The masseter muscle: the silent period and its clinical implications

Standardized solenoid chin taps were delivered downward and upward as parallel as possible to the fibers of the superficial part of the masseter muscle and backward at a right angle to this direction. Each one of the 10 healthy subjects received 10 taps in each direction during isometric masseter muscle activity (clenching the teeth). Taps in all three directions evoked the classic jaw jerk response, M1, a short latency (mean, 8.3 msec) excitation followed by an SP (mean latency, 15.9 msec; mean duration, 42.6 msec). The SP duration varied considerably not only between subjects but also within individuals. Forty-nine percent of the SPs were divided into two parts by an EMG peak, M2, with mean latency 42 msec and mean amplitude 78 microV. Sometimes this peak marked the end of the SP. The SP duration has been claimed to be of diagnostic significance because it is longer in patients with mandibular dysfunction. It is concluded that the large variations of the parameter found in this as well as in other studies make it impossible to establish criteria for a healthy or pathologic SP duration. A carefully obtained history and thorough clinical examination are probably more reliable means to establish a correct diagnosis.     

The silent period in the masseter and the anterior temporalis muscles in adult patients with mild or moderate mandibular dysfunction symptoms

The electromyographical silent period in the masseter and the anterior temporalis muscles during tooth tapping and jaw jerk were studied in patients with fairly mild temporomandibular joint dysfunction symptoms. The length of the silent periods in the patient group did not differ generally from that in a control group. During tooth tapping, however, patients with distinct muscular disorders had shorter silent period duration (7.7 ms) than patients with other symptoms or when compared with control subjects (10.5 and 11.3 ms, respectively). The duration returned to normal after correction of the muscular disorders. This finding suggests that the duration of the silent period is affected by the muscle condition. Patients with obvious muscular disorders of mild to moderate magnitude, thus, may show a shorter silent period duration during tooth tapping.     

A method for standardization of silent period measurements in human masseter muscle

The effect of bite force and stimulus intensity on the electrically induced SP has been studied. The results from the present study and earlier investigations suggest that the following guidelines should be used in future SP studies. (i) The electrical rather than mechanical way of stimulation should be preferred since the stimulus parameters and the receptors stimulated can easily be determined. (ii) The sensory perception threshold (T) of subjects should be used when studying and comparing the SP between individuals rather than the actual values of the electrical stimulation since the value of the sensory perception threshold varies from subject to subject. (iii) Rectified and averaged EMG records should be used rather than single raw traces since it is not possible to measure the duration of SPs from single traces. (iv) The subject should be asked to perform 25% of his/her MBF and the BF should be given as a feedback. (v) A stimulus intensity of about 6T should be used so that small changes in the BF would not affect the SPD. (vi) A jaw separation of about 10 mm should be selected in the subjects with normal overjet (2-3 mm), this being slightly thicker than the average isometric force transducer. The transducer can be brought to this thickness using acrylic resin. (vii) A head rest should be used to keep the head position constant (and hence constant jaw position with respect to the transducer) since the head position alone may change the reflexes elicited in the jaws region.     

The silent period in the masseter and anterior temporalis muscles in young adult subjects unaware of mandibular dysfunction symptoms

In fourteen individuals unaware of mandibular dysfunction symptoms, latency and duration of the silent period in the masseter and anterior temporalis muscles at tooth tapping and jaw jerk were reasonably reproducible after 5 weeks and 5 months. Although the subjects felt no discomfort in the stomatognathic system, in some of them mild to moderate muscle tenderness, TMJ sound and occlusal disharmony were found at clinical examination. The relationship between these symptoms and the silent period was analysed. Muscle tenderness was the only factor to affect the silent period. The subjects with muscle soreness had a shorter duration of the silent period than the subjects without such symptoms.     

An excitatory response preceding the silent period in masseter electromyographs

A masseteric excitatory reflex response preceding the silent period which appears following tapping movement of the jaw, was investigated in order to evaluate its origin. In the pre-anaesthetic state, the latency of response did not change with the intensity of tapping. However, its amplitude increased depending on the intensity of tapping. The response did not disappear even after the anaesthesia. After the anaesthesia the lighter the intensity of tapping was, the longer the latency of the response, coming up to that of the jaw-jerk reflex. The pre-anaesthetic response had an intricate wave form comparing with the post-anaesthetic one. From the above findings it was concluded that the response must be a complex one in nature originating in the muscle spindle of jaw closing muscles and in a certain receptor of the structures surrounding the tooth.     

The variability of EMG silent periods in TMJ patients

The variability of the EMG silent period in jaw closing muscles of TMJ dysfunction patients was examined. Considerable within-subject variability was found in the duration of the silent period. This variability increased with symptom severity. The authors recommend that multiple measurements be averaged to obtain a more accurate estimate of the silent period duration.     

