## Summary

Background:

Multiple risk factors have been associated to sleep bruxism (SB). Nevertheless, there are still many unsolved issues concerning the etiology of SB that have consequences on the clinical management strategies.

Objective:

Systematically review the literature to assess the relationship between risk factors and SB symptoms in adolescents (age 11–19 years).

Search methods and selection criteria:

PubMed, Embase, Scopus, Cochrane Oral Health Group’s Trial Register and Cochrane Register of Controlled Trials, Web of Science, LILACs, and SciELO were searched to identify all peer-reviewed articles potentially relevant to the review.

Data collection and analysis:

The risk of bias was assessed according to the guidelines from the Cochrane handbook for systematic reviews of interventions, with reporting in agreement to the preferred reporting items for systematic reviews and meta-analyses guidelines.

Results:

Four out of the 4546 initially identified articles were selected. According to the grading of recommendations assessment, development and evaluation assessment (GRADE), the magnitude of agreement was almost perfect for all checklist items. Sleep disturbances, and snoring in particular, headache, jaw muscle fatigue, and tooth wear seem to be associated to SB in adolescents from 11 to 19 years old.

Limitations:

Despite the large interest of the scientific community in the field of oral parafunctions, only four articles met the eligibility criteria. Furthermore only associations and not definite cause–effect relationships were highlighted in the selected articles.

Conclusions:

Sleep disturbances presented the strongest association with SB while very few occlusal features had a moderate association. As a common sense the investigation of sleep respiratory disorders could be of great help in the management of SB in adolescents.

## Introduction

Bruxism is a repetitive jaw-muscle activity characterized by clenching or grinding of the teeth and/or by bracing or thrusting of the mandible with circadian manifestations (i.e. sleep or awake bruxism) (1, 2). Sleep bruxism (SB) is a sleep-related motor disorder with several detrimental consequences on the craniofacial complex, including tooth wear, masticatory muscle tenderness and pain, headache, and temporomandibular disorders (TMDs) (3, 4). Prevalence of SB is the highest in childhood at approximately 14–20% (5). It stabilizes at around 8–12% in teenagers and adults and decreases to 3% with aging without gender differences (6). Literature about SB is wide but fragmentary about different age stages and risk factors. As stated by the World Health Organization (WHO) a risk factor is defined as any attribute, characteristic or exposure of an individual that increases the likelihood of developing a disease or injury (7). Different risk factors have been associated to SB, however, there are still many unsolved issues concerning the etiology of SB that have consequences on the clinical management strategies. A higher percentage of SB was found in male adolescents with crowding (38%) (8). Relationship between SB and malocclusion has been investigated for a long time but is still poorly understood (9). Although some dentists suggest that malocclusion may cause SB, a recent review concluded that there is no evidence for a causal relationship between SB and occlusion (9). The etiology of SB is broad and non-specific and thus no single definitive explanation could be provided (10).

The onset of SB episodes during sleep is under the influence of the brainstem arousal-reticular ascending system contributing to the increase of activity and autonomic-cardiac neuronal networks (3). While the presence of SB is a consequence of transient arousal, apnea/hypopnea events induce arousals (11). The association between SB and obstructive sleep apnea syndrome (OSAS) has been claimed (12, 13). However, even if the phenomena are arousals related, apnea events do not induce SB events but only low-to moderate tonic contractions of the masseter muscles (11). Thus, SB and OSAS could not share a common origin but could share a common consequence, since both of them could contribute to painful TMDs via the mechanical overload-induced post-exercise muscle soreness (14–16).

In a recent study, Fernandes et al. (17) showed that painful TMD have a prevalence of 25.5% in a sample of European adolescents and that TMD pain was associated with SB [odds ratio (OR): 1.8]. The same prevalence was reported for a sample of Brazilian adolescents with a significant association between SB and TMD pain (OR: 2.02) (18). Signs and symptoms of TMD occur in healthy individuals and increase with age, particularly during adolescence (19). Considering that adolescents represent the majority of orthodontic patients and that SB is associated to painful TMDs the present systematic review was undertaken to answer the following clinical research questions in adolescents aged from 11 to 19 years suffering from SB:

• Which are the identified risk factors for SB in adolescents?

• Which is the weight of each risk factor?

## Materials and methods

The protocol for this systematic review (CRD42015029241) was registered in the International Prospective Register of Systematic Review (http://www.crd.york.ac.uk/PROSPERO).

