Electromyography and mechanics of mastication in the albino rat.

The masticatory apparatus in the albino rat was studied by means of electromyography and subsequent estimation of muscular forces. The activity patterns of the trigeminal and suprahyoid musculature and the mandibular movements were recorded simultaneously during feeding. The relative forces of the individual muscles in the different stages of chewing cycles and biting were estimated on the basis of their physiological cross sections and their activity levels, as measured from integrated electromyograms. Workinglines and moment arms of these muscles were determined for different jaw positions. In the anteriorly directed masticatory grinding stroke the resultants of the muscle forces at each side are identical; they direct anteriorly, dorsally and slightly lingually and pass along the lateral side of the second molar. Almost the entire muscular resultant force is transmitted to the molars while the temporo-mandibular joint remains unloaded. A small transverse force, produced by the tense symphyseal cruciate ligaments balances the couple of muscle resultant and molar reaction force in the transverse plane. After each grinding stroke the mandible is repositioned for the next stroke by the overlapping actions of three muscle groups: the pterygoids and suprahyoids produce depression and forward shift, the suprahyoids and temporal backward shift and elevation of the mandible while the subsequent co-operation of the temporal and masseter causes final closure of the mouth and starting of the forward grinding movement. All muscles act in a bilaterally symmetrical fashion. The pterygoids contract more strongly, the masseter more weakly during biting than during chewing. The wide gape shifts the resultant of the muscle forces more vertically and moreposteriorly. The joint then becomes strongly loaded because the reaction forces are applied far anteriorly on the incisors. The charateristic angle between the almost horizontal biting force and the surface of the food pellet indicates that the lower incisors produce a chisel-like action. Tooth structure reflects chewing and biting forces. The transverse molar lamellae lie about parallel to the chewing forces whereas perpendicular loading of the occlusal surfaces is achieved by their inclination in the transverse plane. The incisors are loaded approximately parallel to their longitudinal axis, placement that avoids bending forces during biting. It is suggested that a predominantly protrusive musculature favors the effective force transmission to the lower incisors, required for gnawing. By grinding food across transversely oriented molar ridges the protrusive components of the muscles would be utilized best. From the relative weights of the masticatory muscles in their topographical relations with joints, molars and incisors it may be concluded that the masticatory apparatus is a construction adapted to optimal transmission of force from muscles to teeth.     

THE EFFECT OF MASTICATORY PATTERNS AS MEASURED BY TIME-INTENSITY AND ELECTROMYOGRAPHY ON THE PERCEPTION OF BOVINE MUSCLE TENDERNESS

The relationship between perception of tenderness and chewing activity was explored using the Time-Intensity measurement of perception and electromyographic measurements of masseter muscle movement. Semitendinosus muscles from eight treatments of forage fed beef were evaluated by nine individuals. The Time-Intensity parameters of Area Under the Curve and Decrease Area were the most useful for treatment separation based on tenderness. Information was obtained from the electromyogram regarding the number of chews, time to chew and mastication rate. However, more work with EMG measurements is required to accurately study the effect of masticatory patterns on tenderness perception.     

Masticatory muscular activation in elderly individuals during chewing

Background: Old‐age is the last stage of human evolution and, unfortunately, the ageing of the oral cavity and masticatory system seems accelerated. As a consequence, there is a reduction in the amount of food ingested, which can lead to an imbalance in nutrition. Objective: The purpose of this study was to investigate the levels of muscular activation of elderly individuals, during chewing, and to compare with young individuals. Materials and Methods: An electromyographical analysis of the masticatory system in 10 individuals aged between 60 and 75 years (elderly group) and a similar number between 23–30 years old (young group ‐ control) was carried out. The analysis was performed using a MyoSystem‐Br1 electromyographer with differential active electrodes. The test was recorded during functional conditions, and the muscles assessed were the temporalis and masseter. Data were normalised by maximum voluntary contraction (MVC), and the results were analysed using an independent t‐test for comparison between the groups. Results: The normalised electromyographic data obtained showed significant differences in both groups. Comparing the normalised values obtained for MVC, the mean values for the masseter and temporalis muscles of elderly group were statistically lower (p ≤ 0.05) than control group for harder foods, but there were no significant differences for food with the lowest consistency. Conclusion: It can be concluded that elderly individuals show slight hypoactivity of their masticatory musculature during chewing when compared to young individuals.     

