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PubMed1999

The mechanical advantage of the masseter muscle in subjects with different vertical and sagittal facial morphology.

OBJECTIVES: To study the relationships between the mechanical advantage of masseter muscle (the ratio between the muscular and the bite force moment arms) and two parameters describing vertical and sagittal facial morphology. DESIGN: Retrospective analysis of pre-treatment lateral cephalometric radiographs. SETTING AND SAMPLE POPULATION: The Laboratory of Functional Anatomy of the Stomatognathic Apparatus at Milan University, Italy. Four hundred and sixty-eight orthodontic patients (209 males, 259 females) aged between 6 and 50 years. MEASUREMENTS: Mechanical advantage of masseter muscle, posterior-to-anterior facial height ratio, corrected ANB* angle were measured in all cephalograms. RESULTS: Masseter muscle mechanical advantage was, on average, 79.5% (SD 6.1), ranging between 53 and 94%. No sex-related characteristics were found. A moderate effect of age was found (r = 0.19), with a certain increment in the mean mechanical advantage in adults. No relationship with the corrected ANB* angle was found (r = 0.099), while a significant effect of the posterior-to-anterior facial height ratio was detected (r = 0.542). CONCLUSION: The lack of correlation between the mechanical advantage of masseter muscle and corrected ANB* angle might be explained by: (1) when the occlusal point moves depending upon Angle skeletal classification, the ramus width might also modify, and both arms change; (2) the present assessment analysed only the lateral two-dimensional projection of a three-dimensional structure: the retroposition of occlusal point involves also a modification of line slope, and a subsequent reduction of its projection and arm.

Adolescent↗
PubMed1983

Normal variation in skin surface temperature over the temporomandibular joint and masseter muscle.

The range of the temperature at the skin surface overlying the temporomandibular joint (TMJ) and the origin of the superficial belly of the masseter muscle was investigated in 35 individuals with normal TM joints and 29 individuals with normal masseter muscles. The temperature was measured by a thermistor in contact with the skin. The temperature over the TMJ varied between 32.1 degrees C and 35.5 degrees C (average 34.3 degrees C). The difference in temperature between right and left sides was to an average 0.3 degrees C. The temperature over the masseter muscle ranged from 30.9 degrees C to 35.7 degrees C (average 33.2 degrees C). The difference in temperature between right and left sides was an average 0.4 degrees C. It was concluded that there is a fairly wide range of skin temperature over the normal TMJ and origin of the masseter muscle under the experimental conditions prevailing in this study but that the differences between right and left sides are fairly small. It therefore seems possible to use this thermometric method to assess inflammatory processes and/or changed metabolic activity in the TMJ and masticatory muscles.

Adolescent↗
PubMed1995

Inhibitory effect of sympathetic stimulation on activities of masseter muscle spindles and the jaw jerk reflex in rats.

1. To evaluate sympathetic effects on jaw muscles, the discharges of masseter muscle spindle afferents, jaw muscle electromyographic (EMG) activities and blood flow changes were compared in anaesthetized decerebrate rats before and during electrical stimulation of the cervical sympathetic trunk. 2. To eliminate the possibility of efferent control from the trigeminal motoneurones, muscle spindle activity was recorded from the cut peripheral end of the masseter nerve. The absence of a sympathetic component in the masseter nerve was confirmed by the horseradish peroxidase method. 3. Electrical stimulation of the sympathetic nerve at frequencies within the physiological range reduced muscle spindle afferent discharges evoked by passive jaw opening. 4. Sympathetic stimulation also reduced the EMG activity evoked by the jaw jerk reflex, which may reflect a sympathetic effect on spindle afferents. After cessation of stimulation, a transient increase in EMG activity was observed, which may be due to efferent supply from the trigeminal motoneurones. During rhythmical jaw movements, no sympathetic effect on EMG activity was detected. 5. The above sympathetic effect on muscle spindle afferents and EMG activity was independent of blood flow changes.

Action Potentials↗
PubMed1999

Masseter muscle activity during the whole day in children and young adults.

