Stomatognathic Parafunctions and Their Etiology: Literature Review

LITHUANIAN UNIVERSITY OF HEALTH SCIENCES

MEDICAL ACADEMY

FACULTY OF MEDICINE

DEPARTMENT OF ORAL AND MAXILLOFACIAL SURGERY

Marijus Leketas

Stomatognathic parafunctions and their etiology: Literature review.

Master’s Thesis

Supervisor:

Prof. habil. dr. Ričardas Kubilius

Kaunas, 2018 CONTENT

SANTRAUKA ...... 3

ABSTRACT ...... 4

INTRODUCTION ...... 5

MATERIALS AND METHODS ...... 5

RESULTS ...... 7

DISCUSSION ...... 12

CONCLUSIONS ...... 17

CONFLICT OF INTEREST ...... 17

REFERENCES ...... 17

2 SANTRAUKA

Marijus Leketas. “Stomatognatinės parafunkcijos ir jų priežastys. Sisteminė literatūros apžvalga”. Magistro baigiamasis darbas. Darbo vadovas – prof. habil. dr. Ričardas kubilius. Lietuvos sveikatos mokslų universitetas, Medicinos akademija, Medicinos fakultetas, Veido ir žandikaulių chirurgijos klinika. Kaunas, 2018; 22p.

Tikslas ir uždaviniai: Kraniomandibulinių susirgimų atsiradime manoma, jog didžiausią reikšmę turi priežastys, kurių mechanizmas gan aiškus, tai traumos, onkologija, sąkandžio sutrikimai ir kt. Tokia nuomonė vyrauja tarp daugelio gydytojų. Tačiau patyrę šios srities specialistai pastebi, kad egzistuoja dar viena priežasčių grupė, kuri vadinama stomatognatinėmis parafunkcijomis. Gydytojas, kuris nėra įsigilinęs į kraniomandibulinių susirgimų galimas priežastis, dažnai net nežino kas tai yra stomatognatinė parafunkcija ir kokį vaidmenį ji atlieka patologijos atsiradime. Šio darbo tikslas atlikti sisteminę literatūros analizę apie stomatognatines parafunkcijas ir pateikti naują, aiškesnę definiciją bei klasifikaciją. Tyrimo metodika: Literatūros apžvalga buvo atlikta 2017 metų sausio mėnesį naudojant “Medline ir Embase” duomenų bazes. Rezultatai: Atlikus sisteminę literatūros analizę, rezultatai parodė, kad nėra vieningos stomatognatinės parafunkcijos definicijos ir aiškaus sutarimo ar stomatognatinės parafunkcijos yra kraniomandibulinių susrgimų priežastis. Išvados: Tradicinė stomatognatinės parafunkcijos definicija nėra pakankama, kad būtų galima paaiškinti visus procesus, kurie literatūroje apibrėžiami kaip parafunkcija. Be to, literatūroje nėra pateikiama jokia stomatognatinių parafunkcijų klasifikacija. Todėl mes siūlome labiau specifišką stomatognatinės parafinkcijos apibrėžima ir naują klasifikaciją, tikėdami, kad tai padės tyrėjams ir klinicistams gydant ir analizuojant parafunkcijas.

3 ABSTRACT

Marijus Leketas. “Stomatognathic parafunctions and their etiology: Literature review”. Supervisor – prof. habil. dr. Ričardas Kubilius. Lithuanian University of Health Sciences, Medical Academy, Faculty of Medicine, Department of Oral and maxillofacial surgery. Kaunas, 2018; 22 p.

Background and objective: Doctors usualy attribute craniomandibular disorders (CMD) reasons to obvious mechanisms such as traumas, oncology, malocclusion and etc. And only experienced doctors notice that there is one more group of reasons which is called stomatognathic parafunctions (SGP). However, SGP are not unanimously defined and understandable for common doctor. The main aim of this review was to overview what kind parafunctions are analyzed in literature and to suggest a new, broader definition and etiology-based classification of SGP. Material and methods: The literature review was conducted using the resources of Medline and Embase in January 2017. Results: The findings revealed that there is no unanimous definition of parafunctions and no consensus whether or not SGP influence temporomandibular joint pathology or other stomatognathic system structures. Conclusions: The traditional definition of SGP is not sufficient to explain whole range of processes wich are called in literature as parafunctions. Moreover, no classification of SGP exists in literature. Therefore, we suggest more specific definition and a new classification of SGP assuming that they will help researchers and clinicians in treatment and investigation of parafunctions.

