Introduction

Temporomandibular joint disorders (TMDs) are the most common cause of non-infectious and non-dental pain in the face1,2,3. The prevalence of TMD in the population is relatively high and is around 22–30%4. This disorder appears in a wide age range, but its most frequent occurrence is between 20 and 40 years old5. The most common features of TMD include pain disorders in the form of myalgia, headache attributed to TMD and arthralgia in the face, limitation in lower jaw movements, noise during jaw function, disc displacement and degenerative diseases in the temporomandibular joint (TMJ)6,7. The etiology of pain-related TMD is considered multifactorial and is related to biological, psychological, social and environmental factors8,9.

In view of the multifactorial nature of TMD symptoms and their multiplicity, a standard diagnostic system is required that can offer a complete clinical assessment, etiologic analysis, and review of risk factors10,11. To accomplish this goal, various evaluation methods have been employed, such as questionnaires, indicators of patient history, clinical indicators, and diagnostic criteria12,13. The Research Diagnostic Criteria for Temporomandibular Disorders (RDC/TMD) has been the most requested diagnostic protocol for the diagnosis of TMD since its publication in 199214,15. Bruxism is usually considered to be one of the leading causes of TMD disorders15,16. Bruxism is a diurnal or nocturnal parafunctional activity due to repetitive jaw-muscle activity that involves involuntary clenching, grinding, or bracing of the mandibular and maxillary teeth without any conscious awareness and in the absence of neuromuscular protective mechanisms17,18,19,20. The term “parafunction” usually refers to a disordered function. However, new findings indicate that bruxism may not only have negative health consequences but also positive ones. For example, it might play a part in preventing upper airway collapse during sleep, which could help with obstructive sleep apnea21. This disorder is the third most common form of sleep disorder after sleep talking and snoring22. According to the ICSD-R, 85–90 percent of the general population experiences some degree of bruxism during their lifetime, although only 5 percent develop a clinical condition18,23.

Based on the evidence, the relationship between bruxism and TMD disorder is still a controversial issue due to the complexity of the cause and diagnosis of both disorders. In fact, the existence of this relationship as well as the possibility of a cause-and-effect relationship is still unclear24,25. Although there are some studies that show a positive relationship between bruxism and TMD, different results have been obtained due to methodological differences, different interpretation of data, existence of different operational definitions and differences in diagnostic tools for both disorders in different studies. Therefore, in order to detect a more definite relationship, there is a need to synthesize studies by reanalyzing the results of previous studies. Therefore, we have investigated the relationship between bruxism and TMD disorder in this research as a systematic review and meta-analysis.

Methods

In this study, we followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guideline.

PICO question

This review sought to evaluate evidence addressing the PICO (population, exposure, comparison, and outcome) question: In temporomandibular joint disorders (TMDs) patients (P), what is the role of bruxism (E) on symptoms of TMDs (O), when compared to without TMDs (C).

Search strategy

Electronic searches were carried out until 30 March 2022 in PubMed; Web of Science; Scopus; Cochrane; Wiley; ProQuest and Embase and gray literature databases: Google Scholar and Open Gray. The complete search strategies are presented in Table 1S.

Study selection and data extraction

The title and abstract were screened for eligibility and its validation was confirmed by the second author. The inclusion criteria were shown below: (1) cross-sectional, case control and cohort studies (prospective and retrospective) evaluating the association between bruxism and temporomandibular joint disorder; (2) studies that evaluated bruxism and temporomandibular joint disorder through a validated method. English language restrictions were applied in the search. Data were extracted from each included study completely independently by two reviewers (NM and AHT). Disagreements regarding each study were resolved by discussion between reviewers. Reviewers extracted the following items from the included studies: first author(s), location of study; year of publication; study design and duration of follow-up; studied population and age; sample size; gender ratio; bruxism and TMD assessment method, and information on quality assessment (risk of bias).

Risk of bias (quality) assessment

The risk of bias in the included studies was evaluated by two reviewers (AR and AHT) using the Newcastle-Ottawa Scale (NOS)26 for cohort and case-control studies and the adapted NOS for cross-sectional studies27. Each study was classified according to the quality level: low (0–3 stars); fair (4–6 stars); or good (7 or more stars).

Data analysis

The odds ratios (OR) were extracted to quantify the risk of TMD. For the studies in which the OR was not calculated, this index was calculated based on the data provided in the primary studies. A random-effect meta-analysis was performed to synthesize the risk of TMD. The heterogeneity of studies was determined according to the I2 index. Subgroup analysis was performed for variables of table 3S. Eggers and Begg test and funnel plots were used to demonstrate potential publication bias. The analysis was performed using Stata (version 16). Statistical significance was set as p < 0.05.

Results

Literature search and study characteristics

The database search retrieved a total of 1651 articles, of which 821 articles were excluded due to duplication. After title and abstract screening, 621 articles we excluded and the full texts of 209 articles were assessed. Finally, 20 articles with a total of 6607 patients with TMD and 14,186 individuals without TMD were included in the meta-analysis. A flowchart of the study selection process is shown in Fig. 1.

