Characteristics of Grouped Discharge Waveforms Observed in Long-term Masseter Muscle Electromyographic Recording: A Preliminary Study.

We investigated the characteristics of grouped discharge (GD) waveforms obtained from long-term masseter electromyogram (EMG) recording in 6 female temporomandibular disorder (TMD) patients with myofascial pain and 6 healthy females. The EMG measurement was performed from the morning of the experiment day until the subject woke up the next day. We observed a significantly larger number of GD waveforms in the TMD group compared to the control group (p=0.002). Our results indicate that the existence of GD waveforms in masseter EMGs might be a predictor of future TMD with myofascial pain.

Assessment of MRI findings and clinical symptoms in patients with temporomandibular joint disorders.

OBJECTIVES: To investigate the correlations among various temporomandibular joint (TMJ) findings on MRI and the relationships between MRI findings and symptoms. METHODS: 425 patients (850 TMJs) with temporomandibular joint disorders (TMDs) who underwent MRI were enrolled. Oblique sagittal proton density-weighted and T2 weighted images in open- and closed-mouth positions were evaluated. MRI findings included disc configuration, disc position, condylar morphology, bone marrow pattern, and joint effusion. Symptoms included TMJ pain, TMJ noise, and limitation of mouth opening. For statistical analyses, Spearman's rank correlation coefficient and logistic regression analysis were applied. RESULTS: Folded disc, disc displacement without reduction (DDWOR), and osteophytes had significant negative correlations with other normal MRI findings (p < 0.01). DDWOR and marrow edema were associated with TMJ pain. Conversely, osteophytes [odds ratio (OR): 0.52; 95% CI (0.30-0.90)] and combination-type condylar degeneration [OR: 0.45; 95% CI (0.24-0.83)] were associated with decreased risk of TMJ pain. Condylar flattening was positively associated with TMJ noise [OR: 5.25; 95% CI (1.44-19.07)] and negatively associated with limitation of mouth opening [OR: 0.34; 95% CI (0.11-0.99)]. High-grade joint effusion was significantly associated with TMJ pain and noise. CONCLUSIONS: DDWOR and high-grade joint effusion (an indicator of inflammation in the articular cavity) were associated with TMD symptoms. This finding suggests that treatment strategy for DDWOR and decreasing inflammation might lessen clinical TMD symptoms. Condylar degeneration was not associated with indicators of inflammation or TMJ symptoms. These results suggest that patients with TMD symptoms should undergo initial MRI to allow rapid selection of appropriate therapies.

A new occlusal surface design for artificial posterior teeth to achieve high masticatory performance.

PURPOSE: The low masticatory efficiency of denture prostheses impairs the ability of wearers to consume high-fiber foods. Hence, artificial teeth with high masticatory efficiency are required. This study aimed to establish an occlusal surface design for posterior artificial teeth in denture prostheses that is compatible with the existing artificial teeth arrangement and that has high masticatory efficiency for the comminution of raw vegetables. METHODS: A masticatory simulator for occluding complete dentures was used to evaluate the masticatory efficiency of four occlusal surface designs, i.e., with parallel grooves occluding at right angles to the opposing teeth, groove depths of 1 and 0.5mm, and inter-groove distances of 1, 2, and 3mm. Raw carrots, rice, raw lettuce, chicken breasts, and peanuts were used as test foods to evaluate food comminution. RESULTS: Grooved occlusal surface designs with a 1-mm groove depth and a 2- or 3-mm inter-groove distance demonstrated significantly greater masticatory efficiency than the conventional occlusal form (p<0.05). CONCLUSIONS: The superiority of grooved designs over the conventional design was particularly evident for lettuce and raw carrots in this study, both of which are considered difficult foods to chew with complete dentures.

Ipsilateral Molar Clenching Induces Less Pain and Discomfort than Contralateral Molar Clenching in Patients with Unilateral Anterior Disc Displacement of the Temporomandibular Joint.

AIMS: To assess the influence of clenching side and location (ie, tooth) on the bite force necessary to induce pain or discomfort in patients with unilateral anterior disc displacement (ADD) of the temporomandibular joint (TMJ). METHODS: A total of 17 patients with unilateral ADD diagnosed clinically and with magnetic resonance imaging were included. A bite force meter was positioned at each of four experimental positions: the mandibular first premolars and first molars, bilaterally. Subjects were instructed to clench on the bite force meter until they felt pain or discomfort in the orofacial area and to report the location of the pain. Differences in pain location (ADD side versus contralateral side) and in the bite force thresholds for eliciting pain or discomfort were assessed by means of the paired t test and Wilcoxon signed-rank test. RESULTS: Only 18% of subjects (3 out of 17) reported pain in the TMJ with ADD during clenching at the ipsilateral molar, whereas 65% (11 out of 17) reported joint pain during clenching on the contralateral molar (P = .005). At the molars, the mean ± standard deviation bite force necessary to induce pain was 241.2 ± 135.5 N on the side with ADD and 160.9 ± 78.0 N on the contralateral side. The mean molar bite force necessary to induce pain on the side with ADD was significantly higher than that on the contralateral side (P = .002). CONCLUSION: Patients with unilateral ADD had fewer reports of TMJ pain and discomfort when they were molar clenching on the ipsilateral side compared to molar clenching on the contralateral side.