4 INTRODUCTION

There are many different habits in the society – some of them only temporarily influence certain body functions while others irreversibly damage body functions. Practitioners in oral health frequently face the patients with habits that are related with stomatognathic system (SGS). Such habits can lead to dysfunction not only in SGS but in other body systems as well, e.g. musculoskeletal system. The majority of such patients complain about the temporomandibular joint (TMJ) problems. Moreover, they are usually of younger age [1],[2] which is an alarming trend. TMJ pathology is considered as one of the main outcomes caused by stomathognathic parafunctions (SGP) [3]. The precise mechanism and the causes of certain part of TMJ damage are not known. Additionally, the stomatognathic parafunctions as such have no unified definition which reduces the opportunities to investigate the phenomenon in large scale. Therefore, we did a review of literature with the aim to overview which stomatognathic parafunctions are analyzed in literature and to propose a new, broader definition of such parafunctions, which is still lacking in field of dentistry.

MATERIALS AND METHODS

For this review the search in PubMed and Embase was conducted in January 2017. The search strategy included the publications from 1946 to 2017 and was based on two components in the search box: parafunction (defined in three alternative terms: keyword #1 „stomatognathic system parafunction“, keyword #2 „oral parafunction“, and keyword #3 „oral habit“) and pain type (5 alternative terms: keyword #4 „headache“, keyword #5 „TMD“, keyword #6 „muscle pain“, keyword #7 „myofacial pain“, keyword #8 „myofacial pain syndrome“), totalling to 15 search combinations per database. The primary set of eligible articles after removal of duplicates was 421. These articles were scrutinized using following exclusion criteria: review article, intervention study, editorial, or not related with purpose of review. Abstracts were included only if they were explicit about the object and findings of original studies. After reviewing and additional search of 421 articles, 46 met the eligibility criteria – this was a final set of current literature review. The detailed chart of literature search is presented in Figure 1.

5 Figure 1. Flowchart of literature review and its stages.

Figure 1. Flowchart of literature review and its stages.

MEDLINE EMBASE (1946 – 2017) (1946 – 2017)

Stomatognathic Headache Stomatognathic Headache system (65540 hits) system (185573 hits)

TMD parafunctions TMD

n parafunctions

i (6372 hits) (81 hits) (12492 hits)

t (100 hits)

a c oral parafunction Muscle pain oral parafunction Muscle pain

f i (32984 hits) (605 hits) (276667 hits) t (99 hits)

e oral habits Myofascial pain oral habits Myofascial

d I (13766 hits) (2520 hits) (55295 hits) pain myofascial pain (9529 hits) syndrome myofascial (13975 hits) pain syndrome (7271 hits)

Combined with Combined with AND (264 hits) AND (280 hits)

Records after duplicates removed (421 hits )

c Records screened: 421

y Articles assessed for Records excluded: 326 Reasons:

l i eligibility: 95 review article, intervention study,

g editorial, or not related with purpose

E of review.

d Studies included in

u l qualitative synthesis: 46

6 Before the synthesis of studies, we searched for the best available classification of parafunctions according to their etiologies. However, we did not succeed in finding a classification of parafunctions and etiologies. Therefore, in our review all the analyzed articles were approached using following categorization of etiologies and manifestations of parafunctions:  Non-specific etiology (Bruxism, Teeth clenching/grinding, Cheek/lip biting, Nail/pen biting, Thumb sucking, Gum chewing);  Specific etiology: o Occupational (Playing a musical instrument, Singing, Scuba diving); o Psychogenic (Lip grimace, Nasal grimace); o Biomechanical (Position of the tongue in relation to the palate and the mouth floor, Head positions, Additional mandibular movements in time of mouth opening). The parafunctions were reviewed also regarding symptoms and dysfunctions attributed to stomatognathic system problems. They were categorized to for groups:  Temporomandibular joint dysfunction;  Headache;  Facial muscles pain;  Other. The findings of the review are presented in tables with main features and trends commented separately.

RESULTS

In total, 46 studies were included into literature review [Table 1]. The huge majority of the studies under review were conducted in European countries or the United States. More than half of studies investigated the subjects aged below 25 years, mainly including schoolchildren and students, the majority of studies had relatively large samples (100 subjects and more). Less than half of the studies investigated the subjects with specific disorders (mainly temporomandibular).

