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Effect of Occlusal Interference on Habitual Activity of Human Masseter

¹ Department of Dental and Maxillo-Facial Sciences, Section of Orthodontics and Clinical Gnathology, School of Dentistry, University of Naples "Federico II", Via Pansini, 5.I-80131, Italy; and
² Clinic for Masticatory Disorders and Complete Dentures, Center for Oral Medicine, Dental and Maxillo-Facial Surgery, University of Zürich, Switzerland;

Correspondence: ^* corresponding author, michelot{at}unina.it

	ABSTRACT

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It has been suggested that occlusal interference may increase habitual activity in the jaw muscles and may lead to temporomandibular disorders (TMD). We tested these hypotheses by means of a double-blind randomized crossover experiment carried out on 11 young healthy females. Strips of gold foil were glued either on a selected occlusal contact area (active interference) or on the vestibular surface of the same tooth (dummy interference) and left for 8 days each. Electromyographic masseter activity was recorded in the natural environment by portable recorders under interference-free, dummy-interference, and active-interference conditions. The active occlusal interference caused a significant reduction in the number of activity periods per hour and in their mean amplitude. The EMG activity did not change significantly during the dummy-interference condition. None of the subjects developed signs and/or symptoms of TMD throughout the whole study, and most of them adapted fairly well to the occlusal disturbance.

Key Words: electromyography • masseter muscle • occlusal interference • temporomandibular disorders

	INTRODUCTION

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Occlusal interference has been considered as a risk factor for temporomandibular disorders (TMD) (Kirveskari et al., 1992; Ash and Ramfjord, 1995; Dawson, 1998). The proposed causal chain of events suggests that interference triggers masticatory muscle hyperactivity and bruxism, which in turn may result in overload of the masticatory muscle, tenderness, pain, and temporomandibular joint clicking (Ramfjord, 1961). To test this hypothesis, previous investigators have studied the local and peripheral effects arising from the application of several kinds of experimental interference (Christensen and Rassouli, 1995a; Clark et al., 1999). Electromyographic (EMG) studies have shown that the application of occlusal interference may increase the activity of several jaw elevator muscles at rest (Riise and Sheikholeslam, 1982; Christensen and Rassouli, 1995b). Furthermore, these studies recorded the masticatory activity during a short time in an experimental setting. This was a major limitation, since laboratory conditions may influence the amount of muscle activity (Yemm, 1969; Katz et al., 1989). For a better understanding of an individual’s response to occlusal interference, it is necessary that one record long-term masticatory muscle activity in the habitual environment. This has been done in subjects during sleep (Rugh et al., 1984), but not in awake subjects.

The aim of this study was, therefore, to investigate the effects of the application of an acute alteration of occlusion (i.e., interference) on the habitual masseter activity assessed in the natural environment. The effects of the interference on signs and symptoms of TMD were also investigated.

	MATERIALS & METHODS

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Subjects
Female subjects were recruited from among medical first-year students. Of the 32 students who completed a questionnaire about their health conditions and common habits, 20 subjects were selected and underwent a preliminary oral clinical examination (AM) according to the Axis I and Axis II RDC/TMD (Dworkin and LeResche, 1992). Exclusion criteria were: any TMD diagnosis; depression and physical symptoms above the norm; pain in other parts of the body; inflammatory conditions; periodontal diseases; dental prostheses; occlusal wear (> 2 as defined by Clark et al., 1981); previous orthodontic treatment; self-report of clenching or bruxism; absence of one or more teeth, with the exception of third molars; single-contact balancing side and protrusive occlusal interference; slide from retruded contact position to intercuspal position greater than 2 mm; headaches and/or other neurological disorders; nail biting; smoking; habitual use of chewing-gum; and habitual drug intake.

Nine subjects were excluded from the study; therefore, the final sample included 11 females (mean age ± SD = 19.7 ± 1.1 yrs). The subjects signed an informed consent, received C 150 for participation, and were assured that they could leave the study at any time. The study protocol had been approved by the local ethics committee (#139/00).

Study Design
The study was carried out in a double-blind crossover design, all subjects serving as their own control (Fig. 1). Each subject went through 4 different conditions during a six-week period: interference-free condition before the interference application (IFC_before), dummy interference condition (DIC), active interference condition (AIC), and interference-free condition after interference removal (IFC_after). Each active and dummy interference was left in place for 8 days.