Specific diurnal EMG activity pattern observed in occlusal collapse patients: relationship between diurnal bruxism and tooth loss progression.

AIM: The role of parafunctional masticatory muscle activity in tooth loss has not been fully clarified. This study aimed to reveal the characteristic activity of masseter muscles in bite collapse patients while awake and asleep. MATERIALS AND METHODS: Six progressive bite collapse patients (PBC group), six age- and gender-matched control subjects (MC group), and six young control subjects (YC group) were enrolled. Electromyograms (EMG) of the masseter muscles were continuously recorded with an ambulatory EMG recorder while patients were awake and asleep. Diurnal and nocturnal parafunctional EMG activity was classified as phasic, tonic, or mixed using an EMG threshold of 20% maximal voluntary clenching. RESULTS: Highly extended diurnal phasic activity was observed only in the PBC group. The three groups had significantly different mean diurnal phasic episodes per hour, with 13.29±7.18 per hour in the PBC group, 0.95±0.97 per hour in the MC group, and 0.87±0.98 per hour in the YC group (p<0.01). ROC curve analysis suggested that the number of diurnal phasic episodes might be used to predict bite collapsing tooth loss. CONCLUSION: Extensive bite loss might be related to diurnal masticatory muscle parafunction but not to parafunction during sleep. CLINICAL RELEVANCE SCIENTIFIC RATIONALE FOR STUDY: Although mandibular parafunction has been implicated in stomatognathic system breakdown, a causal relationship has not been established because scientific modalities to evaluate parafunctional activity have been lacking. PRINCIPAL FINDINGS: This study used a newly developed EMG recording system that evaluates masseter muscle activity throughout the day. Our results challenge the stereotypical idea of nocturnal bruxism as a strong destructive force. We found that diurnal phasic masticatory muscle activity was most characteristic in patients with progressive bite collapse. PRACTICAL IMPLICATIONS: The incidence of diurnal phasic contractions could be used for the prognostic evaluation of stomatognathic system stability.

Dependence of psychophysical threshold on rate of applied force to the upper first molar in humans.

OBJECTIVE: The study aims to investigate the dynamic perception of a force applied to the upper first molar for different rates of force increase. DESIGN: Six volunteers (four male and two female; mean age, 27.2±2.4 years) with full natural dentition (except for the third molars) participated in this study. The psychophysical threshold for a force applied to the right maxillary first molar and the reaction time corresponding to each threshold were measured for rate of force increase of 103.74, 236.23, 354.58, 478.22 and 584.63 mNs(-1). The physical impulse, which is the integral of force over time, was calculated for each threshold. RESULTS: Psychophysical thresholds in the upper first molar increased with the rate of force increase. The reaction time corresponding to each threshold decreased with increasing force rate. Impulses corresponding to each threshold were independent of force rate. CONCLUSIONS: In the present study, the psychophysical threshold for a force applied to a molar tooth was shown to change depending on the rate of increase of the exerted force. From the viewpoint of the impulse, the dissipated energy necessary to reach the psychophysical sensation threshold was almost constant, regardless of the rate of force increase.

Effect of transient occlusal loading on the threshold of tooth tactile sensation perception for tapping like the impulsive stimulation.

The objectives of this study were (1) to establish a reliable method for detecting the force threshold of the tooth tactile sensation while avoiding experimenter bias and (2) to examine the effect of occlusal force loading on the threshold for impulsive force stimulation in subjects with normal dentition. Twenty volunteers participated in this study (10 males and 10 females; mean age, 26.6 ± 2.9 years). To simulate the bite force during occlusal tapping, a force-loading device was designed to exert impulsive force to the occlusal surface in the direction parallel to the tooth axis. The impulsive force detection threshold of the periodontal sensation was measured before and after loading 98 N of occlusal force on the left upper first molar for 1 min. Transient mechanical loading of the upper first molar caused an increase in the absolute threshold for impulsive force. This increase did not vanish immediately, and the increment of the threshold was maintained during the remainder of the experiment. A computer-controlled method for the evaluation of tooth tactile sensation using impulsive stimulation was established. Transient occlusal force loading parallel to the tooth axis increases the threshold of periodontal sensation for mechanical impulsive stimulation.

Mechanomyographic activity in the human lateral pterygoid muscle during mandibular movement.

