An analysis of exclusively representative population-based studies on adults has shown that only few and inconsistent associations could be detected between malocclusions and clinical signs of temporomandibular disorders (TMD)—and none for functional occlusion factors (occlusal interferences, non–working side contacts, etc). The aim of this study was to analyze associations between morphologic occlusion as well as factors of functional occlusion and subjectively perceived symptoms of TMD—again on the basis of the population-based Study of Health in Pomerania (SHIP), providing a sample of 4310 subjects (out of 7008 subjects yielding a response rate of 68.8%) aged 20 to 81 years, and other international representative studies from the systematic review. Besides occlusal factors also parafunctions and socioeconomic status (SES) were taken into account (including age and sex). Multiple logistic regression analysis was used—adjusted for SES. In this study, none of the occlusal factors were significantly associated with the indication of more frequent subjective TMD symptoms. However, the parafunction “frequent clenching” was connected with subjective TMD symptoms (odds ratio = 3.4). Compared with other population-based studies few and (across studies) inconsistent associations between malocclusions and subjective TMD symptoms could be ascertained. No significant associations of factors of functional occlusion with TMD symptoms were identifiable.
An analysis of exclusively representative population-based studies on adults has shown that only few and inconsistent associations could be detected between malocclusions and clinical signs of temporomandibular disorders (TMD).1,2 For functional occlusion factors (occlusal interferences, non–working side contacts, both with or without wear facets, etc), no associations were found. The aim of this study was to analyze associations between morphologic occlusion as well as factors of functional occlusion and subjectively perceived symptoms of TMD—again on the basis of exclusively population-based studies on adults. These were the representative Study of Health in Pomerania (SHIP) in Germany3,4 and other international representative studies from a systematic review on associations between occlusion and TMD.2 Besides occlusal factors, also age, sex, and socioeconomic status (SES) were taken into account.
MATERIALS AND METHODS
Sample and study area
A representative random sample of 7008 women and men (aged 20 to 79 years) was drawn from three district towns Greifswald, Stralsund, Anklam and 29 communities in the surrounding region, which is part of West Pomerania and is the most northeasterly region of Germany. West Pomerania is the northeastern part of Mecklenburg-West Pomerania, one of the 16 federal German states. The sample selection was done in two steps. First, of the three districts in the region, three cities (17,076 to 65,977 inhabitants) and 12 towns (1516 to 3044 inhabitants) were selected, and of the small towns (less than 1500 inhabitants), 17 out of 97 were drawn at random. Second, from each of the selected communities, subjects were drawn at random from official inhabitant data files proportional to the population size of each community and stratified by age and sex. From the entire population of 212,157 inhabitants, 7008 subjects were sampled, with 292 persons of each sex in each of the 12 five-year age strata. The examinations took place from October 1997 to May 2001. For a full description of the design of the study see John et al3 and for the dental part see Hensel et al.4
Subjective TMD symptoms
To obtain a subjective evaluation of dysfunction symptoms, each participant was interviewed concerning recent unilateral or bilateral symptoms of dysfunction, such as joint sounds like clicking or crepitation, pain in the temporomandibular joints, and pain in the facial muscles (each: sometimes, frequently, always), according to the TMD diagnosis guidelines of the Academy of Orofacial Pain.5 TMD symptoms and their prevalences are shown in Table 1. Together with prevalences of TMD signs, the prevalences of TMD symptoms have been described in detail elsewhere.6 The dependent variable was defined the way that subjects with one or more symptoms belonged to the TMD symptom population. The distribution of TMD symptoms is presented in Table 2.
Subjects with a defined loss of teeth (15+ teeth) were excluded from the analysis because it was not possible to adequately assess their morphologic occlusion (for full definition see Gesch et al1). Thirty-three variables of malocclusion and normal occlusion were clinically investigated (Table 3). An anatomically correct occlusion was considered as normal (ideal) occlusion. A deviation from normal occlusion was regarded as malocclusion.7
Functional occlusion factors
Fourteen factors of functional occlusion comprising occlusal contacts or interferences during mandibular movement and dental attrition were clinically examined (Table 3). Tooth contacts were analyzed by themselves and in combination with wear facets.8 This was done to see whether the person concerned “uses” the “interfering” occlusion (Table 3). Whether tooth contacts occurred on one or both lateral movements of the mandible as well as whether they occurred on one or both sides on protrusion were registered. Parafunctions like sometimes or frequent grinding or clenching were obtained by the dental part of the online interview (Table 3).