On 12 October 2015, a systematic search in the medical literature was performed in order to identify all peer-reviewed articles investigating risk factors related to SB in adolescents aged from 11 to 19 years. The selection procedure was thoroughly described through a detailed flow chart, according to the Preferred Reporting Items for Systematic Reviews and Meta-analyses guidelines (PRISMA) statement and according to guidelines from the Cochrane handbook for systematic reviews of interventions (20, 21) (Figure 1).

Figure 1.

Flow diagram.

Figure 1.

Flow diagram.

In order to retrieve lists of potential articles to be included in the review, the search strategy illustrated in Table 1 was used in the following databases: PubMed, Embase, Scopus, Cochrane Oral Health Group’s Trial Register and Cochrane Register of Controlled Trials, Web of Science, LILACs and SciELO. Title and abstract (TIAB) screening was performed to select articles for full text retrieval. The inclusion and exclusion criteria for admittance in the systematic review were based on the type of study, were dependent on the clinical research questions, and are reported in Table 2. Duplicate articles were removed and the studies were selected for inclusion independently by two of the authors (AB and GR). Disagreements were solved by consensus. The population intervention comparator outcomes (PICO) approach was used to extract data from the included articles independently and in duplicate by two review authors (TC and GR) (22). The authors of the selected articles were contacted to clarify any relevant article queries. The primary outcome was represented by risk factors for SB in adolescents.

Table 1.

Search strategy.

Database Search strategy
MEDLINE, PubMed, Embase, Scopus, Web of Science, SciELO ((bruxer* or sleep brux*) AND (risk or assoc* or relat*) AND (factor*) AND (diagnosis))
LILIACS ((bruxer$or sleep brux$) AND (risk or assoc$or relat$) AND (factor$) AND (diagnosis)) Cochrane Oral Health Group’s Trial Register and Cochrane Register of Controlled Trials ((bruxers patient* or sleep bruxism) AND (risk or assoc* or relat*) AND (factor*) AND (diagnosis)) Database Search strategy MEDLINE, PubMed, Embase, Scopus, Web of Science, SciELO ((bruxer* or sleep brux*) AND (risk or assoc* or relat*) AND (factor*) AND (diagnosis)) LILIACS ((bruxer$ or sleep brux$) AND (risk or assoc$ or relat$) AND (factor$) AND (diagnosis))
Cochrane Oral Health Group’s Trial Register and Cochrane Register of Controlled Trials ((bruxers patient* or sleep bruxism) AND (risk or assoc* or relat*) AND (factor*) AND (diagnosis))
Table 2.

Study selection criteria.

Inclusion criteria Exclusion criteria
Randomized clinical trials (RCTs) Lack of standardized measures for bruxism evaluation
Controlled clinical trials (CCTs) Lack of effective statistical analysis
Cohort studies Case report
Cross-sectional studies Reviews
Case-control studies Abstracts and author debates or editorials
All considered participants were bruxers, with tooth grinding and/ or clenching (chronological age: 11–19-year-old). These patients were identified by using specific questionnaires, clinical analysis of tooth wear, diagnostic criteria of the American Academy of Sleep Medicine (AASM) or polysomnographic (PSG) analyses evaluated accordingly to the PSG criteria for SB. Studies on patients with systemic diseases, syndromes or neurological or psychiatric disorders
Inclusion criteria Exclusion criteria
Randomized clinical trials (RCTs) Lack of standardized measures for bruxism evaluation
Controlled clinical trials (CCTs) Lack of effective statistical analysis
Cohort studies Case report
Cross-sectional studies Reviews
Case-control studies Abstracts and author debates or editorials
All considered participants were bruxers, with tooth grinding and/ or clenching (chronological age: 11–19-year-old). These patients were identified by using specific questionnaires, clinical analysis of tooth wear, diagnostic criteria of the American Academy of Sleep Medicine (AASM) or polysomnographic (PSG) analyses evaluated accordingly to the PSG criteria for SB. Studies on patients with systemic diseases, syndromes or neurological or psychiatric disorders

According to the PRISMA statements and to the CRD (Centre for Reviews and Dissemination, University of York) the evaluation of methodological quality gives an indication of the strength of evidence provided by the study because flaws in the design or in the conduction of a study can result in biases (20, 23). However, no single approach for assessing methodological soundness is appropriate to all systematic reviews (21). The grading of recommendations assessment, development and evaluation criteria (GRADE), are widely adopted by several authors and organizations throughout the world to assess the overall quality and the risk of bias level in a systematic review (24). In order to rate the extent of agreement among data collectors, Kappa statistics were performed (25). Detailed quality assessment and reliability coefficient are illustrated in Table 3.