7 Table 1. Main characteristics and findings of reviewed studies.

Study Country Sample Main findings First group - violinists in the Murcia region of F. J. Rodriguez- Spain Spain, who were examined for TMD. Control group Violin playing appears to be a factor associated with TMD. Lozano, et al. 2010 [31] who did not play any musical instrument. 41-year-old female viola player with facial pain, I. Shargill, et al. 2007 England limited mouth opening, and a history of clicking Viola players are predisposed to TMD. [32] (anterior disc displacement) and lockings. O. Kovero, et al. Finland 31 music students. Intense violin playing may have a predisposing role in the etiology of TMDs in adolescence. 1996 [33] J. J. Taddey, 1992 United 3000 vocalists. TMD symptoms appear to be significant in trombone, trumpet, tuba, violin, and viola musicians. [35] States A. Steinmetz, et al. 30 musicians undergoing CMD treatment with Pain in the shoulder and/or upper extremity was the most frequent symptom reported by 83% of subjects, Germany 2009 [34] oral splints. followed by neck pain (80%) and pain in the teeth/TMJ regions (63%) C. Rocha, et al. 2012 Brazil 100 popular singers (50 men and 50 women). Popular singers reported presence of body pains mainly proximal to the larynx [36] A. Koob, et al. 2005 TMD-related symptoms seem to be at the greatest risk of developing pain in the masticatory muscle system Germany 296 active divers, aged 18 to 65 years, [37] and/or the temporomandibular joint during or after the dive. F. Lobbezoo, et al. 536 scuba divers (mean +/- SD age = 40.4 +/- 11.9 TMD pain is a common complaint among scuba divers who were free of such complaints before they started Netherlands 2014 [38] years; 34.1% women) diving actively. H. S. Chung, et al. Korea 9 conscious human adults. Lip exercise can promote the activities of the lips, tongue and digastric muscles. 1989 [21] A. B. Lipira, et al. United Patients exhibiting face grimace had worse velopharyngeal closure than those without (79.1% versus 70.7%; p 88 outpatients 2011 [43] States = .035). 20 Class III subjects (9 males, 11 females, 19.2 +/- J. Primozic, et al. Tongue posture is significantly lower in Class III subjects and is associated with the dentoalveolar Slovenia 4.6 years) and 20 Class I subjects (6 males, 14 2012 [26] characteristics of the maxilla and mandible. females, 17.4 +/- 1.7 years). L. G. Ries, et al. 2008 Brazil 40 female volunteers. Individuals with TMD presented greater postural asymmetry. [44] C. M. Visscher, et al. 10 persons, without craniomandibular or cervical The influence of head posture upon the kinematics of the mandible is probably a manifestation of differences Netherlands 2000 [24] spine disorders. in mandibular loading in the different head postures. H. Panek, et al. 2012 Poland 303 healthy students (mean age 18.8 years). TMDs were more prevalent in the presence of bruxism than in other oral parafunctions. [5] N. M. Farsi, 2003 Saudi 1940 schoolchildren. 505 with primary, 737 with The most common symptoms were headache (13.6%) and pain on chewing (11.1%). Nail biting was the most [16] Arabia mixed and 734 with permanent dentition. common oral parafunction (27.7%) while bruxism was the least common (8.4%). M. C. Carra, et al. 604 children aged 7–17 yrs, seeking orthodontic Canada Sleep- and wake-time parafunctions are frequently associated with signs and symptoms suggestive of TMDs. 2011 [6] treatment. S. Marklund, et al. 308 dental students examined at the start of Self-reported bruxismshowed association with the 1-year period prevalence of myofascial signs and Sweden 2008 [45] their dentistry course and re-examined after 1 year. symptoms in the jaw-face region. A. Gavish, et al. 2000 248 randomly selected high school girls, aged 15- Statistically significant associations were found between intensive gum chewing and muscle sensitivity Israel [15] 16 years. (P<0.001) and joint noises (P<0. 05). J. A. Kieser, et al. New Juvenile bruxing was a self-limiting condition which does not progress to adult bruxism and which appeared 150 bruxers between the ages of 6 and 9 years. 1998 [46] Zealand to be unrelated to TMJ symptoms. S. I. Kalaykova, et al. 260 outpatiens. 12- to 16-year-old adolescents Diurnal clenching may be a risk factor for intermittent TMJ locking while age may be a risk factor for anterior Netherlands 2011 [7] (52.3% female). disc displacement with reduction. 