Masticatory muscle activity during maximum voluntary clench in different research diagnostic criteria for temporomandibular disorders (RDC/TMD) groups
From: Man Ther. 2008 Oct;13(5):434-40.
EMG can be used for a deeper understanding of the pathologies of several dysfunctional patients, for instance of those with temporomandibular joint disorders (TMJ). Tmj is a complex disease, and its nature has not been completely understood yet. A large part of tmj patients report pain in the masticatory muscles, and present symptoms and signs of muscular alteration. EMG assessment of their masticatory function is being used for diagnosis, to monitor the progression of the disease, and to measure the effect of treatment. In particular, when used as a diagnostic test to differentiate between patients with tmj and neck disorders, standardized EMG of the masticatory muscles was found to have a sensitivity of 0.86, with a 0.92 specificity.
Among the methods developed for the classification of patients with tmj, the research diagnostic criteria for temporomandibular disorders (research diagnostic criteria for temporomandibular disorders) had been proposed to produce reproducible case definitions, to investigate the time course of the disease, and to assess treatment efficacy. The research diagnostic criteria for temporomandibular disorders uses a two-axis system, taking into consideration physical diagnosis, pain-related disability, and psychological status. The physical findings (axis I) can be coordinated with the assessment of psychological distress and psychosocial dysfunctions associated with orofacial disability (axis II). Axis II considers the pain-related disability and the psychological status (depression, anxiety, vegetative symptoms), with subjective reports of pain intensity, activity limitations, and non-specific physical symptoms.
Patient assessment uses a history questionnaire (demographics, general health, specific orofacial pain and symptoms, non-specific health complaints); standard scales are used to estimate pain and orofacial disability. Patient examination comprises both metric (maximum mouth opening, lateral excursion and protrusion) and non-metric (temporomandibular joint sound, spontaneous and provoked orofacial pain and tenderness) items. According to axis I findings, three patient categories are obtained: muscle disorders; disc displacements; and arthralgia, arthritis and arthrosis. Some additional patients do not fall into these groups, and have prevalent axis II findings, where psychological distress mingles with more or less specific somatic symptoms.
A satisfactory between-examiners reliability has been reported for most research diagnostic criteria for temporomandibular disorders measurements, while poor reliability was found for non-metric assessments. Quantitative and objective evaluations are therefore needed also for tmj diagnosis.
In the present study, tmj patients were categorized according to the research diagnostic criteria for temporomandibular disorders, and the quantitative EMG characteristics of their masticatory muscles were analyzed. We wanted to see if patients in the different research diagnostic criteria for temporomandibular disorders groups had some objective differences in the EMG characteristics of their masticatory muscles during standardized teeth clenching. Patient data were also compared to those collected in control subjects without TMJ alterations.
Indeed, EMG is not universally considered a useful tool for tmj diagnosis: if well-standardized methods were not used, the problems in EMG reliability and validity hinder its clinical validity. For instance, technical artifacts (instrumental noise); differences due to facial type, age, sex, thickness of subcutaneous fat; cross talk from different muscles. Therefore, a correct EMG assessment should be performed only with standardized (normalized) potentials, thus removing most of biological and technical noise. In standardization recording, dental contact effect was excluded by making the subjects clench on two cotton rolls positioned on mandibular molars.
When well-standardized protocols are used, surface EMG of the head muscles has been reported to be an effective method for the functional assessment of the stomatognathic apparatus, with a good repeatability. Also, standardized EMG indices recorded in MVC had already been used as a diagnostic test to differentiate between patients with tmj and neck disorders.
In the present study, the quantitative EMG characteristics of the masticatory muscles of tmj patients during standardized teeth clenching were found (1) to differ from those recorded in healthy control subjects without TMJ alterations, and (2) to allow a differentiation among different diagnostic categories defined according to the research diagnostic criteria for temporomandibular disorders.
Three categories of tmj patients without mixed dysfunctions and neck disorders were analyzed; two of them were individualized according to RCD/tmj axis I, while the last one comprised patients who did not fall into any axis I diagnosis but presented prevalent symptoms and signs of research diagnostic criteria for temporomandibular disorders axis II.