Changes in size and shape of the craniofacial skeleton during growth may be related to the masticatory muscle function in daily life. The purpose of this study was to measure the masseter muscle activity during the whole day in children and to investigate the differences between children and young adults. Fifteen children (7.8-13.0 years of age) and 30 young adults (20.3-34.7 years of age), who had acceptable occlusions without any remarkable skeletal discrepancy or temporomandibular disorder, were used as the subjects. In both children and young adults, most high-amplitude bursts of masseter muscle appeared mainly during mealtime, whereas a substantially larger number of low-amplitude ones were widely distributed throughout the whole day. The number and total duration of bursts of masseter muscle activity during the whole day was greater in children than in young adults, although significant differences were not found between the sexes. During daytime and sleep, both the number and total duration of bursts were greater in children. During mealtime, no significant differences in the number of bursts were found between children and young adults, however, the duration of bursts tended to be longer in children. It is concluded that the masseter muscle activity during the whole day is greater in children than in young adults.

Adolescent↗
PubMed2000

[Pseudoprognathism frequency and masseter muscle activity in operated cleft lip patients].

BACKGROUND: As a consequence of a growth inhibition of middle third of the face, cleft lip surgery could lead to a pseudoprognathism. However, this is not always the case. Also, when facial architecture is changed, the activity of muscles such as the masseter could be disturbed. AIM: To study masseter electromyographic activity in operated cleft lip patients. MATERIAL AND METHODS: Forty two patients with cleft lip (25 male, aged 13 to 35 years old) were studied. All were operated during their first year of life. An electromyography of masseter muscle was performed and from a teleradiography, ANB and Gonion angles were obtained. Forty healthy subjects conformed the control group. RESULTS: The sample was divided in two groups. A group of 20 patients had a significantly lower EMG voltage than control subjects (662 +/- 228 and 1511 +/- 340 microV respectively, p < 0.001). Only seven patients had an anteriorly inverted bite with a mean ANB of + 1.7 and a mean molar relation of -4.8, figures that confirm the presence of pseudoprognathism. CONCLUSIONS: Eighty three percent of cleft lip patients do not have cephalometric parameters of pseudoprognathism and half of the patients have a lower masseter activity.

Adolescent↗
PubMed1983

Masseter muscle fatigue in man objectively quantified by analysis of myoelectric signals.

Localized muscle fatigue in the masseter muscle was studied with a method based on power spectrum analysis of myoelectric signals. Under the influence of fatiguing contractions, a gradual shift of the spectral curve occurred; the rate of change was taken as a measure of the development of fatigue. The fatigue was dependent on the bite force. The existence of a threshold value of force, below which significant myoelectric fatigue changes do not develop, was shown.

Adult↗
PubMed1993

Sexual dimorphism in the trigeminal motor neurons innervating the mouse masseter muscle.

The extrafusal and intrafusal muscle fibers in the masseter muscle are innervated by motoneurons in the trigeminal motor nucleus (Mo 5). In the present study, we found that the number of trigeminal motor neurons in the male mouse was significantly larger than that in the female. However, we could not detect any significant difference between male and female mice in the number of sensory neurons in the trigeminal mesencephalic nucleus (Me 5). This is the first report on sexual dimorphism in masticatory motoneurons of mammals.

Animals↗
PubMed2002

Non-nociceptive upper limb afferents modulate masseter muscle EMG activity in man.

