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PubMed1983

Histologic changes in rat masticatory muscles subsequent to experimental increase of the occlusal vertical dimension.

Histologic changes in various masticatory muscles were studied in Wistar rats from 12 hours to 84 days after an experimental increase of the occlusal vertical dimension. The increases applied (1 to 2 mm) were based on the amount of interocclusal space (1 mm) in the rat. A sequence of tissue reaction from early acute inflammatory destruction of myofibers to healing with myofiber regeneration was most pronounced in the deep masseter muscle of the 2 mm group (12 hours to 21 days). A similar sequence was also observed to a lesser degree in the superficial masseter and anterior temporal muscles, while no changes were detectable in the posterior temporal, medial pterygoid, and anterior digastric muscles of this group. Variable severity of tissue response in different masticatory muscles might be attributable to the amount of stretching of myofibers as a result of an increased occlusal vertical dimension. On the other hand, degenerative atrophy of myofibers in the lateral pterygoid muscle occurred within 2 days with no sign of inflammation, and these condensed myofibers persisted as late as 84 days. Based on these findings, it was suggested that displacement of the mandibular condyle would exert an important influence on the prognosis after an increase in the occlusal vertical dimension. Only a transient acute inflammation occurred in the deep and superficial masseter muscles in the 1 mm group. Except for a small number of condensed myofibers in the lateral pterygoid muscle, degenerative changes of myofibers were not revealed. Therefore, increasing the occlusal vertical dimension within the extent of the interocclusal space might be regarded as appropriate for prosthodontic practices.

Animals↗
PubMed1996

Evaluation of masticatory muscles of edentulous patients by computed tomography and electromyography.

On account of its tissue resolution capacity, computed tomography (CT) is an excellent method for the examination and quantitative analysis of the masticatory muscles. The aim of the present study was to evaluate the influence of the long edentulous period by CT on the structure of the main masticatory muscles in 10 edentulous patients and to study their functioning by electromyography (EMG) in connection with the renewal of the dentures. CT scanning was performed before prosthodontic treatment and EMG recordings of the masseter and temporal muscles before treatment and 4 weeks and 6 months after insertion of the new dentures. The results suggest, however, that a long edentulous period is visible not only in the functioning of the masticatory muscles, in terms of decreased EMG activity, but also as decreased density of the muscles which implies muscle atrophy, as seen by CT in the masseter and medial pterygoid muscles.

Adult↗
PubMed1996

[Histopathological effects of overextended opening of the mouth on the temporomandibular joint and masticatory muscles].

The purpose of this study is to explore the effects and the pathogenicity of overextended mouth opening on the temporomandibular joint (TMJ) and the masticatory muscles. In this experiment, the animal model of overextended opening was, established by overly opening the miniature pigs' mouths for certain periods with mouth gag, and then the TMJ and the masticatory muscles were studied histopathologically. The results showed that the histosytes of the TMJ and the masticatory muscles had suffered injury, proliferation, atrophy and degeneration from the overextended opening, and the extents of deterioration became greater with the increase of frequency and duration of overextended opening. Among the injured tissues, the cartilage of the condyle, the posterior and anterior portions of the articular disc, the anterior extenting tissue of the disc, the lateral perygoid muscles and the temporalises underwent the most obvious histopathological changes. The result of this study confirms that overextended mouth opening is one of the pathogenic factors of CMD, and it has great significance for the clinical diagnosis, treatment and prevention for CMD.

Animals↗
PubMed2003

An adolescent case of anterior open bite with masticatory muscle dysfunction.

In adolescence an open bite is likely to be induced by various causes, and most patients with open bite show masticatory muscle dysfunction. The orthodontic correction of an open bite may lead to the recovery of harmonious stomatognathic function. This article reports treatment for a 12-year and 10-month-old female patient with an anterior open bite. She complained of occlusal disturbances due to an anterior open bite. Before the eruption of the upper second molars, she recognized the presence of occlusal contacts between the upper and lower anterior teeth. Before treatment, the activity of the masticatory muscles was relatively low during clenching. An upper lingual arch with an accessory spring to eliminate the premature contact and to improve the anterior open bite plus conventional fixed appliances for tooth alignment was selected as the treatment plan. Immediately after improvement of the open bite, the muscle activity became slightly greater than that at the initial stage. The total treatment time was 13 months. After treatment, an acceptable occlusion was achieved, the masticatory muscles were working harmoniously, and their activity was much higher than that at the initial stage. After a 2-year retention period, an acceptable occlusion was maintained without recurrence of the open bite, indicating a long-term stability of occlusion. The result of this treatment indicated that correction of open bite is of great importance for prevention of skeletal impairment and recovery of harmonious stomatognathic function lost by an unstable occlusion.