The male:female ratio of the current study was approximately 1 man to 6–7 women. Indeed, tmj is more frequently found in women than in men. Nevertheless, the sex distribution between the four groups (three patient groups and the control group) did not significantly differ. Age was significantly different between groups (psycogenous patients significantly older than the single other patient subgroups and control subjects); the effect was statistically controlled using an analysis of covariance. Previous investigations found no significant effects of sex on normalized EMG indices; when the effect of age was ruled out, found that the differences in EMG indices remained highly significant between tmj patients and patients with neck pain.
Overall, during teeth clenching, normal subjects had the largest standardized muscular activity and the largest symmetry in their temporalis muscles. Myogenous patients were similar to the control subjects in both muscular activity and symmetry, while arthrogenous and psycogenous patients had significantly smaller values. Myogenous and arthrogenous patients were not subdivided into diagnostic subgroups; indeed the EMG indices were somewhat more homogenous in the patient groups than in the control subjects. The only exception was temporalis POC.
The EMG indices were chosen among those that, in a previous study, significantly discriminated between patients with tmj and neck disorders, but in the current experiment, only two of them gave significant differences. In the previous study, only a mean POC (masseter plus temporalis) was available, while, in the current study, both indices were calculated. Apparently, in differentiating among different diagnostic research diagnostic criteria for temporomandibular disorders categories, the temporalis muscle asymmetry is more useful than the masseter muscle asymmetry. The lack of significant differences in TC may arise from its somewhat reduced value in the present reference subjects, who were all dental patients. In contrast, the normal subjects all had healthy occlusal conditions.
Previous investigations found that the masticatory muscles of symptomatic tmj patients were less efficient and become more easily fatigued when compared to those of healthy subjects. Overall, the contraction of masticatory muscles elicited reduced electric potentials, the masticatory efficiency was lessened, and the maximum bite force was significantly reduced. In the current study, bite force was not measured, but EMG activity during MVC may be considered as a useful approximation, pointing to a reduced muscular force in two patient groups of three.
Apparently, in no previous investigation, the objective characteristics of masticatory muscles were compared among subgroups of patients categorized according to the research diagnostic criteria for temporomandibular disorders.
Previous studies based on questionnaires found limited differences among subgroups of tmj patients classified according to research diagnostic criteria for temporomandibular disorders axis I. The objective and quantitative evaluation of muscle function provided by EMG allows to circumvent some limitations of questionnaires based on self-report. Also, in the current study both diagnostic axes of research diagnostic criteria for temporomandibular disorders method were used for patient classification. Indeed, the use of physical characteristics only does not consider the effect that pain produces on the psychological status of the patient.
In both arthrogenous and psycogenous patients, a significant reduction in the standardized muscular activity was found. In particular, in psycogenous patients, MVC on cotton rolls (the standardization recording) was made with significantly larger EMG potentials than MVC in intercuspal position. Therefore, psycogenous patients had a functionally unstable occlusion. Clenching on the cotton rolls reduced the proprioceptive inputs from this unstable occlusion, and allowed the patients to contract more efficiently their masticatory muscles. Even if the actual role of occlusion in the development of signs and symptoms in patients with tmj is still controversial, in some patients altered occlusal conditions may be a factor in triggering abnormal muscular activity.
EMG indices could not differentiate between normal subjects and myogenous tmj patients. In myogenous patients, the proprioceptive inputs generated when the occlusal surfaces came into contact (MVC in intercuspal position) did not modify the EMG activity of masseter and temporalis muscles relative to the standardization recording (MVC on cotton rolls).
In conclusion, surface EMG of masticatory muscles allowed a fast and simple assessment of the functional and dysfunctional characteristics of the analyzed tmj patients, permitting an objective discrimination among different non-overlapping research diagnostic criteria for temporomandibular disorders subgroups. This evaluation could assist conventional clinical assessments. Nevertheless, the analyzed individuals represent a convenience sample, and the extrapolation of the present results to a wider population, as well as to different tmj diagnostic groups, should be done with caution.
