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PubMed2006

A method for studying jaw muscle activity during standardized jaw movements under experimental jaw muscle pain.

This paper describes a method for studying superficial and deep jaw muscle activity during standardized jaw movements under experimental jaw muscle pain. In 22 healthy adults, pain was elicited in the right masseter muscle via tonic infusion of 4.5% hypertonic saline and which resulted in scores of 30-60 mm on a 100-mm visual analogue scale. Subjects performed tasks in five sessions in a repeated measures design, i.e., control 1, test 1 (during hypertonic or isotonic saline infusion), control 2 (without infusion), test 2 (during isotonic or hypertonic saline infusion), control 3 (without infusion). During each session, subjects performed maximal clenching and standardized jaw tasks, i.e., protrusion, lateral excursion, open/close, chewing. Mandibular movement was recorded with a 6-degree-of-freedom tracking system simultaneously with electromyographic (EMG) activity from the inferior head of the lateral pterygoid muscle with fine-wire electrodes (verified by computer tomography), and from posterior temporalis, the submandibular muscle group and bilateral masseter muscles with surface electrodes. EMG root mean square values were calculated at each 0.5 mm increment of mandibular incisor movement for all tasks under each experimental session. This establishes an experimental model for testing the effects of pain on jaw muscle activity where the jaw motor system is required to perform goal-directed tasks, and therefore should extend our understanding of the effects of pain on the jaw motor system.

Adult↗

PubMed1988

Masticatory muscles in the muscular dystrophic mouse. Aspects of the age-related progression of the disease.

Cross-sections of normal and dystrophic digastric and masseter muscles from 7- and 35- to 40-week-old mice were studied in the light microscope. Comparisons of mean cell size, cell size variance and number of centrally positioned nuclei in a given number of fibers were carried out. The masseter muscle seems at both ages to be far more affected by the disease than the digastric muscle. However, the progression of the disease from 7 to 40 weeks is more pronounced in the digastric muscle than in the masseter muscle.

Aging↗

PubMed2005

Jaw movement alters the reaction of human jaw muscles to incisor stimulation.

The changes in the minimum time to consciously react (reaction time) and the order of jaw muscle recruitment to precisely controlled axial stimulation of the incisors during controlled jaw movements are not known. To this end, ten subjects were recruited to investigate the reaction time of bilateral temporalis and masseter muscles and bite force. Stimuli were delivered axially to the upper central incisors during active jaw closing and opening, and under static conditions. The results showed that the reaction time was increased an average of 35% during both jaw opening and closing movements when compared with static jaw conditions. The left temporalis was recruited approximately 10 ms before the right temporalis, whereas no significant side differences were found between the masseter muscles. The masseter muscles were recruited an average of 20 ms before the temporalis muscles during jaw closing, but no difference existed during opening. Under static conditions the reaction time in the bite force was approximately 16 ms longer than the left temporalis, but was not significantly different from the reaction time of any of the other muscles, indicating that, under the static conditions tested, the left temporalis was more often responsible for initiation of the mechanical reactions in the jaw. Because of active compensation, no force measurements were made during jaw movement. This study is a prerequisite for investigations into the modulation of reflexes during jaw movement, because a response to a stimulus commencing after the minimum reaction time may not be entirely reflex in origin.

Adult↗

PubMed1983

Changes in electromyogram power spectra of facial and jaw-elevator muscles during fatigue.

Endurance and changes in electromyogram (EMG) power spectra were investigated during a fatiguing static contraction at 50% of the maximum EMG amplitude in two jaw-elevator muscles (masseter and temporalis) and five facial muscles (frontalis, corrugator supercilii, zygomaticus major, orbicularis oris, and buccinator). Relatively high endurance was found in orbicularis oris, frontalis, and corrugator supercilii muscles; intermediate endurance was found in zygomaticus major, buccinator, and temporalis muscles; and low endurance was found in the masseter muscle. The last muscle showed a relatively fast linear decrease of the median frequency of the power spectrum. The other muscles showed a much slower, exponential decrease. The median frequency appeared to reflect reliably the changes in the shape of the power spectra during fatigue. Large differences between the shape of power spectra of different muscles in the unfatigued state were found. These, however, were unrelated to endurance and degree of spectral shift during fatigue.

