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PubMed2002

Evidence for functional partitioning of the rabbit digastric muscle.

The rabbit digastric muscle has a single belly that opens and retracts the mandible. It does not contain connective tissue partitions, and all fibers arise from the same tendon and insert into a single broad site. Historically, it was assumed that the muscle functioned as a single unit. Since we had preliminary evidence that this might not be the case, we carried out five small studies in rabbits. First, we showed that electromyographic (EMG) activity varies between recording sites within the muscle during the masticatory cycle induced by repetitive stimulation of the sensorimotor cortex. We found that EMG activity in the caudal region sometimes began before the anterior EMG during mastication when the jaw swung to the side of the muscle, but the two regions became active at the same time during other patterns. We next showed that separate branches of the mylohyoid nerve enter the anterior, intermediate and caudal regions of the digastric. However, a separate study showed that the motor endplates were distributed across a continuous sheet, consistent with a single anatomical partition. We then stimulated single nerve branches to deplete glycogen. By comparing the optical density of fibers labeled by the periodic acid-Schiff method for glycogen, we were able to show that the three branches innervate separate regions of the muscle. Finally, we applied either FluoroGold or Fast Blue dyes to the central cut ends of the branches to label the cell bodies of the three pools of motoneurons. These were found within the middle and caudal thirds of the trigeminal motor nucleus, but there appeared to be no spatial separation of the three pools or double labeling of cells. We conclude that the digastric muscle contains two and possibly three functional subregions. The fact that the motoneurons are intermingled suggests that the distribution of motor commands to the three pools is not based on their location.

Animals↗
PubMed2005

Soft tissue mechanics of the temporomandibular joint.

Direct measurement of temporomandibular joint (TMJ) tissue deformation requires animal experimentation. Most of the available data pertain to the mechanical strain on the bone surfaces around the joint. However, bone is rarely the first joint tissue to show injury, being affected after damage to collagenous tissues such as the disc or capsule. Capsular ligaments guide or limit movement, while the intra-articular disc may also distribute joint loads. However, these tissues are difficult to visualize dynamically and not suitable for strain gage attachment, so in vivo deformations are poorly understood. Using pigs as the best nonprimate model for human TMJ function, we implanted differential variable reluctance transducers to measure antero-posterior strain in the lateral aspect of the intra-articular disc. The results were compared to previously published data on the TMJ capsule. Passive manipulation in anesthetized animals indicated that opening, protrusion, and contralateral movements caused the disc to elongate. On the contrary, closing, retrusion and ipsilateral movements caused disc shortening. These strains are opposite to those observed in the capsule and are expected on anatomical grounds. Surprisingly, disc strain during mastication differed from that during manipulation. The disc elongated during jaw closure, more on the retruding balancing side (16% +/- 1) than on the working side (8% +/- 2). This anomalous behavior may reflect compressive loading, such that the disc elongates as a result of the Poisson effect rather than condylar movement. Because the capsule also elongates during the power stroke, especially on the balancing side, both disc and capsule are maximally loaded on the same side at the same moment.

Animals↗
PubMed2005

Anatomically based modelling of the human skull and jaw.

We present here an anatomically based model of the human masticatory system that provides a framework for simulating the complex chewing process. The initial motivation for creating this model was the desire to have a computational model of the human jaw that can be used to simulate the action of simple bites, and to calculate the stresses and forces on the teeth that are involved. The model created also provides a platform that can be used to investigate other features of the masticatory system. To construct this global model, individual models of the bones of the skull and jaw were created from generic data sets. Geometric models of the muscles of mastication were also created and attached to the appropriate bones. To complete this initial model, representations of the crowns of the teeth were created and a basic model of the temporomandibular joint (TMJ) was included. The finite element method was used to solve for the stresses and strains created by the loading conditions during a clenching simulation involving the mandible bone. The model presented here is also discussed in relation to a model of the entire musculo-skeletal system being developed as part of the Physiome Project.

Computer Simulation↗
PubMed1982

Sarcomere length and EMG activity in some jaw muscles of the rabbit.