Table 3.

Quality assessment according to simplified GRADE checklist.

Study limitation (risk of bias) Source, year % Agreement
ITANI 2013 CARRA 2011 OHAYON 2001 VAN SELMS 2013
1. Was random sequence generation used (i.e. no potential for selection bias)? 1.0
2. Was allocation concealment used (i.e. no potential for selection bias)? 1.0
3. Was there blinding of participants and personnel (i.e. no potential for performance bias)? 1.0
4. Was there blinding of outcome assessment (i.e. no potential for detection bias)? 1.0
5. Was an objective outcome used? 1.0
6. Were more than 80% of participants enrolled in trials included in the analysis (i.e. no potential reporting bias)? 0.5
7. Were data reported consistently for the outcome of interest (i.e. no potential selective reporting)? 1.0
8. No other biases reported? (i.e. no potential of other bias) 1.0
9. Did the trials end as scheduled (i.e. not stopped early)? 1.0
Study inter-rater reliability     0.94
Study limitation (risk of bias) Source, year % Agreement
ITANI 2013 CARRA 2011 OHAYON 2001 VAN SELMS 2013
1. Was random sequence generation used (i.e. no potential for selection bias)? 1.0
2. Was allocation concealment used (i.e. no potential for selection bias)? 1.0
3. Was there blinding of participants and personnel (i.e. no potential for performance bias)? 1.0
4. Was there blinding of outcome assessment (i.e. no potential for detection bias)? 1.0
5. Was an objective outcome used? 1.0
6. Were more than 80% of participants enrolled in trials included in the analysis (i.e. no potential reporting bias)? 0.5
7. Were data reported consistently for the outcome of interest (i.e. no potential selective reporting)? 1.0
8. No other biases reported? (i.e. no potential of other bias) 1.0
9. Did the trials end as scheduled (i.e. not stopped early)? 1.0
Study inter-rater reliability     0.94

Risk of bias. From left to right were reported GRADE checklist point, source and year of publication and percentage of agreement. In the last column was reported the inter-rater reliability, showing a 94% agreement in the ratings given by reviewers. Inter-reliability or concordance is the degree of agreement among raters. It gives a score of how much homogeneity, or consensus, there is in the ratings given by reviewers.

### Statistical analysis

Statistical analysis was performed using the R statistical package (version 3.0.1, R Core Team, Foundation for Statistical Computing, Vienna, Austria). To improve the power of risk factors estimates associated with SB symptoms, articles in which a multiple regression analysis was performed (adjusted for variables statistically associated with SB symptoms) were selected for the review process. Several data extracted from the selected studies were processed in order to obtain either suitable data for the analysis or for presentation in an evidence table; only statistically significant risk factors were included (P value < 0.05).

The primary outcomes were risk factors associated to SB in adolescents, calculated as the standardized OR effect size. This effect size was the result of the OR differences between sleep bruxers and controls. Each OR was then weighted by the inverse of its variance and adjusted for small sample bias.

Non-overlapping 95% confidence interval (CI) was considered statistically significant. Based on recommendations of the Cochrane Collaboration (20), one author (GC) converted the standardized relative risk into a natural log OR. In order to be considered eligible for the final review process, articles had to include OR analysis for investigated risk factors.

Empirical evidence suggests that relative effect measures are, on average, more consistent than absolute measures (26, 27). OR is the main way to quantify how strongly the presence or absence of a risk factor is associated with the presence or absence of a disease in a given population.

## Results

The search strategy yielded four relevant publications. All selected studies were cross-sectional and randomized (28–31). The article selection process is illustrated in the PRISMA flow diagram (Figure 1). Sample size in individual studies ranged from 278 to 98.411 subjects, with a total of 114.639 subjects. Mean age in the evaluated samples ranged from 11 to 19 years. In all the included studies, SB patients were identified by using specific questionnaires following the diagnostic criteria of the American Academy of Sleep Medicine (AASM) (32).Only one author used clinical examination and specific questionnaires (28). Table 4 summarizes the characteristics of each of the four included studies.

Table 4.

Characteristics of included studies: risk factors associated with SB symptoms.

