And signs and symptoms of temporomandibular dysfunction in adults with sleep bruxism
João Vicente Rosar a, Taís de Souza Barbosa b, Ilo Odilon Villa Dias a, Fernanda Yukie Kobayashi b, Yuri Martins Costa c, Maria Beatriz Duarte Gavião b, Leonardo Rigoldi Bonjardim c, Paula Midori Castelo d
Abstract
Objective
The purpose was to evaluate the effect interocclusal appliance therapy on bite force (BF), sleep quality and salivary cortisol levels in adults with SB diagnosed by polysomnography. As a secondary aim, signs and symptoms of temporomandibular dysfunction (TMD) were evaluated.
Design
Forty-three adults (19–30 y/o) were divided into two groups: experimental group (GSB), composed of 28 subjects with SB, and control group (GC), without SB and TMD (n = 15). GSB was treated with stabilization interocclusal splint and evaluated at time intervals: before (baseline), one month (T1) and two months (T2) after therapy began, to collect data related to BF, sleep quality (Pittsburgh Sleep Quality Index), salivary cortisol levels and TMD. GC was also examined three times and received no therapy. Data were analysed by means of normality tests, t-test/Mann-Whitney and One-way ANOVA repeated measures (Tukey post-test). Two-way ANOVA test for repeated measures was applied to verify the effect time*group interaction on the variance of each dependent variable (α = 0.05).
Results
GSB showed an increase in BF and a positive effect on muscular symptomatology, range of mandibular movements and sleep quality; in GC these parameters did not differ. Cortisol concentration decreased between baseline and T1 in GSB (F(1,31) = 4.46; test power = 62%; p = 0.017). The variance observed for BF, TMD and sleep quality among time points was dependent on the group (moderate effect size: partial Eta square >0.16; test power >80%).
Conclusions
The results suggested that short-term interocclusal appliance therapy had a positive effect on BF, temporomandibular symptomatology, sleep quality and salivary cortisol levels in adults with SB.
1. Introduction
Bruxism is a repetitive muscular activity, characterized by the clenching or grinding of the teeth and/or prosthesis; this condition has two distinct circadian manifestations, i.e., it occurs during sleep (named sleep bruxism − SB) or while the subject is awake (awake bruxism) (Lobbezoo et al., 2013). In addition, bruxism may be classified as a primary (idiopathic) or secondary condition; the primary form occurs in the absence of a previous medical history, while the secondary form may be associated with other neurological, psychiatric or sleep disorders, and with the use of some drugs and medicines (Kato, Rompré, Montplaisir, Sessle, & Lavigne, 2001).
The prevalence of SB is difficult to estimate due to its subjective characteristics, limitations of the diagnostic techniques and variation over time (cyclical character), although some studies have shown that its prevalence may range between 14 and 20% in children aged approximately 11 years old, and 13% in young adults aged 18 to 29 years, decreasing with age (Lavigne & Montplaisir, 1994; Manfredini, Winocur, Guarda-Nardini, & Lobbezoo, 2013). It is also important to consider that only 5 to 20% of individuals with SB are aware of their condition (Bader & Lavigne, 2000). According to Aloé, Gonçalves, Azevedo, and Barbosa (2013), the main signs and symptoms of SB and its consequences include teeth grinding, tooth wear, masseter hypertrophy, muscle sensitivity, poor sleep and daytime sleepiness.
Studies have suggested an association between SB and TMD, with the former being a possible risk factor for development of signs and symptoms of the latter. However, based on studies found in the literature, it is difficult to establish a causal relationship, mainly due to the lack of diagnostic specificity of SB, poor quality of studies (Macedo, Silva, Machado, Saconato, & Prado, 2007; Manfredini & Lobbezoo, 2010) and confounding factors known to be common to both conditions, such as sleep disturbances, anxiety, stress and obstructive sleep apnea (Ohayon, Li, & Guilleminault, 2001; Manfredini, Guarda-Nardini, Marchese-Ragona, & Lobbezoo, 2015).