Fig. 1
figure 1

PRISMA 2020 flow diagram for systematic reviews.

The publication year of the included studies ranged from 2003 to 2020. Of these, 11 studies were cross-sectional and 9 were case-control studies. Eleven studies have been conducted in European countries, 4 studies in North American, 4 studies in South American countries, and one study in Asian countries. In 18 studies, TMD status was determined using the RDC/TMD method, and in the other two studies, using the DC/TMD method. A self-report method was used in 15 included studies, and a clinical evaluation method was used in 5 studies.

The further features of eligible studies and risk of bias assessment using the NOS are summarized in Table 2S.

The association between bruxism and TMDs

The meta-analysis suggested that any type of bruxism is a risk factor for TMD in both adjusted and unadjusted results (unadjusted OR = 2.42, 95% CI: 2.01–2.82, and adjusted OR = 1.86, 95% CI: 1.44–2.29). The statistical heterogeneity was significant (I2 = 83%, p < 0.001, and I2 = 76.9%, p < 0.001) (Fig. 2A, B).

Fig. 2: The Foest Plot of the association between bruxism and TMD by type of model and other subgroups.
figure 2

A Unadjusted, B Adjusted.

Analysis based on the type of bruxism, in the unadjusted model; There is a significant association between sleep bruxism and TMD (OR = 2.12, 95% CI: 1.81–2.43). The results indicate a higher probability (OR = 2.12) of TMD for patients with sleep bruxism as compared to without sleep bruxism individuals. Also, there is a significant association between awake bruxism and TMD (OR = 2.72, 95% CI: 2.11–3.33). The results indicate a higher probability (OR = 2.72) of TMD for patients with awake bruxism as compared to without sleep bruxism individuals. However, the meta-analysis of articles that considered both types of bruxism together and did not differentiate between them, and did not specify the type of bruxism, did not show a significant association between bruxism and TMD (Fig. 2A).

Analysis based on the type of bruxism, in the adjusted model; There is a significant association between sleep bruxism and TMD (OR = 1.96, 95% CI: 1.52–2.41). Also, there is a significant association between awake bruxism and TMD (OR = 2.09, 95% CI: 1.16–3.03). However, the meta-analysis of articles that considered both types of bruxism together and did not differentiate between them, and did not specify the type of bruxism, did not show a significant association between bruxism and TMD (Fig. 2B).

Subgroup analyses

In the unadjusted model for sleep bruxism; the subgroup analysis on the study design revealed an estimated for case-control study with an OR = 2.62, 95% CI: (1.70; 3.54) (I2 = 37.1%), and an OR = 1.94, 95% CI (1.70; 2.19) for cross-sectional study (I2 = 0.0%). A subgroup analysis on the TMD diagnosis criteria resulted in an OR = 2.15, 95% CI: (1.82; 2.48) for RDC/TMD (I2 = 39.5%). Also, subgroup analysis on the bruxism diagnosis criteria indicated an OR = 2.12, 95% CI: 1.81–2.43 in self-report method (I2 = 36.9%). The subgroup analysis on quality of study for those studies revealed an estimated fair quality with an OR = 3.47, 95% CI: 0.40; 6.55, (I2 = 0.0%) an OR = 2.11, 95% CI: 1.79; 2.43 for those with a good quality (I2 = 42.6%).

In the unadjusted model for awake bruxism; In subgroup meta-analyses according to study design, In the four case-control studies, awake bruxism was associated with 3.91–fold (95% CI 2.47–5.36) increased risk of TMD (I2 = 40.8%). In three cross-sectional studies, awake bruxism was associated with 2.24–fold (95% CI 1.81–2.67) increased risk of TMD (I2 = 37.9%). Subgroup analysis by TMD diagnosis criteria indicated that awake bruxism was associated with TMD in RDC/TMD diagnosis criteria (OR = 2.73, 95% CI: 2.10–3.36, I2 = 68.1%). In subgroup analysis by bruxism diagnosis criteria, the combined OR estimate was 2.72 (95% CI: 2.10–3.33, (I2 = 62%)) for clinical examination, no study has assessed self-reported waking bruxism. In subgroup analysis by quality of study, the combined OR estimate was 2.55 (95% CI: 2.00–3.10, (I2 = 56.6%)) for good quality, only one study was in fair quality. Other subgroup analysis presented in Table 3S.

Publication bias

Publication bias was assessed according to the overall pooled analysis. Begg’s test and Egger’s test and the symmetrical funnel plot for unadjusted and adjusted model (Fig. 3a, b) indicated a publication bias in this meta-analysis.

Fig. 3: A funnel plot illustrating publication bias in the included studies.
figure 3

A Unadjusted, B Adjusted.

Discussion

This is the first systematic review summarizing the evidence from observational studies the association between bruxism and temporomandibular joint disorders. This systematic review and meta-analysis of 20 observational studies examined the risk of TMD in people with bruxism. In overall, bruxism, whether in sleep or wakefulness, can increase the risk of TMD by about 2 times, this finding is also constant in most subgroups.