Chewing pattern and muscular activation in open bite patients

Different studies have indicated, in open bite patients, that masticatory muscles tend to generate a small maximum bite force and to show a reduced cross-sectional area with a lower EMG activity. The aim of this study was to evaluate the kinematics parameters of the chewing cycles and the activation of masseters and anterior temporalis muscles of patients with anterior dental open bite malocclusion. There have been no previous reports evaluating both kinematic values and EMG activity of patients with anterior open bite during chewing. Fifty-two young patients (23 boys and 29 girls; mean age±SD 11.5±1.2 and 10.2±1.6years, respectively) with anterior open bite malocclusion and 21 subjects with normal occlusion were selected for the study. Kinematics parameters and surface electromyography (EMG) were simultaneously recorded during chewing a hard bolus with a kinesiograph K7-I Myotronics-Usa. The results showed a statistically significant difference between the open bite patients and the control group for a narrower chewing pattern, a shorter total and closing duration of the chewing pattern, a lower peak of both the anterior temporalis and the masseter of the bolus side. In this study, it has been observed that open bite patients, lacking the inputs from the anterior guidance, that are considered important information for establishing the motor scheme of the chewing pattern, show narrower chewing pattern, shorter lasting chewing cycles and lower muscular activation with respect to the control group.     

Influence of occlusal stabilization splint on the asymmetric activity of masticatory muscles in patients with temporomandibular dysfunction

The aim of present study was to evaluate the symmetry of masticatory muscles' activity at various clenching levels in the intercuspal position in patients with functional disorders and in healthy subjects. The purpose was also to determine the effect of full-arch maxillary stabilization splint on the asymmetry of masticatory muscle activity in patients with temporomandibular dysfunction. In this study 6 TMD patients and 12 healthy subjects were investigated. Surface EMG recordings were obtained from left and right anterior temporal, left and right masseter and from the sub-mandibular group in the region of the anterior belly of the digastric muscle on the left and right side during clenching with the maximum 100% voluntary contraction (MVC) as well as during clenching at 50% and 25% of the maximum activity in the position of maximal intercuspation of teeth. In order to quantify asymmetrical masticatory muscle activity, the asymmetry index (AI) was calculated for each subject and for each muscle from the average anterior temporal, masseter and digastric potentials recorded during each test (100% MVC, 50% MVC and 25% MVC). In the group of patients EMG recordings were repeated during and after the splint therapy. The asymmetries of masticatory muscle activity was present in both groups, but in the group of TMD patients the asymmetry indices for anterior temporal muscle at 100% MVC (p = 0.049) and 50% MVC (p = 0.031) were significantly higher. Results have shown that the use of splint suppressed the asymmetry of all muscles, as during the splint therapy the asymmetry indices were lowered. After the therapy, the level of temporal muscle symmetry during submaximal clenching in the intercuspal position increased significantly (p = 0.046). This investigation points out that electromyography may be a valuable method of documenting that asymmetric activity of masticatory muscles improves after occlusal splint therapy in patients with TMD.     

Complexity of ruminant masticatory evolution

The evolution of robust jaws, hypsodont teeth, and large chewing muscles among grazing ruminants is a quintessential example of putative morphological adaptation. However, the degree of correlated evolution (i.e., to what extent the grazer feeding apparatus represents an evolutionary module), especially of soft and hard tissues, remains poorly understood. Recent generation of large datasets and phylogenetic information has made testing hypotheses of correlated evolution possible. We, therefore, test for correlated evolution among various traits of the ruminant masticatory apparatus including tooth crown height, jaw robustness, chewing muscle size, and characters of the molar occlusal surfaces, using phylogenetic and nonphylogenetic comparative methods as well as phylogenetic evolutionary model selection. We find that the large masseter muscles of grazing ruminants evolved with the inclusion of grass in the diet, an increase in the proportion of occlusal enamel bands oriented parallel to the chewing stroke, and possibly hypsodonty. We suggest that the masseter evolved under two evolutionary regimes: i) selection for higher masticatory forces during chewing and ii) flattening of the tooth profile, which resulted in reduced tooth guidance and, thus, a requirement for more chewing muscle activity during each chewing stroke, in agreement with previous research. The linear jaw metrics (depth of the mandibular angle, mandibular angle width, and length of the superficial masseteric scar) all show correlated evolution with hypsodonty and the proportion of enamel bands oriented parallel to the chewing stroke. We suggest that changes in the shape of the mandible represent the combined effects of selection for a reorientation of the chewing stroke, so as to emphasize horizontal translation of the teeth, and accommodation of high‐crowned teeth. Our analyses show that the ruminant feeding apparatus is an evolutionary mosaic with its various components showing both correlated and independent evolution. J. Morphol. 275:1093–1102, 2014. © 2014 Wiley Periodicals, Inc.     