Interocclusal appliances are frequently used in the treatment of SB, and they are primarily prescribed to protect against tooth wear and to reduce any associated muscle sensitivity (Macedo et al., 2007). One of the mechanisms proposed to explain this decrease in muscle sensitivity is that the use of interocclusal appliances would reduce masticatory muscle activity (Dubé et al., 2004). However, their effectiveness in reducing muscular symptoms and whether this condition is in fact important in the genesis of muscular pain are questionable issues, because there is no scientific evidence to support these assumptions, mainly due to the lack of adherence to the treatment, difficulty with following-up patients for a longer period (Harada, Ichiki, Tsukiyama, & Koyano, 2006) and insufficient sample sizes of previous studies (Macedo et al., 2007). The placebo effect, produced either by behavioral or functional changes, also has to be considered (Dubé et al., 2004, Pomponio, 2010). In children, therapy with a rigid appliance showed no improvement in anxiety levels or tooth wear when compared with the control group (Restrepo, Medina, & Patiño, 2011); that is, the device did not seem to prevent the occurrence of bruxism episodes.
A previous study found higher electrical activity of the masseter and temporal muscles at the rest in individuals with SB, and the authors suggested that this finding would be a consequence of the multiple contractions that occurred at night (Li, Lin, Teng, & Li, 2008). Studies with adults diagnosed with SB, who were treated with occlusal appliances showed higher (Jain, Mathur, Abhishek, & Kothari, 2012) and lower maximum bite force (Alkan, Bulut, Arici, & Sato, 2008) after therapy, as well as lower electromyographic activity (Amorim, Vasconcelos Paes, de Faria Junior, de Oliveira, & Politti, 2012) in short-term use. These controversial results reinforce the need for further studies to show the effectiveness of therapy in muscle function and in decreasing the painful symptomatology. In addition, most of the previous studies failed to obtain a homogeneous sample of bruxers diagnosed by polysomnography − considered the gold standard criteria for SB; or included a limited sample size. The follow-up of a control group (without SB) is also important.
The hypothesis tested in this study was that individuals with SB, who used an interocclusal appliance would present improvements in masticatory muscle strength, TMD symptomatology, sleep quality, and salivary cortisol levels as a stress marker. Therefore, the primary objective of this study was to evaluate the effect of interocclusal appliance use on maximal bite force, sleep quality and salivary cortisol levels in adults with SB diagnosed by polysomnography. As a secondary objective, remission of TMD signs and symptoms was evaluated.
2. Material and methods
2.1. Study design
This study was approved by the Ethics Committee of Piracicaba Dental School, University of Campinas, Protocol No. 750.187. Terms of verbal and written consent were obtained from the subjects, who were informed about the procedures, possible discomforts or risks and the possible benefits of the study.
This is a longitudinal exploratory study with interventional and two-arm parallel design, registered in the Brazilian Clinical Trials Registry (ReBEC; http://www.ensaiosclinicos.gov.br/), protocol no. RBR-3nkwyv. The report of this study was based on the recommendations of SPIRIT protocol (Standard Protocol Items: Recommendations for Interventional Trials) (Chan et al., 2013).
2.2. Sample
One hundred and forty-two adults of both sexes, students of the University of Chapecó (UNOCHAPECÓ) (Brazil), were invited and examined for possible inclusion. After anamnesis and clinical examination, and based on inclusion and exclusion criteria, a convenience sample of 43 adults between 19 and 30 years old was selected, divided into two groups: SB Group (GSB) with 28 subjects diagnosed with the condition, and control group (GC) with 15 subjects with no signs or symptoms of SB (Fig. 1).
Fig. 1. Flowchart of sample selection and allocation in groups.
The sample size was calculated based on previous studies (Alkan et al., 2008, Jain et al., 2012) that had found an increase and decrease in bite force after interocclusal appliance therapy. The difference in bite force means of 120N (Jain et al., 2012) and 380N (Alkan et al., 2008), respectively, a test power of 0.80 and bilateral alpha level of 2.5% were considered; an increase of 10% in the total number was also calculated to compensate possible losses. Finally, a minimum sample of 15 subjects in each group was required to conduct the study.
2.3. Anamnesis
The subjects were interviewed regarding personal data, sociodemographic characteristics, medical and dental histories, and other data; the following were the exclusion criteria: presence of a systemic disorder that could compromise the masticatory system (e.g., neurological disorders, epilepsy, cerebral palsy, among others); systemic disorder or current use of drugs that could interfere, directly or indirectly, with muscle activity or salivary secretion (e.g., hypertension, cancer, rheumatoid arthritis, diabetes, dyslipidemia, xerostomia, use of antihistamines, benzodiazepines, antidepressants, anxiolytics, syrups, anti-inflammatory agents, corticoids, homeopathy, among others); inappropriate behavior and/or refusal to cooperate with dental procedures and data collection. The inclusion criterion was: healthy university students with complete permanent dentition.