The activity of the lateral pterygoid muscle has been regarded to be related to the pathological condition of the temporomandibular joint (TMJ) in the craniomandibular disorders. Because the lateral pterygoid muscle is a deep muscle, a needle electrode is necessary for EMG recordings. The purpose of this study was to establish a non-invasive method for the evaluation of muscle activity of the lateral pterygoid muscle using mechanomyogram (MMG). In three male subjects, surface electromyogram (EMG) in the left masseter muscle, left anterior and posterior belly of the temporal muscle, left anterior belly of the digastric muscle and needle EMG of the inferior head of the lateral pterygoid were recorded during mandibular movement tasks simultaneously with the MMG derived from a condenser microphone in the external ear canal. There were significant positive correlations between the needle EMG signal of the lateral pterygoid muscle and the MMG signal for the tasks of static jaw opened position of 30 mm of interincisal distance (p=0.000, R(2)=0.725), static jaw opened position of 40 mm of interincisal distance (p=0.000, R(2)=0.753), 5mm protruded mandibular position (p=0.000, R(2)=0.653), the most protruded mandibular position (p=0.000, R(2)=0803). On the contrary, for the task of maximal clenching, there was no significant correlation between the EMG signal of the lateral pterygoid muscle and the MMG signal. These results suggest that the activity of the lateral pterygoid muscle could be evaluated by the MMG signals recorded in the external ear canal, unless jaw closing major muscles show active contraction.

A custom-made sealing screw cap for a closed, hollow obturator.

This paper describes a closed, hollow obturator with a nondetachable screw cap to seal the access hole that also allows cleaning of the inside of the bulb. A small screw cap was fabricated from a cobalt-chromium alloy. This screw was screwed to the access hole prepared at the top of the hollow obturator and when removed allows the inside of the obturator to be cleaned through the access hole. At the end of the screw, a preformed cobalt-chromium wire was attached that works as the safety retention mechanism of the screw. This ensures the screw in not aspirated or swallowed if it becomes unscrewed and falls from the prosthesis during wear in the patient's mouth. This device enables the patient to clean the inside of the closed obturator at home.

The design of non-occlusal intraoral appliances on hard palate and their effect on masseter muscle activity during sleep.

This study aimed to reveal whether masseter muscle activity during sleep is affected by the difference in design of non-occlusal intraoral appliances on hard palate. Eight healthy Japanese participants were selected and wore each of the four types of appliances (horse shoe, thin, thick and medium thick) during sleep for one week with a one week interval without appliance during sleep. A masseter muscle electromyograph (EMG) was recorded during sleep. The EMG activities were analyzed by calculating the number of bursts per hour, episodes per hour, and bursts per episode. The EMG parameters with the thick type appliance were significantly lower than the baseline condition without appliance. In this study, it is suggested that a thick type appliance has an active effect on suppression of masseter muscle activity.

Effect of adhesive primer developed exclusively for heat-curing resin on adhesive strength between plastic artificial tooth and acrylic denture base resin.

Despite progress in the development of denture base resin and artificial tooth materials, dental clinics are still plagued with artificial teeth falling off the denture base--due to poor bond strength--after denture delivery. Against this background, this study sought to examine the effect and durability of an adhesive primer developed exclusively for heat-curing resin on the adhesive strength of heat-curing denture base acrylic resin to plastic artificial tooth. Test specimens were divided into four groups according to the treatment method of the artificial tooth's test bonding surface: air abrasion, adhesive primer application, adhesive primer application after air abrasion, and pretreatment only (control). After heat curing of acrylic resin onto the bonding surface, shear test was performed for two storage periods: 24-hour versus 100-day water storage. From the results obtained, it was revealed that the evaluated adhesive primer was significantly effective in increasing adhesive strength between artificial tooth and acrylic resin, although specimens were stored in water for 100 days.

A new method for evaluating the threshold of periodontal ligament mechanoreceptor by slow speed mechanical stimulation.

OBJECTIVES: This study aimed to establish a reliable method for detecting the threshold for perception of force applied to tooth and to report the basic properties of force threshold in the normal dentition subjects. BACKGROUND: The perception of mechanical stimulation exerted to tooth is formed by input from periodontal ligament mechanoreceptors and intradental mechanoreceptors. Periodontal ligament mechanoreceptors respond to a wide range speed of stimulus, whereas intradental mechanoreceptors are activated only by a rapid stimulation. Reliable properties of perception have not been reported because of the difficulties to regulate velocity and degree of the stimulus. METHODS: Eighteen healthy subjects were observed in this study (mean age: 27.2 +/- 5.7 years). A loading device to generate slow speed loading was fabricated and measured as follows: (i) the fluctuation of the force threshold within a day, (ii) day-to-day fluctuation, (iii) changes in the force threshold by a transient mechanical loading. RESULTS: In the normal dentition, it was observed that the fluctuation within a day and day to day of the pressure sense showed no significant differences; however, a transient mechanical loading caused the sensitivity of the periodontal ligament to decrease significantly. CONCLUSIONS: The device examining the force threshold resulted from periodontal ligament mechanoreceptors was useful in evaluating the sensitivity of the periodontal ligament.