8 A. G. Glaros, et al. United 20 individuals without temporomandibular The precise behavioral meaning of clenching varies across individuals. 2006 [47] States disorder. 135 examined 15-year-old subjects, 103 G. E. Carlsson, et al. Sweden completed a questionnaire and 84 were examined Nocturnal tooth grinding is related to demand for treatment of TMD. 2004 [48] clinically at the 10-year follow-up (at age 25). R. Miyake, et al. 3557 Japanese university students, aged Chewing on one side and tooth clenching caused an increased risk of TMJ noise TMJ pain and impaired mouth Japan 2004 [49] between 18 and 26 years. opening. 557 patients (127 men, 430 women) affected by A. Michelotti, et al. Italy myofascial pain or disc displacement or Daytime clenching/grinding was a significant risk factor for myofascial pain and for TMJ disc displacement. 2010 [8] arthralgia/arthritis/arthrosis. A. G. Glaros, et al. United 235 patients seeking care at a general medical Results from the logistic regressions provide convergent validity on the importance of oral parafunctions, 2012 [12] States clinic. specifically tooth contact and clenching, to facial/head pain. A patient diagnosed with migraine with aura, M. Melis, et al. 2006 United with concomitant temporomandibular joint and A headache related to dental occlusion and dental parafunctions seems to be able to mimic a primary [50] States masseter muscle pain, was treated by the use of a migraine headache. dental appliance. G. Kobs, et al. 2005 Lithuania 307 subjects (140 males und 167 females). Intensive clenching can predominantly lead to pathologic phenomena in the muscles or joints. [51] A. P. Vanderas, et al. Parafunctional and some structural and psychological factors may increase the probability of the child Greece 314 children, aged 6-8 years. 2002 [52] developing the signs and symptoms of CMD. A. P. Vanderas, 1995 United 386 children aged 6-10 years with and without Significant correlations were found between parafunctions and difficulties in opening wide and pain on [53] States unpleasant life events. movements. M. Nilner, 1983 [11] Finland 440 randomly selected school children. Correlations were found between recurrent headaches and oral parafunctions. M. J. van der TMD pain patients, one group 303 patients and No clinically relevant relationships were found between different types of self-reported oral parafunctions Netherlands Meulen, et al. 2006 [4] the other group 226 patients. and TMD pain complaints. E. Winocur, et al. Oral parafunctions (except chewing gum) were significantly associated between themselves and suggest a Israel 323 girls, aged 15-16 years. 2001 [54] behavioural pattern of "jaw hyperactivity". S. G. Cortese, et al. Dysfunctions and parafunctions showed a high prevalence and were significantly associated with TMD Argentina 133 patients (average age: 12±3 years old). 2009 [9] symptoms. M. Quinteromarmol- The physiomechanical conditions in the oral cavity of the patients of upheavals temporomandibular are Mexico 130 out patients. Juarez, et al. 2009 [55] considered a risk factor condition. E.Winocur, et al. Adolescent females had a higher prevalence of TMD signs and symptoms and carried out oral habits more Israel 314 adolescents (136 males and 178 females). 2006 [19] intensively. Parafunctional habits and gender may be risk factors of TMD. N. Farsi, et al. 2004 Saudi 1976 children aged 3-15 years old. Oral parafunctions have a significant role in the aetiology of TMD. [56] Arabia G. Corvo, et al. 2003 106 patients with TMJ disorders who were being Italy Malocclusions and parafunctions are considered factors fundamental to early diagnosis of TMJ problems. [57] treated in the orthognathic ambulatory. G. E. Carlsson, et al. The results indicated that some signs and symptoms (bruxism, oral parafunctions, TMJ clicking, and deep bite) Sweden 402 subjects 7, 11, and 15 years old. 2002 [58] might predict TMD signs and symptoms in a long-term perspective. 59 children without TMJ dysfunction and 123 children with TMJ dysfunction in the mixed S. Sari, et al. 2002 Turkey dentition. In the permanent dentition 89 children The association of parafunctions and TMJ dysfunction is present in the mixed and the permanent dentitions. [59] without TMJ dysfunction and 123 children with TMJ dysfunction.

