INTRODUCTION

Temporomandibular disorders (TMDs) are a heterogeneous group of conditions affecting the temporomandibular joint (TMJ) and/or the jaw muscles.1 These disorders have a multifactorial etiology, providing that a number of risk factors interact at the individual level and determine the onset of clinical signs and symptoms.2 Through a compelling insight on the mutual interactions of pain, bruxism, and psychosocial factors, the pathophysiology of TMDs may be further clarified.3 In general terms, and in line with other similar musculoskeletal disorders, the onset of signs and symptoms may be the result of an altered equilibrium between those factors that load the musculoskeletal system and others that influence the resistance to load.4

In spite of early suggestions,5 the role of dental malocclusion in the etiology of TMDs has been diminished over the years,6 also due to the wide use of dedicated approaches for the study of multifactorial biological models.7 In this regard, multiple variable models have shown that the various dental malocclusion features have much lower importance than believed in the past to explain, among the others, the presence of TMJ inflammatory-degenerative disorders,8 myofascial pain of jaw muscles,9 and pain in the TMJ.10

Notwithstanding that there are still some claims that malocclusion traits may be associated with the onset of temporomandibular disorders, there are still some issues to be clarified.11,12 In particular, it must be pointed out that the role of dental occlusion cannot be considered negligible because it may determine the patterns of load distribution on the temporomandibular joints, thus acting as a factor that could influence the overall resistance of the musculoskeletal system.

Thus, the potential role of malocclusion in TMD patients is still worthy of exploration. This is especially true if one considers the high prevalence of purported malocclusion traits,13,14 the clinical relevance of which should be assessed by evaluating their potential association with functional disturbances. For instance, focus should be put on the relationship between the various malocclusion features and positional abnormalities of the TMJ discs, as identified by joint click sounds, which are an early sign of altered TMJ biomechanics. Based on this premise, the present investigation was performed to assess the association of several dental malocclusion features with TMJ clicking in a population of temporomandibular disorder patients.

The primary hypothesis under testing was that dental malocclusion features are associated with TMJ clicking, irrespective of the joint side. The secondary hypothesis was that side-specific associations between clicking joints and dental malocclusion features exist.

MATERIALS AND METHODS

RESULTS

Prevalence of Occlusal Features in Patients With TMJ Clicking

The prevalence of the different occlusal features in the TMJ clicking group ranged between 52.9% in subjects with an anterior open bite to 65.3% in subjects with posterior crossbite, while prevalence of the various occlusal features in the no-TMJ clicking group ranged between 50.4% and 57.8% (Table 1). The differences between the groups were generally not statistically significant, with the exception of the RCP-MI slide ≥2 mm (P  =  .003), mediotrusive interferences (P  =  .027), and posterior crossbite (P  =  .035), the prevalence of which was greater in the TMJ clicking group than in the no-TMJ clicking group.

Table 1. Prevalence (as Percentage) of the Various Malocclusal Features in Each Group and Univariate Inferential Analyses^a

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Primary Hypothesis Testing

Mediotrusive interferences (P  =  .015) and RCP-MI slide ≥2 mm (P  =  .001) were the only two occlusal factors that were associated with TMJ clicking in the multiple logistic regression analysis. The adjusted ORs for the presence of TMJ clicking were generally low for both variables (1.63 and 1.89 for mediotrusive interferences and RCP-MI slide ≥2 mm, respectively). Moreover, the amount of variance in the prevalence of TMJ clicking that was predicted by the final model was as low as 4.5% (R²  =  0.045) (Table 2).

Table 2. Occlusal Features Remaining in the Final Multiple Logistic Regression Model Predicting TMJ Clicking (n  =  442)^a

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Secondary Hypothesis Testing

Regarding the association of the side-specific mediotrusive/laterotrusive interferences and RCP-MI slide with side-specific TMJ clicking, only laterotrusive interferences on the left side of the dental arch (P  =  .029) were significant predictors for TMJ clicking in the right side. The final regression model also included the variables RCP-MI slide ≥2 mm (P  =  .052) and laterotrusive interferences on the right side (P  =  .092), with a low correlation value for TMJ clicking (R²  =  .033). On the contrary, RCP-MI slide ≥2 mm was the only predictor for TMJ clicking in the left side (P  =  .006), with an R² value of 0.024 (Table 3).

Table 3. Malocclusion Features Remaining in the Final Multiple Logistic Regression Models Predicting Either Left or Right TMJ Clicking (n  =  442)^a

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DISCUSSION

Through the analysis of a large sample of TMD patients, the present study showed no clinically relevant associations between several malocclusion features and the presence of TMJ clicking.