Effect of Postnatal Myostatin Inhibition on Bite Mechanics in Mice

As a negative regulator of muscle size, myostatin (Mstn) impacts the force-production capabilities of skeletal muscles. In the masticatory system, measures of temporalis-stimulated bite forces in constitutive myostatin KOs suggest an absolute, but not relative, increase in jaw-muscle force. Here, we assess the phenotypic and physiologic impact of postnatal myostatin inhibition on bite mechanics using an inducible conditional KO mouse in which myostatin is inhibited with doxycycline (DOX). Given the increased control over the timing of gene inactivation in this model, it may be more clinically-relevant for developing interventions for age-associated changes in the musculoskeletal system. DOX was administered for 12 weeks starting at age 4 months, during which time food intake was monitored. Sex, age and strain-matched controls were given the same food without DOX. Bite forces were recorded just prior to euthanasia after which muscle and skeletal data were collected. Food intake did not differ between control or DOX animals within each sex. DOX males were significantly larger and had significantly larger masseters than controls, but DOX and control females did not differ. Although there was a tendency towards higher absolute bite forces in DOX animals, this was not significant, and bite forces normalized to masseter mass did not differ. Mechanical advantage for incisor biting increased in the DOX group due to longer masseter moment arms, likely due to a more anteriorly-placed masseter insertion. Despite only a moderate increase in bite force in DOX males and none in DOX females, the increase in masseter mass in males indicates a potentially positive impact on jaw muscles. Our data suggest a sexual dimorphism in the role of mstn, and as such investigations into the sex-specific outcomes is warranted.     

Flavor-Enhanced Modulation of Cerebral Blood Flow during Gum Chewing

Background

Flavor perception, the integration of taste and odor, is a critical factor in eating behavior. It remains unclear how such sensory signals influence the human brain systems that execute the eating behavior.

Methods

We tested cerebral blood flow (CBF) in the frontal lobes bilaterally while subjects chewed three types of gum with different combinations of taste and odor: no taste/no odor gum (C-gum), sweet taste/no odor gum (T-gum), and sweet taste/lemon odor gum (TO-gum). Simultaneous recordings of transcranial Doppler ultrasound (TCD) and near infrared spectrometer (NIRS) were used to measure CBF during gum chewing in 25 healthy volunteers. Bilateral masseter muscle activity was also monitored.

Results

We found that subjects could discriminate the type of gum without prior information. Subjects rated the TO-gum as the most flavorful gum and the C-gum as the least flavorful. Analysis of masseter muscle activity indicated that masticatory motor output during gum chewing was not affected by taste and odor. The TCD/NIRS measurements revealed significantly higher hemodynamic signals when subjects chewed the TO-gum compared to when they chewed the C-gum and T-gum.

Conclusions

These data suggest that taste and odor can influence brain activation during chewing in sensory, cognitive, and motivational processes rather than in motor control.     

Electromyographic evaluation of masseter and anterior temporalis muscles in rest position of edentulous patients with temporomandibular disorders, before and after using complete dentures with sliding plates

Objectives: This study was performed with the purpose of investigating electromyographic (EMG) activity of the anterior temporalis and masseter muscles in edentulous individuals with temporomandibular disorder (TMD), before and after using sliding plates on complete dentures in the mandibular rest position. Background: Edentulous patients may present TMD, which is characterised by pain in temporomandibular joints, masticatory and neck muscles, uncoordinated and limited mandible movements, joint sounds and an altered occlusal relationship. It is imperative to offer treatment in order to re‐establish stomatognathic system structures before submitting the individual to any definitive restorative treatment. Materials and methods: The patients were edentulous for at least 10 years. EMG recordings were made before the insertion of the dentures (0 months) and also after using the sliding plates at the fourth month, 9th month and 12th month, using computerised electromyography K6‐I/ EMG Light Channel Surface. EMG evaluations of the muscles were performed under the following clinical conditions: rest position with dentures (R1), rest position without dentures (R2), rest position with dentures post‐activity (chewing) (R3), rest position without dentures post‐activity (chewing) (R4). Results: All patients obtained remission of muscular fatigue and reduced pain in stomatognathic system structures. Temporalis muscle showed significant increase in EMG activity compared with initial values (p < 0.01). Masseter muscles showed significantly lower mean values (p < 0.01) compared with initial values. Conclusion: The sliding plates allowed the process of neuromuscular deprogramming, contributing to muscular balance of the masticatory system, and are therefore indicated to be used before the fabrication of definitive complete dentures in patients with TMD.     

