Oral parafunction takes many forms, including tooth-to-tooth behaviors (such as clenching, grinding, and pressing of the teeth together), teeth-separated behaviors (such as bracing or guarding the jaw), soft tissue behaviors (such as cheek biting or tongue posturing), and others (such as musical instrument placement or telephone cradling with the jaw). Among these parafunctions, teeth clenching and grinding, nail and object biting, and gum chewing are the most commonly reported [29–32]. While all oral parafunctions were once regarded as a result of malocclusion (e.g., sleep bruxism was the body’s response to correcting an occlusal discrepancy; bracing the jaw was due to not having a comfortable centric occlusion), no causal relationship between any occlusal feature and oral parafunction has ever been supported by any evidence. In contrast, stress reactivity and habitual behavior have varying but sufficient levels of support as important features of oral parafunctions, and each of these belongs to the domain of psychosocial factors [33–36]. In addition, nocturnal bruxism has been identified as a sleep-related movement disorder with a multifactorial etiology.

The importance of oral parafunctional behaviors to both TMD and orthodontic treatment is becoming increasingly supported by research. Multiple studies have found significant associations between daytime oral parafunction (typically, clenching, and grinding) and myofascial pain [33, 35–37]. In addition, diurnal parafunctional activities can be a risk factor for disc displacement as well [37]. One proposed mechanism for parafunctional behaviors contributing to TMD is that such behaviors may overload the dentition and masticatory system [38, 39]. More specifically, the mechanism has been postulated to involve damage of muscle fibers [40] or to a reduction of blood supply [41]. Because most oral parafunction occurs at a low intensity level, but often for prolonged periods, the hypotheses of “overload” to the masticatory system and “damage” to muscle fibers clearly need better evidence and explanation regarding mechanism; at present, however, we do not know how parafunction exerts its apparently pathologic effects. A malocclusion coupled with a particular pattern of clenching or grinding might aggravate a disc displacement with reduction. However, not everyone with such malocclusions engages in parafunctional behaviors as described here, and those behaviors generally emerge independent of the malocclusion. The presence of the particular occlusal features will often be seen by some clinicians as causing the patients to do the particular parafunctional behavior, but this represents attribution bias, not causation.

The mechanism by which parafunction may affect the dentition and masticatory system (whether by “overload” or other mechanism) is equally relevant for orthodontic diagnosis, treatment, and posttreatment retention. According to the belief of an “occlusion-centered” etiology for masticatory system overload, an orthodontist may focus on the evaluation of centric condylar position, dental or skeletal discrepancies, and occlusal interferences [42, 43]. However, during orthodontic treatment, teeth are moved considerably, and this induces a stream of constantly changing occlusal interferences and continuous bouts of occlusal instability. As a consequence of the continuous change in the occlusal pattern, the achievement of any “ideal centric” condylar position cannot be guaranteed.

Finally, during posttreatment retention, attention is given again to the potential presence of occlusal interferences, and sometimes a limited occlusal adjustment is suggested to finalize and stabilize the orthodontic treatment results [44, 45]. This entire process often has the unfortunate side effect of inducing patients and their dentists to focus their attention on the necessity of an ideal and perfect intercuspation and on checking for potential occlusal interferences. The increased attention of the patients on their teeth can be detrimental if, during the posttreatment retention phase, even a little relapse of the achieved “ideal” occlusion occurs, inducing worries and fears. Interestingly, empirical data exist in support of this observation; a recent study showed that individuals with a history of TMD developed symptoms and adapted less well to the introduction of an active occlusal interference when compared to subjects without a TMD history [46].

In summary, oral parafunctions are behaviors that are not caused by malocclusion of any type. Rather, they are a function of mental states such as anxiety, stress reactivity, or simple habit; in the case of sleep bruxism, it is part of the sleep disorder phenomenology. Therefore, it is not the kind of malocclusion a person has but rather what the person does with it (e.g., parafunction) that matters in terms of symptoms that appear to be caused by the occlusion but are not.