9 E. Winocur, et al. Israel 248 girls, aged 15-16 years. There was an association between generalized joint laxity and parafunction. 2000 [60] L. Westling, et al. Sweden 74 female patients. Parafunctions and trauma were associated with increased symptoms and signs in individuals with lax joints. 1990 [61] Normal controls; TMJD patients diagnosed with A. G. Glaros, et al. United disk displacement only; TMJD patients diagnosed TMJD patients diagnosed with myofascial pain and arthralgia engage in significantly higher levels of 2005 [62] States with myofascial pain only; and TMJD patients parafunctional tooth contact than non-TMJD individuals. diagnosed with both myofascial pain and arthralgia. A. Michelotti, et al. Very high level of oral parafunctions were associated with a significant increase of masticatory muscle pain Italy 200 medical students. 2012 [63] and headache. 96 subjects diagnosed with myofascial pain, A. G. Glaros, et al. United Parafunctional behaviors, especially those that increase muscle tension, and emotional states are good myofascial pain and arthralgia, disk displacement or 2005 [64] States predictors of jaw pain levels in patients with TMD and healthy control subjects. no TMD symptoms. S. E. Widmalm, et al. United 525 4- to 6-year-old African-American and Parafunction is the cause or the consequence of pain, or a third factor is causing both pain and increased 1995 [65] States Caucasian children, mean age 5.1±0.65 (SD). prevalence of oral parafunctions. M. Wieckiewicz, et Poland 456 students. 264 (58%) women and 192 (42%) Emotional burden and excitability are factors that predispose to muscular disorders. al. 2014 [66] men.

10 Looking to the etiology of stomatognathic parafunctions, of 46 analyzed studies, 13 analyzed specific and 33 – non-specific etiologies (typical parafunctional habits) [Table 2]. Of note, the specific etiologies included occupational, biomechanical or psychogenic factors that in general are not considered as parafunctions in literature though they are the habits as such. In contrast, habits related with non-specific etiology such as bruxism or teeth clenching are the main parafunctions described in literature.

Table 2. Overview of the studies by parafunctions, their etiology, related symptoms and dysfunctions.

Symptoms and dysfunctions Etiology Habit Facial muscles Temporomandibular joint Headache Other pain dysfunction Bruxism Bruxism [5], [16], [6], [15] - [15], [45] unrelated to TMJ [46]

Teeth clenching / Jaw movements [7], [47], [48], [49], [8] [12], [50] [51], [52], [8] grinding problems [53] Unknown (non- Jaw movements Cheek / lip biting [52] - [11] specific) problems [53] Nail / pen biting - - - No pathology [4]

- Thumb sucking - - -

Gum chewing [54] - - -

All habits (without [9], [55], [19], [56], [57], [63], [64], [65], [63] - specification) [58], [59], [60], [61], [62] [66]

Playing a musical [31], [32], [33], [35] - - CMD [34] instrument Occupational Body and larynx Singing [35] - - pain [36]

Scuba diving [37], [38] - - -

The position of the tongue in relation to the Dentoalveolar - - - palate and the mouth deformities [26] floor Biomechanical Head positions [44], [24] - - -

Additional mandibular movements in time of - - - - mouth opening

Lip grimace - - [21] - Psychogenic Speech problems Nasal grimace - - - [43]

In studies that analyzed parafunctions related with specific etiology, the main causes of disorder were occupational (8 studies), while biomechanical and psychogenic causes of parafunctions were much less investigated. Among the studies analyzing non-specific causes of parafunctions, the majority examined bruxism, teeth clenching or grinding (17 studies in total). Other parafunctions were approached only in very rare cases. Additionally, 14 studies analyzed stomatognathic parafunctions without specification.

Overview of the symptoms and dysfunctions that are most frequently attributed to parafunctions in analyzed studies revealed that the most common problem is temporomandibular joint dysfunction. It is common in cases with both specific (especially occupational) and non-specific etiology. The facial muscles pain predominated over headache and they were prevailing in cases with non-specific etiology of parafunctions.

Other craniomandibular disorders were analyzed in studies under review only relatively rarely. There was one single study that found the absence of any pathology related with parafunction – it analyzed nail or pen biting in patients with TMD [4].

