An electromyographic study of the temporalis muscle in normal persons during selected positions and movements of the mandible

Comparison of muscle response in patients treated with rigid and flexible fixed functional appliances

AIM

The present study was undertaken to evaluate and compare muscle activity after the treatment with rigid and flexible fixed functional appliance.

MATERIAL AND METHOD

The study was conducted on 14 skeletal Class II malocclusion patients in the age group of 13-17 years, divided into two groups comprising 7 patients in each group. Group I was treated with a rigid fixed functional appliance (MPA IV), and Group II was treated with a flexible fixed functional appliance (Churro Jumper). Masseter and anterior temporalis muscle activities were recorded using needle electromyography (EMG) at postural rest, saliva swallowing, and clenching during five intervals (T0 to T4) during fixed functional appliance treatment. Unpaired t-test, Mann-Whitney U, and Wilcoxon sign rank test were applied for statistical analysis, and a P value of <.05 was considered statistically significant.

RESULTS

Group I (MPA IV) showed a significant increase in EMG activity during postural rest position (P = 0.003, P = 0.001), swallowing (P = 0.013, P = 0.005), and clenching (P = 0.001, P = 0.002) in masseter and anterior temporalis muscle, respectively. Group II (Churro jumper) also showed a significant increase in EMG activity during postural rest position (P = 0.000, P = 0.000), swallowing (P = 0.001, P = 0.000), and clenching (P = 0.001, P = 0.000) in masseter and anterior temporalis muscle, respectively.

CONCLUSION

Both rigid (MPA IV) and flexible (Churro Jumper) fixed functional appliances caused a significant increase in EMG activity of masseter and anterior temporalis muscle during postural rest position, swallowing, and clenching in 6 months of the observation period, but the flexible appliance (Churro Jumper) showed more significant increase.

Assessment of lower incisor alveolar bone width using cone-beam computed tomography images in skeletal Class III adults of different vertical patterns

Objective

This study was performed to investigate the alveolar bone of lower incisors in skeletal Class III adults of different vertical facial patterns and to compare it with that of Class I adults using cone-beam computed tomography (CBCT) images.

Methods

CBCT images of 90 skeletal Class III and 29 Class I patients were evaluated. Class III subjects were divided by mandibular plane angle: high (SN-MP > 38.0°), normal (30.0° < SN-MP < 37.0°), and low (SN-MP < 28.0°) groups. Buccolingual alveolar bone thickness was measured using CBCT images of mandibular incisors at alveolar crest and 3, 6, and 9 mm apical levels. Linear mixed model, Bonferroni post-hoc test, and Pearson correlation analysis were used for statistical significance.

Results

Buccolingual alveolar bone in Class III high, normal and low angle subjects was not significantly different at alveolar crest and 3 mm apical level while lingual bone was thicker at 6 and 9 mm apical levels than on buccal side. Class III high angle group had thinner alveolar bone at all levels except at buccal alveolar crest and 9 mm apical level on lingual side compared to the Class I group. Class III high angle group showed thinner alveolar bone than the Class III normal or low angle groups in most regions. Mandibular plane angle showed negative correlations with mandibular anterior alveolar bone thickness.

Conclusions

Skeletal Class III subjects with high mandibular plane angles showed thinner mandibular alveolar bone in most areas compared to normal or low angle subjects. Mandibular plane angle was negatively correlated with buccolingual alveolar bone thickness.

Electromyographic analysis of anterior temporalis and superficial masseter muscles in mandibular angle fractures--a pilot study

INTRODUCTION

Fracture of the mandible occurs more frequently and the surgical anatomy of the mandible and adjacent structures is extremely important in understanding the pattern of fracture, the displacement of fractured fragments, and factors necessary for uncomplicated healing. In the field of dentistry, surface electromyography, electrokinesiography, and more recently, TMJ sound analysis have been particularly important developments. Previous electromyographic studies involving anterior temporalis and superficial masseter have been conducted in mandibular condylar fractures and in orthognathic and cosmetic procedures of the jaws.

MATERIALS AND METHODS

This experimental study was undertaken to measure the electrical activity of the anterior temporalis and superficial masseter muscles in mandibular angle fracture cases (n = 6) and the changes in the electrical activity over a period of 6 months.

RESULTS

The study shows that muscle activity increases significantly over a period of time but never matches the muscle activity of the normal subjects of same sex and age in a follow-up of 6 months post-trauma.

DISCUSSION

Electrodiagnostic testing is a potentially valuable tool for the management of patients who have suffered craniomaxillofacial trauma. Electromyography is being employed in clinical practice, and allows the inclusion of quantitative data on the qualitative aspects of a diagnosis. These data are often of significant importance in the correct management of therapy and patient follow-up, particularly if the subject is at risk of developing a different and/or more serious disease.