View larger version (20K): [in this window] [in a new window]   Figure 1. Description of the study design. IFC = interference-free condition; DIC = dummy interference condition; AIC = active interference condition.

Occlusal Interference
Dental impressions (Extrude XP and Wash, Kerr, Romulus, MI, USA) were poured with stone (Vel-Mix Stone type IV, Kerr, Scafati, Italia), and the casts were mounted in the intercuspal position in a semi-adjustable articulator (Panadent 1210; Panadent Co., Grand Terrace, CA, USA). Contact points between first molars were marked on the subject in the intercuspal position by means of marking paper (Accufilm II, Parkell, Farmingdale, NY, USA), and then reproduced on the casts by means of a pencil.

A strip of gold foil (length x width x height = 2 x 8 x 0.250 mm; weight = 0.05 g) was placed on the lower first molar of the preferred chewing side (nine right-sided and two left-sided subjects) on the occlusal contact and carefully adapted to the tooth anatomy on the dental cast. This active interference disturbed the intercuspal position, but did not create interference during lateral or protrusive mandibular excursions. To create the dummy interference, we adapted a second strip to the vestibular surface of the same tooth without interfering with the intercuspal position.

One of the authors (RM), using composite (Revolution, Kerr, Orange, CA, USA), then glued both kinds of interference, the active and the dummy, to the tooth at AIC_day-1 and DIC_day-1, respectively.

EMG Recordings
The activity of the masseter muscle ipsilateral to the interference side was recorded by means of a portable EMG recorder (Gallo and Palla, 1995), which also measured (every 5 min) the impedance between the electrodes to detect a possible disconnection, and recorded it in the data file. Two surface EMG electrodes (model 13L20, Dantec, Skovlunde, Denmark; 6 mm diameter) were attached, always by the same examiner who was blind to the interference condition (AV), over the skin of the masseter muscle on the preferred chewing side. They were aligned along the muscle fibers with 20 mm between them (Gallo et al., 1999). Electrode repositioning was achieved by means of a plastic template (Farella et al., 1999). Recording time was set between 10 a.m. and 7 p.m. Subjects were asked to eat between 1 and 2 p.m., as well as to avoid chewing gum, physical exercise, sleeping, and electromagnetic fields. The first 3 min of each recording included standard tasks: 3 maximal voluntary contractions (MVC) with 10-second intervals, 3 swallows, 10 pullings of the wire, and 10 self-touchings of the electrodes. Off-line analysis included calculation of the number per hour (N/hr), and net duration (Dur) and mean amplitude (A_mean) of activity periods (APs) (Gallo et al., 1999). The individual maximal voluntary contraction (MVC) was determined as the mean of 6 measurements obtained during the interference-free conditions (IFC_before, IFC_after). To define the threshold for activity periods, we analyzed the first 3 min of each EMG recording. Mean value (± SD) for simulated artifacts (i.e., touching and pulling of the electrodes) was 7.2 ± 3.5% MVC. Based on this result, the threshold was set at 10% MVC. Based on this threshold, the swallowing activity (22.2 ± 7.1% MVC) was not cut off.

Clinical Protocol
After 3 consecutive days of baseline EMG recordings (IFC_before), the active or dummy interference was applied, with each subject being assigned, by means of a balanced block randomization, to the AIC (n = 5) or DIC (n = 6), each with a duration of 8 days. The allocation sequence was prepared and sealed before enrollment and assignment (LMG). The masseter activity was recorded during days 1-2-3-5-8. After a one-week washout period, the interference was applied in reverse order. After the second washout period, EMG activity was monitored for 3 additional consecutive days, again with the teeth free of any foil strips (IFC_after). A total of 16 recording days was obtained from each subject (Fig. 1).

Pain, Occlusal Discomfort, TMD, and Occlusal Contacts
Muscle pain, headache, current stress level, and occlusal discomfort were assessed by means of 100-mm visual analog scales (VAS) (Michelotti et al., 2000). VAS ratings were collected at the beginning of each recording day. The numbers and distributions of occlusal contacts were assessed by means of plastic strips 0.05 mm thick (Model No. 691, Hawe Neos Dental, Bioggio, Switzerland). Tooth contact was defined as the subject’s ability, during firm biting, to maintain the strip between opposing teeth against a strong pull (Bakke et al., 1990).