2.4. Clinical examination
The clinical examination was performed by one trained examiner, Dentist, Specialist and Master in Orthodontics (JVR). The exam was carried out in a dental environment, under adequate lighting for the diagnosis of dental wear facets using: clinical mirror, oral retractors, gauze, tweezers, exploratory and periodontal probes, triple syringe and individual protection equipment. The conditions of the lips, gingiva, tongue, palate, jugal mucosa, lip and presence or absence of teeth were also checked.
The following conditions were also considered exclusion criteria: tooth loss (except for third molars); soft tissue abnormalities; toothache report; active periodontitis (presence of periodontal pockets and involvement of the supporting tissues); caries lesions; use of orthodontic appliance; use of dental prosthesis (fixed or removable partial). Subjects with moderate to severe malocclusions, diagnosed by using the Orthodontic Treatment Need Index (IOTN) (scores 5 or 6 − severe and extreme need for orthodontic treatment) were also excluded (Uçüncü & Ertugay, 2001).
The presence of signs and symptoms of TMD were evaluated by clinical exam (Axis I) and specific questionnaire (Axis II) for analysis of psychological status and perception of pain according to Research Diagnostic Criteria (RDC) protocol (Dworkin & LeResche, 1992; Yap, Tan, Hoe, Yap, & Jaffar, 2001). The exam was performed after examiner calibration in a pilot study with 12 volunteers, who were not included in the final sample (the intra-examiner Kappa coefficient obtained was above 0.9 for all items of RDC/Axis I). The presence of signs and/or symptoms of TMD was an exclusion criterion for the GC, but not for GSB.
The clinical findings of the RDC/Axis I were also analyzed by means of the Temporomandibular Index (TMI), which evaluates the severity of TMD, considering three domains: functional, muscular and joint indexes, with scores ranging from zero (no clinical sign) to one (presence of clinical sign) (Pehling et al., 2002). For the functional index, twelve items related to mandibular movements are considered: unassisted and assisted opening with or without pain, right and left range of motion, protrusion and mandibular opening pattern. The muscle index measures pain associated with bilateral digital palpation of selected intraoral and extra-oral masticatory muscles at a total of 20 sites. The articular index is composed of eight items and measures pain evoked by digital palpation of two sites for each TMJ and the occurrence of noise in each joint (click and/or crepitus). The TMI index is calculated by the arithmetic mean of the three indexes; the closer the score is to 1, the greater the severity of signs and symptoms of TMD.
2.5. Sleep bruxism diagnosis
The diagnosis of SB was established by three groups of criteria:
•
report of sounds and nocturnal tooth grinding, at least three days a week, mainly reported by the room partner, either associated with, or without the report of pain and/or facial and/or cervical muscles fatigue, joint and/or dental discomfort upon awakening and fractured restorations or cusps;
•
presence of wear facets on enamel or dentine, polished and shiny between opposing teeth, detected during clinical examination and mandibular excursions;
•
results of the polysomnography exam (Lobbezoo et al., 2013). The electromyographic analysis of SB episodes was performed, based on the American Association of Sleep Medicine criteria (AASM, 2014, Maluly et al., 2013), by a trained and calibrated technician. The number of rhythmic masticatory muscle activity (RMMA/SB) episodes per hour and by type (phasic, tonic or mixed) in the different stages of sleep was considered, and the diagnosis was classified as positive when the index was above 2 episodes per hour of sleep; when above or equal to 4 episodes/hour, the patient was classified as having high frequency SB, as described in the study of Carra, Huynh, and Lavigne (2012). In GC, individuals should not present any sign or symptom of SB.
If the subjects reported the presence of any sound or grinding the teeth and presented any clinical sign of tooth wear, they were considered eligible for polysomnography, which would provide a definitive diagnosis of SB (Lobbezoo et al., 2013). The polysomnographic exams were carried out at the Center of Sleep Disturbances, Unimed Hospital, Chapecó (SC), Brazil (Meditron, Sonolab 620, Meditron Eletromedicina Ltda., Brazil). All exams were performed at 9 pm, and the subjects were monitored and followed-up for one night.