Change in bite force and electromyographic activity of masticatory muscle in accordance with change of occlusal plane

PURPOSE

The purpose of the present study was to evaluate effects of occlusal plane on masticatory function (biting force, masticatory muscle activity, biting efficiency) after bimaxillary orthognathic surgery.

PATIENTS AND METHODS

The subjects of the present study consisted of a group of 55 adults who had undergone bimaxillary surgery more than 6 months earlier. Lateral cephalographs, bite force, and electromyographic measurements of the anterior temporal [EMG(t)] and masseter muscles [EMG(m)] were recorded before and after bimaxillary surgery. Statistical analyses were performed.

RESULTS

In the increased occlusal plane group, the frequency of decreased EMG(t) was significantly high. The frequency of increased EMG(t) was also significantly high in the decreased occlusal plane group. A negative correlation was found between the postoperative occlusal plane angle and the biting force efficiency change. No significant difference was found between the group that moved from an abnormal to a normal range and the group that moved from a normal to an abnormal range. The occlusal plane change was significantly greater in the decreased EMG(t) group than in the increased EMG(t) group.

CONCLUSIONS

The value of EMG(t) was related to the changes in the occlusal plane, and the biting efficiency was affected by the postoperative occlusal plane angle. However, normalization of the occlusal plane might not play a major role in masticatory function.

Effect of unilateral posterior crossbite on the electromyographic activity of human masticatory muscles

Studies dealing with the electromyographic activity of masticatory muscles in patients with unilateral posterior crossbite are infrequent. The purpose of this study was to assess the electromyographic pattern of masticatory muscles at rest position, during swallowing, and during mastication, in 30 subjects with right posterior crossbite and to compare them to 30 normocclusive subjects. The 2 groups were matched according to age, gender, skeletal Class I, and mesofacial growth pattern. Electromyographic activity of right and left anterior temporalis, posterior temporalis, masseter, and anterior digastric muscles was recorded at rest position, while swallowing water, and while chewing. Disposable bipolar surface electrodes were used in both groups. Data were compared between groups and between right and left sides within each group. The results revealed that the posterior temporalis of the non-crossbite side was more active than that of the same side in subjects with crossbite at rest position and during swallowing. The activity of both anterior digastrics was higher in the crossbite subjects during swallowing. During chewing the right masseter muscle was less active in the crossbite patients than in normocclusive subjects. The results obtained during chewing indicate a bilateral masticatory pattern in both groups.

Effect of osseointegrated implants on the coordination of masticatory muscles: a pilot study

STATEMENT OF PROBLEM

The neuromuscular network of masticatory function is, in part, coordinated with afferent information provided by the periodontal ligament (PDL). Osseointegrated implant-supported prostheses lack this PDL-derived proprioceptive feedback mechanism.

PURPOSE

This pilot study was designed to address the hypothesis that implant patients acquire different patterns of functional coordination. Patients with implant-supported prostheses were characterized in regard to masticatory muscle tenderness and fatigue as well as changes in the coordinated activities of masticatory muscles during chewing and maximal occluding force. Results were compared with those of patients with natural teeth and interpreted to assess the functional outcome of implant therapy.

MATERIAL AND METHODS

Fifty-seven volunteers (25 partially edentulous patients restored with implantsupported fixed prostheses; 32 control patients) were evaluated. A comprehensive set of clinical examinations was performed, including occlusal analysis and examination of masticatory muscle and TMJ. EMG recordings of 5 volunteers from each group were further evaluated. EMG activities of the masseter and anterior temporalis were recorded during habitual chewing and voluntary maximal occluding force.

RESULTS

There were essentially no differences in the clinical evaluations between volunteers in the implant and control groups and no significant alterations in the masticatory muscle coordination for habitual chewing. During the maximal occluding force measurement, EMG recordings revealed a unique masticatory muscle coordination pattern in the implant group with a tendency to activate the working and nonworking side muscles simultaneously.

CONCLUSION

Patients with implant-supported prostheses appeared to be well adapted to perform habitual masticatory functions. However, during a nonhabitual function such as maximal occluding force, our pilot data revealed a less coordinated masticatory muscle activity in the implant patients.

Masticatory muscle activity in children and adults with different facial types

The purpose of this study was to examine the relationship between the duration of masticatory muscle activity during daytime and vertical craniofacial morphology in children and adults. Thirty children (12 boys and 18 girls) and 30 adults (20 men and 10 women) with normal anteroposterior skeletal relations, acceptable occlusions, and without any temporomandibular joint disorders, were selected as subjects. Activities of the masseter, temporal, and digastric muscles were recorded for 3 hours during daytime, excluding the periods for meals, sleep, and hard-exercise, using a portable electromyogram recording system. A lateral cephalogram was taken of each subject at the intercuspal position to divide the subjects into 3 different facial types, ie, low, average, and high angle groups. Masseter, temporal, and digastric muscle activities mainly consisted of low-amplitude bursts during daytime. Children exhibited longer duration of temporal muscle activity, whereas the masseter muscle presented longer duration of activity in adults. The activities of masseter and digastric muscles were significantly related with the vertical facial type in both children and adults, although temporal muscle activity presented no significant relationship with the craniofacial morphology. The duration of masticatory muscle activity during daytime showed a significant difference between children and adults, indicating a close association with vertical craniofacial morphology in children and adults.