Adolescent↗

PubMed2001

Masticatory muscle function and transverse dentofacial growth.

Numerous experimental and clinical studies have shown an association between masticatory muscle function and craniofacial growth. The present series of studies focuses on the influence of masticatory muscle function on some basic mechanisms in craniofacial growth as well as on the effect of the functional capacity of the masseter muscle on the width of the maxillary dental arch in human subjects. The animal experimental model used to study the influence of masticatory muscle function was a non-invasive one that produced a decreased functional demand of the masticatory system in growing rats by feeding them a soft diet. The influence of reduced masticatory muscle function on the transversal dimensions of the premaxilla, maxilla (including the dental arch) and the calvaria was studied on dry skulls using a high precision non-contact measuring system. Moreover, a methodological study was carried out to study the influence of one preparation method on the dimensions of dry rat skulls, using axial contact radiographs. The effect of different functional demands on sutural dimensions and sutural bone apposition was studied on frontal undecalcified sections of the rat snout using automatic image analysis. For the quantification of sutural dimensions contact microradiographs were used, while for sutural bone apposition the bone level at certain time intervals was marked with calcein. In the clinical part of this thesis the relationship between maxillary dental arch width and masseter muscle thickness in humans was studied using ultrasonography. Masticatory muscle function was found to influence the transverse growth of the skull at areas under direct muscle influence as well as the dental arch width in regions with molars under eruption. The methodological study indicated that direct comparisons between measurements on fresh specimens and those on dry rat skulls are not permissible, since the dimensions of the dry rat skull are smaller than those of the original fresh specimens. The dimensions and morphology of the facial sutures as well as the sutural bone apposition were negatively affected by reduced masticatory function. The experimental findings of the present study might indicate some of the underlying mechanisms of the clinical findings, where subjects with thicker masseter muscles were found to have a broader maxillary dental arch.

Adolescent↗

PubMed1987

Cortically induced masticatory rhythm in masseter motoneurons after blocking inhibition by strychnine and tetanus toxin.

In the first series of experiments, we studied whether or not strychnine (STR)-resistant inhibition of masseter motoneurons (MASS . MNS) was involved in their rhythmical inhibition that occurs during masticatory activity, induced by repetitive stimulation of the cortical masticatory area (CMA) in the cat. After systemic STR injection, repetitive CMA stimulation induced rhythmically alternating activity in the masseteric and anterior digastric nerves with a shorter cycle time than before STR-administration. The short-latency IPSPS in the MASS . MNS evoked by single shocks applied to the CMA were abolished. In contrast, repetitive CMA stimulation still induced a rhythmical alternation of EPSPS and IPSPS in the MASS . MNS, although the IPSPS were significantly reduced in amplitude. In the second series, we attempted to abolish the STR-resistant component of the rhythmical IPSP with tetanus toxin (TT). This was injected into one superficial masseter muscle of the guinea pig. In the majority of animals, repetitive CMA stimulation induced a tonic EMG superimposed by rhythmical bursts in the TT-intoxicated masseter muscle. Repetitive CMA stimulation induced a rhythmical sequence of EPSPS and superimposed spikes in the MASS . MNS innervating the TT-intoxicated masseter muscle in paralyzed guinea pigs. It was concluded that: (1) the cortically-evoked short-latency inhibition of MASS . MNS is STR-sensitive, as is part of the rhythmical inhibition during CMA-induced mastication; and (2) rhythmical inhibition is not essential for the central generation of the rhythmical activity in the MASS . MNS.

Action Potentials↗

PubMed1990

Body posture and hand strength of patients with temporomandibular disorder.

The aim of this study was to observe the difference between patients of craniocervical muscle pain and nonpatients in head-neck posture, masticatory muscle activity, and the force exerted by the hand. Fifty-one patients and 28 nonpatients were observed. The electric activity of the masseter muscles was recorded when the subjects were doing pinching or grasping with the jaw in positions of rest, clenched, and clenched with gauze. Measurement of right and left tilting or extension and flexion of the head and neck was made from photographs of frontal and lateral views. It was found that the pinching and grasping force was much stronger in men than in women and in nonpatients than in patients with pain. The pinching and grasping force was more powerful with the teeth clenched. Clenching with gauze did not increase, but more often decreased the strength of the hand. The activity of the masseter muscle during clenching was about 10 to 26 times that of the resting activity. The activity decreased slightly when clenching with pinching or grasping. Patients were more likely to have a stretched neck with more extension of the head. Their masseter muscle activity and hand force were significantly weaker than that of the nonpatients.