Effect of bolus hardness on the chewing pattern and activation of masticatory muscles in subjects with normal dental occlusion

The aim of the study was to evaluate the effect of bolus hardness on the kinematic of mastication and jaw-elevator muscle activity in subjects with normal dental occlusion and function. The mandibular motion and the surface EMG envelope of the masseter and temporalis anterior muscles were assessed in twelve subjects during mastication of a soft and hard bolus of the same size. When chewing the hard bolus, the chewing pattern in the frontal plane was significantly higher and wider, with smaller closure angle and higher peak velocity than when chewing the soft bolus. EMG peak amplitude of both the masseter and anterior temporalis muscles was higher for the side of the bolus but the contralateral side increased its activity significantly more than the ipsilateral side when the hardness of the bolus increased (for the masseter, mean ± SD: 130.4 ± 108.1% increase for the contralateral side and 29.6 ± 26.9% for the ipsilateral side). Moreover, the peak EMG activity for both muscles occurred more distant from the closure point with hard bolus. The increased activity of the contralateral side may help maintaining the mandibular equilibrium, with indirect participation to the power stroke generated by the chewing-side masseter. The results provide kinematic and EMG adaptations to bolus hardness in healthy subjects and can be used as normative data in the development of methods for early diagnosis of impaired chewing function.     

Nocturnal masseter electromyographic activity of complete denture wearers

doi: 10.1111/j.1741‐2358.2011.00528.x Nocturnal masseter electromyographic activity of complete denture wearers Objective: Collection of normative data on activity patterns of the masseter during sleep in elderly denture wearers by portable electromyography (EMG) recorders. Background: Complete denture wearers might suffer from orofacial pain caused by myoarthropathies of the masticatory system. Indeed, denture instability may precipitate parafunctional habits and consequently muscle soreness and/or temporomandibular pain. Materials and methods: We collected normative masseter EMG data during sleep in 15 complete denture wearers (five women, 10 men, 56–88 years) by portable recorders in their natural environment. Activity periods (AP) were signal portions including subthreshold intervals ≤5 s. Signal amplitude was expressed in per cent of maximum voluntary contraction (%MVC). For this reason, maximum bite force was assessed. Twenty age‐matched dentate controls were also recorded for the maximum bite force. Results: We found 157.2 ± 86.5 AP per night, i.e. 24.0 ± 12.2 AP/h. Mean amplitude was 15.1 ± 4.3%MVC. AP lasted 6.8 ± 4.1 s, and their time integral was 126.3 ± 112.5%MVC?s. Maximum bite force was 116.8 ± 69.6 N in the edentulous, significantly lower than in controls (344.8 ± 150.4 N). Conclusions: Healthy complete denture wearers showed intermittent periods of nocturnal masseter activity of very low intensity and short duration.     

Reconstruction of the cranial musculature and masticatory function of the Pleistocene panamerican ground sloth Eremotherium laurillardi (Mammalia,Xenarthra, Megatheriidae)

Cranial musculature, dental function and mandibular movement patterns in Eremotherium laurillardi were reconstructed from the examination of crania and dentitions. Size, shape and pattern of muscle divisions were reconstructed from the examination of bony rugosities indicating muscle attachments. Details of masticatory muscle structure and function were based on dissections of the tree sloths Bradypus and Choloepus. Among sloths, masticatory muscles in E. laurillardi demonstrate a different synergist–antagonist pattern, reflecting greater emphasis on mediolateral mandibular movements. Eight cranial character complexes (anterior facial, zygomatic arch, superficial masseter, deep masseter–zygomaticomandibularis, pterygoid, temporal, occipital and occlusal) determined by interrelated contributions of each component made to group functions were identified. An elongate anterior face and predental spout in E. laurillardi allowed protrusion of a long narrow tongue at small degrees of gape, reflecting a probably ancestral xenarthran condition. Gape minimisation, in conjunction with the mediolaterally directed masticatory stroke in E. laurillardi, was a unique solution to increase masticatory efficiency by permitting molariform tooth shearing surfaces to remain in or near occlusion for a greater percentage of each chewing cycle.     