Muscle response to the twin-block appliance: an electromyographic study of the masseter and anterior temporal muscles

An electromyographic study was performed on 10 young growing girls in the age group of 9 to 12 years with Class II Division 1 malocclusion and retruded mandible, who were under treatment with Twin-block appliances. Bilateral EMG activity of elevator muscles of the mandible (ie, anterior temporalis and masseter) was monitored longitudinally with bipolar surface electrodes to determine changes in postural, swallowing, and maximal voluntary clenching activity during an observation period of 6 months. The changes were noted at the start of treatment (0 month), within 1 month of Twin-block insertion, at the end of 3 months, and at the end of 6 months. The results revealed a significant increase in postural and maximal clenching EMG activity in masseter (P <.01) and a numeric increase in anterior temporalis activity during the 6 month period of treatment. The increased electromyographic activity can be attributed to an enhanced stretch (myotatic) reflex of the elevator muscles, contributing to isometric contractions. The main force for Twin-block treatment appears to be provided through increased active tension in the stretched muscles (motor unit stimulation) and from initiation of myotatic reflex activity and not through passive tension (viscoelastic properties) of jaw muscles. The results of this study reaffirm the importance of full-time wear for functional appliances to exert their maximum therapeutic effect by way of neuromuscular adaptation.

Adaptations of the masticatory system after bilateral fractures of the mandibular condylar process

PURPOSE

The objective of this investigation was to evaluate the adaptations that occur in the masticatory system after treatment of bilateral fractures of the mandibular condylar process.

PATIENTS AND METHODS

Twenty-two patients (15 men and seven women) with bilateral condylar process fractures treated by open reduction and rigid internal fixation (n = 6), closed therapy (n = 14), or a combination of these techniques (n = 2) were compared with 22 sex- and age-matched controls. Measures of mandibular range of motion, bite force, muscle activity, estimated joint forces, and skeletal morphology were determined at 6 weeks, 6 months, and 1, 2, and 3 years after treatment in all subjects. Various statistical tests were used for comparing differences between patients and controls.

RESULTS

There was no significant difference in the patients' morphologic measures for the open and closed reduction procedures; therefore, all of these patients were tested as a single group. After treatment, patients had significantly increased mandibular plane and gonial angles and decreased facial axis angles. They also showed a significant reduction in posterior facial height and moment arm length for the masseter and pterygoid muscles. Anterior and posterior temporalis muscle direction also was significantly different between patients and controls. Patients had significantly limited mobility during the first year after fracture. Bite forces were lower for patients at all times and tooth positions, with a significant difference at 6 weeks after treatment. Patients had a tendency to use proportionally higher temporalis muscle activity during maximum biting; however, the differences were not statistically significant, probably because of the small sample size. The estimated joint force magnitudes were essentially identical between patients and controls; however, the direction of the patients' joint forces were more posteriorly directed for both incisor and molar bites.

CONCLUSION

The results of this study suggest that early reduction in mandibular range of motion, bite force, and the distribution of masticatory muscle activity assist in preventing overloading of the bilaterally fractured mandibular condylar processes.

Forward head posture: its structural and functional influence on the stomatognathic system, a conceptual study

An extensive conceptual analysis to establish the primary role a forward head posture plays in the appearance of some craniomandibular dysfunctions and internal derangements of the temporomandibular joints, associated to craniocervical postural disturbances. The analysis is based on findings contributed by scientific investigations in the field of dentofacial orthopedics and dysfunction. Special emphasis has been put on the influence of forward head posture on the craniofacial growth as it can determine a morphoskeletal and neuromuscular pattern leading to a dysfunctional condition. A correlation is established between Class II Occlusion, forward head posture, and craniomandibular dysfunction. The concept of craniocervical postural position is defined, as well as its close relation to the mandibular postural position.