Adult↗

PubMed1994

Muscle strength and mineral densities in the mandible.

Bone mineral density (BMD) in the femoral neck and lumbar spine was measured for 355 postmenopausal 48- to 56-year-old women and the BMD in five different regions in the mandible for 77. All 355 women were also classified according to the size of the masseter muscle. Both skeletal measures and the BMD of the buccal cortex distally from the foramen mentale were compared with the size of the masseter muscle. This study indicates that functional stress, caused by the masseter muscle, is involved in maintaining bone mineral density in edentulous regions of the mandible. Those individuals who are physically active or are bruxists may lose less mineral, after extractions of teeth, from those regions of the jaw bones where the muscles are attached.

Absorptiometry, Photon↗

PubMed1996

A chronic organized masseter abscess causing trismus resolved by hemi-masseter myotomy.

We present a 62-year-old patient with a chronic organized abscess of the right masseter muscle that developed following parotidectomy. This patient's chronic abscess originated from an infection in the masticator space from the second molar. Cases of repeated parotitis should lead one to suspect a masticator space infection. In this case, the cicatricical masseter muscle produced severe trismus. Total eradication of the abscess and myotomy of the masseter resolved the problem completely. We should pay attention to the dental condition in case of masticator space infection, especially in case of masseter muscle infection mimicking parotitis.

Abscess↗

PubMed2005

[Computed tomographic examination of muscle volume, cross-section and density in patients with dysgnathia].

PURPOSE: The individual jaw position is determined by the masticatory muscle among other factors. Before surgical treatment of malocclusions, thorough evaluation of the muscles is required to estimate the relapse risk. MATERIALS AND METHODS: By means of computer tomography, lateral radiographs of the skull and denture models, the relationships between morphological parameters of the masticatory muscles and the jaw bone were analyzed. Furthermore, possible causes for the extent of the malocclusion are described. RESULTS: A patient group with deep overbite was found to have significantly higher muscle densities (measured in Hounsfield units [HU]) in the medial pterygoideus muscle (59.89 +/- 3.91 HU to 48.94 +/- 4.14 HU, p < 0.01), masseter muscle, and genioglossus muscle (p < 0.05) in comparison to open bite patients. Significant differences of the muscle cross-section were measured in the masseter muscle between patients with retroclined maxillary incisors and with an open bite (5.4 +/- 0.7 cm (2) to 3.8 +/- 0.4 cm (2), p < 0.05). CONCLUSION: The results show a correlation between different jaw positions and masticatory muscles. They also suggest that the function of each muscle may be different. Additional examinations of the muscle structures are required for verification of the influence of the masticatory muscles on facial morphology.

Bone Density↗

PubMed1991

Development of masticatory muscles and oral behavior from suckling to chewing in dogs.

1. At the age of 20 postnatal days, more than a half of the dogs (13/20) showed incisor eruption. At the age of 25 postnatal days, half of the dogs (10/20) showed molar eruption. 2. At the age of 24 postnatal days, half of the dogs (10/20) started chewing behavior. 3. EMG amplitudes of temporal muscle were larger than that of masseter muscle until 26.5 days after birth, but they were reversed at 26.5 days after birth. 4. This suggests that in dogs the earlier teeth erupted, the earlier the dominant oral behavior was changed from suckling to mastication, and concomitantly the dominant muscle for the oral behavior was changed from the temporal to the masseter muscle.

Animals↗

PubMed1998

Experimental jaw-muscle pain does not change heteronymous H-reflexes in the human temporalis muscle.

