A definition and a system for the generally accepted classification of bruxism is provided by an international expert consensus: Therein bruxism is characterized by various repetitive muscle activities such as grinding or clenching of the teeth and/or tension or displacement of the mandible1. The current revision of the consensus paper recommends dividing bruxism into its two circadian manifestations2. Awake bruxism (AB) is awake masticatory muscle activity characterized by repeated or sustained tooth contact and/or mandibular tightening or advancement. SB comprises a masticatory muscle activity of the masseter muscles during sleep. The muscle activity is determined to be rhythmic (phasic), non-rhythmic (tonic), or a mixed form. The powerful displacement of the mandible and the accompanying friction of the tooth surfaces causes an audible grinding sound. Both activities are not considered as movement disorders according to the consensus paper, and SB is also not considered a sleep disorder in otherwise healthy individuals. However, the intensity of SB activity can be viewed on a continuum, with the presence of an extremely frequent and untreated form of SB leading to disorders. In adults, the prevalence of SB ranges from 5.5–15.0% depending on its underlying diagnostic method3–5.
The etiology of SB is multifactorial2. Certain neurotransmitters such as serotonin6 or specific genetic conditions7 are discussed at the biological level. The presence of certain diseases like Parkinson’s disease8 and obstructive sleep apnea syndrome9 is also associated with an increased rate of SB. Exogenous factors such as alcohol, caffeine, and tobacco also seem to explain an increase in the likelihood of occurrence10. Numerous studies examine the influence of psychosocial factors such as stress11,12 and maladaptive management of stress13,14, negative emotions such as anxiety15, or mental illness such as depression on SB11. There are both positive and negative consequences of SB with respect to pathological symptom appearance2. It is debated whether SB has an impact on the development of painful temporomandibular dysfunction (TMD)16,17. Other studies show that the occurrence of SB and the implied muscle activity have positive effects on sleep-related diseases such as obstructive sleep apnea syndrome18 or gastroesophageal reflux19.
Depending on the underlying diagnostic method, SB is classified as possible, probable, or definite SB1,2. Possible SB is based on a positive subjective self-report. Probable SB is based on a positive dental clinical examination, with or without a positive self-report. Definite SB is based on a positive instrumental finding, with or without a positive self-report and with or without a positive clinical finding. Various instrumental and non-instrumental methods are used to diagnose SB2. Non-instrumental methods register possible and probable SB and include self-assessment protocols and clinical examinations. In research, questionnaires are most commonly used to assess SB20. For instance, typical questions come from the Research Diagnostic Criteria for Temporomandibular Disorders (RDC/TMD) protocol21: "Have you been told or noticed yourself that you clench or grind your teeth during sleep?". The clinical examination includes a medical history in the form of an interview and a systematic examination of the teeth22. Here, the criteria of the American Academy of Sleep Medicine (AASM) are examined23: (A) presence of regular or frequent teeth grinding sounds during sleep; (B) presence of one or more of the following clinical symptoms: (1) abnormal wear of teeth consistent with the above reports of teeth grinding during sleep; (2) temporary morning pain or fatigue in the jaw muscles; and/or temporary headache; and/or restricted mouth opening upon awakening consistent with the above reports of teeth grinding during sleep.
Instrumental methods encompass extraoral and intraoral devices that measure current SB-activity. Extraoral devices involve portable electromyography (EMG) measurement that record the current activity of the masseter muscles24–26. Advanced measurement includes the use of polysomnography (PSG), which records other sleep parameters (distribution of sleep phases, micro arousals, and respiratory-related events) in addition to masticatory muscle activity during sleep. This procedure is considered the gold standard in determining SB, even though the procedure involves high time and financial costs27,25. The diagnostic accuracy rises when SB activity is examined via audio and video recording28. SB activity can be operationalized and summarized in episodes by specific patterns of masticatory muscle activity27. Intraoral devices allow the measurement of the actual grinding of teeth by placing a medium between the upper and lower jaws in order to operationalize the effect of grinding. A typical medium is a thin synthetic diagnostic sheet29. A recent study presented the validity of a novel diagnostic sheet made of biocompatible material that can determine SB30. Different colored layers are revealed on the surface due to teeth grinding and ablation of material. The pattern is evaluated by specific software and the strength of the current SB activity is summarized with a sum value.
SB can thus be determined in different ways and the comparability of the diagnostic methods is questionable. Relationships between psychological distress and SB are not consistent. This could possibly be related to the choice of diagnostic procedure. Possible SB correlates positively with both subjective31 and objectively measured stress (cortisol in saliva)32. Probable SB correlates positively with manic, depressive, and anxious symptoms33–36, as well as with negative coping strategies37 or less positive coping strategies14. Objectively measured stress (salivary cortisol) also positively correlates with probable SB33,36. Looking at definite SB, there is no significant association between the number of SB episodes and anxiety (measured by the STAI) and the expression of stress coping mechanisms38. Also, there is no significant difference between subjects with definite SB, classified by EMG, and chronic stress39. Depression, as measured by the Beck Depression Inventory, does not significantly correlate with the presence of definite SB and does not correlate with the amount of SB episodes11. In contrast, others show that subjects with definite SB (n = 15) have significantly higher anxiety levels40.
Previous research shows that the relationship between SB and psychological distress varies depending on the respective type of SB diagnosis. As a result, it is still unclear to what extent SB correlates with psychological distress. Furthermore, it is problematic that in most studies SB is examined according to only one procedure and, to date, a direct comparison between the different diagnostic procedures is still missing. Moreover, the question arises whether specific aspects of SB that are assessed either instrumentally (objectively) or non-instrumentally (subjectively) show different degrees of correlation with psychological distress. In summary, the aim of the present study was to analyze the relationship with psychological distress in more depth by applying the three recognized and, additionally, a novel diagnostic procedure for the estimation of SB in the same sample. Based on the existing evidence base, the following assumptions are formulated and tested in this paper: (1) The procedures are consistent with each other in the distribution of SB and non-SB diagnoses; (2) there is a correlation between parameters of the instrumental examination (PSG), as well as the diagnostic slide and psychological stress; and (3) there is a correlation between parameters of non-instrumental methods (self-report, clinical examination) and psychological stress.