Sociodemographic parameters like age, sex, education, and professional training were determined from the general question section of the online interview. Socioeconomic items such as net income were registered by means of a self-administered questionnaire (Table 3, for definitions see Gesch et al1).
When subjects stated a history of “inflammatory joint diseases” (such as chronic polyarthritis) or “one or more accidents involving cranial injury in the past 12 months” in the medical questions section of the online interview, they were excluded (n = 316).
Clinical examinations were performed by eight calibrated examiners. Prebaseline orthodontic certification of the dentists was based on the examination of 30 pairs of dental casts showing complex symptoms of occlusal deviation that was repeated after several days. The findings recorded by the specialist were the gold standard. Intraexaminer and interexaminer agreement were tested by Cohen's ;gk.9
The results were within the range of “strong agreement” (;gk 0.66–0.81). Regarding examiner agreement for the clinical examinations, the examiners were compared for corresponding prevalences of the recorded malocclusions and the results presented in biannual Data Safety and Monitoring Committee reports. In case of distinct deviation, the examiner concerned was recalibrated. The dental interview was conducted by two trained dental assistants. With the informed consent of the subjects, the data input quality was tested by means of tape recordings and comparison of the acoustic and computerized data in a 10% sample of all interviews. In 18 controlled tape recordings, only one input error per dental assistant was found in 1217 single inputs per subject. For further description of quality management procedures see Hensel et al.4,10 All measurements and evaluations were performed blinded with respect to the identity of the study participants.
The data obtained were recorded online on a standardized data entry sheet and later statistically processed, edited, and analyzed at the Institute of Epidemiology and Social Medicine at the University of Greifswald, using the SAS System (SAS Institute Inc., Cary, NC). Statistical test results with P < .05 are considered statistically significant.
At stage 1, potential occlusal factors for TMD symptoms were screened with several multivariate logistic regression models (SAS PROC LOGISTIC) including age, sex, socioeconomic factors, and one of the occlusal factors in each model (thus further called “univariate models”). Also interactions of occlusal factors with sex were tested to detect possible risk differences between sexes. Results and prevalences of occlusal factors and other factors are shown in Table 3. At stage 2, all independent variables and interactions between sex and occlusal factors were simultaneously entered into a multivariate logistic regression model. With the stepwise backward method, first all nonsignificant interactions and afterward all nonsignificant single independent variables were eliminated (SAS SLSTAY = 0.05, SLENTRY = 0.20). The results were adjusted for sociodemographic and socioeconomic variables as in the univariate models. Intervariable correlations were examined for potential multicollinearity between variables.
In this area of research it has been recommended that, for an odds ratio (OR) to be relevant, it would need to be statistically significant and, for it to be clinically noticeable, it should represent a doubling (OR > 2)11,12 or halving (OR < 0.5) of risk, although prevalence of disease and base risk must be taken into account for a final decision of clinical relevance. An OR of less than 1 indicates that the presence of the factor is associated with reduced risk, an OR more 1 indicates increased risk.
Of the 7008 subjects initially sampled, 741 dropped out, either because they moved away (n = 615) or because of death (n = 126), reducing the sample size to 6267. A response rate of 68.2% for men and 69.4% for women, resulted in a sample of 4310 subjects. The overall response rate was 68.8% for the ages of 20 to 81 years and 71.3% for the age groups 20 to 74 years. An analysis of nonresponders found that the main reasons for nonparticipation were disinterest (39.7%), health problems (23%), adequate available medical care (11.6%), lack of time (16.7%), fear of examination results (3%), and others (6%).4 Twenty subjects were unable to undergo the oral examination. Finally, 4290 adults could get investigated anamnestically for symptoms of temporomandibular dysfunction and clinically for different types of malocclusions as well as for factors of functional occlusion. Sex and age distribution of the sample are given in Table 4.
