INTRODUCTION

Two main questions about temporomandibular disorders (TMD) in relation to malocclusion/orthodontic treatment seem to be of interest. The first concerns correlations between TMD and different kinds of functional or morphologic malocclusions. The other seeks to determine whether the severity and prevalence of TMD are influenced or even caused by orthodontic treatment.

The concept of TMD usually includes a wide variety of signs and symptoms, such as pain from the temporomandibular joints (TMJs) or jaw muscles, pain on mandibular movement, joint sounds, and locking/luxation of joints, as well as restricted mandibular movement. Often, TMD have been evaluated on the basis of variation in rather insignificant signs and symptoms, for instance, variation in muscle sites that are tender to palpation, without correlation to experienced problems.

The etiology of TMD is usually considered multifactorial. Untreated malocclusions, unstable occlusion, stress and other psychologic factors, trauma, individual predisposition, and structural conditions have been suggested as possible etiologic factors. The view of the influence of occlusion on the development of TMD has varied from none to considerable. There are still controversies concerning canine protection, lack of non–working side interferences on lateral mandibular movements, and the importance of establishing an ideal occlusion via orthodontic treatment. Does any particular concept of ideal occlusion still represent an ideal with regard to oral function, or is it time to re-evaluate and modify the norm?

The aim of this systematic literature review was to answer the following question: Are signs and symptoms of TMD related to malocclusion or orthodontic treatment?

MATERIALS AND METHODS

Methods for the Literature Review

This review was part of a project at the Swedish Council on Technology Assessment in Health Care, which focused on malocclusions and orthodontic treatment from a health perspective. As a first step, the literature was searched in the Medline and Cochrane Library databases from 1966 to May 2003. A later update was made in January 2005. Key words such as “experimental,” “animal,” “letters,” “editorials,” and “short communications” were excluded. Studies of cleft lip and palate, craniofacial syndromes, and treatment with orthognathic surgery were not included. Human studies in Danish, English, Finnish, Norwegian, or Swedish were considered. The search strategy is presented in Table 1. A review team of three persons made the searches interactively, with support from an expert in informatics.

Table 1. Search Strategy

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The reference lists and abstracts of the papers were examined by at least two members of the evaluation team. The full text of a paper was obtained if at least one of the reviewers thought that the paper addressed the issue in question. If obviously relevant literature was missing from the reference lists, further database searches were carried out using key words from known papers. Textbooks and reports from authorities as well as review papers could be used as background material, but not in the evaluation of systematic evidence.

All selected papers were read and evaluated independently by two persons according to a protocol that was previously designed by the whole group of eight reviewers. This protocol included evaluation of study design and study population, including presence of a control group; the variables used; the extent and character of dropouts; statistical analysis; and testing of validity and reliability. In the event of disagreement between the two reviewers, the article was discussed among the whole group to reach a consensus. If necessary, additional advice from, for instance, statisticians could be obtained. The evidence grade of the included studies was judged to be strong, moderately strong, or limited. It is important to note that “insufficient scientific knowledge” refers only to the question at hand and does not mean that other kinds of scientific knowledge are lacking.

The following criteria for inclusion of studies were used:

• Studies that addressed the questions of untreated malocclusions or orthodontic treatment in relation to signs and symptoms of TMD

• All kinds of clinical studies in humans, including qualitative research

• Papers written in English or Scandinavian languages

• Studies in children and adolescents; studies in adults were accepted when long-term follow-up of treatment and untreated malocclusions had been performed

RESULTS

The search resulted in 58 papers. Twenty-eight papers6–33 were excluded because they did not contribute to the evidence regarding the question at hand and are listed in Table 2. Twenty-one papers34–54 were found to contribute limited evidence (Table 3). Nine papers55–63 were judged to give moderately strong evidence (Table 4).