Electromyographic heterogeneity in the human temporalis and masseter muscles during static biting, open/close excursions, and chewing

The human temporalis and masseter muscles are not activated homogeneously during static bite force tasks. In this study, we studied the possible existence of regional differences in these muscles under dynamic conditions. Electromyographic (EMG) activity was recorded by means of bipolar fine-wire electrodes. Six electrodes were inserted into the temporalis muscle and three into the masseter muscle. Recordings were made during maximal effort intercuspal and incisal static clenches, open/close excursions from both the intercuspal and incisal positions, and unilateral gum and licorice chewing on right and left sides. The EMG peak amplitudes and the peak occurrences were compared. During the static clenches and the open/close excursions, no differences could be demonstrated between the regions of the temporalis muscle. However, during the chewing tasks, the anterior and posterior regions behaved differently. Throughout almost all tasks, both superficial and deep parts could be distinguished in the masseter muscle. A further division of the deep masseter was task-dependent. In both the temporalis and masseter muscles, maximal activity (100%) was reached during intercuspal clenches. The average activity declined to 35% of the maximal activity in the temporalis muscle, to 47% in the deep, and to 86% in the superficial masseter during incisal clenches. During all chewing tasks, the EMG peak activity of the anterior temporalis and the superficial masseter muscles was higher in the working than in the balancing condition. The general finding was that different regions were preferentially activated, according to task. The detailed regional specialization previously observed during static bite force tasks could not be demonstrated in the present study.

The relationship between jaw-muscle mechanical advantage and activity levels during isometric bites in humans

Mathematical analyses of mandibular biomechanics often assume a close relation between the relative activity levels of the jaw-adductor muscles and their mechanical advantage. However, this relation has not been clearly demonstrated. This study examined the relation between muscle activity levels, force generation, and mechanical advantage in 48 (23 male, 25 female) normal subjects. Mechanical advantage in the sagittal plane was calculated from standard lateral cephalograms. Activity for the anterior and posterior temporalis and masseter muscles was recorded from surface electrodes during incisor and molar bites. Muscle activity was standardized by calculating a least-squares regression for electromyographic versus occlusal force. At each bite position the more efficient muscles were recruited at higher rates but muscle size was more important in determining efficiency than was mechanical advantage. Differences in jaw-muscle recruitment rates between males and females were also best explained by differences in muscle size. Decreased muscle recruitment rates at molar bite positions matched increased mechanical advantage for the anterior temporalis and masseter. However, posterior temporalis recruitment patterns seemed to reflect jaw position more closely than mechanical advantage.

Statistics of differential Lissajous EMG for normal occlusion and Class II malocclusion

Differential Lissajous electromyography (DL-EMG), a method of representing the simultaneous EMG values of two paired muscles as a single, continuous figure, was used in chewing tests with a sample of Japanese women having normal occlusion compared with a like group of Japanese female adolescents, having Class II, Division 1 malocclusion. For the normal sample, the DL-EMG figures described by the coordinated activity of the bilateral temporal and masseter muscles during chewing gum mastication typically appeared as repetitions of loops. Statistical data were gathered on the total EMG voltages, and on the location, rotational direction, and orientations of the DL-EMG loops when graphed onto Cartesian coordinates. These data were compared with those taken from the Class II sample. Significant differences were observed in the latter group for the EMG voltage levels of the balancing-side masseter muscles. The DL-EMG figures of the Class II group also showed irregularities in the shapes, locations, rotational directions, and orientations of the loops, indicative of irregular masticatory patterns. These data suggest that with sufficient control of factors such as age and sex, electromyography and especially DL-EMG techniques can be useful for characterizing masticatory differences between various occlusal groups.

Reliability of surface electromyography of the masseteric and anterior temporal areas

This was studied in 37 patients suffering from myofascial pain-dysfunction. Bipolar surface electrodes were used to record tonic, resting EMG activity for 2 consecutive trials. This protocol was repeated at a second recording session 2 weeks later. A custom-made, plastic template was constructed for each subject during the first visit in order to relocate the electrodes accurately at the second recording session. Correlation coefficients were calculated for each muscle area. Within-session (same day) r values ranged from 0.7620 to 0.8884 for the masseteric area and from 0.8686 to 0.9109 for the anterior temporal area. Across-session (different day) r values ranged from 0.5645 to 0.6503 for the masseteric area and 0.3309 to 0.4844 for the anterior temporal area. The lower correlation between different day recordings could reflect a methodological shortcoming in relocating the electrodes, particularly on the anterior temporal area. The greater variability recorded from the anterior temporal area could also reflect the dynamic role of the temporalis muscle in maintaining the postural rest position of the mandible.