TMD examinations were performed independently by two blind examiners (AM, MF) calibrated in TMD diagnosis at the following days: IFC_{before, day-1}, AIC_day-8, DIC_day-8, and IFC_{after, day-1}.

Occlusal contacts were assessed by one of the authors (RM) at IFC_{before, day-1}, at DIC_day-1, and at AIC_day-1 immediately after the application of the interference, at DIC_day-8 and AIC_day-8 before the removal of the interference, at DIC_day-8 and AIC_day-8 immediately after the removal of the interference, and at IFC_{after, day-1}.

Statistics
Normality of data was checked by the Kolmogorov-Smirnov test (K-S). We used analysis of variance (ANOVA) for repeated measurements with post hoc multiple comparisons to test whether the EMG variables varied under the different occlusal contact conditions, while we used the Friedman’s test to analyze whether this was the case for the VAS ratings, the tenderness scores, and occlusal contacts (SPSS 10.0, Chicago, IL, USA). The significance level was set at p < 0.05 (two-tailed). A power analysis for repeated-measurements ANOVA was carried out with the alpha error set at 0.05, the beta error at 0.2 (i.e., 80% power), and with a 15% change in EMG variables being considered clinically relevant. With ten subjects per group, the design achieved 76 to 88% power to test the factor ‘dental condition’.

	RESULTS

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The whole study was carried out over a six-month period (February to July), and no subject withdrew at any stage.

The number of activity periods per hour and their mean amplitude (K-S; p > 0.05) were significantly reduced (0.001 < p < 0.01) under the active-interference condition, whereas their mean duration (K-S; p > 0.05) did not vary significantly (p > 0.05) among the 3 different occlusal conditions (Table).

View larger version (25K): [in this window] [in a new window]   Figure 2. Number of events/hr (mean and standard error of the mean, N = 11) for conditions IFCbefore, DIC, AIC, and IFCafter. Student’s t test; * = p < 0.05; ** = p < 0.01.

View larger version (17K): [in this window] [in a new window]   Figure 3. Plots of N/hr for events with amplitudes 10–20, 20–30, and 30–40% MVC vs. event duration, for the conditions IFCbefore, DIC, AIC, and IFCafter. Data presented in the charts (dots = mean values, N = 11) were averaged across subjects, conditions, and recording days. Two-factor (dental condition, amplitude level) repeated-measurements analysis of variance was computed on the log values of areas under the curves (AUC) of each individual data point (Kolmogorov-Smirnov test; p > 0.05): The conditions differed with statistical significance (F = 12.1; p < 0.001), with lower values during AIC (Student-Newman-Keuls test; p < 0.001). At higher amplitude levels, a significant negative trend of AUC was found for the activity periods recorded during AIC (test for linear trend; p < 0.001). N/hr = number of activity periods per hour; MVC = maximal voluntary contraction; IFC = interference-free condition; DIC = dummy interference condition; AIC = active interference condition.

None of the subjects developed TMD across the study, and mean scores for temporomandibular joints (TMJs) and muscle tenderness (K-S; p < 0.05) did not change (p > 0.05) from baseline (mean ± SD; 0.1 ± 0.1 for TMJs and 0.3 ± 0.4 for muscle tenderness).

The number of occlusal contacts (K-S; p < 0.05) at baseline was 13.7 ± 2.4. It decreased significantly immediately after the application of the active interference (2.7 ± 2.8; p < 0.001) and increased gradually during AIC up to 4.4 ± 2.2, still being lower (p < 0.01) than the baseline value and remaining unchanged throughout DIC. At the end of the study, the number of occlusal contacts did not differ from baseline (13.0 ± 2.2; p > 0.05).

The amount of dental discomfort was almost negligible (mean ± SD; 0.5 ± 0.1 mm VAS) after the application of the dummy interference, but it increased significantly after the application of the active interference (55.6 ± 19.8 mm VAS; p < 0.001), fading during AIC up to 9.8 ± 4.6 mm VAS.