Table 2. Excluded Papers

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Table 3. Studies with Low Evidence Value

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Table 4. Studies of Associations Between TMD and Malocclusions^a

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The differences in TMD between those with and without malocclusion were small. Subjects with untreated crossbite, crowding of teeth/high PAR (Peer Assessment Rating) index value (overall severity of malocclusion), or large overjet showed a higher prevalence of signs and symptoms of TMD.57586162 Associations between malocclusions and TMD varied over time in some longitudinal studies and sometimes even disappeared.55–58 Different kinds of malocclusions were correlated to TMD in the studies mentioned above. Other studies failed to identify significant associations between malocclusion and TMD.55585963 For that reason, no conclusions could be drawn about associations between specific types of malocclusion and TMD. Studies that followed patients to about 30 years of age showed a general reduction in signs and symptoms of TMD and an overall low prevalence at that age.585962 The prevalence of clicking remained fairly unchanged, whereas prevalence of locking of the TMJ decreased. Especially when recordings related to muscle involvement such as tender muscles or reported muscle pain and headache were used, a considerable decrease was seen in prevalence during early adult life. Differences regarding TMD between those with and without malocclusion tended to diminish. Moreover, some studies that followed subjects from adolescence to adulthood observed large individual variations over time in prevalence of TMD.586162

In some studies that compared treated malocclusions with untreated controls, a slightly lower prevalence of TMD was found.5661 Differences were small, and the findings referred to different types of malocclusions. In other studies, no difference was found between subjects with treated versus untreated malocclusions.5863 The influence of different orthodontic treatment methods as well as differences between treatment and no treatment tended to vanish over time.5558

DISCUSSION

The restrictions imposed by the authors concerning the number of databases and languages in searching the literature may have yielded fewer articles than would a more comprehensive search. However, the strength of the evidence in a systematic review is probably more dependent on the quality of the included studies than on the degree of comprehensiveness.64 One of the major problems when interpreting associations between malocclusion and TMD is whether the recorded variables represent a severe enough condition to motivate patients to seek treatment for the functional problems. There seems to be a great disproportion between dysfunction that was recorded as “moderately severe” or “severe” and the estimated treatment need for TMD. The prevalence of moderately severe to severe TMD has been found to be about 20% to 30%, whereas the treatment need seems to be about 5%.5061 A lot of indirect measures, such as muscle palpation and painless clicking/popping from the TMJs, are used to identify TMD. There are many doubts about whether this gives a proper representation of the condition. Signs and symptoms are often dealt with separately, often with greater emphasis on the recorded signs than on reported symptoms.

Another difficulty when evaluating associations between malocclusions and TMD is the previously mentioned individual fluctuation of signs and symptoms of TMD when subjects are followed longitudinally.586162 Prevalence as well as severity may vary between recordings made on different occasions.

The studies that evaluated the effects of orthodontic treatment on TMD gave heterogeneous results, with some studies finding small positive effects in treated subjects and others finding no differences or differences between groups that tended to diminish with age.555661–63 None of the studies indicated that orthodontic treatment caused TMD.

In studies that included variables other than malocclusion, items such as psychologic health and muscular endurance showed associations with TMD that were at least as obvious as that between TMD and malocclusion.2862 Muscle strength and muscle endurance have previously been associated with craniofacial morphology. Vertically growing faces are often characterized by low muscle strength, whereas the opposite is true regarding subjects with parallel maxillary and mandibular planes.51 The etiologic importance of malocclusion on the development of TMD must be viewed with this in mind. Muscular resistance to overload may be more important than the presence of a certain malocclusion. It is interesting to note that deep bites tend to be more common among subjects without TMD.62

CONCLUSIONS

1. Associations between specific types of malocclusions and development of significant signs and symptoms of TMD could not be verified.

2. There is still a need for longitudinal studies.

3. Observations should be made before treatment as well as by long-term observation of untreated and treated subjects.

4. The differentiation of patients into control and study groups is a constant problem.

5. A proper observation period appears to be from the late teenage years to young adulthood (age about 30 years).

6. Individual variations in TMD over time must be controlled in the study design.

7. The main focus in the observations should be on TMD signs and symptoms of such a magnitude that treatment of TMD seems likely to be demanded.