A review of masticatory muscle function

Current reports of the actions of the major muscles of mastication are reviewed for clenching tasks in centric occlusion and eccentric jaw positions, mandibular opening, and unilateral chewing. The relationship of the mandible to the maxillae in the intercuspal position and, consequently, the relationship of the condyle to the fossa of the temporomandibular joint may be controlled by the clinician. Similarly, the number and site of tooth contacts in the intercuspal position and in eccentric jaw positions may be altered. These alterations all produce alteration of muscle activity during functional and parafunctional acts. Some of these alterations are now predictable. Nevertheless, much more study needs to be done before the outcomes of morphologic changes on the muscular system can be accurately predicted. Even now, however, clinicians must understand the nature of clenching and chewing muscle activity and how it can be altered to a more favorable situation for each individual. This review indicates that: The elevator muscles demonstrate maximum activity when even bilateral occlusal contacts occur during clenching in the intercuspal position. The elevator muscles are activated together in the intercuspal zone of tooth contact during chewing when the occlusal contacts are balanced bilaterally in this intercuspal position. Increasing the number of eccentric tooth contacts increases the muscle activity during both chewing and clenching. The medial pterygoid muscle is variable in its action. This variability appears to be related to the chewing pathway and the type of occlusion. Its action is enhanced during laterally directed chewing actions of the mandible. The inferior head of the lateral pterygoid muscle has a reciprocal role with the medial pterygoid muscle during chewing and contributes to forward and lateral bracing of the condyle of the mandible. Although the superior head of the lateral pterygoid muscle appears to be active during mandibular closing, the significance of this finding is not fully understood.

Nocturnal electromyographic evaluation of masseter muscle activity in the complete denture patient

Nocturnal oral activity was evaluated in 12 complete denture wearers by means of EMG measurements of the masseter muscle. Patients who had worn dentures for at least 6 months were selected. EMG levels were compared when subjects slept with and without the dentures in the mouth. Three subjects appeared to have reduced EMG values when sleeping with the dentures. However, no overall group trends or significant differences were obtained. High variability in nightly EMG values could not be explained by a post hoc analysis of patient oral symptoms or denture characteristics. Efforts should be directed at improved methodology to study the specific mechanism of the effect of denture wearing on nocturnal muscle activity levels. Additional knowledge is needed on the occurrence and effects of parafunctional habits in the edentulous patients.

Electromyographic relaxed mandibular position in long-faced subjects

Previous articles have presented conflicting results concerning the relationship of EMG rest position in subjects with varying craniofacial morphology. This study sought to resolve the issue by studying long-faced subjects and controlling head position. Our findings showed that long-faced subjects have large EMG rest positions in contrast to the results reported by Wessberg et al. We were not able to resolve the differences between these two studies by variation in head position or lack of consistent sample population. The results of this study and earlier studies question the clinical practice of using EMG rest as a starting point to establish vertical dimension of occlusion.

An electromyographic analysis of the temporalis function of normal occlusion

Electromyographic (EMG) activity was recorded from the anterior, middle, and posterior regions of the temporalis muscle in ten subjects with normal occlusion of the teeth and with the mandible at rest and during exertion of increased biting force, using bipolar intramuscular electrodes. Results show that the posterior part of the temporalis muscle maintains the mandibular posture. Although there is no statistically significant difference in EMG recordings between the three divisions of the temporalis muscle during intercuspal biting, individual variations in EMG pattern exist. During exertion of increased biting force the EMG activity increases proportionally in all parts of the muscle. During retruded biting force the posterior temporalis predominates. The EMG activity of the temporalis muscle is correlated to the form and position of the mandible.

Predicted pattern of human muscle activity during clenching derived from a computer assisted model: symmetric vertical bite forces

A computer assisted three-dimensional model of the jaw, based on linear programming, is presented. The upper and lower attachments of the muscles of mastication have been measured on a single human skull and divided into thirteen independent units on each side--a total of 26 muscle elements. The direction (in three dimensions) and maximum forces that could be developed by each muscle element, the bite reaction and two joint reactions are included in the model. It is shown for symmetrical biting that a model which minimizes the sum of the muscle forces used to produce a given bite force activates muscles in a way which corresponds well with previous observations on human subjects. A model which minimizes the joint reactions behaves differently and is rejected. An analysis of the way the chosen model operates suggests that there are two types of jaw muscles, power muscles and control muscles. Power muscles (superficial masseter, medial pterygoid and some of temporalis) produce the bite force but tend to displace the condyle up or down the articular eminence. This displacement is prevented by control muscles (oblique temporalis and lateral pterygoid) which have very poor moment arms for generating usual bite forces, but are efficient for preventing condylar slide. The model incorporates the concept that muscles consist of elements which can contract independently. It predicts that those muscle elements with longer moment arms relative to the joint are the first to be activated and, as the bite force increases, a ripple of activity spreads into elements with shorter moment arms. In general, the model can be used to study the three-dimensional activity in any system of joints and muscles.

Electromyographic activity of anterior temporal area pain patients and non-pain subjects

Anterior temporal area and non-specific facial muscle activity were recorded from 11 patients with unilateral anterior temporal area muscle pain and from 11 matched asymptomatic individuals at various mandibular openings. No significant differences were observed (1) in temporal area EMG activity between pain and non-pain sides and (2) between temporal area and non-specific facial muscle EMG activity between patient and non-patient groups. In relation to increased vertical mandibular opening from centric occlusion: (a) anterior temporal area EMG activity decreased to a minimum level (with further opening, anterior temporal area EMG did not significantly change); and (b) non-specific facial muscle EMG activity decreased to a minimum level (with further opening, non-specific facial muscle EMG increased).