TMDs are multifactorial clinical conditions that affect the masticatory muscles, the TMJ, and related structures28. RDC/TMD and its more comprehensive version DC/TMD are among the limited measurement tools that can be used to make TMD diagnosis more accurate and reliable. Due to their high accuracy and characteristics for evaluating psychological aspects, these criteria are considered most suitable for evaluating TMDs29. During the course of this systematic review, both RDC/TMD and DC/TMD tools were considered to measure TMDs.

According to the goal of this systematic review, TMDs may be connected to bruxism. This relationship is still debated in dentistry, probably due to the etiology and the method of diagnosis of both disorders27. Bruxism increases the likelihood that a person will develop TMDs29,30, as evidenced by the current study. The present meta-analysis shows that any type of bruxism is a risk factor for TMD, regardless of controlling for confounding factors (The adjusted OR = 1.86 and unadjusted OR = 2.42 results were both significant). Consequently, the relationship between TMD and bruxism is unaffected by age or gender.

Bruxism and TMD were also not clearly linked in some studies, possibly because of the limited age groups (children and adolescents) and different diagnostic tools31. Additionally, the prevalence of TMDs increases with age, possibly because adolescents and adults express their pain more effectively than children32,33. Consequently, the examination of TMD disorders in adults can be very different from that in children34. Both adjusted and unadjusted models suggest that bruxism during wakefulness was more likely to be associated with TMD than bruxism while asleep (adjusted 2.09 AB (awake bruxism vs 1.96 SB (sleep bruxism); unadjusted 2.72 AB vs 2.12 SB). This relation is backed up by the results of many studies35,36,37. Different etiologies of awake and sleep bruxism can contribute to this outcome. As the name implies, awake bruxism is a non-functional habit, while sleep bruxism is a sleep-related movement disorder. It is also reasonable to assume that the true percentage of individuals with bruxism could be somewhat higher since not all individuals are aware of their parafunction35,38. A small number of studies have found no significant difference between the types of bruxism. Due to patients’ orientation towards awake bruxism, the condition has become a kind of individual habit that can be actively reduced during dental examinations or by following medical advice38.

The present systematic review categorized studies based on their methods of bruxism assessment into 2 categories: self-reporting (including questionnaires and interviews) and clinical evaluation (including clinical examination of teeth, evaluation of dental wear, and polysomnography (PSG). In studies based on self-report methods, bruxism was associated with TMD more frequently than in studies based on clinical assessments. Possibly this is due to the fact that a self-report of bruxism is regarded as “possible” bruxism, whereas a clinical examination is considered “probable” bruxism35. However, self-reporting of bruxism may be affected by various factors that are linked to their personal beliefs about what causes pain and tooth wear, as well as the viewpoints conveyed by their dentist39. According to many studies, self-reporting and clinical examination are clearly related to TMD symptoms, whereas bruxism based on PSG has a less favorable relationship to TMD, or in some cases even a negative relationship40. There is likely to be a negative correlation because of pain during PSG, which leads to a reduction in bruxism41. Further research is required to elucidate why certain individuals with sleep bruxism develop TMD, while others do not42. Report of the absence of a relationship between TMD and sleep bruxism diagnosed by PSG examination, can support the notion that TMD and sleep bruxism are two distinct rather independent conditions43.

In an adjusted model of cross-sectional studies, bruxism is statistically significantly related to TMD risk during sleep and awake (OR = 1.89, CI = 1.48–2.31; and OR = 2.10, CI = 1.15–3.02, respectively). It is possible that the data related to bruxism in case-control studies are not remembered correctly since the data is retrospective. A cross-sectional study examines both the exposure and the outcome simultaneously, so the relationship observed in this type of study might be statistically significant.

The strengths of this study are searching important databases without restrictions on publication year and the high quality of the included studies increase the validity for our findings. As a result of evaluating all the valid articles, the current research’s reported results on the relationship between sleep bruxism and increased risk of TMD are important and have valid scientific evidence.

The current study has the limitation that the statistical heterogeneity between TMD and bruxism was significant in both adjusted and unadjusted results. Subgrouping was used to control heterogeneity. Finally, several included studies did not adjust for age and sex which may have had further unobserved confounding effect on risk estimates. Although, all studies included in the meta-analysis are of moderate to good quality. In addition, the results of the subgroup analysis based on the quality level show that there is no significant difference between fair and good quality levels. However, it should be considered that this study was conducted on observational studies. Observational studies are subject to inherent biases such as selection bias, measurement bias, and confounding. These biases can affect the accuracy of the study results and may lead to overestimation or underestimation of the associations. our findings tend to suggest an association rather than causation. Therefore, the conclusions drawn from our study should be interpreted with caution. Additional research, such as randomized controlled trials, is needed to confirm our findings.

Conclusion

According to available data, bruxism is positively related to TMDs; the presence of bruxism increases the risk of developing TMDs in the future. Bruxism must be diagnosed early and its causes eliminated for a person not to suffer from TMDs. The person suffering from bruxism should be informed about the consequences of the condition and prescribed occlusal appliances if necessary. Further studies are needed with designs that offer a better quality of evidence and valid findings, such as cohort and case-control studies with representative samples, rigid eligibility criteria and control of confounding factors and missing data.