DISCUSSION

Many authors have claimed that parafunction habits such as bruxism, clenching of teeth, cheek biting, finger or pencils sucking, etc. are significant for the etiology of TMJ dysfunction [5], [6], [7], [8], [9]. However, it is only one group of parafunctions, and while summarizing the studies under review we noticed that the parafunctions can be classified by their etiology. In previous literature, the parafunctions relate mainly to non-specific or unknown etiology – this applies to most common parafunctions such as bruxism or clenching of teeth etc [15]. However, we also found the studies where oral habits result in certain dysfunctions or symptoms, such as muscle pain or headache [64]. We looked for SGS features that influence the emergence of dysfunctions. Therefore, we suggested a classification based on origin of such disorder by adding three specific etiologies – occupational, biomechanical and psychogenic. The issue of etiology will be discussed more in detail. Since habits are the largest group representing stomatognathic parafunctions, we want to discuss the etiology of parafunctional habits and their impact on SGS dysfunctions. Manfredini et al. conducted a literature review to overview the causes for parafunctional habits. They presented two hypotheses: the first states that causes of parafunctional habits are of central origin, and they are influenced by the limbic system, which is responsible for emotions and their expression. The second hypothesis states that parafunctions are influenced

12 by impaired periodontal receptor activity [10]. However, over time the first theory became more accepted because it has been demonstrated that parafunctional habits still remain even in the absence of occlusal contacts. There are no recent theories in literature proposed to explain the causes of parafunctional habits. We suggested that a parafunction is not just a habit. It can involve any movement of SGS or even any movement of head and neck which influences the emergence of dysfunction. Therefore, the type of etiology is playing a significant role in identification and, possibly, treatment of parafunctions.

Regarding non-specific etiology, there is a lot of research conducted on it. For instance, in early 1980s it was proven that recurrent headaches are closely related to parafunctional habits such as lip or cheek biting, nail biting, teeth clenching or bruxism [11]. Latest research supports it: scientists of Kansas University demonstrated that tooth contact and clenching were significantly associated with head pain [12]. In contrast, Capurso et al. [13] found that parafunctions, namely parafunctional habits and dysfunctions of craniomandibular system, are not statistically significantly associated with each other, but they can co-exist and impede each other's progress. A similar conclusion was made by other authors Westling et al. who stated that there is no correlation between parafunctional habits and craniomandibular disorders and they are not a causal factor; instead, stomatognathic parafunctions are of great significance to TMJ hypermobility [14]. The opposing opinion was presented by some other authors who mentioned in their work that constant gum chewing influences the emergence of sounds and changes of TMJ. They also noted that there is no correlation between bruxism and TMJ dysfunction [15]. Moreover, such authors as Michelotti et al. and others claimed that there is a strong correlation between TMJ dysfunction and bruxism or teeth clenching [5], [8], [16]. Some scientists put forward an opinion about bruxism whereby they claim that the more a person believes in the influence of bruxism on TMJ pathology, the stronger this influence is [17]. Such dissent suggests that there is no consensus as to what parafunctional habit results in which dysfunction. However, almost all authors analyzing parafunctions have noticed that teenagers make up the biggest risk group [6-8], [18], [16]. Parafunctional habits are more widespread and common among women [19]. This indirectly shows that there may be some social or psychological features which predispose the occurrence parafunctions. The second group of parafunctional etiologies includes habits related to occupational activities which affect SGS. Playing a musical instrument is known to be a cause of damage in SGS or certain body area. Playing the violin or viola are most frequently associated with negative impact for SGS in literature [30], [31], [32], [33]. That most probably happens because an instrument is held on one's shoulder being supported by the head, which is tilted during the playing time. In addition, musicians tend to push the mandible towards the contralateral TMJ. Not a single study has been found to present an opposing opinion. Thus, playing violin negatively affects stomatognathic system and therefore this habit can be considered as parafunction. Moreover, playing wind instruments such as clarinet or oboe [34] as well as playing string instruments other that violin and viola [35] also have been associated with negatyve effects on SGS. However, it is not known what the