Adolescent↗
PubMed2002

Human masticatory muscle volume and zygomatico-mandibular form in adults with mandibular prognathism.

Although several investigators have reported associations between masticatory muscles and skeletal craniofacial form, there is no agreement on the association. We tested the hypothesis that masticatory muscle volume correlates with the size and form of the adjacent local skeletal sites. For this purpose, we investigated the morphological association of the cross-sectional area and volume of temporal and masseter muscles with zygomatico-mandibular skeletal structures using computerized tomography (CT) in 25 male adults with mandibular prognathism. Muscle variables significantly correlated with widths of the bizygomatic arch and temporal fossa but not with the cranium width. Masseter volume significantly correlated with cross-sectional areas of the zygomatic arch and mandibular ramus. Masseter orientation was almost perpendicular to the zygomatic arch and mandibular antegonial region. The zygomatic arch angle significantly correlated with the antegonial angle. The results of the study suggest that the masticatory muscles exert influence on the adjacent local skeletal sites.

Adolescent↗
PubMed1979

Motor nuclear representation of masticatory muscles in the rat.

Representation of the masticatory muscles within the motor trigeminal nucleus was studied in rats by the horseradish peroxidase (HRP) method and the antidromic field potential method. The motor trigeminal nucleus of the rat could be divided cytoarchitecturally into a dorsolateral and a ventromedial division. Within the dorsolateral division, the temporal muscle was represented dorsomedially, the masseter muscle dorsolaterally and laterally, and the lateral and medial pterygoid muscles ventrolaterally. Within the ventromedial division, the anterior digastric muscle was represented dorsomedially and the mylohyoid muscle ventrolaterally. Distribution of antidromic field potentials evoked by stimulation of the mylohyoid and masseteric nerves coincided with the results from the HRP investigation.

Animals↗
PubMed2000

Computerized 3-dimensional study of the embryologic development of the human masticatory muscles and temporomandibular joint.

PURPOSE: In this study, the development of human embryonic temporomandibular joint (TMJ) and masticatory muscles were investigated by using computed 3-dimensional reconstructions. MATERIALS AND METHODS: Sixteen human embryos and fetuses, ranging from 6.5 to 107 mm crown-rump length, were examined. RESULTS: At 10 weeks, a band of mesenchyme extending from the attachment of the lateral pterygoid muscle to the condylar process was observed to pass through the medial side of the condylar process to attach to the malleus. The temporal, masseter, and pterygoid muscles develop from the so called "temporal muscle" primordium, and the temporal muscle was in continuity with the masseter muscle until 14 weeks of fetal life. CONCLUSIONS: The study shows that the muscles of mastication arise from a single primordium. It also confirms the presence of a ligamentous attachment between the lateral pterygoid muscle and the malleus.

Computer Simulation↗
PubMed1993

Effect of jaw opening on masticatory muscle EMG-force characteristics.

This study was designed to evaluate the way in which changes in vertical jaw opening affected the relative contributions of various masticatory muscles to bite force production. EMG activity was recorded simultaneously from the masseter, and anterior, middle, and posterior temporalis muscles, during controlled isometric biting at different force levels and vertical jaw openings. EMG-force characteristics were compared between muscles and bite openings. All but the posterior temporalis muscle displayed significant increases in muscle activity with increased bite force production; the masseter muscle demonstrated the largest activity increments. Statistically significant changes in muscle function due to jaw opening were demonstrated only for the masseter muscle, though similar trends were observed for the anterior and middle temporalis muscles. Minimum increases in muscle activity associated with increases in bite force occurred between 9 and 11 mm of opening, measured at the first molar, for all three muscle groups. The results of this study suggest that changes in masticatory muscle length resulting from vertical jaw opening cause alterations in muscle contractile properties, but the relative contributions of various masticatory muscles toward bite force production may also be affected by biomechanical factors and neural control adaptations.

Biomechanical Phenomena↗
PubMed2003

Effects of masticatory muscle function on craniofacial morphology in growing ferrets (Mustela putorius furo).

Studying the effects of masticatory muscle function on craniofacial morphology in animal models with different masticatory systems is important for further understanding of related issues in humans. Forty 5-wk-old male ferrets were equally divided into two groups. One group was fed a diet of hard pellets (HDG) and the other group was fed the same diet but softened with water (SDG). Lateral and dorsoventral cephalograms were taken on each group after 6 months. Cephalometric measurements were performed by digital procedures. For SDG ferrets, the hard palate plane was more distant from the cranial base plane, and canines were more proclined compared with HDG ferrets. The SDG ferrets were also found to have smaller interfrontal and interparietal widths, and a slenderer zygomatic arch than the HDG ferrets. In the mandible, the coronoid process was generally shorter and narrower for the SDG ferrets. The effects of the altered masticatory muscle function on craniofacial morphology in growing ferrets seemed to differ from those previously reported in other animal models studied under similar experimental conditions. Such differences in the effects are presumably related to the differences in the mode of mastication, craniofacial anatomy and growth pattern in different animal models.