Source, year Trial type, sample size Children age Risk factors SB diagnostic means Outcome points
ITANI 2013 Cross-sectional 13–18 years Lifestyle Questionnaire Snoring—always
n = 98411  Sleep status  Snoring—often
Depressive status  Snoring—sometimes
Gender female
Smoking
Nocturnal awakening
Enjoyed much < usual activities
Difficulty initiating sleep
Early morning awakening
Drinking alcohol
Naptime ≥15, <60 min
Naptime ≥60 min
Having usual depressive feelings
CARRA 2011 Cross-sectional 7–17 years Sleep bruxism Questionnaire and Grade 12th (7th ref.)
n = 278  Wake bruxism clinical examination Jaw muscle fatigue
Tooth clenching  Tooth wear
Sleep-disordered breathing  Daytime sleepiness
Feeling unrefreshed (morning)
Difficulty wakening (morning)
Loud breathing during sleep
Distracted by external stimuli
Uncomfortable dental occlusion
Difficulties in yawning
Dental Class II
Posterior crossbite
OHAYON 2001 Cross-sectional 15–18 years Sociodemographic informations Questionnaire Age 15–18 (≥ 65 ref.)
n = 12464  Sleep disturbances  OSA syndrome
Psychoactive substances
Psychological disorders
VAN SELMS 2013 Cross-sectional 10–22 years Sociodemographic informations Questionnaire Jaw pain/tense feeling
n = 3486  Ethnic background  Feeling blue
Orofacial pain
Joint sounds
Orthodontic treatments
Stress
Feeling blue
Smoke
Alcohol
Source, year Trial type, sample size Children age Risk factors SB diagnostic means Outcome points
ITANI 2013 Cross-sectional 13–18 years Lifestyle Questionnaire Snoring—always
n = 98411  Sleep status  Snoring—often
Depressive status  Snoring—sometimes
Gender female
Smoking
Nocturnal awakening
Enjoyed much < usual activities
Difficulty initiating sleep
Early morning awakening
Drinking alcohol
Naptime ≥15, <60 min
Naptime ≥60 min
Having usual depressive feelings
CARRA 2011 Cross-sectional 7–17 years Sleep bruxism Questionnaire and Grade 12th (7th ref.)
n = 278  Wake bruxism clinical examination Jaw muscle fatigue
Tooth clenching  Tooth wear
Sleep-disordered breathing  Daytime sleepiness
Feeling unrefreshed (morning)
Difficulty wakening (morning)
Loud breathing during sleep
Distracted by external stimuli
Uncomfortable dental occlusion
Difficulties in yawning
Dental Class II
Posterior crossbite
OHAYON 2001 Cross-sectional 15–18 years Sociodemographic informations Questionnaire Age 15–18 (≥ 65 ref.)
n = 12464  Sleep disturbances  OSA syndrome
Psychoactive substances
Psychological disorders
VAN SELMS 2013 Cross-sectional 10–22 years Sociodemographic informations Questionnaire Jaw pain/tense feeling
n = 3486  Ethnic background  Feeling blue
Orofacial pain
Joint sounds
Orthodontic treatments
Stress
Feeling blue
Smoke
Alcohol

Risk factors associated with SB symptoms. From left to right were reported source and year of publication, trial type and sample size, age range, risk factors, SB diagnostic means and outcome points.

### Quality analysis

According to the GRADE guidelines, among the selected sample, the methodological quality was moderate/almost perfect for all studies (28–31). The inter-rater reliability or the percentage of agreement among the selected article reviewers accordingly to the simplified GRADE checklist, was almost perfect (94%) (Table 3). The most important sources of bias were the absence of allocation concealment and the lack of adequate blinding procedures for all included studies.

### Study results

Table 5 summarizes the results of each article reviewed, by the type of study and risk factor analyzed.

Table 5.

Summary of results: risk factors associated with SB symptoms.