Historically, dental occlusion was assigned a central role in the etiology and management of TMDs because dental professionals had achieved a better know-how and had seen more TMD patients than other professionals. Over the years, however, a growing body of evidence has been gathered in support of a diminished role of occlusal abnormalities and misalignments in the etiology of TMDs.21 In particular, findings from studies adopting multifactorial models of disease suggested that dental occlusion features are poorly associated with muscle and TMJ pain,7,9,10 thus confirming that other factors (ie, bruxism activities, psychosocial factors) are fundamental for pain symptoms to occur.22 On the other hand, there is some orthodontic and maxillofacial surgery literature suggesting the existence of a possible skeletal predisposition to TMJ disc displacement due to peculiar features of facial morphology.23,24 Thus, the possibility that certain occlusal features may be associated with an increased risk for disc displacement was worthy of investigation.

In the present investigation, the presence of a click sound as diagnosed according to the RDC/TMD,17 the strength of association between occlusal variables, and TMJ clicking in a population of TMD patients was low (Table 2). In particular, the amount of variance for TMJ clicking explained by the malocclusion features was very poor (4.5%) and suggestive of the fact that other factors need to be investigated to find more reliable predictors of disease-specific signs and symptoms. The only variables that were seen to be significantly associated with TMJ clicking in the multivariate models were the presence of mediotrusive interferences and a RCP-MI slide ≥2 mm. However, neither of these malocclusion features reached OR values for the presence of TMJ clicking that can be considered clinically relevant, ie, greater than 2, as suggested in previous publications.7

Notwithstanding that, it must be pointed out that both malocclusion features have a common characteristic, ie, they are a sign of occlusal instability, thus being potentially associated with orthopedic instability at the level of the TMJ. Some early literature studies suggested that this condition may be considered a risk factor for the development of an anterior TMJ disc displacement.25,26 Thus, the present findings of an association between dental instability and TMJ clicking, although with little clinical relevance (Table 2), may support at least in part the hypothesis that a stable occlusion is important to keep the physiologic relationship between joint structures. Further studies adopting magnetic resonance imaging (MRI) as the standard of reference to depict the TMJ disc status are needed to confirm these findings and the actual relationship with disc displacement. In particular, the clinical relevance of the presence of TMJ clicking in patient care should be further appraised in light of recent suggestions that disc displacements that are predicted by clinical RDC/TMD assessment show good to excellent agreement with MRI findings.27

Based on those findings, attempts to define the potential influence of side-specific interferences on side-specific TMJ clicking were also made in the present study. This was to answer the clinical research question, “Is side-specific (ie, right or left joint) TMJ clicking associated with a specific type of dental interference pattern (ie, RCP-MI slide, right or left mediotrusive/laterotrusive interferences)?” Actually, no relevant predictive model was identified. The TMJ clicking in the right side was associated with RCP-MI slide ≥2 mm and laterotrusive interferences on both left and right sides of the dental arch, while TMJ clicking in the left side was associated with RCP-MI slide ≥2 mm only.

In any case, the adjusted ORs of those factors for side-specific TMJ clicking were also below the threshold for clinical relevance, and the amount of variance for right or left click sounds explained by those occlusal features was very low (2.4%–3.3%, Table 3). Thus, the presence of those variables characterizing occlusal instability would be not enough to differentiate subjects with and without TMJ clicking in a population of TMD patients. This finding is in line with observations that factors other than occlusal/orthopedic instability must be considered for defining accurately the pathophysiology of TMJ click sounds and disc displacement.28 For instance, prolonged joint loading, especially in the form of jaw clenching, is a potential factor for overload of the joint structures, and its inclusion in anatomically-oriented regression models may help predict TMJ disorders with an increased accuracy.29

Taken together, findings from this investigation lend support to the literature suggesting that it is not possible to define predictable, clinically relevant models for TMDs that are based on the analysis of dental occlusion alone.6,21 It is likely that the few malocclusion features that were seen to be associated with TMDs, even if weakly, represent a small portion of the complex picture of factors that should be entered in a multifactorial model for disease.

CONCLUSIONS

Findings from the present investigation suggest that:

• In a population of TMD patients, the contribution of dental malocclusion features to predict TMJ clicking is minimal, with no clinical relevance.

• Based on these findings, orthodontic correction of dental malocclusion for managing TMJ click sounds potentially associated with disc displacement should not be performed.