Animal Feed↗
PubMed1990

Distribution of the chain of stretch receptors on masticatory muscles of the rat.

The existence of stretch receptors on the fasciae of masticatory muscles of the rat was demonstrated by means of methylene blue vital staining. A single cord of connective tissue on each side extends over the surfaces of the superficial masseter, temporal, and medial pterygoid muscles. The complex arborizations of unmyelinated nerve terminals originated from branches of the buccal nerve gathered closely within the cord forming the chain of stretch receptors. The chain was lengthened during the jaw closing phase, when the firing rates of stretch receptors probably increased. Results suggested that the chain of stretch receptors signaled position or movement of the mandible. The same stretch receptors were also detected on the masticatory muscles of some other rodents.

Animals↗
PubMed2006

Masticatory muscles and the skull: a comparative perspective.

Masticatory muscles are anatomically and functionally complex in all mammals, but relative sizes, orientation of action lines, and fascial subdivisions vary greatly among different species in association with their particular patterns of occlusion and jaw movement. The most common contraction pattern for moving the jaw laterally involves a force couple of protrusor muscles on one side and retrusors on the other. Such asymmetrical muscle usage sets up torques on the skull and combines with occlusal loads to produce bony deformations not only in the tooth-bearing jaw bones, but also in more distant elements such as the braincase. Maintenance of bone in the jaw joint, and probably elsewhere in the skull, is dependent on these loads.

Animals↗
PubMed2000

Effects of spring-loaded posterior bite-block appliance on masticatory muscles.

The effects of spring-loaded posterior bite-blocks on masticatory muscles were investigated to evaluate the correlation between masticatory muscles and craniofacial form in long-faced children. The appliance was used in 10 subjects (6 girls and 4 boys) with a chronological mean age of 10.40 +/- 1.12 years for the treatment of skeletal anterior open bite. Electromyographic activity of the anterior and posterior temporal and masseter muscles was recorded before and after treatment during postural position, maximal biting, chewing, swallowing, postural position with the appliance in the mouth, and maximal biting with the appliance in the mouth. After treatment, increases in SNB and overbite (P <.05, P <.001, respectively) and decreases in ANB, SNGoAr, and overjet parameters were found to be statistically significant (P <.05, P <.01). When the measurements related to muscle activity were examined, increases in anterior temporal postural (P <.05), anterior temporal chewing (P <.01), masseter chewing (P <.05), posterior temporal chewing (P <. 05), and masseter swallowing (P <.01) were found to be statistically significant. A positive correlation was found between ANB and anterior temporal postural and a negative correlation between SNGoAr and masseter swallowing. The increase in muscle activities was considered to occur as a result of the appliance used.

Cephalometry↗
PubMed1997

Controllability of temporomandibular joint loading by coordinative activities of masticatory muscles: a two-dimensional static analysis.

Masticatory and bite forces, when applied to the teeth, generate tremendous compressive energy in the temporomandibular joint (TMJ). Excessive 'TMJ loading', if left untreated, deteriorates articular functions. Normally, it is controlled, to a certain extent, by stomatognathic means. In an attempt to clarify this control mechanism, we analyzed the relationship between TMJ loading and the activities of the masticatory muscles, by employing a static two-dimensional jaw model. This comprises two rigid bodies, the upper and lower jaws, including three dominant muscles, i.e. the masseter, the anterior portion of the temporalis and the lateral pterygoid. Static equilibrium analyses determined that TMJ loading can be minimized, under controlled bite conditions, by pointing the loading vector in a direction solely indicated by individual morphological factors, such as the position and orientation of the masseter and the temporalis. This theoretically optimum direction of TMJ loading was also anatomically acceptable, because the load is applied exactly to those portions of the articular disk and mandibular head that can most easily sustain it. Interestingly, this factor was absolutely independent of both the activities of the lateral pterygoid and the direction of bite force. Consequently. TMI loading can be minimized, by coordinating the activities of the masseter and the anterior portion of the temporalis.

Bite Force↗
PubMed1998

[Biomechanical characteristics of masticatory muscles of rat after using bite plate].

OBJECTIVE: The purpose of this study is to evaluate the changes on biomechanical characteristics of masticatory muscles of rat after using bite plate. METHODS: 30 growing rats were selected to wear the bite plate for four weeks. The changes of contractive force and time of masticatory muscles were measured by method and technique of biomechanics. RESULTS: The biomechanical characteristics of masticatory musclse changed significantly. The contractive tension and contraction time of masseter muscles increased shortened respectively, while that of the digastric muscles increased and prolonged. CONCLUSION: This suggests that the orthopedic mechanism of the bite plate is to alter the original masticatory muscle dynamics and finally to reach the goal of functional jaw orthopedics.