Trait anxiety, somatosensory amplification, and hypervigilance are three psychological constructs with specific relevance to the focus of this chapter. While trait anxiety has been studied for decades with respect to its strong relationship to health and disease, somatosensory amplification and hypervigilance were identified more recently as core constructs involved in functional disorders. Functional disorders are also known as idiopathic pain syndromes when pain is the primary symptom, and as medically unexplained symptoms in other contexts. A primary characteristic of such disorders is that the extent of illness greatly exceeds the extent of objectively measured disease.

Trait anxiety refers to a general pattern of physical dysregulation and worry that is an inherent characteristic of an individual [47–49]. Somatosensory amplification refers to the tendency of the individual to perceive a given somatic sensation as intense, noxious, and disturbing [50]. And, hypervigilance is an increased awareness of the discrepancy between the perceived sensation vs. what is expected as “normal”, with a heightened attention typically focused on weak sensations. While each construct can occur independently of the others, together they appear to constitute a substantial triad linking a variety of psychosocial states with a variety of clinical disorders that overlap with TMD and problematic occlusions.

Trait anxiety might predispose to somatosensory amplification [51]. A number of studies have shown that somatosensory amplification is correlated with several indices of general distress including anxious and depressive symptoms [52–54]. Anxiety appears to influence the perception of orthodontic pain [55, 56], and individuals with prolonged pain during orthodontic treatment exhibit higher anxiety scores as compared to individuals with pain of short duration [57]. Orthodontic pain perception has been found to be significantly greater in individuals with high trait anxiety and somatosensory amplification [58]. Perhaps either of these processes could underlie pain complaints during orthodontic treatment that are misdiagnosed as TMD.

Moreover, anxiety and somatosensory amplification could jointly contribute to a more pronounced attention bias toward a potentially threatening stimulus represented by occlusal interferences. It is likely that individuals with high anxiety and/or somatosensory amplification are also hypervigilant against a perceived discrepancy in perceived vs. expected sensation. Such individuals may exhibit an increased awareness of their occlusion, reacting to those somatic sensations with emotional affect and cognitions that intensify them and transform them into more alarming, threatening, and disturbing sensations [51].

Individuals with high trait anxiety also exhibit greater amounts of parafunction [59]. Indeed, high levels of trait anxiety indicate an anxious personality disposition, which can be associated with a high rate of environmental scanning and reduced ability to switch attention away from the threatening stimulus. So, it could be hypothesized that patients presenting with high-frequency parafunctional activity are occlusally hypervigilant and are more disturbed by the occlusal interferences and changes induced by orthodontic therapy, resulting in pain and dysfunction. Occlusal hypervigilance may be explained by the Generalized Hypervigilance Hypothesis, according to which hypervigilance is a “perceptual habit” that involves subjective amplification of a variety of aversive sensations, not just painful ones [60]. According to this hypothesis, if attention is habitually focused on sensations of a particular type, their amplification increases and became autonomous [61]. Moreover, it has also been reported that some myofascial pain patients have high levels of somatosensory amplification that is characterized by a general bodily hypervigilance to unpleasant sensations [62]. This hypothesis could explain why, during the orthodontic treatment process, which creates several occlusal interferences throughout a long period, some patients do not adapt to the occlusal changes. These individuals may develop TMD signs and symptoms, which are then misdiagnosed as being “caused” by the constantly changing occlusion during orthodontic treatment.

The general model for functional disorders in Fig. 4.1 can be applied more specifically to the orofacial region, as shown in Fig. 4.2; the lower section (in regular font) is modified after Ohrbach & McCall [63]. Indeed, an occlusal interference in individuals who exhibit somatic hypervigilance, symptom amplification, or catastrophizing can induce a muscle hyperactivity that becomes persistent, which in turn leads to additional hypervigilance, symptom amplification, or catastrophizing, and ultimately more distress. Health care utilization that results in a focus on structure as the source of the individual’s distress will reinforce the hypervigilance, symptom amplification, catastrophizing, and distress. Collectively, these processes can lead to TMD pain, masticatory dysfunction, or both. Different levels of these factors might account for the high interindividual variability in perception of pain or discomfort observed in orthodontic patients during treatment [64, 65]. As a consequence, orthodontic practitioners should be aware of the psychological characteristics of their patients, and they should try to recognize those individuals who may represent yellow flags for irreversible dental treatments. This is especially true when patients report having multiple bad experiences with previous orthodontists, but now they have selected you to solve their problems.