2.6. Interocclusal appliance treatment
Maxillary and mandibular stone casts were obtained from patients of the GSB; the casts were mounted on semi-adjustable articulators in maximal intercuspation for further fabrication of rigid full-arch maxillary stabilization-type splints. Wax up was done on the maxillary cast and adjusted in eccentric movements to provide a disocclusion of 2 to 3 mm. It was further processed in heat polymerized resin, and simultaneous and symmetric contacts were obtained in maximum intercuspation (Okeson, 2013). The plate was then polished to remove any irregularities before delivery, and patients were instructed to wear the appliance every night.
The appointments occurred once a month and when additional adjustments were required. The researcher contacted the patient weekly to ensure adherence to the protocol. No other recommendation was made, such as sleep hygiene, to prevent confounding factors.
2.7. Maximum bite force
The maximum unilateral bite force was evaluated by using a digital gnatodynamometer (model DDK, Kratos Equipamentos Industriais Ltda., Cotia, SP, Brazil) with fork connected to a digital appliance that provided the maximum force values in Newtons (N), as previously described (Scudine et al., 2016).
The gnatodynamometer has a fork force with the following dimensions: 12 mm high, 15 mm depth and 15 mm width, which provides an accurate measure of the force generated by each pair of teeth. During the examination, the subject remained seated, with the head in a relaxed position; the fork was placed between the maxillary and mandibular arches, on the first permanent molars, which are the teeth that generate the higher force (Ferrario, Sforza, Serrao, Dellavia, & Tartaglia, 2004).
The subjects were previously trained and instructed to bite the fork with maximum force; after this, they were asked to repeat the exam on each side of the dental arches (left and right). The measurement was repeated twice, and the maximum value was considered for each side (approximation of 0.1N).
2.8. Sleep quality
Sleep quality was assessed by using the Pittsburgh Sleep Quality Index, previously validated for the Brazilian Portuguese language by Bertolazi et al. (2011). This instrument was used to evaluate the sleep quality during a month and consisted of 19 items for self-report and an additional five questions that had to be answered by roommates; the latter information was used only for clinical purposes.
The nineteen questions were distributed among seven components, with a score ranging from 0 to 3. The components were: subjective sleep quality (C1), sleep latency (C2), sleep duration (C3), efficiency of habitual sleep (C4), sleep disturbances (C5), use of sleep medication (C6) and diurnal dysfunction (C7). Overall score was calculated by the sum of the scores obtained from these components, ranging from 0 to 21, with the highest score indicating poorer sleep quality. An overall score higher than five points indicated higher difficulty in at least two components or moderate difficulty in more than three components (it is important to point out that the use of sleep medications was considered an exclusion criterion).
2.9. Salivary cortisol levels upon awakening
Two stimulated saliva collections were obtained from each subject: in the morning, on a week day previously scheduled by the researcher (immediately after waking up and 30 min after waking), in fasting. All samples were collected at home, using salivettes (Salivette®, Sarstedt, Germany); the subjects were asked to gently move the swab with the tongue sufficiently to enable the roll to be soaked with saliva. The rolls were stored in the refrigerator until delivery to the researcher (on the same day). In the laboratory, samples were centrifuged (at 3500 rpm, 10 min, 4 °C) and stored at −80° C until analysis.
The volunteers were instructed to wake up between 6 and 8 am, not to brush their teeth before collection, and drink no beverages with caffeine/alcohol or practice physical exercise on the day before.
The salivary cortisol levels were determined in duplicate by using commercial, highly sensitive enzyme immunoassay kits (product no. 1-1102, Salimetrics®, State College, PA, USA) by a researcher blinded to the subjects’ diagnoses, Pharmacist (IOVD). Whole centrifuged saliva (25 μl) was added to each well of a microtiter plate and read at 450 nm in a microplate reader (Stat Fax 2100, Awareness Tech. Inc., Palm City, FL, USA). Controls and standards were read in the same plate. Enzyme-linked immunosorbent assays (ELISAs) were performed according to the manufacturer’s instructions. The minimum concentration of cortisol that could be distinguished from 0 was 0.003 μg/dL21 (Castelo, Barbosa, Pereira, Fonseca, & Gavião, 2012).
Morning collections were performed to express the cortisol awakening response, which represents a discrete and dynamic part of the circadian cortisol secretory cycle with physiological significance (Clow, Thorn, Evans, & Hucklebridge, 2004). Further, the area under the curve against time (AUC) was estimated by the trapezoid formulae respective to the ground level (according to Gordis, Granger, Susman, & Trickett, 2006).