13 mechanism underlying SGS damage by those instruments is. Playing other instruments was mentioned as an etiological factor for SGS dysfunction only in a couple of papers. Singing is another professional habit that might result in the damage of stomatognathic area [36] and the larynx is mostly shown to be affected. Singers complain about pain in larynx as well as in the area of chest. Another professional habit which can be attributed to this group is scuba diving. The search yielded a few papers dealing with divers having TMD [37], [38]. The authors of those articles claim that scuba diving often causes TMD and pain in the area of face muscles. The mechanism behind this damage is not yet known. However, TMD has been suggested to appear due to a snorkel held in a mouth for prolonged period of time and additional mandibular movements. The third group of parafunctional etiologies include specific biomechanical causes – abnormal posture and biomechanics of craniomandibular apparatus. In 1989, a sliding cranium theory was proposed to explain the relationship between the head-neck complex and the craniomandibular system [23]. It was proven that head position affects the relationship of the upper and lower jaws, thereby changing the occlusion and masticatory muscle tone on different sides of the face. Visscher et al. in their study showed that head position influences the kinematics both in the motion and position of mandible at rest. According to this study, „the opening movement path of the incisal point with the head held in a military posture is shifted anteriorly relative to its path in a natural head posture. In a forward head posture its path is shifted posteriorly. During lateroflexion, the movement path of the incisal point deviates to the side the head has moved to. In repose occlusal plane orientation can vary from 2 to 5 mm, depending on the head position“ [24]. However, Olivo et al. conducted a review of articles about associations between head or spinal position and TMJ dysfunction and concluded that there is no credible link therein, and if there is a link, then the mechanism is not clear [25]. However, an additional review revealed some data on the position of the tongue posture which can affect the changes of certain parts of SGS [26]. They pointed out that position of the tongue in relation to mouth floor and hard palate affects dental arches and the relationship between lower and upper maxillary alveolar process. They discovered that there is a statistically significant difference between the lower and upper tongue positions. There is also some evidence that the lower position of the tongue, i.e. the closest distance between the bottom of the mouth and the tongue, has an effect on the formation of Angle Class III. The higher the position of the tongue is, the closer the tongue is to the hard palate, and as a result, the occlusion passes in to Angle class I. In this group, it is important to mention the biomechanical disorders of craniomandibular apparatus which could also be called parafunctions. However, there has not been found sufficient evidence in literature to prove it. According to the literature, pathological movements occur on jaw openings in the presence of TMJ problems and they can be used in TMJ pathology diagnosis [27], [28] and observation of treatment process [29]. Finally, the fourth group of SGS was identified, i. e. the parafunctions of psychogenic origin. It can be supposed that face grimaces may be assigned to such parafunctions. Some research demonstrates that face

14 grimaces can be due to damages in central nerve system [20]. Moreover, studies show that facial grimaces affect other parts of SGS, leading to various dysfunctions. For example, grimacing of lips affects the activity of tongue and digastric muscle, as well as there is a slight influence on the activity on masseter muscle [21]. As there emerges an extra muscle activity, it in turn may affect various pathologies related to those muscles. Another study found that nasal grimacing has a significant impact on the quality of speech sounds and is considered as one of the possible etiological factors of resonance disease [22].

In summarizing all four highlighted etiologies we can conclude that parafunctions falling into one of the groups have certain influence on the emergence of stomatognathic system disorders, which is evident from the aforementioned examples. The extent to which parafunctions affect those disorders are difficult to estimate by data from literature because scientific articles lack detailed analysis of parafunctions. Only a few parafunctions (such as bruxism) are given a thorough examination. However, many more of them have not been detailed so far. It is difficult to explain such low interest in the area of SGS. However, we think that the absence of a unanimously defined, thorough and clear definition in the area of SGS might be the reason.

Thus, our literature review revealed that in scientific articles the oral habits are quite commonly considered as parafunctions. However, in field of dentistry there is no single definition of parafunctions that can be universally applied. The lack of definition for parafunctions as such reduces the opportunities to investigate the phenomenon at large scale in research. Therefore, we did a review of literature with the aim to overview which stomatognathic parafunctions are analyzed in literature and to propose a new, not just a broader, but more precise definition of such parafunctions. Current definitions of parafunction are not consistent. For instance, a general medical definition of parafunction states that it is 'disordered or perverted function' [39], [40] or 'Abnormal or disordered function' [41]. However, the term of parafunction is mainly used in dentistry and its definition is more specific, but still is not universal. Thus, Medical Dictionary for the Dental Professions states that parafunction is 'movements of mandible that are outside normal function (e.g., bruxism)' [41], while Mosby's Dental Dictionary refers to 'habitual movements (e.g., bruxism, clenching, and rocking of teeth using teeth for tools) that are normal motions associated with mastication, speech, or respiratory movements and that result in worn facets and other problems associated with occlusal trauma' [42]. Moreover, the parafunctions can be referred to as parafunctional habits or oral habits [42] which shifts the term of oral habits fully towards abnormal range. In any case, even such specific definitions of parafunction are still not fully precise, and our review has demonstrated this. From our perspective, in any case the parafunction is not a normal condition but a certain problem related to body organs. In contrast, oral habits are not necessarily parafunction, e.g. smoking is an oral habit, but there is no evidence that it can lead to SGS dysfunction, even though it negatively affects other body