Source, year Author conclusions Bruxers Control P value Outcome points
n (N) n (N)
ITANI 2013 Smoking habit, alcohol consumption, and depressive feelings were significantly associated with disorders of sleep-related bruxism 98 (5216) 1754 (93195) <0.001 Snoring—always
126 (5216) 2258 (93195) <0.001 Snoring—often
414 (5216) 7403 (93195) <0.001 Snoring—sometimes
406 (5216) 7255 (93195) <0.001 Snoring—seldom
1422 (5216) 45976 (93195) <0.001 Gender female
189 (5216) 3381 (93195) <0.001 Smoking
513 (5216) 9160 (93195) <0.001 Nocturnal awakening
186 (5216) 3317 (93195) <0.001 Enjoyed much < usual activities
653 (5216) 11664 (93195) <0.001 Difficulty initiating sleep
245 (5216) 4382 (93195) <0.001 Early morning awakening
609 (5216) 10885 (93195) <0.001 Drinking alcohol
984 (5216) 17581 (93195) <0.001 Naptime ≥ 15, <60min
2120 (5216) 37873 (93195) <0.001 Naptime ≥ 60 min
681 (5216) 12159 (93195) <0.001 Having usual depressive feelings
1082 (5216) 19323 (93195) <0.001 Grade 10th (7th ref.)
1018 (5216) 18194 (93195) <0.001 Grade 11th (7th ref.)
924 (5216) 16501 (93195) <0.001 Grade 12th (7th ref.)
CARRA 2011 Sleep parafunctions are frequently associated with signs and symptoms suggestive of TMDs, and with sleep and behavioural problems 6 (58) 3 (220) <0.01 Jaw muscle fatigue
8 (58) 4 (220) <0.001 Tooth wear
4 (58) 4 (220) <0.01 Daytime sleepiness
7 (58) 9 (220) <0.01 Frequent headache
13 (58) 22 (220) <0.01 Feeling unrefreshed (morning)
16 (58) 23 (220) <0.01 Difficulty wakening (morning)
14 (58) 20 (220) <0.01 Loud breathing during sleep
22 (58) 36 (220) <0.01 Distracted by external stimuli
19 (58) 39 (220) <0.01 Uncomfortable dental occlusion
5 (58) 4 (220) <0.01 Difficulties in yawning
35 (58) 86 (220) <0.05 Dental Class II
9 (58) 74 (220) <0.05 Posterior crossbite
OHAYON 2001 Patients with sleep-disordered breathing have a higher number of risk factors for sleep bruxism 17 (568) 773 (11896) <0.01 Age 15–18 (≥ 65 ref.)
27 (568) 8 (568) <0.01 OSA syndrome
VAN SELMS 2013 For sleep bruxism, associations were found with orofacial pain and depressive mood 82 (624) 474 (2862) <0.001 Jaw pain/tense feeling
239 (624) 1378 (2862) <0.01 Feeling blue
Source, year Author conclusions Bruxers Control P value Outcome points
n (N) n (N)
ITANI 2013 Smoking habit, alcohol consumption, and depressive feelings were significantly associated with disorders of sleep-related bruxism 98 (5216) 1754 (93195) <0.001 Snoring—always
126 (5216) 2258 (93195) <0.001 Snoring—often
414 (5216) 7403 (93195) <0.001 Snoring—sometimes
406 (5216) 7255 (93195) <0.001 Snoring—seldom
1422 (5216) 45976 (93195) <0.001 Gender female
189 (5216) 3381 (93195) <0.001 Smoking
513 (5216) 9160 (93195) <0.001 Nocturnal awakening
186 (5216) 3317 (93195) <0.001 Enjoyed much < usual activities
653 (5216) 11664 (93195) <0.001 Difficulty initiating sleep
245 (5216) 4382 (93195) <0.001 Early morning awakening
609 (5216) 10885 (93195) <0.001 Drinking alcohol
984 (5216) 17581 (93195) <0.001 Naptime ≥ 15, <60min
2120 (5216) 37873 (93195) <0.001 Naptime ≥ 60 min
681 (5216) 12159 (93195) <0.001 Having usual depressive feelings
1082 (5216) 19323 (93195) <0.001 Grade 10th (7th ref.)
1018 (5216) 18194 (93195) <0.001 Grade 11th (7th ref.)
924 (5216) 16501 (93195) <0.001 Grade 12th (7th ref.)
CARRA 2011 Sleep parafunctions are frequently associated with signs and symptoms suggestive of TMDs, and with sleep and behavioural problems 6 (58) 3 (220) <0.01 Jaw muscle fatigue
8 (58) 4 (220) <0.001 Tooth wear
4 (58) 4 (220) <0.01 Daytime sleepiness
7 (58) 9 (220) <0.01 Frequent headache
13 (58) 22 (220) <0.01 Feeling unrefreshed (morning)
16 (58) 23 (220) <0.01 Difficulty wakening (morning)
14 (58) 20 (220) <0.01 Loud breathing during sleep
22 (58) 36 (220) <0.01 Distracted by external stimuli
19 (58) 39 (220) <0.01 Uncomfortable dental occlusion
5 (58) 4 (220) <0.01 Difficulties in yawning
35 (58) 86 (220) <0.05 Dental Class II
9 (58) 74 (220) <0.05 Posterior crossbite
OHAYON 2001 Patients with sleep-disordered breathing have a higher number of risk factors for sleep bruxism 17 (568) 773 (11896) <0.01 Age 15–18 (≥ 65 ref.)
27 (568) 8 (568) <0.01 OSA syndrome
VAN SELMS 2013 For sleep bruxism, associations were found with orofacial pain and depressive mood 82 (624) 474 (2862) <0.001 Jaw pain/tense feeling
239 (624) 1378 (2862) <0.01 Feeling blue

Risk factors associated with SB symptoms. From left to right were reported source and year of publication, author conclusions, sample size (bruxers and control groups) and level of significance for each outcome point. Snoring, jaw muscle fatigue, tooth wear, daytime sleepiness and frequent headache were significantly associated to SB onset in adolescents.