The effect of age on the function of the masticatory system--an electromyographical analysis

Background: Old age is the last stage of human life and, unfortunately, the ageing rhythm of the oral cavity and masticatory system seems to be rather accelerated. Hence, there is a reduction in the amount of food ingested, establishing the imbalance of nutritional phenomena. Objective: To assure a better quality of life for the elderly, it is necessary to understand their masticatory system. Materials and methods: An electromyographical analysis of the masticatory system in 10 individuals aged between 60–75 years (group 1) and 10 between 23–30 years old (group 2) was carried out. The analysis was performed using a MyoSystem‐Br1 electromyographer with differential active electrodes. The test was registered during positioning and functional conditions, such as chewing, and the muscles assessed were the temporalis and masseter. Data were normalised by maximum voluntary contraction (MVC), and the results were analysed statistically using independent t‐test during the comparison between groups. Results: Statistical significance (p ≤ 0.05) was found in the analysis of various activities, in which the elderly showed greater electromyographical activity values relative to their MVC during the different mandibular positions, such as left laterality. While the elderly subjects revealed values of 30% relative to their MVC, the young revealed 10%. However, during masticatory activities, the elderly revealed values of up to 79%, and the young of 108%, relative to their MVC, such as when they are chewing peanuts. Conclusion: It could be concluded that the elderly show hyperactivity of masticatory musculature during posture maintenance and a slight hypoactivity of this musculature during chewing when analysed side by side with young individuals.     

The effect of different attachment systems with implant‐retained overdentures on maximum bite force and EMG

doi: 10.1111/j.1741‐2358.2010.00389.x The effect of different attachment systems with implant‐retained overdentures on maximum bite force and EMG Objective: To compare the effect of different attachment systems with implant‐retained overdentures on maximum bite force and muscle activity using electromyography (EMG). Background: Denture retention and stability is of considerable interest in prosthetic dentistry. Materials and methods: Thirty‐five patients were examined: 15 edentulous patients treated with mandibular implant‐retained overdentures (MIRO) and maxillary dentures (MCD) (two implant‐ball attachment) (BC); 10 edentulous patients treated with MIRO and MCD (four implants‐bar attachments) (BRC); 10 patients with edentulous mandibular treated with MIRO and maxillary fixed partial dentures (MFPD) (two implant‐ball attachments) (BF). Before implant placement all patients received new dentures. After using these dentures for 3 months the maximum bite force and electrical activity of masseter muscle were measured. Two or four implants were then inserted into the intraforaminal region. After osseointegration periods, patients were treated with MIRO which duplicated their dentures and after three months the measurements were repeated. The data were collected and statistically analysed. Results: Muscle activity and chewing ability increased in the second period of measurements. Also chewing time was significantly decreased at the first measurements. The highest muscle activity was observed in the group of patients treated with group BF. Conclusion: The EMG values of the masseter muscle significantly increased when an implant attachments was used in the overdenture.     

下颌起排法全口排牙临床效果研究

目的:研究通过比较不同的排牙方式制作的全口义齿对咀嚼次数、咀嚼效率以及患者的主观感受的影响。方法:78例无牙颌患者按照牙槽嵴高度分为正常组和低平组,正常组有患者32例,低平组有患者46例,每组分别戴用上颌起排法和下颌起排法制作的全口义齿,比较咀嚼次数、咀嚼效率和满意度的差别。结果:在戴用上颌起排法制作的全口义齿时,正常组的咀嚼次数高于低平组,差异具有统计学意义(t=3.60,P0.05);戴用下颌起排法制作的全口义齿后,低平组的咀嚼次数高于采用上颌起排法的全口义齿,差异具有统计学意义(t=4.41,P0.05);在戴用上颌起排法制作的全口义齿中,正常组的咀嚼效率高于低平组,差异具有统计学意义(t=5.72,P0.05);而在戴用下颌起排法制作的全口义齿后,低平组的咀嚼效率高于采用上颌起排法的全口义齿,差异具有统计学意义(t=7.16,P0.05)。正常组认为使用下颌起排法制作全口义齿咀嚼能力、稳固感好于上颌起排法,差异均具有统计学意义(P0.05);低平组认为使用下颌起排法制作全口义齿咀嚼能力、稳固感和舒适感好于上颌起排法,差异均具有统计学意义(P0.05)。结论:对于牙槽嵴低平无牙颌患者使用下颌起排法制作的全口义齿,能够有效地增强咀嚼效率,提高义齿的稳定性。     

Socio-medical condition and oral functional status in an older institutionalised population