15 systems. Such oral habits can‘t be considered as parafunctions and therefore these terms should not be used synonimously. Similarly, if we consider a parafunction as 'movements of mandible that are outside normal function (e.g., bruxism)' [41] or like 'habitual movements' [42] then such habit as mouth breathing (not a movement, but rather a stable though not correct position of mandible) would not be classified as a parafunction, though it may lead to SGS dysfunction. This demonstrates that parafunction not necessarily is a movement as defined above. After all, the Latin word 'parafunctio' includes a function, not simply the movements. Also, the parafunction not always should result in worn facets and other problems associated with occlusal trauma, as defined by Mosby dictionary [42]. An example could be a lip biting, affecting TMJ function, but hardly resulting in occlusal trauma. If we use occlusal trauma as a criterion for parafunction, the TMJ dysfunction or muscles pain or even headache are not sufficient conditions for possible parafunction. In contrast, our review showed that TMD, muscle pain and headache are most common outcomes and symptoms of parafunction. TMJ pathology is considered as one of the main outcomes caused by stomathognathic parafunctions in other studies as well [3]. The precise mechanism and the causes of certain part of TMJ damage are not known.

Since the currently used definitions allegedly describe parafunctions inaccurately, they require a more accurate and extended definition. Our review of literature provided a wider scope of causes for SGS damages which can also be called parafunctions. Thus, not only habits can be attributed to parafunctions, but certain disordered and/or abnormal human body states and functions as well. The suggestion of new definition was as a by-product of the literature review where we found that the notion of parafunctions varies quite widely. Our concept of stomatognathic parafunctions differs from others in a more specific approach. Based on scientific publications, arguments, and inaccuracies of current definitions of parafunctions, we suggest a broader definition of stomatognathic parafunction: „The parafunction of stomatognathic system is nothing more than an additional recurrent disordered or abnormal function leading to dysfunction of any part of stomatognathic system“. As long as this process is not a dysfunction, the parafunction has no anatomical structure changes.

We conducted a thorough literature review of scientific articles on stomatognathic parafunctions. It was comprehensive in that it included the studies with both patients and relatively healthy subjects. This is important because parafunctions are the type of disorder which may exist out of the scope of health care, since the people with parafunction may not seek health care. Also, our review included not only most common parafunctions such as bruxism or teeth clenching, but more rare conditions as well. Moreover, we used the etiology-based approach and based on the findings we proposed more exact and specific definition of stomatognathic parafunctions, which could lead to better understanding of this condition.

16 However, our review also has some limitations. Among those it should be noted that some studies under review did not provide strong evidence from clinical studies and therefore certain aspects of parafunctions were covered only by one or several studies. Our clinical practice suggests that there may be even more parafunctions than conventionally accepted, but to prove this, we need additional and more detailed research. After all, a new research on the issue of parafunctions may also have impact on proposed classification. This would also depend on clinical applications including presumably other unknown causes of stomatognathic system dysfunctions.

CONCLUSIONS

Based on the present literature review on the influence of stomatognathic parafunctions on stomatognathic system, we can conclude that stomatognathic parafunctions affect stomatognathic system. However, there is no unanimous opinion as to what part of stomatognathic system is affected by which kind of parafunction and to what extent. To resolve unclarity and lack of common agreement on the issue a further research is needed. With this paper we do not expect a sudden universal shift in the paradigm of parafunctions, but rather we hope for a more thorough and intense research by other scientists and clinicians using our suggested new insights and a fuller, more specific definition as well as a new classification of stomatognathic parafunctions. The end result of the new findings would be a better counseling and treatment of the suffering patient.

CONFLICT OF INTEREST

There are no potential conflicts of interest.

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