The four articles included in the review examined effects of various risk factors on SB: lifestyle (29), sleep status (28, 29, 31), depressive and stressed status (29, 30, 31), grade (29), tooth wear (28), sleep disordered breathing (28, 31), TMDs (28, 30), socio-demographic information and ethnic background (30, 31), orthodontic treatment (30), and smoke and alcohol intake (30). Table 6 shows the results of the review process.

Table 6.

Effect size by descending order.

Source, year Risk factors Odds ratio (95% CI)
ITANI 2013 Snoring—always 12.6 (11.0–14.4)
CARRA 2011 Jaw muscle fatigue 10.5 (2.4–45.2)
CARRA 2011 Tooth wear 8.8 (2.5–30.8)
ITANI 2013 Snoring—often 7.9 (6.9–9.1)
CARRA 2011 Daytime sleepiness 7.4 (1.6–33.6)
CARRA 2011 Frequent headache 4.3 (1.4–12.9)
CARRA 2011 Feeling unrefreshed (morning) 3.4 (1.5–7.7)
CARRA 2011 Difficulty wakening (morning) 3.3 (1.5–7.2)
CARRA 2011 Loud breathing during sleep 3.1 (1.3–7.2)
CARRA 2011 Distracted by external stimuli 2.8 (1.4–5.6)
ITANI 2013 Snoring—sometimes 2.7 (2.4–3.1)
CARRA 2011 Uncomfortable dental occlusion 2.5 (1.2–5.0)
ITANI 2013 Snoring—seldom 2.3 (1.9–2.6)
CARRA 2011 Difficulties in yawning 2.2 (1.3–3.6)
CARRA 2011 Dental Class II 2.0 (1.1–3.8)
OHAYON 2001 Age 15–18 (≥ 65 ref.) 1.8 (1.2–2.7)
OHAYON 2001 OSA syndrome 1.8 (1.2–2.6)
VAN SELMS 2013 Jaw pain/tense feeling 1.5 (1.2–1.9)
ITANI 2013 Gender female 1.5 (1.4–1.6)
ITANI 2013 Smoking 1.5 (1.3–1.8)
ITANI 2013 Nocturnal awakening 1.4 (1.2–1.6)
VAN SELMS 2013 Feeling blue 1.4 (1.1–1.7)
ITANI 2013 Enjoyed much < usual activities 1.4 (1.2–1.7)
ITANI 2013 Difficulty initiating sleep 1.3 (1.1–1.4)
ITANI 2013 Early morning awakening 1.3 (1.1–1.5)
ITANI 2013 Drinking alcohol 1.2 (1.1–1.4)
ITANI 2013 Naptime ≥ 15, <60min 1.2 (1.1–1.4)
ITANI 2013 Naptime ≥ 60 min 1.2 (1.1–1.3)
ITANI 2013 Having usual depressive feelings 1.2 (1.1–1.4)
ITANI 2013 Grade 10th (7th ref.) 0.8 (0.7–0.9)
ITANI 2013 Grade 11th (7th ref.) 0.7 (0.6–0.8)
ITANI 2013 Grade 12th (7th ref.) 0.7 (0.6–0.8)
CARRA 2011 Posterior crossbite 0.4 (0.2–0.9)
Source, year Risk factors Odds ratio (95% CI)
ITANI 2013 Snoring—always 12.6 (11.0–14.4)
CARRA 2011 Jaw muscle fatigue 10.5 (2.4–45.2)
CARRA 2011 Tooth wear 8.8 (2.5–30.8)
ITANI 2013 Snoring—often 7.9 (6.9–9.1)
CARRA 2011 Daytime sleepiness 7.4 (1.6–33.6)
CARRA 2011 Frequent headache 4.3 (1.4–12.9)
CARRA 2011 Feeling unrefreshed (morning) 3.4 (1.5–7.7)
CARRA 2011 Difficulty wakening (morning) 3.3 (1.5–7.2)
CARRA 2011 Loud breathing during sleep 3.1 (1.3–7.2)
CARRA 2011 Distracted by external stimuli 2.8 (1.4–5.6)
ITANI 2013 Snoring—sometimes 2.7 (2.4–3.1)
CARRA 2011 Uncomfortable dental occlusion 2.5 (1.2–5.0)
ITANI 2013 Snoring—seldom 2.3 (1.9–2.6)
CARRA 2011 Difficulties in yawning 2.2 (1.3–3.6)
CARRA 2011 Dental Class II 2.0 (1.1–3.8)
OHAYON 2001 Age 15–18 (≥ 65 ref.) 1.8 (1.2–2.7)
OHAYON 2001 OSA syndrome 1.8 (1.2–2.6)
VAN SELMS 2013 Jaw pain/tense feeling 1.5 (1.2–1.9)
ITANI 2013 Gender female 1.5 (1.4–1.6)
ITANI 2013 Smoking 1.5 (1.3–1.8)
ITANI 2013 Nocturnal awakening 1.4 (1.2–1.6)
VAN SELMS 2013 Feeling blue 1.4 (1.1–1.7)
ITANI 2013 Enjoyed much < usual activities 1.4 (1.2–1.7)
ITANI 2013 Difficulty initiating sleep 1.3 (1.1–1.4)
ITANI 2013 Early morning awakening 1.3 (1.1–1.5)
ITANI 2013 Drinking alcohol 1.2 (1.1–1.4)
ITANI 2013 Naptime ≥ 15, <60min 1.2 (1.1–1.4)
ITANI 2013 Naptime ≥ 60 min 1.2 (1.1–1.3)
ITANI 2013 Having usual depressive feelings 1.2 (1.1–1.4)
ITANI 2013 Grade 10th (7th ref.) 0.8 (0.7–0.9)
ITANI 2013 Grade 11th (7th ref.) 0.7 (0.6–0.8)
ITANI 2013 Grade 12th (7th ref.) 0.7 (0.6–0.8)
CARRA 2011 Posterior crossbite 0.4 (0.2–0.9)