2.10. Follow-up evaluation
For GSB, all variables were evaluated three times: baseline, T1 − one month and T2 − two months after therapy began.
GC was also evaluated three times: baseline, T1 − one month and T2 − two months after the first examination.
2.11. Statistical analysis
Data were statistically analyzed using the SigmaPlot 11.0 software (Systat Software, Germany) and Statistica 12.0 software (Dell, Tulsa, USA), considering an alpha level of 5%.
Descriptive statistics consisted of means and standard deviation, medians and interquartile ranges. Normality was checked by using the Shapiro-Wilk test and Quantile-quantile-plot graphs (QQ-plot); non-normal distribution variables were transformed by using the natural or exponential logarithm. At baseline, the homogeneity of the groups was tested for age (unpaired t-tests) and sex (Fisher’s exact test).
The answers of two questions of the RDC-Axis II were explored and the Chi-square test was used to verify whether the frequencies of positive responses to painful symptomatology in the face region upon awakening (Question 15e. Does your jaw ache or feel stiff when you wake up in the morning?) differed between baseline, T1 and T2. In addition, the self-perception of facial pain reported during the interview by means a visual numerical scale (Question 7. “How would you rate your facial pain on a 0 to 10 scale at the present time, that is right now, where 0 is ‘no pain’ and 10 is “pain as bad as could be”?) was compared between baseline, T1 and T2 for both groups.
Two-way repeated measures ANOVA test was used to verify the interaction effect between time (baseline, T1 and T2) and group (GSB and GC), providing the effect size (partial Eta square) and test power parameters. The variances of the differences between the levels of the factor within-subjects and homogeneity (Mauchly’s sphericity test and Levene’s test, respectively) were also tested. When necessary, the Greenhouse-Geisser correction was applied. By means of the One–way repeated measures ANOVA test (with Tukey post-test), the variance between baseline, T1 and T2 was tested for each group.
3. Results
Sample characteristics according to the demographic and clinical data are shown in Table 1. Age (t = 1.186; p = 0.2420) and the frequencies of males and females (p = 0.6960) between groups did not differ at baseline.
Table 1. Demographic and clinical data at baseline.
GSB, sleep bruxism group; GC, control group; SD, standard deviation; IOTN, Index of Orthodontic Treatment Need; RDC, Research Diagnostic Criteria; SB, sleep bruxism.
In the GSB, 82% of the subjects presented muscular TMD, three subjects were diagnosed as having disc displacement with reduction and two subjects did not have TMD. In addition, 68% of them were diagnosed as having high frequency of sleep bruxism.
Table 2 shows the results of the statistical analysis applied to the clinical data gathered during the trial. At baseline, no difference in bite force, sleep quality index and salivary cortisol concentrations was observed between groups (p > 0.05).
Table 2. Clinical variables evaluated during the trial (raw data).
GSB, sleep bruxism group; GC, control group; SD, standard deviation; IQR, interquartile range; AUC, area under the curve; TMI, Temporomandibular Index; NS, non-significant.
a ≠ b ≠ c in the same column (p<0.05; One-way ANOVA repeated measures and Tukey post-test.
†
Two-way repeated measures ANOVA test (Tukey post-test).
A decrease in the perception of pain in the mandibular region upon awakening (p = 0.0025) was observed in GSB during the treatment, while it remained stable in GC (p = 1.00) among the time points. On the other hand, the self-perception of facial pain reported during the interview by means of a visual numerical scale (Axis II, Question 7) did not differ among observations for both groups (p > 0.05).
During the trial, GSB showed an increase in the bite force magnitude and a positive effect on the functional, muscular and temporomandibular indexes and sleep quality, while in GC these parameters did not differ between baseline, T1 and T2.
The interaction effect between time (baseline, T1 and T2) and group (GC/GSB) was assessed using Two-way repeated measures ANOVA test. A significant interaction between group and time was observed for bite force, functional, muscular and TMI indexes and sleep quality, indicating that these variables showed different behaviors between groups during the treatment. Effect size and power test parameters were also gathered, as shown in Table 2; interaction graphs correspond to Figs. 2–5.
Fig. 2. Interaction effect of group and time (baseline, one month and two months) for the left bite force.
Fig. 3. Interaction effect of group and time (baseline, one month and two months) for the right bite force.
Fig. 4. Interaction effect of group and time (baseline, one month and two months) for the temporomandibular index.
