Introduction

Studies of the relationship between features of dental occlusion and temporomandibular disorders (TMD) has been a topic in the dental literature since the early decades of past century [3]. On the one hand, the majority of investigations showed that such a relationship, if existing, is actually weak, thus, claiming for the need to search for other more relevant risk factors for TMD (e.g., psychosocial features, clenching-type bruxism, hormones, genetics) [14, 29, 31]. On the other hand, some professionals still advocate the need to assess dental occlusion of TMD patients in detail and correct it accordingly [28]. The latter viewpoint, despite not being supported by any recent literature reviews or consistently published data [21], is actually quite common among clinicians working in the field of restorative and functional dentistry.

This means that the paradigm shift from peripheral (i.e., occlusal, morphological) to central theories of TMD pathophysiology has not been fully accomplished and that further investigations are needed. In particular, possible areas of interest should be identified within common beliefs that have not actually been explored yet. For example, in the field of orthodontics and functional dentistry, the presence of asymmetry in dental occlusion has always been considered a condition needing treatment in order to restore physiological jaw function [1, 2]. Clinically, an easy-to-assess asymmetry is the presence of different Angle classes on the two interarch sides. In theory, such asymmetry could have potential consequences on both the frontal and sagittal occlusal plane and may be associated with functional demand for adaptation. In practice, it is surprising that the investigations on its relationship with temporomandibular joint (TMJ) disorders are actually scarce [4, 5, 9].

The relationship between occlusion and TMD has been explored as far as several single and multiple occlusal risk factors are concerned [11, 15, 23, 24], but there seems to be lack of data concerning Angle class dental asymmetry. Thus, the present investigation aims to answer the clinical research question of whether there is an association between the presence of asymmetric molar or canine Angle classes on both sides and the presence of TMD. Thus, the frequency of the various Angle classes in a population of TMD patients was compared in a cross-sectional investigation with a matched group of non-TMD individuals.

Materials and methods

Study sample and design

Participants in the study were two groups of age- and sex-matched individuals who were divided into either a study (TMD) or a control group. The study group comprised patients seeking TMD treatment at the Temporomandibular Disorders Clinic, while the control group included subjects seeking dental care at the School of Dentistry of the University of Padova, Italy and had no current or past pain at the TMJ and/or jaw muscles.

TMD patients (N = 90) were recruited consecutively and included in the study if they met the following criteria: age between 20 and 40 years; presence of an evaluable interarch canine and/or molar relationship; no history of orthodontic treatment; and absence of any past major (i.e., more extensive than single crowns) prosthetic treatments or occlusal rehabilitations (i.e., occlusal adjustments). The control group (N = 58) was recruited according to the same criteria. The presence of asymptomatic joint click sounds was not considered an exclusion criterion for the control group. As described in detail below, standardized guidelines were adopted to assess TMD patients as well as to screen controls for the absence of TMD signs and symptoms [26]. All participants were assessed by the same two trained examiners.

The study protocol was reviewed and approved by the Institutional Review Board of the University of Padova, and each participant gave written consent to take part to the investigation.

TMD assessment

In the TMD group, clinical assessment for TMD was performed according to the Diagnostic Criteria for Temporomandibular Disorders (RDC/TMD) guidelines [26] by the same two trained examiners, under the supervision of the leading investigator with expertise in TMD clinical assessment and research methodology [12, 16].

The assessment focused mainly on diagnosing the presence of joint pain (i.e., DC/TMD diagnosis of arthralgia), disc displacement (i.e., any DC/TMD diagnosis of disc displacement with or without reduction), arthrosis (i.e., DC/TMD diagnosis of temporomandibular joint [TMJ] osteoarthrosis, viz., and degenerative joint disease), and muscle pain (i.e., DC/TMD diagnosis of myofascial pain). Selected imaging techniques were prescribed, when needed, to support the clinical assessment.