Associations between anterior temporal, masseter, and orbicularis oris muscle activity and craniofacial morphology in children

In a sample of children with Class I (n = 18), Class II, Division 1 (n = 25) and Class II, Division 2 (n = 12) malocclusions, the relationship between jaw muscle activity and specific craniofacial variables has been quantified. Muscle data were obtained from the temporal, masseter, and orbicularis oris muscles at rest, in maximum intercuspation, and during clenching, jaw opening, and swallowing. Contrasts in means of the three skeletal groups identified differences in both the craniofacial and muscle variables. Orbicularis oris muscle amplitude was greater at rest and in maximum intercuspation in Class II, Division 2 subjects. Anterior temporal amplitude during clenching was significantly lower in Class II, Division 1 subjects. A principal component analysis reduced the data base, and three canonical correlations were identified at the 0.05 level of significance. Significant canonical correlations could not be found between the cephalometric data and clench, swallow, or jaw-opening tasks, which suggests no dependency between these factors. Two canonical correlations were found for the rest position data. A linear combination of masseter and orbicularis oris activity and incisor separation was correlated (r1 = 0.758) with a linear combination of mandibular unit length, upper and lower central incisor angulation, and SNOP. The second rest canonical correlation (r2 = 0.733) involved the anterior temporal muscle and a series of sixteen cephalometric variables. For the maximum intercuspation data, orbicularis oris amplitude was correlated with a linear combination of upper incisor angulation and SNOP variables (r1 = 0.727). The multivariate statistical analysis extracted a clinically significant association between orbicularis oris muscle activity and incisor position.

Muscle activity during function and its correlation with craniofacial morphology in a sample of subjects with Class II, Division 1 malocclusions

To evaluate the relationship between craniofacial morphology and jaw muscle activity in a sample of skeletal Class II, Division 1 malocclusions, electromyographic and cephalometric analyses were undertaken in a sample of twenty adolescent children. For each subject, electromyographic activity was recorded from the left anterior temporal, masseter, and orbicularis oris muscles at rest, in maximum intercuspation, and during clenching. In addition, mandibular displacement was measured simultaneously with a kinesiograph, and twenty-seven anatomic points were digitized from a lateral head film. A computer-based system was used to sample the data and to determine correlation coefficients between selected parameters. Five significant correlations were found (at the 99 percent confidence level) for the rest position data. High anterior temporal amplitude was correlated with flat palatal planes and large ramus heights; high masseter amplitude correlated with upright maxillary incisors. Anterior positions of the mandible at rest correlated with flat occlusal planes and small pogonions. In contrast, significant correlation coefficients could not be identified for the maximum intercuspation recordings. However, low orbicularis oris muscle amplitude during clenching was correlated with high lower face height, overerupted mandibular molars, and elongated maxillary incisors. This interdependence of jaw muscle activity and craniofacial morphology in Class II, Division 1 subjects suggests a contribution from the musculature to the development of the dentition.

Histochemical and morphological muscle-fibre characteristics of the human masseter, the medial pterygoid and the temporal muscles

An extensive histochemical and quantitative analysis of various portions of the human masseter, the medial pterygoid and the temporal muscles was performed in young adult males with normal intermaxillary relationships and complete dentition. There was marked and locally radical intramuscular variability in the muscle-fibre composition. Each muscle and the subunits of the muscles exhibited a characteristic fibre pattern--both the relative frequency and the diameter of the various fibre types differed significantly between the different portions. The fibre pattern was quantitatively different to that of the human lateral pterygoid muscle and both quantitatively and qualitatively dissimilar to that of the human digastric muscle and that of normal limb and trunk muscles. A large proportion of the fibres were ATPase intermediate fibres and must be regarded as being a part of the normal fibre population of the human mandibular elevator muscles. Type IIA fibres were rare. As muscle-fibre differentiation is considered to be influenced by motoneurone function, it can be assumed that the complex fibre pattern of the jaw-closing muscles is related to the unique function of the human mandibular locomotor system. A functional specialization is suggested providing optimal jaw control. Compared with the lateral pterygoid muscle (with predominantly type I fibres) and the digastric muscle (with predominantly type II fibres), the heterogeneous fibre composition of the jaw-closing muscles probably reflects their more complicated activity pattern and functional requirements. The marked difference between the type I and the type II fibre diameters, type II fibres generally being smaller, might reflect evolutionary changes in the masticatory habits, such as adaptation to refined and soft food. The individual variability in fibre composition suggests various levels of utilization and varying ability to adapt to jaw-muscle hyperactivity, to resist fatigue.