Muscle pain generally has an inhibitory effect on voluntary orofacial motor function. However, it is not known whether muscle pain causes direct or indirect changes in motoneuron excitability. In this study a monopolar needle stimulation technique was used to evoke the direct motor response (M-response) in the left masseter muscle and the heteronymous H-reflex in the left temporalis muscle as an indirect measure of motoneuron excitability. Series of 20 repeated electrical stimuli were delivered at 50% of maximal voluntary contraction (MVC) before, during, and after periods with experimental jaw-muscle pain in 11 healthy subjects. Pain was induced by standardized infusion of hypertonic (5%) saline into the mid-portion of the right masseter muscle. The mean pain intensity rating on a 100-mm visual analog scale was 42+/-5 mm. The short-latency responses (less than 6 ms) could be evoked in all subjects. Analysis of the latency and amplitude of the temporal H-reflex indicated no significant effect of jaw-muscle pain. The amplitude of the masseteric M-response was significantly smaller in the postpain condition than in the pain conditions (ANOVA, P=0.018), but no differences were found between the prepain and postpain conditions. In nine subjects, poststimulus periods (mean offset latency, 69.6+/-8.6 ms) with significantly (more than 50%) suppressed EMG activity were detected in the ipsilateral masseter muscle following the M-response (mean offset latency, 5.5+/-0.2 ms). These reflex responses did not show a systematic change during the pain conditions. In conclusion, acute contralateral jaw-muscle pain does not seem to modulate the motoneuron excitability as measured by the heteronymous H-reflex.

Adult↗

PubMed2003

Activation of peripheral excitatory amino acid receptors decreases the duration of local anesthesia.

BACKGROUND: Postsurgical wound infiltration with the -methyl-d-aspartate receptor antagonist ketamine and bupivacaine can significantly prolong the duration of local anesthesia. One possible mechanism for this effect is that increased glutamate concentrations, caused by tissue damage, sensitize nociceptive primary afferent fibers through activation of peripheral excitatory amino acid receptors. METHODS: The effect of intramuscular injection of hyper-tonic glutamate (1,000 mm), dextrose (1,400 mm), glutamate (1,000 mm) with the broad spectrum excitatory amino acid receptor antagonist kynurenate (100 mm), or isotonic saline (155 mm) on the duration of masseter muscle afferent fiber blockade after lidocaine (37 mm [1%], 10 microl) infiltration, on muscle edema formation and on muscle blood flow was examined. RESULTS: Injection of either glutamate or dextrose significantly shortened the duration of lidocaine blocks compared with isotonic saline; however, block duration was significantly shorter after glutamate than after dextrose. Injection of glutamate, but not isotonic saline, dextrose, or glutamate with kynurenate, significantly decreased the mechanical threshold of muscle afferent fibers. Injection of glutamate, dextrose, or glutamate with kynurenate produced equivalent large, long-lasting (> 60 min) edemas with high initial peak extracellular water content. Peak extracellular water decreased more rapidly when kynurenate was coinjected with glutamate. Both glutamate and dextrose significantly increased muscle blood flow for 30 min after injection. Glutamate-induced increases in blood flow were attenuated by kynurenate. CONCLUSIONS: These results suggest that shortened lidocaine block durations observed after glutamate injection into the masseter muscle result from sensitization of afferent fibers as well as increases of peak extracellular water content and blood flow in masseter muscle. These effects of glutamate are mediated in part through activation of peripheral excitatory amino acid receptors.

Anesthetics, Local↗

PubMed1996

A method for the study of single jaw adductor muscle motor units in the rat.

Single motor units of the superficial masseter muscle were stimulated by impaling axons in the third division of the trigeminal nerve. Glass micropipette electrodes were driven into the nerve as it runs along the floor of the cranial cavity while passing small current pulses through the electrode to activate axons. Stable penetrations were maintained for as long as 45 min and twitch tensions were recorded for motor units that generated as little as 159 mg (1.6 mN) of force within a superficial masseter muscle capable of producing 20 g of twitch force. These are the first measurements of single motor-unit forces measured directly from the tendon of a whole jaw adductor muscle.

Action Potentials↗

PubMed2002

Modeling rabbit temporomandibular joint torques during a power stroke.