Occlusal evaluation

All subjects underwent an assessment of dental occlusion, partly based on protocols adopted in previous studies [19]. The assessment focused on the assessment of bilateral canine and molar Angle class relationship. For statistical purposes, all cases of the so-called head-to-head canine or molar relationship have been considered as class II, and patients were thus grouped as follows, based on the presence of symmetrical or asymmetrical dental class on the two sides: absence of bilaterally evaluable class; presence of bilateral class I; presence of bilateral class II; presence of bilateral class III; presence of class I and class II on the two opposite sides; presence of class I and III on the two sides; presence of class II and III on the two sides. All the above groupings were performed separately for the canine and molar classes (e.g., individuals with molar class II may have canine class I in the case of ectopic first or second bicuspid).

Statistical analysis

Within the whole study sample, the prevalence of each of the above specific Angle class groups was compared between subjects with or without the different TMD diagnoses by means of the phi (ϕ) coefficient. The ϕ coefficient is a measure of the degree of association between two binary variables and is similar to the correlation coefficient in its interpretation. The values range from −1.0 to +1.0, indicating different levels of negative or positive correlation. As a general rule for correlation analyses, values higher than 0.7 are considered supportive of a strong positive correlation [20].

Results

Within the whole sample of 148 individuals, TMJ pain was diagnosed in 58, disk displacement in 96, arthrosis in 17, and muscle pain in 46 individuals. As for molar Angle class, it was symmetrical in 86 individuals (i.e., n = 44 class I, n = 22 class II, n = 20 class III) and asymmetrical in 62 subjects (i.e., n = 22 class I–II, n = 9 class I–III, n = 4 class II–III, n = 27 not evaluable class on one or both sides). Canine class was symmetrical in 106 subjects (i.e., n = 58 class I, n = 24 class II, n = 24 class III) and asymmetrical in 42 individuals (i.e., n = 17 class I–II, n = 13 class I–III, n = 7 class II–III, n = 5 not evaluable class on one or both sides).

Molar Angle class was not correlated with any TMD diagnoses (Table 1). In particular, symmetrical class I, II, or III was found in 32 of 58 subjects with TMJ pain, and in 54 of 90 subjects without TMJ pain (ϕ = 0.120; p = 0.906), while the prevalence was 56 of 96 in subjects with and 22 of 52 without disc displacement (ϕ = 0.216; p = 0.332), 8 of 17 in subjects with and 79 of 131 without arthrosis (ϕ = 0.217; p = 0.325), and 24 of 46 in subjects with and 62 of 102 without muscle pain (ϕ = 0.192; p = 0.484).

Tab. 1 Prevalence of the different combinations of molar Angle classes in the various diagnostic groups
Full size table

Canine Angle class was correlated with TMJ arthrosis (Table 2), with a 6 of 17 prevalence of symmetrical class in subjects with and 100 of 131 without arthrosis (ϕ = 0.389; p = 0.001). Correlation values with the other TMD diagnoses were low, and not significant: symmetrical class was found in 46 of 58 subjects with TMJ pain, and in 60 of 90 subjects without TMJ pain (ϕ = 0.178; p = 0.584), while the prevalence was 73 of 96 in subjects with and 33 of 52 without disc displacement (ϕ = 0.238; p = 0.212), and 33 of 46 in subjects with and 73 of 102 without muscle pain (ϕ = 0.165; p = 0.673).

Tab. 2 Prevalence of the different combinations of canine Angle classes in the various diagnostic groups
Full size table

Discussion

The role of dental occlusion as the main risk factor for disorders of the temporomandibular joint and jaw muscles has been progressively dismantled over the past decades [30], but it seems to be still alive in some clinical practitioners’ minds, as a quick look at dental websites or nonscientific journals can easily unveil. The reasons for this missed paradigm change from a dentally to a centrally oriented management of TMD and orofacial pain have been addressed several times [10, 13, 25]. In short, it may have to do with the unproven transfer of some prosthodontics and/or orthodontic dogmas into the TMD field, as if some occlusal treatment goals prosecuted by prosthodontists or orthodontists were fundamental to prevent damage to the TMJ [6].