Results of the univariate logistic regression analysis
The univariate analysis revealed that the dependent variable TMD symptoms were associated with one factor of morphologic occlusion (spacing) and parafunctions (clenching, grinding) but none of the factors of functional occlusion. It was not possible to calculate any ORs for unilateral or bilateral open bite above three mm because of the small number of observations (Table 3).
Interactions between sex and a single occlusal factor were not significant throughout, apart from unilateral non– working side interference and unilateral scissors-bite (total buccal crossbite), but in these cases the occlusal factor was not significant itself. An exception was the variable spacing (meaning primary spacing, not because of tooth loss).
Results of the multivariate regression analysis
Although grinding was significantly associated with TMD symptoms in the univariate logistic regressions (P < .001), it proved nonsignificant when considered simultaneously with the other variables of the multivariate logistic regression (Table 5). On the other hand, wear facets in dental restorations were not significant in the univariate, but in the multivariate logistic regression (OR = 0.7, Table 5; for a detailed description of the prevalences of wear facets see Bernhardt et al13). As in the univariate logistic regressions, the dependent variable TMD symptoms were significantly associated with spacing with an OR of 0.6 (prevalence 27.3%), with frequent clenching (OR = 3.4, 5.2%), and with the sex by spacing interaction. Only subjects who were aware of frequent clenching, more frequently stated one or more subjective TMD symptoms compared with those without this parafunction.
On the other hand, the occlusal factors spacing and wear facets in dental restorations were connected with less frequently perceived TMD symptoms. Normal occlusion was not significantly associated with TMD symptoms. The same was true for the sociodemographic and socioeconomic parameters. The ORs of none of the significant variables of the multivariate logistic regression analysis were between one and two. They were either below or, solely in the case of the nonocclusal variable frequent clenching, higher than two.
The only significant sex by spacing interaction showed a differential association between spacing and TMD symptoms for men and women. Women with this morphologic occlusion showed less frequent symptoms of TMD compared with those without spacing. All intervariable correlations between the occlusal variables of the final multivariate logistic models were <0.25 (Pearson), showing no potential redundancies.
In view of the random selection method and the response rate of 68.8%, this study can be considered representative of the adult population aged 20 to 81 years in the region under survey and of small and medium cities in a rural setting. It is thus also representative of the subjective functional status of the population's stomatognathic system as well as the malocclusions, factors of functional occlusion, parafunctions, and socioeconomic parameters. For limitations of the study see Gesch et al.1
Besides univariate logistic regressions, multivariate logistic regression methods were used to find out whether associations exist between malocclusions and functional occlusion factors and symptoms of TMD and how strong these are. Multivariate logistic models identifying subjects with one or more symptoms of TMD were generated incorporating just one statistically significant morphologic occlusion and one functional occlusion factor as well as one parafunction for the entire study population (Table 5). Containing all significant variables simultaneously in the multivariate model after backward selection, none of the occlusal factors (malocclusions, functional occlusion) under survey was—in sense of a risk marker—significantly associated with subjective TMD symptoms (Table 5). In fact, the only two significant occlusal factors, primary spacing (not caused by tooth loss) and facets in dental restorations, were even connected with few TMD symptoms. Only the parafunction frequent clenching was significantly and clinically relevant (more often) associated with TMD symptoms.
In a systematic review2 of exclusively population-based representative studies on associations between morphologic and functional occlusion and signs or symptoms of TMD in adults, we also found only few significant occlusal factors, although limitations of the reviewed studies should be considered.2
Concerning malocclusions, similar tendencies emerged from the review. Mohlin14 stated no associations between malocclusions and subjective TMD symptoms, except for a positive relation with the number of rotated posterior teeth. Szentpetery et al15 found no significant correlation between morphologic occlusion and the anamnestic dysfunction index.16 However, only bivariate correlations were calculated. Dworkin et al17 determined no statistically significant difference among TMD cases and controls with regard to the malocclusions under survey, except for anterior crossbite that, however, occurred more frequently in community controls. De Kanter18 stated that malocclusions (eg, overbite, overjet, Angle classification, crowding) were not directly statistically relevant related to TMD symptoms. Unfortunately, in all studies the strength was not given in form of an association (OR) or correlation. Only significance or P values were mentioned.