Little information exists regarding the effects of changes in mandibular form as a result of orthognathic surgery on torques produced about the temporomandibular joint (TMJ). In this study, we have modeled torques produced about the working side TMJ by selected compartments of the rabbit masseter muscle based on published electromyographic activity. The masseter muscle is composed of multiple subregions or compartments that have unique mechanical actions. In a previous study, forces were elicited by electrical stimulation of each compartment and were recorded by a multiaxis force transducer attached to the anterior mandible. Torques were calculated using mandibular lever arms measured from the center of the TMJ. We have extended this modeling to include variations in mandibular width, length, or height to determine the torques that would be generated with variations in mandibular form. Three superficial masseter compartments on the working side and one posterior deep compartment from the balancing side masseter were examined using data collected from a companion study. It was found that the working and balancing side compartments were synergists for pitch torque components but were antagonists for roll and yaw. In modeling an increase of each mandibular dimension by 20%, nonuniform changes in compartment-generated torques were found. The largest increase was found for the posterior superficial masseter yaw torque component. The effects of changing mandibular form on torques produced about the TMJ may be greater than predicted by previous models that assumed a single line of force produced by each jaw muscle.

Analysis of Variance↗

PubMed1985

Clinical, radiographic, and electromyographic study of patients with internal derangement of the temporomandibular joint.

Fifteen patients with internal derangement of the temporomandibular joint (TMJ) were examined clinically, radiographically, and electromyographically. Electromyographic recordings were also obtained from 11 subjects without signs or symptoms associated with their TMJs or masticatory musculature. All the patients with internal derangement demonstrated interferences on the ipsilateral side. This was interpreted as the result of disc displacement producing a reduced joint space and, consequently, a decreased vertical dimension on the symptomatic side. Slow opening and closing mandibular movements without clenching could be performed by healthy persons without noticeable EMG activity in the temporalis and masseter muscles. In association with disc displacement, electromyographic activity of the temporalis and masseter muscles occurred when the condyle slid over the posterior band of the disc and could be interpreted as an arthrokinetic reflex caused by distraction. Continuous muscle activity could be provoked by TMJ disc displacement and ceased when the disc position was normalized on mouth opening, only to occur again every time the disc became displaced on mouth closure. Anterior disc displacement without reduction (closed lock) could cause spastic activity in the temporalis muscle on the affected side. Spastic activity of the masseter and temporalis muscles occurring on the same side as a joint with anterior disc displacement hinders or inhibits the condylar movement necessary to achieve reduction.

Action Potentials↗

PubMed2005

Masseter myodegeneration as a cause of trismus or dysphagia in adult horses.

The medical records of eight horses with histological evidence of myodegeneration of the masseter muscles were examined. While they were alive their most common clinical signs had included difficulty in eating or opening their mouths, weight loss, difficulty in moving, and noticeable atrophy of the masseter muscles. The serum activities of muscle enzymes were abnormally high in all of the horses. Whole blood and/or liver selenium and vitamin E concentrations were less than the reference ranges in some of the horses. The lesions varied with the stage of the disease and consisted of swelling and discoloration, or muscle atrophy and fibrosis. Histologically, the muscle changes ranged from acute to subacute degeneration, with regenerative changes accompanying ongoing degeneration, to chronic degeneration with fibrotic replacement of muscle tissue. There were changes in the masseter muscle of all the horses, but some had widespread lesions in skeletal muscle, and a few also had myocardial lesions.

Animals↗

PubMed2003

Effects of interocclusal appliances on EMG activity during parafunctional tooth contact.

To test the hypothesis that a flat plane interocclusal appliance affects the electromyographic (EMG) activity of the temporalis and masseter muscles in pain-free individuals, maxillary splints were fabricated for 20 individuals who reported no history, signs or symptoms of myofascial pain or arthralgia as determined by two trained, independent examiners. Subjects were instructed to establish light tooth contact, maximum clenching, and moderate clenching with/without the splint in place (as determined by random assignment) while EMG data from the left and right temporalis and masseter muscles were recorded. A 5-min biofeedback training session to relax the masticatory muscles was followed by a repetition of the tooth contact/clenching tasks with/without the splint in place. With the splint in place, the activity of the temporalis muscles decreased for all tasks, significantly for the left and right temporalis under maximal clenching and for the right temporalis under moderate clenching. In contrast, the activity of the masseter muscles increased under light and moderate clenching (significantly for the left masseter under moderate clenching) and decreased slightly under maximal clenching. The effectiveness of interocclusal appliances may be due to mechanisms other than redistribution of adverse loading.

Bruxism↗