Animals↗
PubMed1989

Short-term effect of a stabilization splint on the asymmetry of submaximal masticatory muscle activity.

Ten healthy subjects continuously wore equilibrated maxillary full-arch stabilization splints in the retruded position for 7 days. The muscular activity balance of the masseter muscles during submaximal isometric clenching at 10% and 50% of the maximum voluntary contraction (MVC) did not change immediately on insertion of the splint, but was improved at the 50% level after 7 days (P less than 0.05). While the muscular balance of the anterior temporal muscles was not affected, either immediately on splint insertion, or after wearing it for 7 days, temporal muscle activity at 10% of the MVC was greater on the side to which the mandible moved from the retruded contact position (RCP) to the inter-cuspal position (ICP), both before (P less than 0.025) and after (P less than 0.01) wearing the splint. Splint removal after 7 days resulted in increased awareness of interferences in the ICP and increases in masseter muscle asymmetry (10%, P less than 0.025; 50%, P less than 0.05) when the electromyograms in the ICP after splint removal were compared with those on the stabilization splint before removal. After wearing the splint, the masseter muscle activity at the 10% level was greater on the side where premature contacts were present in the RCP (P less than 0.01). The use of masticatory muscle asymmetry indices in the evaluation of splint treatment for craniomandibular dysfunction is indicated since submaximal masticatory muscle activity is related to occlusal stability, premature contacts in the RCP and the direction of lateral slides from the RCP to the ICP.

Adaptation, Physiological↗
PubMed1990

Changes in the main masticatory muscles in CT after mandibular condyle fracture.

The aim was to study changes in the main masticatory muscles after mandibular condyle fracture as seen by CT, comparing the densities and sizes of the masseter, medial pterygoid and lateral pterygoid muscles on the fractured side with those on the non-fractured side. More or less permanent muscular changes are detectable several years after the accident, especially in the lateral pterygoid muscle, which is attached to the condylar head. This muscle was statistically significantly smaller (p less than 0.001) on the fractured side. Examination of the masticatory muscles immediately after the fracture and at intervals afterwards could reveal more about the pathogenesis of the changes in these muscles.

Adolescent↗
PubMed1995

Asymmetry of masticatory muscle activity during intercuspal maximal clenching in healthy subjects and subjects with stomatognathic dysfunction syndrome.

The present study investigated the asymmetry of masticatory muscle activity during maximal intercuspal clenching in healthy subjects and subjects with stomatognathic dysfunction syndrome. Stomatognathic dysfunction syndrome, unilateral mastication and the asymmetry of masticatory muscle activity appear to be related to each other. The asymmetry of masseter muscle activity was greater as stomatognathic dysfunction became increasingly severe. Because fatigue and pain are produced more quickly by unilateral clenching than bilateral clenching, clenching under conditions of left and right muscular imbalance can further aggravate stomatognathic dysfunction. It is suggested the asymmetry of masseter muscle activity during maximal clenching correlates with the onset of the stomatognathic dysfunction syndrome. Stomatognathic dysfunction syndrome is closely related to the asymmetry of masseter muscle activity and only slightly related to the asymmetry of temporal muscle activity. The asymmetry of anterior temporal muscle activity appears to have little clinical significance.

Adolescent↗
PubMed1989

[Effect of bite plane on electromyographic activity of masticatory muscles. 1. Muscle activity during tapping movement].

The purpose of this study is to examine the influence of the insertion of bite plane on masticatory muscle activity during tapping movement, and find out useful parameters for examination and diagnosis of stomatognathic dysfunction. The subjects in this study were twenty normal subjects with intact dentition and no signs of mandibular dysfunction as the control group, and twelve subjects with unilateral temporomandibular joint pain during jaw opening or closing movement as the patient group. Electromyographic activity in the bilateral anterior temporal and masseter muscles and mandibular movement using the Mandibular Kinesiograph were recorded simultaneously during tapping movement. The usefulness of the 61 items used in this study was evaluated. The results were summarized as follows: 1. In the patient group, a significant correlation between the jaw opening degree during tapping movement and the maximal jaw opening one without pain was recognized. 2. The total muscle activity in the patient group decreased significantly by the insertion of the bite plane. 3. Many subjects of the patient group showed a significant difference between the right and left activity of each muscle. 4. A significant difference between the two groups was found in the duration of muscle activity before and after Silent period, those coefficients of variation and the difference between those durations of the right and left muscle activity. This study suggests that the above factors are very useful parameters for examination and diagnosis of temporomandibular joint dysfunction.

Humans↗