Activity of the temporal and masseter muscles in class II, division 1 malocclusions. An electromyographic investigation

A quantitative analysis of EMG activity in the masticatory muscles was performed in homogeneous groups of boys with Class II, Division 1 malocclusion and normal occlusion. Integrated EMG recordings from the temporal and masseter muscles were analyzed during maximal biting in the intercuspal position (centric occlusion) and during the chewing of peanuts. The results of the investigation revealed the following: (1) During maximal biting in intercuspal position the boys with Class II malocclusion exhibited less EMG activity in the masseter and temporal muscles than the boys with normal occlusion. In the Class II boys the reduction in EMG activity was most apparent for the masseter muscle. (2) During chewing the Class II subjects showed less EMG activity in the masseter muscle than the normal occlusion subjects. For the temporal muscle, no differences were found between the two occlusion groups. (3) High positive correlations were found between the EMG activity during maximal biting and chewing for both muscles in the two occlusion groups. The impaired muscle activity found in the Class II cases may be attributed to a diverging dentofacial morphology and unstable occlusal contact conditions.

An electromyographic and roentgen cephalometric study of occlusal morphofunctional disharmony in children

In a sample of children with dual bite and symptoms in the masticatory system, the activity of the anterior temporal and masseter muscles was studied electromyographically during clench and light tapping in the posterior occlusal contact position and centric occlusion and in protrusive bite on the incisors. The findings indicated that the optimal muscle function during maximal clench and tapping with displayed in the posterior occlusal contact position, which in the present subjects was situated, on the average, 0.5 to 1 mm. anterior to the centric jaw relation. It should therefore be recommended that, in orthodontic treatment of children with dual bite, the intercuspal position (centric occlusion) should be established in the posterior occlusal relationship. This is in accordance with the opinion of several authors who have emphasized the use of the retruded contact position (the centric jaw relation) as key reference position in orthodontic treatment and occlusal rehabilitation. These authors have further emphasized the importance of stable occlusal contacts in centric relation, centric occlusion, and the various jaw excursions in obtaining and maintaining a harmonious function between the dentition, the temporomandibular joints, and the neuromuscular system.

A functional analysis of the primate masticatory system and the origin of the anthropoid post-orbital septum

Study of the dry weights of primate and non-promate masticatory musculature reveals a significant relationship between the Anterior Temporalis/Masseter ratio and the relative development of the anterior dentition. Available dietary information demonstrates that species emphasizing incisal preparation of food have a high AT/M index; species emphasizing molar occlusion have a low AT/M index. Utilizing this information, a model is presented of the origin of the anthropoid post-orbital septum. Frugivory or extensive incisal preparation of food is causally related to the development of the post-orbital septum, because diet can then create selection pressures for an increasingly tendinous and enlarging anterior segment of the temporalis muscle which requires additional bony areas of orgin in the anterior temporal fossa. Cenozoic climatic oscillations leading to increasing seasonality may have been the triggering element in this model, because seasonality creates periods in which the availability of fruit is relatively predictable.

Condylar translation and the function of the superficial masseter muscle in the rhesus monkey (M. mulatta)

The relationship between translation of the mandibular condyle during symmetrical mandibular rotation, i.e., symmetrical jaw depression and elevation, and the function of the superficial masseter muscle was examined in light of relative torque and the length-tension relationship for muscle. Lateral cephalograms of live adult rhesus monkeys (Macaca mulatta) were analyzed using two models: (1) Model A, normal symmetrical jaw rotation accompanied by condylar translation; and (2) Model B, mandibular rotation about an axis fixed at the position of the condyles during centric occlusion. The decrease in relative torque and the excursion of the superficial masseter at mouth-open positions are significantly greater in Model B than in Model A. Symmetrical rotation of the jaw about a fixed axis would result in a 35% greater loss of maximum producible tension at maximum gape than rotation associated with condylar translation. These results suggest that condylar translation during mandibular depression and elevation functions to minimize reduction in relative torque and excursion of superficial masseter muscle, thereby maintaining optimal potential for exerting maximum tension during jaw closure.

Integrated actions of masticatory muscles: simultaneous EMG from eight intramuscular electrodes

In 29 normal persons with complete dental arches, the muscular activity of the temporalis, masseter, medical pterygoid, anterior belly of the digastric, mylohyoid and geniohyoid muscles was studied electromyographically with bipolar fine wire electrodes during various mandibular movements--both resisted and unresisted. Action potentials were recorded on FM magnetic tape and each experiment was also videotaped. Temporalis muscle was active during centric closing of the jaw with either contact of the teeth, or against resistance; during free lateral movements to the ipsilateral side, either against resistance or occlusal contact; during incisor gum chewing, molar gum chewing on ipsilateral or contralateral sides, during normal mastication; and during forceful centric occlusion. Activity occurred in the masseter and medial pterygoid muscles during the following movements; closing the jaw slowly either without occlusal contact or with occlusal contact and against resistance; free lateral movement to contralateral side, either against resistance or with occlusal contact; protraction of the jaw either without occlusal contact or with occlusal contact; swallowing either saliva or water; incisor gum chewing with either the ipsilateral or contralateral molars; normal mastication; and during forceful centric occlusion. Activity occurred in the digastric, mylohyoid and geniohyoid muscles during the following movements; opening of the jaw either slowly or maximally against resistance; closing the jaw against resistance; free lateral movement to ipsilateral and contralateral sides, either against resistance or with occlusal contact; protraction against resistance of the jaw either without or with occlusal contact; swallowing saliva and water; and protraction of the tongue. They work in antagonism (reciprocally) during gum chewing and normal mastication.