Significant results of included studies. From left to right were reported source and year of publication, risk factors and effect sizes. The effect size was the result of the OR differences between bruxers and controls. OR is the main way to quantify how strongly the presence or absence of a risk factor is associated with the presence or absence of a disease in a given population. For each risk factor was reported the 95% confidence interval. Snoring showed the greater relationship, while Posterior crossbite showed the lower relationship with SB symptoms.

Patients snoring every night presented the strongest association with SB (OR: 12.6; 95% CI: 11–14.4) although OSAS showed mild association (OR: 1.8; 95% CI: 1.2–2.6). High association with SB was found for frequent headache (OR: 4.3; 95% CI: 1.4–12.9) and bad sleep hygiene: daytime sleepiness (OR: 7.4; 95% CI: 1.6–33.6), feeling unrefreshed at awakening (OR: 3.4; 95% CI: 1.5–7.7) and difficulty in waking up (OR: 3.3; 95% CI: 1.5–7.2), although patients reporting nocturnal awakenings showed mild association (OR: 1.4; 95% CI: 1.2–1.6). Other important risk factors for SB were jaw muscle fatigue (OR: 10.5; 95% CI: 2.4–45.2) together with tooth wear (OR: 8.8; 95% CI: 2.5–30.8), although jaw pain showed mild association (OR: 1.5; 95% CI: 1.2–1.9). Uncomfortable occlusion (OR: 2.5; 95% CI: 1.2–5) and dental Class II malocclusion (OR: 2; 95% CI: 1.1–3.8) showed moderate association with SB, although the presence of posterior crossbite showed weak relationship with SB symptoms (OR: 0.4; 95% CI: 0.2–0.9). Smoke (OR: 1.5; 95% CI: 1.3–1.8) showed moderate association with SB, while alcohol intake (OR: 1.2; 95% CI: 1.1–1.4) showed weak association. Depressive status (OR 1.4; 95% CI: 1.1–1.7) showed mild association with SB. Female subjects presented mild association (OR: 1.5; 95% CI: 1.4–1.6) with SB.

## Discussion

The present review evaluated the existing literature related to risk factors and SB symptoms in adolescent population (age 11–19 years).

Accordingly to the GRADE tool (24), the quality of evidences emerging from the selected articles was excellent. The analysis of possible sources of bias revealed the lack of some methodological features: a limitation of all studies was represented by allocation concealment and lack of adequate blinding procedures for all included studies.

The systematic review of the existing scientific literature confirmed a probable multifactorial model for SB. Sleep disturbances, and snoring in particular, headache, jaw muscle fatigue and tooth wear seem to be associated to SB in adolescents from 11 to 19 years old.