The sarcomere length of the rabbit pterygoid, temporal and digastric muscles was determined in seven mandibular positions. Six positions are representative of mastication, one of gnawing. Jaw displacement has a different effect in jaw openers (digastric and lateral pterygoid) and closers. The sarcomeres of the openers are long when the jaw is in molar occlusion and shorten relatively little at jaw opening. In the jaw closers they are very short in molar occlusion and become strongly extended when the mouth is open. Timing of electrical activity and sarcomere shortening corresponds closely. All muscles become active at sarcomere lengths of 2.6-3.1 micrometers. In the jaw closers, sarcomeres shorten till 2.1-2.3 micrometers during activity; in the jaw openers they do not shorten beyond 2.6-2.7 micrometers. Hence the openers remain close to optimum length (2.7 micrometers) while the closers shorten to lengths where possibly only 60% of the maximal isometric force can be exerted. During the initial phase of jaw closing, the medial pterygoid and temporalis shorten at speeds estimated to approach 50% of the presumed maximum speed. During the rest of the closing and power stroke, contraction takes place under near-isometric conditions, with little loss of force due to shortening. The jaw openers act at low shortening velocities.

Analysis of Variance↗
PubMed1988

Effect of altered masticatory function on [3H]-thymidine and [35S]-sulfate incorporation in the condylar cartilage of the rat.

The response of the condylar cartilage to alterations in compressive joint reaction forces in vivo has been little studied. In an attempt to reduce or eliminate the occlusal forces resulting from mastication or incision, male weanling rats were fed a soft diet requiring little chewing and/or had their incisors clipped every other day. Incorporation (dpm/micrograms DNA) of [3H]-thymidine and [35S]-sulfate was significantly decreased relative to controls in the incisor-clipped group, but not in the soft-diet group. Animals having both treatments also exhibited significantly lower incorporation values than controls, suggesting the importance of incision for loading at the mandibular joint. These data corroborate in vitro studies which suggest that compressive forces can affect mitotic activity and synthesis of proteoglycans in the condylar cartilage. However, additional factors, both hormonal and biomechanical in nature, may be important in the in vivo environment.

Animals↗
PubMed1989

Effect of eating cheese on Ca and P concentrations of whole mouth saliva and plaque.

A study was undertaken to examine the release of calcium and phosphate from cheese during mastication. Unstimulated saliva was collected for baseline analysis in the initial study followed by saliva collection after chewing different cheeses with and without biscuits. In the second study, volunteers who had abstained from tooth cleaning for 24 h had plaque samples taken from two quadrants, they then chewed cheese in their own personal eating manner, and a second sample of plaque was taken within 5 min. The results showed that the calcium ion concentration of the oral fluids rose from a mean of 30 micrograms/ml to between 200 and 540 micrograms/ml, depending on the type of cheese, but the phosphate concentration fell below baseline. The release of both ions tended to be less when the cheese was eaten with a biscuit. In the second study a highly significant rise in plaque calcium concentration was shown after eating cheese, but no consistent change in phosphate level was found. Acidic soft drinks, following eating, tended to reduce the plaque calcium levels, but no consistent change was found if tea or coffee was taken following the cheese consumption. It is suggested, from these findings, that cheese eaten alone at the very end of a meal raises plaque calcium and might be effective in reducing dental caries.

Beverages↗
PubMed2003

External carotid artery territory ischemia impairs outcome in the endovascular filament model of middle cerebral artery occlusion in rats.