Relations between occlusal interference and jaw muscle activities in response to changes in head position

The jaw muscles responded to changes in the head position. Electromyographic responses to head positions were classified as either of two types--balanced and unbalanced. The balanced type of electromyographic responses of participants with normal occlusion changed to the unbalanced type after being set with an overlay to make a premature contact artificially, and returned to the balanced type after removal of the overlay. The unbalanced type of electromyographic response of participants with occlusal interference turned to the balanced type after occlusal adjustment.

Muscles of mastication in small children: an electromyographic analysis

The temporalis, masseter, and depressor muscles of fifteen normal children were studied electromyographically to determine their activities during the position of resting tonus, during various movements of the mandible, and during the act of swallowing. Analysis of our results led to the following conclusions: 1. Resting tonus of the mandible does not depend on neuromuscular activity in the three portions of the temporalis, masseter, and depressor muscle group; they were inactive bilaterally. 2. Opening of the jaw either slowly or maximally, and against resistance, is brought about by the contraction of the depressor muscle group. 3. Closing of the jaw quickly is brought about mainly by greater contraction of the masseter muscle, and slightly by contraction of the temporalis. 4. Closing the jaw quickly with occlusal contact of the teeth is effected by the temporalis and masseter muscles. 5. Protraction of the jaw is performed by contraction of the masseter muscles. The depressor muscle group contracts to maintain the slight depression followed by the protraction. 6. Swallowing saliva and water is accompanied by contractions of the depressor muscle group. 7. Incisor chewing of gum and peanuts mainly recruits marked contraction of the masseter muscle. Some slight-to-moderate activity is achieved by the temporalis. The depressor muscle group acts in antagonism with the elevator muscles. 8. Chewing gum on the right and left molar sides and normal mastication are effected by the contraction of the temporal and masseter muscles. The suprahyoid muscle group acts in antagonism with the elevator muscles. 9. Normal children with deciduous teeth have patterns that are similar to those of normal adults.

The human mandible: lever or link?

The mammalian mandible, and in particular the human mandible, is generally thought to function as a lever during biting. This notion, however, has not gone unchallenged. Various workers have suggested that the mandible does not function as a lever, and they base this proposition on essentially two assertions: (1) the resultant of the forces produced by the masticatory muscles always passes through the bite point; (2) the condylar neck and/or the temporomandibular joint is unsuited to withstand reaction forces during biting. A review of the electromyographic data and of the properties of the tissues of the temporomandibular joint do not support the non-lever hypothesis of mandibular function. In addition, an analysis of the strength of the condylar neck demonstrates that this structure is strong enough to withstand the expected reaction force during lever action. Ordinarily the human mandible and the forces that act upon it are analyzed solely in the lateral projection. Moments are then usually analyzed about the mandibular condyle; however, some workers have advocated taking moments about other points, e.g., the instantaneous center of rotation. Obviously it makes no difference as to what point is chosen since the moments about any point during equilibrium conditions are equal to zero. It is also useful to analyze the forces acting on the mandible in the frontal projection, particularly during unilateral biting. The electromyographic data suggest that during powerful unilateral molar biting the resultant adductor muscle force is passing between the bite point and the balancing (non-biting side) condyle. Therefore, in order for this system to be in equilibrium there must be a reaction force acting on the balancing condyle. This suggests that reaction forces are larger on the balancing side than on the working side, and possibly explains why individuals with a painful temporomandibular joint usually prefer to bite on the side of the diseased joint.

Effects of the tonic neck reflex on the jaw muscles of the rat

Effects of the tonic neck reflex on the jaw muscles were studied in rats with both ear labyrinths destroyed immediately after decerebration. Electric activities of the jaw muscles increased or decreased in response to rotation, tilting, and flexion of the head. The electromyographic responses to head position were abolished after the first three cervical nerves were cut. We concluded that the tonic neck reflex has an influence on the jaw muscles.

Mechanical simulation of human chewing motions

To study the cause of dental attrition frequently found among primitive peoples, a cam-actuated, motor-driven device that could simulate human chewing motions was built. Casts of the dentition could be mounted in this machine and worn down by moderate pressures during chewing action.