Recent electrophysiological data obtained in anaesthetized rats evidenced jaw muscle excitatory responses to the electrical stimulation of type II limb somatosensory afferents. In the present work, we describe an inhibitory reflex evoked in human masseter muscles by stimulation of non-nociceptive fibres travelling in the median and radial nerves (MED and RAD, respectively). Eighteen healthy volunteers participated in the study. Subjects were seated on a comfortable chair, with the complex head-mandible-neck-trunk and the limbs securely fixed to the chair. Attempts were made to minimize possible interferences due to the activation of afferents other than the stimulated ones. The subjects were instructed to contract masseter muscles at a submaximal level and to maintain a stable level of muscle contraction during all trials. EMG voluntary activity was recorded from both masseter muscles by means of coaxial needle electrodes before and after the electrical stimulation of MED and/or RAD at intensities below pain threshold. In all subjects, MED stimulation induced bilaterally a marked depression of masseter EMG activity, which occurred at a latency of 23.6 +/- 2.1 ms and lasted 27.8 +/- 6.6 ms. RAD stimulation also induced a marked reduction in masseter EMG activity, but this effect was clearly observed in 9 out of 18 subjects, and it showed latency (30.2 +/- 7.5 ms) and duration (44.9 +/- 5.4 ms) significantly longer in comparison with the MED-induced effect. All subjects exhibited the inhibitory period in masseter EMG following the simultaneous stimulation of both nerves; this one appeared at a latency not significantly different (25.3 +/- 5.9 ms) and lasted much more (37.4 +/ - 8.2 ms) than EMG depression evoked by MED stimulation. The duration of masseter muscle inhibition, induced by MED and/or RAD stimulation, was inversely related to the level of EMG activity, while latency was not related to it. Significant increases in the inhibitory period duration were also observed by increasing stimulus intensity, within a subthreshold range for the activation of nociceptive fibres. In all cases, the inhibitory period was followed by a later excitatory rebound activity, whose latency and duration depended on the duration of the preceding EMG inhibition and on the background level of masseter activation. In conclusion, results evidenced that the activation of arm somatosensory fibres modulates masseter muscle activity in normal man. This might lead to a coordination between limb and masticatory muscle activity, which is required in several complex motor acts.

Action Potentials↗
PubMed2005

Morphologic characteristics of masseter muscle in cleidocranial dysplasia: a report of 3 cases.

Cleidocranial dysplasia (CCD) is a rare autosomal dominant skeletal dysplasia. In order to define the morphologic characteristics of the masseter muscle incidental to bone abnormalities, we present 3 cases of CCD with the masseter muscle thickness and maxillofacial bone abnormalities, using computed tomography (CT) and panoramic radiographs. In CCD patients (a) the masseter muscles were less thick than in age- and sex-matched control subjects, (b) the zygomatic arch was discontinuous with the hypoplastic zygomatic bone, (c) the ascending ramus of the mandible had parallel-sided borders, and (d) the coronoid process pointed upwards and/or posteriorly. We have concluded the masseter muscles are less thick than normal, alongside the maxillofacial bone abnormalities in CCD patients.

Adult↗
PubMed1992

Histochemical characterization of masseter muscle fibres in a biopsy study of normal young women.

Muscle fibres from biopsies of the anterior part of the masseter muscle (pars superficialis) were histochemically characterized in 13 healthy female students. They were 21-28 yr old with a full complement of teeth, and normal facial and occlusal relations. Before surgery, normal masseter muscle function was demonstrated by bite-force measurements and recordings of electromyographic activity. After staining for myosin ATPase activity, the relative mean areas of muscle fibres were: type I 82.9%, type IM 9.5% and type II 8.3%. The intraindividual (18-155% of mean) and interindividual (0-216% of mean) variation of the fibre size was large. The type I fibres had the largest diameter (10-80 microns, mean: 39 microns), the type II fibres (6-47 microns, mean: 21 microns) and the IM fibres (8-54 microns, mean: 28 microns) the smallest. The biopsy technique and the histochemical characterization will be suitable for reference in women with functional changes or diseases of the masseter muscle.

Adult↗
PubMed1983

Microprobe analysis of human masseter muscle biopsies.

Biopsies of the human masseter muscle were made using an intraoral approach. Sections were cut in a cryostat and freeze-dried. Using the myosin ATPase method at pH 9.3 the recognized heterogeneity of the muscle with respect to type I and type II fibers was confirmed. Sections were examined in an SEM by the method of energy dispersive X-ray analysis using a computer assisted semi-quantitative method (bulk analysis-thick sections) to determine the distribution of Na, P, S, Cl and K and their relative masses. Element concentrations in descending order were K, S, P, Na and Cl. Some element ratios were compared with results in the literature for other muscle (biochemical and microprobe analysis) and found to be generally in reasonable agreement. The results form a basis for the study of a prominent and accessible masticatory muscle in altered states.