The association of SB and sleep disturbances appeared to be stronger for snoring than for any other disorder (28, 29). Subjects reporting snoring every night had a very strong association with SB (OR: 12.6; 95% CI: 11–14.4). Bad sleep hygiene seems to be a high risk factor for SB, considering that patients with daytime sleepiness, feeling not refreshed in the morning, frequent headache and difficulty at wakening presented more episodes of SB respect to control groups (28).

Several studies have suggested that SB could be associated with OSAS (11, 12). Physiological evidences support the presence of SB as a consequence of transient arousal while apnea–hypopnea events induce arousals. However, apneic events are not always followed by SB/tooth grinding, suggesting that an apneic event, per se, does not seem to be a direct cause of a SB/tooth grinding episode (11). Several studies suggested the hypothesis that SB may represent a way to reinstate the airway patency following an obstructive respiratory event during sleep, although this hypothesis needs further investigation (13, 33). Results from the present review seem to support the association between SB and OSAS in adolescents since snoring, daytime sleepiness, frequent headache are both SB and OSAS risk factors (34). However, if we consider OSAS as potential risk factors for SB, Ohayon et al. (31) reported only a mild association (OR: 1.8; 95% CI: 1.2–2.6).

Jaw muscle fatigue and tooth wear showed a strong correlation with SB, even if their role as potential risk factors was described only in one article (28). Tooth wear is a complex, multifactorial phenomenon involving the interplay of biological, mechanical, chemical, and tribological factors. Moderate tooth wear has been reported in 51% of adolescents in Birmingham (UK) (35). In Brazil, among 295 12-year-old adolescents, the prevalence of dental wear was 26.90% (36). From a clinical point of view tooth wear is not necessarily a direct consequence of SB (34) but it represents a strong risk factor for SB in adolescents. Uncomfortable occlusion and dental Class II malocclusion showed moderate association with SB. The orthodontic treatment of patients presenting those occlusal features could lead to a reduction of SB insurgence risk in adolescent patients. However, over the years the role of dental morphology and occlusion as a cause of bruxism has been greatly reduced and, to date, there are no scientific studies supporting any cause-effect relationship (9, 34, 37–42).

Depressive status turns out to be related with SB in a moderate way. The probability of depression rises from around 5% in early adolescence to 20% by the end of that time (43, 44). Previous studies reported a strong association between SB, painful TMDs, and moderate to high level of depression (45). The significance of these connections is poorly understood and a causal relationship cannot be established. However, the use of diagnostic criteria for TMD in the orthodontic clinical setting could be of help in identifying adolescent patients suffering from depression in order to refer the patient to a proper intervention before starting the orthodontic treatment.

Smoking is a moderate SB risk factor in adolescents, confirming what previously stated by other authors about SB and direct exposure to smoke in adolescents and adult population (34, 46).

Oral parafunctions predict TMDs (47) and considering the association between SB and painful TMDs in adolescents (17–19) and that SB seems to precede the insurgence of TMDs via the mechanical overload-induced post-exercise muscle soreness (14–16), the identification of possible risk factors associated to SB could be very important to improve the quality of life of adolescent orthodontic patients. In this light the clinician should investigate for sleep disturbances, and snoring in particular, should identify possible headache, jaw muscle fatigue, and tooth wear. Identification of possible depression patients should be performed too. From a dental point of view particular attention should be devoted to patient concerns related to unstable occlusion and to patients presenting Class II malocclusions.

This review revealed the need for methodologically well designed and well conducted studies. Questionnaires are often used to investigate possible risk factors and to investigate about the presence of SB. There is a need for further evidence-based longitudinal studies with standardized and validated diagnostic criteria and polysomnography or validated portable devices (48, 49), in order to obtain more accurate data.

## Limitations

Only four cross-sectional articles met the inclusion criteria and none of them highlighted any cause–effect relationship. Thus, we have not yet evidence about SB risk factors in adolescents.

## Conclusions

The present review didn’t highlight any cause–effect relationship related to the insurgence of SB in adolescents. Thus a definite conclusion cannot be drawn. However sleep disturbances presented the strongest association with SB while very few occlusal features had a moderate association. From a clinical point of view the investigation of sleep respiratory disorders seems to be of great help in the management of SB in adolescents.

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## Author notes

Correspondence to: Andrea Bargellini, Department of Surgical Sciences, Specialization School of Orthodontics, Dental School, University of Torino, Via Nizza 230, 10126 Torino, Italy. E-mail: bargelli@ipsnet.it