Fig. 5. Interaction effect of group and time (baseline, one month and two months) for the Pittsburgh Sleep Quality Index.
The effect sizes found for the interactions were considered moderate (partial Eta square >0.16), except for the left bite force (partial Eta square = 0.11) (Cohen, 1988), and the power of the tests calculated for each variable were all considered strong (above 80%).
Although articular index and salivary cortisol AUC showed no significant time*group interaction, salivary cortisol AUC showed a significant decrease between baseline and T1 in GSB (F(1,31) = 4.46; power of the test = 62%; p = 0.017), which was not observed in GC.
4. Discussion
This longitudinal, controlled study followed-up young adults with SB, diagnosed by polysomnography, who were treated by means of interocclusal appliances, as well young adults without SB nor signs and symptoms of TMD. The main finding was the positive effect on the maximum bite force, signs and symptoms of TMD and sleep quality in GSB, with a moderate effect size and strong power of the statistical test, denoting the strength of the results found; salivary cortisol levels also showed alterations during the trial, while in the GC no change was observed in the studied variables.
Use of the TMI index made it possible to evaluate the functional, muscular and articular aspects of TMD, scoring the severity of the condition during therapy. The treatment with interocclusal appliances showed positive effects on functional and muscular indexes; that is, it was observed an increase in the mandibular range of motion and a decrease in the number of painful muscle sites on palpation, which was relevant for the sample evaluated, considering that 82% of the subjects were diagnosed as having myofascial pain. The articular index showed no significant alteration, possibly due to the reduced number of subjects diagnosed with articular alterations (n = 3). Positive effects of the interocclusal appliance on signs and symptoms of TMD have been reported in the literature, and therefore, it has been recommended for the treatment of TMD (Fricton et al., 2010; Karakis, Dogan, & Bek, 2014; Vilanova, Gonçalves, Pimentel, Bavia, & Rodrigues Garcia, 2014).
In the present study, the interocclusal appliance therapy showed a significant effect on the perception of pain or tiredness in the face upon awakening, but not on the intensity of facial pain during the interview (daytime). Similarly, Pomponio (2010) compared the pain perception (by using a visual analogue scale) among groups who wore an occlusal appliance, a non-occlusal device and the controls (untreated bruxers). The intensity of orofacial pain decreased significantly in both groups treated with intraoral devices for 45 days, with and without occlusal covering, while the intensity did not vary in the untreated group. A previous randomized clinical trial found an improvement in pain intensity and quality of life in adults with SB after wearing an interocclusal appliance, whether associated with facial massage, or not, with a large effect size (Gomes et al., 2015). Overall, although controversial, the above-mentioned results suggested that wearing the interocclusal appliance could reduce pain intensity and provide comfort to the patient with SB in a short-term treatment, with a similar or greater effect than a placebo as observed previously (Raphael, Marbach, Klausner, Teaford, & Fischoff, 2003; Gomes et al., 2015, Pomponio, 2010). According to Bertram, Rudisch, Bodner and Emshoff (2001), who showed that a short-term use of stabilization-type splints decreases asymmetry indices of masseter muscle sites, these appliances can reestablish neuromuscular symmetry and provide a stable occlusal relationship.
SB may interfere with the function of the masticatory muscles, especially if concomitant with orofacial pain (Camparis & Siqueira, 2006). The present study found significant increase in maximal bite force on both sides (left/right) during the therapy, and this variation may be related to the improvement in the self-perception of orofacial pain/fatigue upon awakening, in addition to the reduction in signs and symptoms of TMD, which requires further investigation. In a comparative study between the effects of rigid intraoral appliances vs. antidepressant drugs in adult bruxers, Alkan et al. (2008) found a reduction in bite force in the appliance group, while the magnitude of bite force increased in the antidepressant group after three months of treatment. However, it is important to consider the limited sample size of the groups evaluated (n = 5). In another study, with a large number of individuals (n = 50), an increase in bite force was observed after 12 weeks of rigid appliance therapy; however, the control group was examined only at baseline, which may limit the interpretation (Jain et al., 2012). Multiple evaluations of a control group that does not receive any intervention are necessary to control a phenomenon named “regression toward the mean”, which occurs when an extreme variable appears in the first measurement; it tends to be close to the mean in the second measurement.
