not support the notion that bruxism is a cause of TMDs.

The negative evidence can be broadly classified into two groups. First, patients’ self-assessment scores exhibited low reliability, despite the use of clear factors for diagnosis. Although polysomnography (PSG) is needed for a definitive diagnosis of bruxism, the procedure is costly, and doctors tend to rely on self-reports from individual patients. Indeed, many patients with self-reported bruxism did not meet criteria for diagnosis when examined via PSG.[14] Furthermore, bruxism is characterized by attrition of the teeth and teeth marks on the buccal mucosa, and a previous study reported no significant difference between self-reported and PSG-identified bruxism in terms of these clinical findings.[15] Second, bruxism appears to have no adverse effects in terms of physiology: in humans experiencing bruxism for 2 h/day over 3 days, pain tends to be acute, local, and to dissipate within 5 min.[16] These results suggest that bruxism may not result in damage to the muscle at a cellular level. These findings therefore raise concern regarding the true cause of TMDs. Many patients with TMDs do not exhibit malocclusion, suggesting that factors other than bruxism may cause or worsen TMDs.

A previous study has reported that while the morphology of samples from isometric contraction and passive stretch groups appeared normal, samples from eccentrically exercised rabbit skeletal muscles exhibited abnormally large fibers when viewed in cross section. When rabbit skeletal muscle was subjected to continuous isometric contraction, which is equivalent to masticatory muscle contraction during bruxism, no damage to the muscle tissue was observed.[17] In addition, continuous eccentric contractions, which are equivalent to opening the mouth, resulted in damage to fiber structures in rabbit skeletal muscle.[17] This exercise style is the reverse of the action experienced during bruxism. These results suggest that involuntary mouth opening might influence TMDs.


Based on previous scientific observations, we hypothesize that involuntary mouth opening might cause TMDs. Therefore, we suggest that involuntary mouth opening might be a new etiology for TMDs. As there is currently no published evidence regarding the association between TMDs and involuntary mouth opening, we suggest that both baseline and epidemiological studies be performed to evaluate this hypothesis.

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Conflicts of interest

There are no conflicts of interest.


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