Normal occlusion was not significantly associated with symptoms of TMD compared with subjects with malocclusions. Normal occlusion thus occurred with similar frequency in subjects with and without TMD symptoms. A total of 2.6% of the study population exhibited this form of morphologic occlusion. The other population-based studies did not mention anything about a possible association between normal occlusion and TMD symptoms. A “protective” association of the kind frequently assumed in orthodontics was likewise not investigated or described and was equally not confirmed in our sample.
The nonexistence of associations between the variables of functional occlusion and subjective TMD symptoms in this study was also confirmed by other representative studies. In none of the studies of the systematic review2 were such associations described.14,15,18,19 Mohlin14 found no associations between functional occlusion and subjective TMD in 20- to 45-year-old women (men were not examined). Jenni et al19 showed no significant difference in the prevalence of occlusal interferences between amnestical dysfunction groups and subjectively symptom-free persons.16 De Kanter18 observed in his multivariate analysis that functional occlusion factors were not related to subjective temporomandibular dysfunctions.
In this study the (nonocclusal variable), frequent clenching was the only significant variable of all independent variables under survey (malocclusions, factors of functional occlusion, parafunctions, SES, Table 3) with an OR above one (OR = 3.4), thus showing that subjects with this factor significantly more frequently complained about symptoms of TMD than persons without this parafunction. In addition, it was the only variable that was associated with subjective TMD symptoms if one considers an OR of ≥2 being the threshold for clinical relevance as recommended.11,12 At 5.2% in this study, 222 subjects showed this kind of parafunction. Also, the representative studies of the review2 have examined parafunctions; however, only Szentpetery et al15 described correlations with the anamnestic dysfunction index according to that of Helkimo.16
Because associations between age as well as sex and TMD were described in other population-based studies,18 we adjusted the univariate and multivariate analyses for age and sex (as well as socioeconomic parameters). Also, in our sample6 nearly all prevalence figures for symptoms of TMD were higher for women than for men in all age groups, although not all significantly higher. Significant age differences existed for subjectively perceived joint sounds and pain in the facial muscles but not for subjective joint pain.6 Regarding sex and TMD symptoms, only the significant interaction between sex and spacing pointed to a different risk for men and women. Men did not show a significant association between spacing and TMD symptoms, but in women a protective association occurred. No sociodemographic or socioeconomic factor was significantly associated with temporomandibular dysfunction symptoms. Except for age and sex, the population-based studies mentioned, either did not investigate or did not mention any other sociodemographic or socioeconomic parameters.
For the variables under research here and in other population-based studies the following conclusions are drawn:
None of the occlusal factors under research, ie, none of the malocclusions and none of the functional occlusion factors, were significantly associated with the indication of more frequent subjective TMD symptoms.
The parafunction “frequent clenching” was significantly and clinically relevant connected with subjective TMD symptoms.
Only a small part of the variance of subjective symptoms of TMD could be explained by the numerous occlusal and nonocclusal variables investigated here. Additional occlusal and especially nonocclusal factors must play a role.
As compared with other representative population-based studies of the systematic review, only few and (across studies) inconsistent associations between malocclusions and subjective TMD symptoms could be ascertained. No significant associations of factors of functional occlusion with TMD symptoms were identifiable.
This work was part of the Community Medicine Research net (CMR) of the University of Greifswald, Germany, which was funded by the Federal Ministry of Education and Research (grant ZZ9603), the Ministry of Cultural Affairs, and the Ministry of Social Services of the Federal State of Mecklenburg-West Pomerania. The CMR coordinates several research projects, which share data of the population-based Study of Health in Pomerania (SHIP; http://www.medizin.uni-Greifswald.de/cm).
Corresponding author: Dr. Dietmar Gesch, Department of Orthodontics, Center of Oral Health, University of Greifswald, Rotgerberstraße 8, D-17487 Greifswald, Germany (firstname.lastname@example.org)