A positive association between SB and insomnia was found in a representative sample of 1042 subjects diagnosed by polysomnography (Maluly et al., 2013), although it remains unclear the direction of this cause-and–effect relationship. Sleep hygiene measures and behavioral techniques have been proposed to prevent SB episodes, such as avoiding the use of electronic appliances, drinking alcohol and caffeine derivatives, and relaxing techniques at night and before bedtime (Aloé, Gonçalves, Azevedo, & Barbosa, 2003; van der Zaag et al., 2005). Poor sleep and daytime sleepiness are common complaints reported by bruxers (Aloé et al., 2003); therefore, considering that occlusal appliances have a positive effect on temporomandibular symptomatology and 82% of the GSB subjects showed muscular TMD, it is expected that a beneficial effect on sleep quality may also be observed.
The subjects included reported a better quality of sleep after one month of therapy, and this improvement remained stable until the second month. This result corroborates those found by Vilanova et al. (2014), which showed an improvement in measures of sleep quality in women with myofascial pain after interocclusal therapy. An increase in the duration of slow-wave sleep (stages 3 and 4) and the deep sleep phase was also observed in subjects who wore the appliance for 8 weeks (Sjoholm, Kauko, Kemppainen, & Rauhala, 2014). Taken together, the improvement in sleep quality observed with this therapy may be attributed to a reduction in the signs and symptoms of TMD, as shown with the reduction in TMI scores, although the placebo effect should also be considered (Dubé et al., 2004, Pomponio, 2010).
Recent studies pointed out the importance of neurotransmitters of the central nervous system (dopamine, serotonin and norepinephrine) in the genesis of bruxism episodes during sleep (Lobbezoo & Naeije, 2001; Carra, Huynh, & Lavigne, 2012), while psychosocial and peripheral aspects have lost attention as causal factors. However, due to its multifactorial nature, the evaluation of stress as a possible predisposing factor to certain diseases and conditions, such as SB, may be interesting. A recent study (Dharmadhikari et al., 2015) performed magnetic resonance spectroscopy to evaluate regions of the brain in nine adults with possible SB, who wore occlusal devices, and observed an association between SB and disturbances in the GABAergic and glutamatergic systems of the brain, those which are involved in anxiety-related behavioral disorders.
Very few studies that evaluated the relationship between bruxism and stress biological markers have been reported. The evaluation of saliva, free of any discomfort that could confound the analysis, is a convenient and useful parameter in the study of acute and chronic stress. Salivary cortisol has been shown to be altered in adults (Karakoulaki, Tortopidis, Andreadis, & Koidis, 2015) and children with SB (Castelo et al., 2012), suggesting a possible relationship with the genesis of rhythmic muscle movements. In spite of the lower power of the test (62%), the present findings showed a significant decrease in salivary cortisol levels after one month of therapy, which was concomitant with the improvement in sleep quality. Since sleep deprivation is considered a stressor and induces elevation of cortisol or corticosterone levels in humans and rodents, respectively (Palma, Tiba, Machado, Tufik, & Suchecki, 2007), taken together the results of our study suggested a positive effect of occlusal appliance therapy on sleep quality and, consequently, cortisol secretion regulation.
As limitations of the present study and suggestions for future studies, we did not diagnose awake bruxism, which is a different condition from SB and represents a challenge in the study of bruxism behavior. Wear quantification of the occlusal splints would be an interesting aspect to be evaluated, since it may be related to the presence of signs and symptoms of TMD and the frequency of bruxism.
In addition, multiple collections of saliva, on separate days, would better describe the cortisol secretory behavior. As saliva was collected at home, the lack of adherence to the protocol and collection of insufficient saliva led to sample losses and limited the results found for salivary cortisol. For ethical reasons, a group of untreated SB was not considered. Nevertheless, it is important to highlight the strengths of the study: the diagnosis of SB using polysomnography (considered the gold standard), adequate sample size and follow-up of a control group (without SB).
5. Conclusions
The results suggested that the short-term treatment with interocclusal appliance therapy had a positive effect on the self-report of muscle pain/tiredness upon awakening, muscular symptomatology and range of mandibular movements, bite force, sleep quality and salivary cortisol levels in adults with sleep bruxism.
Acknowledgement
This study received financial support from the co-operative agreement between UNOCHAPECÓ-UNICAMP Dinter (Universidade Comunitária da Região de Chapecó and Faculdade de Odontologia de Piracicaba − UNICAMP; Doutorado Interinstitucional/Dinter, Process No. 7626-0).
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