The subjects did not report any adverse events.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS & METHODS RESULTS DISCUSSION REFERENCES

 
The application of active occlusal interference in healthy females influenced the daily pattern of habitual activity of the masseter muscles. However, contrary to the ‘hyperactivity’ hypothesis (Ramfjord, 1961), the number of activity periods dropped in the first two days following the application of the interference, and increased gradually thereafter, up to the levels recorded during the non-interference conditions. The reduction in the number of activity periods was more pronounced at higher contraction levels (between 20 and 40% of MVC).

The increase in the numbers of activity periods after the third recording day paralleled the gradual decrease in the perception of occlusal discomfort. This might be due to the subjects’ adaptation to the occlusal disturbance and/or to the decrease of the height of the interference. The significant increase in the number of occlusal contacts from the application of the active interference to the time immediately before its removal seems to indicate a reduction in the height of the interference, either through an intrusion of the tooth pair with the interference, or through wear of the gold foil. The absence of shiny facets on the gold foil points toward the first hypothesis.

The most likely explanation for the reduction in the number and amplitude of activity periods, during the active-interference condition, is an avoidance behavior developed in response to occlusal discomfort. The observation that the high-level AP decreased more than the low-level ones may further corroborate this hypothesis. Indeed, APs with higher EMG levels should produce higher occlusal contact forces and, therefore, greater discomfort.

It may be that the reaction to an occlusal disturbance is different in subjects who are occlusally hypervigilant (Palla, 2001), or who have or have had a TMD history. Indeed, it has been reported that subjects without a TMD history adapted fairly well to experimental occlusal interference, whereas subjects with a TMD history showed a significant increase in clinical signs (Le Bell et al., 2002). Long-term EMG recordings in TMD patients might help to clarify this point.

In general, the coping strategy with a stressor is highly variable and depends on both environmental and individual factors (Ando, 2002). The females investigated in the present study had normal Axis II RDC/TMD profiles. Psychologically distressed women might react differently to the introduction of an occlusal disturbance.

It must be stressed that the EMG changes observed could not be related to an impairment of chewing function, because the subjects were asked not to chew outside a definite time lapse (i.e., 1 hr) that was discarded from the EMG analysis.

To our knowledge, this is the first long-term study investigating the effect of occlusal interference on the EMG activity of the masseter during awake subjects; therefore, our findings cannot be directly compared with those of previous studies. Nevertheless, analysis of our data is consistent with that obtained in bruxists during sleep, showing a decrease in EMG activity (Rugh et al., 1984). It must be mentioned, however, that the opposite has also been reported (Kobayashi, 1982).

A limitation of EMG recordings obtained by means of portable recorders is the occurrence of electrical movement artifacts. By setting the threshold at 10% MVC, we were able to decrease the number of these artifacts, which were supposed to be randomly distributed across the different conditions of the study, without adding systematic variations. The disadvantage of this approach, however, is that changes in very low-level EMG activity (below 10% MVC) could not be detected. This activity might play some role in the responses to experimental interference. Indeed, an increase in masseter and temporalis postural activity has been reported after the insertion of a 0.5-mm-thick interference in centric occlusion (Riise and Sheikholeslam, 1982).

Unlike other studies (Randow et al., 1976; Riise and Sheikholeslam, 1982, 1984; Magnusson and Enbom, 1984), none of the subjects investigated reported TMD symptoms, and in no case was an RDC/TMD diagnosis made throughout the study. Positive VAS ratings for headache occurred in only three subjects, and not during the AIC period. Methodological differences concerning the collection of TMD data, or the inclusion criteria, can account for these conflicting findings.

In conclusion, the introduction of experimental interference in the sample investigated reduced their masseter EMG habitual activity in the natural environment. None of the subjects developed any sign or symptom of TMD.

	ACKNOWLEDGMENTS

 
This study was supported by the fund MM06181419-PRIN 2000 from the Italian Ministry for University and Research. A special thanks to the Kerr Co. (Scafati, Italy) and Mr. Mauro Morino for providing some of the materials used in this research.

Received for publication June 29, 2004. Revision received February 20, 2005. Accepted for publication April 22, 2005.

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Journal of Dental Research, Vol. 84, No. 7, 644-648 (2005)
DOI: 10.1177/154405910508400712


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