Adult↗
PubMed1995

Craniomandibular disorders, edentulousness and the clinically estimated size of the masseter muscle.

A total of 355 postmenopausal women were classified according to the degree of edentulousness, and anamnestic information on masticatory dysfunction symptoms was recorded. Five masticatory muscles were palpated, and the subjects were classified according to size of the masseter muscle. This simple classification for the clinical inspection of the masseter muscle was tested. In agreement with previous reports, decreasing functional exercise of the jaws was found to diminish the size of the masticatory muscles making them more disposed to dysfunction symptoms. The three-level classification of size of the masseter muscle is useful in normal clinical practice for determining the condition of this muscle.

Chi-Square Distribution↗
PubMed1998

Body position effects on EMG activity of sternocleidomastoid and masseter muscles in healthy subjects.

This study was conducted in order to determine the effects of body position on integrated electromyographic (IEMG) activity of sternocleidomastoid and masseter muscles in 20 healthy subjects. EMG recordings at rest and during swallowing of saliva and maximal voluntary clenching were performed by placing surface electrodes on the sternocleidomastoid and masseter muscles (contralateral to the habitual side of sleeping of each subject), in the following body positions: standing, seated, supine, and lateral decubitus position. Significant higher EMG activities were recorded in the sternocleidomastoid muscle in the lateral decubitus position, whereas significant lower EMG activities were recorded in the masseter muscle in the supine position. This finding supports the idea that there may exist a differential modulation of the motor neuron pools of the sternocleidomastoid and masseter muscles of peripheral and/or central origin. Significant differences in the EMG pattern as well as in the levels of EMG activities upon variations in body positions were observed between healthy subjects and patients with myogenic craniomandibular dysfunction reported by Palazzi, et al.

Adolescent↗
PubMed1994

Relationship between attachment of the superficial masseter muscle and craniofacial morphology in dentate and edentulous humans.

Previous studies have described the importance of the interaction between the masticatory muscles and the craniofacial skeleton in the control of craniofacial growth. This study describes the attachment and orientation of the superficial masseter muscle and its relationship with craniofacial morphology in dentate and edentulous subjects. Data were obtained from lateral cephalometric radiographs of a total of 31 cadavers in which the superficial masseter muscle had been defined with liquid barium. The results provide evidence that the morphology of the superficial masseter muscle in the gonion region differed significantly between dentate and edentulous subjects, with the masseter being 2.7 mm wider and the gonion-anterior muscle border distance being 4.0 mm greater in dentate subjects. The complex relationship between craniofacial morphology and the dimensions and inclination of the superficial masseter muscle were most clearly evident in dentate subjects where the position of the anterior border was related to ramus dimensions and mandibular and occlusal plane angles. The association is much less clear in edentulous subjects where normal function was disturbed. In general, age was not a significant determinant of variation in superficial masseter muscle dimensions and orientation.

Age Factors↗
PubMed2005

Transneuronal tracing of vestibulo-trigeminal pathways innervating the masseter muscle in the rat.

Previous studies reported that the activity of trigeminal motoneurons innervating masseter muscles is modulated by vestibular inputs. We performed the present study to provide an anatomical substrate for these physiological observations. The transynaptic retrograde tracer pseudorabies virus-Bartha was injected into multiple sites of the lower third of the superficial layer of the masseter muscle in rats, a subset of which underwent a sympathectomy prior to virus injections, and the animals were euthanized 24-120 h later. Labeled masseteric motoneurons were first found in the ipsilateral trigeminal motor nucleus following a 24-h postinoculation period; subsequent to 72-h survival times, the number of infected motoneurons increased, and at > or =96 h many of these cells showed signs of cytopathic changes. Following 72-h survival times, a few transynaptically labeled neurons appeared bilaterally in the medial vestibular nucleus (MVe) and the caudal prepositus hypoglossi (PH) and in the ipsilateral spinal vestibular nucleus (SpVe). At survival times of 96-120 h, labeled neurons were consistently observed bilaterally in all vestibular nuclei (VN), although the highest concentration of infected cells was located in the caudal part of the MVe, the SpVe, and the caudal portion of PH. The distribution and density of labeling in the VN and PH were similar in sympathectomized and nonsympathectomized rats. These anatomical data provide the first direct evidence that neurons in the VN and PH project bilaterally to populations of motoneurons innervating the lower third of the superficial layer of the masseter muscle. The MVe, PH, and SpVe appear to play a predominant integrative role in producing vestibulo-trigeminal responses.