Objectives: To investigate the association between the general medical condition, the socio-economic status, and some factors related to the functional status of the stomatognathic system. Design: A survey in an elderly population. Subjects: 257 older adults, with a mean age of 83.7 years. Setting: Residential homes for the elderly. Intervention: Examination of the medical records on the overall health and the drugs consumed, a structured interview on the socio-economic status, the complaints for xerostomia, the subjective chewing difficulties, and a clinical evaluation of the number of natural teeth and the number of posterior occluding pairs of teeth contacts (premolars and molars). Results: Multiple pathology and polypharmacy were recorded. 25% of the residents had no occluding posterior tooth contact (natural or prosthetic) and 62% were edentulous. 43% of the residents reported complaints for xerostomia, and 46% for chewing difficulties when eating specific food types. Xerostomic feelings and chewing problems were not related to age. Chewing difficulties were not related to the number of natural teeth, but to the number of posterior occluding teeth contacts, natural or prosthetic (less than two). From all medical conditions examined, only the psychiatric disorders were significantly related to dental status (p<0.05). Moreover, the number of remaining natural teeth was related to socio-economic status, while the number of posterior occluding teeth contacts was also inversely related to the duration of institutionalisation (p<0.05). Conclusions: Psychiatric disorders, low socio-economic status and increased duration of institutionalisation were most closely related to poor dental status. The presence of more than two posterior occluding teeth contacts, natural or prosthetic, benefit the very old patient in terms of reduced subjective chewing difficulty. A formal oral care delivery system for the institutionalised elderly, and particularly for those suffering from psychiatric disorders, is imperative.     

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.     

Medial pterygoid muscle activity during the closing and compressive phases of human mastication

The lack of specific data correlating activity in the human medial pterygoid muscle with displacement of the jaw during mastication, and the hint of possible differences in function between certain mammalian species, prompted a study of unilateral mastication in six adult subjects. Muscle activity in the medial pterygoid, masseter, and anterior temporal muscles was recorded simultaneously with three-dimensional movement of an incisor point on the mandible. Signals from muscles and displacement transducer were sampled by a disc-based computer system programmed to analyze data averaged over 30 chewing cycles on each side and in some instances over 30 open-close and clench cycles. Patterns of medial pterygoid activity were consistent for the group as a whole, demonstrating activation of both muscles early in the closing cycle with strong ipsilateral muscle activity before and throughout the intercuspal phase of mastication. By contrast contralateral activity ceased during the crushing phase of the cycle, reappearing in some subjects just before the end of intercuspation. Medial pterygoid activity mirrored masseter and anterior temporal activity only during certain phases of the closing cycle, suggesting that these muscles should be considered as being selectively coactivated with, rather than synergists of, the major elevators of the jaw. The muscles were active during horizontal components of movement of the incisor teeth in chewing, but were inactive during the open-close and clench task despite vigorous contraction of the masseter muscles. Overall, the observations complement previous reports of medial pterygoid muscle activity in humans. They also confirm, for these muscles at least, a general similarity between man and the little brown bat, a relationship hitherto suspected but unsubstantiated.     

Measurement of the size and orientation of human masseter and medial pterygoid muscles

To gain a better understanding of biting and chewing performance, the size and orientation of the masseter and medial pterygoid muscles in living humans were studied. Twenty-seven young males having complete dentition, class I dental occlusion and normal muscle and jaw function were examined using magnetic resonance images of the head between the zygomatic arch and hyoid bone. The sections were parallel to the palatal plane, and the thickness was 3 mm without a gap. A computer software program (Medical Dental Image, MDI) was developed to identify and calculate the area of each cross section of the muscle, and the volume of the muscle was then estimated. The axis of the muscle was determined by connecting the centroids of the sections in the lower and upper 1/3 of the whole muscle. The effective muscle cross section area was then calculated by resectioning the muscle perpendicularly to the muscle axis. It was found that the mean masseter muscle volume was around 31 cm3, and that the mean medial pterygoid muscle volume was 11 cm3. Their mean effective cross section areas were around 6.2 cm2 and 3.5 cm2, respectively. The axis of the masseter muscle was more perpendicular to the palatal plane and parallel to the sagittal plane than was the medial pterygoid muscle. The results suggest that the use of magnetic resonance images (MRI) is an effective noninvasive measurement technique for determining the size and orientation of masseter and medial pterygoid muscles. This technique can be employed in future studies on human bite force evaluation and masticatory function.