In the literature, applying multiple variable models to the study of abnormalities of dental occlusion and TMD has repeatedly shown, at best, a weak association between the two conditions [31]. Notwithstanding that, it could be argued that data drawn from investigations adopting complex statistical approaches are not so easy to manage for the average practitioners. Thus, it was recently suggested that descriptive studies may be even more useful to provide a take-home message on the topic [17]. In the field of orthodontics, asymmetry of dental occlusion, grossly identified by the presence or absence of the same canine and molar Angle class on the two sides, has always been considered a condition requiring treatment [7, 22, 27, 32], but its actual correlation with disorders of the stomatognathic system has never been assessed.

Within these premises, this investigation had a simple design, focusing on the hypothesis that asymmetry of dental occlusion, viz., different Angle classes on the two sides, is correlated with TMD. In general, findings refute the study hypothesis, with a minor exception (i.e., asymmetric canine class and TMJ arthrosis). In short, the presence of the same Angle class on the left and right side has a similar prevalence than asymmetric class in both TMD cases and controls. Among the several possible combinations of Angle classes (i.e., bilateral class I, II, III, or asymmetrical presence of class I–II, I–III, II–III), there are no patterns that correlate with any of the TMD diagnoses under investigation, neither at the canine or the molar level. The only exception, viz., the correlation between the canine class and arthrosis, is likely due to the very small size of the arthrosis group. The significant correlation is indeed due to the arthrosis diagnosed in three of the only five subjects with a not evaluable canine class on one side due to teeth migration or rotation. Despite being potentially interesting, the functional meaning and clinical relevance of this finding has to be explored in future studies.

This study has several limitations, mainly related with the single variable analysis and the absence of any attempts to discriminate between dental- and skeletal-based asymmetries as well as with the absence of information on other relevant associated factors with TMD (e.g., psychological assessment). The single variable analysis might have been a limitation to the validity of findings in the case that some potentially relevant correlations were found, since further exploration of data via multiple variable approaches, which best depict biological models, should have been performed [23]. However, with the above minor exception, the data do not support the need for going deeper into their analysis, given the absence of correlations between dental asymmetries and TMD here described. On the other hand, the inclusion of unspecifically diagnosed asymmetries may be a shortcoming from an orthodontic perspective. While an evaluation of the patients’ skeletal features as well as an assessment of compensation factors associated with mandibular deviation and asymmetric Angle class should have added some important information for further discrimination between patients’ subgroups from an orthodontic perspective, it should also be pointed out that this study provides the first data on the topic and, as such, they could be used to refine methodological strategies in the near future.

To our knowledge, despite the many studies investigating the association of temporomandibular disorders with several purported abnormalities of dental occlusion (e.g., cross bite, open bite, deep bite, large anterior overjet, dental midline discrepancy, interferences during jaw movements, slides from centric relation to maximum intercuspation), the presence of asymmetries has never been assessed. The findings of this investigation must be interpreted with caution, but it could be suggested that, based on the weak, if existing, correlation with TMD, orthodontic indications for correcting dental asymmetries should not be justified with the need to either prevent or treat disorders of the TMJ and the jaw muscles. This is particularly important in the light of the number of papers focusing on the possible strategies, instead of the reasons, to correct mandibular asymmetries [8]. As already pointed out in the TMD and orofacial pain literature, risk factors for diseases are currently searched outside, rather than inside, the dental field (OPPERA summary). Within the occlusion–TMD field, important clinical implications have come from recent studies suggesting that the correction [18] and even the failed attempt of correction of malocclusions [16] is neutral as far as the effects on TMD is concerned. This means that, from an orthodontic perspective, studies on the prevalence of natural occlusion features in orthodontically untreated adult populations are fundamental to unveil the natural course of purported malocclusions and their association with the incidence of signs and symptoms of disease. Future studies on the topic are encouraged to adopt a careful approach to evaluate the pathology-related indications for orthodontic treatment.

Conclusions

The present investigation was designed to obtain greater understanding into the possible association between the presence of asymmetric molar or canine Angle classes on two sides and the presence of temporomandiblar disorders. Within the limits of this study, it can be suggested that such an association does not exist and that the role of dental asymmetries as a factor correlated with the presence of TMD signs and symptoms is minimal.