Animals↗
PubMed1996

Masseter muscle activity during the whole day in young adults.

This study was conducted to evaluate the biting function during the entire day in young adults by measuring the masseter muscle activity with a 24-hour electromyography (EMG) system. With this system, the masseter muscle activity for 24 hours was recorded in 20 male and 10 female young adult subjects with acceptable occlusion. For the male subjects, means of the number and total duration of bursts over level 2, which is one-fourth of the maximum discharge voltage during maximum clenching, were 1938 times and 357 seconds. For the female subjects, the values were 2356 times and 419 seconds. A large percentage of such high amplitude bursts as level 3 or 4 was observed during meals in both groups. These results indicated that most of the strong bursts of the masseter muscle appeared only during meals and a number of low amplitude bursts were observed during the entire day, although masseter muscle activity during the entire day in young adults was less than expected. It is anticipated that the nature of masticatory muscle activity revealed in this study might provide valuable information about the association between the biting function and the craniofacial structure or malocclusion.

Activity Cycles↗
PubMed1997

Anatomical organization of efferent neurons innervating various regions of the rabbit masseter muscle.

Previous studies have shown that the masseter muscle is supplied by motoneurons located in the anterodorsal region of the trigeminal motor nucleus and by an additional group of efferent neurons located in cell group k. The present experiments were performed on nine rabbits and were designed to establish the locations of neurons innervating the different regions of this muscle. Retrograde labeling with two fluorescent tracers (FluoroGold and Fast Blue) was applied to the central ends of cut branches of the masseter nerve. Serial coronal sections of the brainstem were viewed with fluorescence microscopy. The labeled cells were counted in all animals, and three-dimensional reconstructions of their distribution were made in five cases. In each successful experiment, labeled neurons were seen in the anterodorsal region of the trigeminal motor nucleus and in the two dorsal cell columns of cell group k (k1 and k3). Within-animal comparisons of the median position of populations innervating two distinct muscle regions in five rabbits showed that there were no significant differences in either the dorsoventral or rostrocaudal axes. However, in each case, there was a small but significant difference (83-173 microm) in the mediolateral axis within the motor nucleus but not within cell group k. Even in this axis, there was a 94-99% overlap of the two populations. Comparisons of the neuronal cross-sectional area showed that the deep regions were innervated by a larger proportion of small neurons from both nuclei than were the superficial and intermediate regions. Our results suggest that there is no simple topographical arrangement of motoneurons that corresponds to the peripheral pattern of nerve supply to the different regions of the masseter muscle.

Animals↗
PubMed2005

Laser application effects on the bite strength of the masseter muscle, as an orofacial pain treatment.

OBJECTIVE: The present research studies the effects of AsGaAl (low-intensity laser) on the bite strength of the masseter muscle in order to evaluate the contribution of laser therapy in patients with orofacial pain. BACKGROUND DATA: Studies on laser therapy suggest its usefulness in the treatment of temporomandibular disorders. This paper presents the effects of low-intensity laser in the contraction of the masseter muscle in patients with neuromuscular discomfort. METHODS: Fifteen patients of both genders, ages 19-29, suffering from pain in the masseter muscle, were exposed to laser application (AsGaAl) applied from a 2-mm distance. RESULTS: All patients showed improvement in muscle contraction strength of about 2.51-3.01 kgf on the right and left masseter muscle. CONCLUSIONS: These results suggest that low-level laser application is an effective tool for the treatment of patients with orofacial pain.

Adult↗