TMD chronic pain and masseter silent period in psychiatric patients on antidepressive therapy

Summary  The aim of the study was to evaluate the long-term effects of antidepressive therapy on chronic pain and related disability, and masseter silent period in psychiatric depressive patients with temporomandibular disorders (TMD). The study included hospitalized psychiatric depressive patients on antidepressive therapy protocol (tetracyclic antidepressant-maprotiline and anxiolytic-diazepam) ( n  = 30) and non-psychiatric patients seeking prosthodontic treatment (control group, n  = 38). TMD were diagnosed by Research Diagnostic Criteria for temporomandibular disorders proposed by Dworkin and LeResche. The surface electromyography was recorded from left and right masseter muscles and masseter inhibitory reflex (masseter silent period) was recorded after mechanical stimulation. The incidence of TMD appearance was very similar, of aproximately 40% in both group of patients. The results of the study also indicated a higher prevalence of joint related TMD, a lower prevalence of muscular subtype of TMD and a lower grade of chronic pain and related disability in the psychiatric group of patients on antidepressive therapy in comparison with findings in the control group. In the patients on antidepressive therapy with TMD masseter silent period was not prolonged, while in the control group of patients with TMD the prolongation of the silent period was observed. The study provided evidence that long-term, combined therapy (maprotiline and diazepam) in psychiatric depressive patients significantly modulated signs and symptoms of TMD in comparison with the control group.     

The silent period in the masseter and the anterior temporal muscles at repeated registrations

Three dental nurses took part in an investigation of the reproducibility of the EMG silent period. The latency and duration of the silent period after tooth tapping and jaw jerk were found to be rather stable within the same day and between recordings every other day for 10 days. No differences in the SP variables between right and left side were found. Differences in the latency and duration of the silent period between the masseter and anterior temporal muscles during tooth tapping and jaw jerk were registered. The findings suggest functional difference between the two muscles.     

The comparative efficacy of relaxation training and masseter EMG feedback in the treatment of TMJ dysfunction

The present series of single case studies was designed to evaluate the comparative efficacy of masseter EMG feedback with progressive relaxation training in the treatment of TMJ dysfunction. Study 1 employed a multiple baseline design across the symptoms of jaw pain, jaw tension and shoulder pain in a patient with chronic TMJ problems. The results indicated that relaxation training was the treatment responsible for improvements in subjective ratings of jaw pain and tension, whereas masseter EMG feedback provided little additional benefit. Trapezius EMG feedback was necessary to provide improvements in ratings of shoulder pain. Study 2 consisted of four single case designs in which masseter EMG feedback and relaxation training were compared with four TMJ patients. Results indicated that relaxation produced clear improvements in pain and tension ratings for two patients, a combination of EMG feedback and relaxation training resulted in slight improvements in one patient, and neither treatment procedure was effective for the final patient. The possibility of wide-scale application of relaxation training with TMJ patients and the need for further controlled research of masseter EMG feedback were discussed.     

Influence of vertical dimension on masseter muscle electromyographic activity in patients with mandibular dysfunction

Occlusal splints were adjusted to different vertical heights and used to single out the influence of vertical dimension of occlusion in increments on BT-EMG activity of the masseter muscle in patients with mandibular dysfunction. The vertical dimension of least EMG activity was determined for each of 60 patients, who were randomly divided into three groups according to the vertical dimension at which the occlusal splint was adjusted: group No. 1, 1 mm from occlusal vertical dimension; group No. 2, mean vertical dimension, 4.25 mm; group No. 3, mean vertical dimension, 8.25 mm. Results showed a significant reduction of masseter BT-EMG activity (series A and B) at the end of the 3-week treatment period for patients in group Nos. 2 and 3 in comparison with group No. 1. Furthermore, the short-term use of occlusal splints with a vertical height that exceeded the so-called physiologic interocclusal distance did not result in an increase in masseter BT-EMG activity. This study suggests that an increase of vertical dimension of occlusion to or near the vertical dimension of least EMG activity by means of occlusal splints can be an effective way to obtain a reduction in masseteric muscle activity.     

Correlations of electromyographic silent period duration and the Helkimo dysfunction index.

Fifty-eight patients seen for consultation concerning mandibular dysfunction symptoms were examined utilizing the Helkimo index of dysfunction and electromyographic silent period recordings from masticatory muscles. The data collected were analyzed for correlations between the two parameters. Several significant correlations were noted between certain variables of the dysfunction index and the duration of the silent period. The mean duration of the silent period increased with increasing dysfunction index value. However, the clinical significance of this correlation is not established as long as we do not know the validity of neither the dysfunction index nor the the EMG silent period duration.