BACKGROUND AND PURPOSE: Middle cerebral artery occlusion (MCAO) by an intraluminal filament is a widely accepted animal model of focal cerebral ischemia. In this procedure, cutting of the external carotid artery (ECA) is a prerequisite for thread insertion. However, the implications of ECA transsection have not yet been described. METHODS: After 90 minutes of filament MCAO or sham surgery, rats were evaluated for up to 14 days in terms of body weight development, core temperature, and motor performance. Repeated in vivo MRI of the head and neck was performed for quantification of brain edema and infarct volume. The temporal muscles were histologically analyzed postmortem. RESULTS: In 47% of all rats, ischemic tissue damage to the ipsilateral ECA area, including temporal, lingual, and pharyngeal musculature, was detectable by MRI. Histology of temporal muscles confirmed acute ischemic myopathy. Animals with ECA territory ischemia (ECA-I) showed delayed body weight development and poorer recovery of motor function. There was no difference in the extent of brain edema or final cerebral lesion size between ECA-I-affected and unaffected rats. CONCLUSIONS: Filament MCAO was complicated by the consequences of ECA ischemia in approximately half of all rats. Impaired mastication and swallowing functions restricted ingestion and resulted in postsurgical body weight loss and worse motor performance. Impaired cerebral microperfusion resulting from dehydration and reduced spontaneous motor activity resulting from reduced food and water uptake might have contributed to poorer neurological recovery in ECA ischemic rats. Thus, adverse effects caused by extracerebral ischemia with potential impact on outcome have to be considered in this stroke model.

Animals↗
PubMed2001

Influence of dentition status on physical disability, mental impairment, and mortality in institutionalized elderly people.

The loss of teeth is known to influence the mastication of foods and nutritional status. Therefore, we hypothesize that poor dentition status can impair the systemic health of the aged. To clarify the influence of dentition status on deterioration in physical ability, mental impairment, and mortality, we conducted a six-year prospective cohort study of the institutionalized elderly living in 29 of the 30 institutions for the elderly in Kitakyushu, Japan. Bivariate analysis revealed that worse dentition status at baseline led to significantly worse physical and mental impairment, and higher mortality. In a multiple logistic regression analysis, the physical ability of edentulous subjects without dentures significantly deteriorated compared with that of dentate subjects with 20 or more teeth. The six-year mortality rate of the edentulous subjects without dentures was significantly higher than that of the subjects with 20 or more teeth. Poorer dentition status, especially edentulousness without dentures, may therefore be related to deterioration in the systemic health of the aged.

Aged↗
PubMed2001

Condylar motion in patients with reduced anterior disc displacement.

The influence of reduced anterior disc displacement on condylar motion has not been fully examined in young adults. Reduced anterior disc displacement was hypothesized to inhibit condylar motion. Using a six-degrees-of-freedom jaw-tracking system, we recorded bilateral condylar motion during maximum open-close jaw movement and gum-chewing on both sides in ten young adults with unilateral reduced anterior disc displacement and in ten control subjects without temporomandibular disorders. The bilateral condylar motion during both maximum open-close jaw movement and chewing on the disc-displacement side was inhibited in the test group. The condylar motion on the disc-displacement side during chewing on the non-disc-displacement side was also inhibited in the test group. These results suggest that the limitation of condylar motion on the disc-displacement side may influence condylar motion on the non-disc-displacement side during maximum open-close jaw movement, and mastication on the disc-displacement side in young adults.

Adolescent↗
PubMedDate not supplied

Reproducibility of the electromyogram.

Electromyographic recordings of masseter muscles were taken during mastication without the introduction of an experimental variable. The amplitude, duration, and sequence of the onset of muscle bursts were used to evaluated the recordings. Amplitude showed the most variation during the same session, at different sessions, and between three different areas of the masseter muscle. Onset showed the least variation during the same session and at different sessions; duration showed the least variation between areas.

Adult↗
PubMed1979

A preliminary scanning electron microscope examination of wear striation direction on primate teeth.

Wear striations experimentally produced on initially unworn teeth were examined at high magnification using a scanning electron microscope. Certain characteristics of individual wear striations on these teeth indicate the direction of motion that produced them. Other striations on worn teeth of American Indians and the Paleocene primate Phenocolemur show similar characteristics and correspond to mandibular movement during mastication.

Animals↗
PubMed1981

Direct measurement of loads at the temporomandibular joint in Macaca arctoides.

This paper describes a novel technique of directly measuring loads at the condyle in Macaca arctoides and provides recordings on one animal while measuring loads at the TMJ. This study is the first to actually measure loads directly at the articulating surfaces of the condyle. The data collected in this study confirm Hylander's data that the TMJ is a stress-bearing joint. With this pilot study, observations which can be made are: 1) The loads at the condyle are minimal during posterior chewing of both soft and hard foods (1-3 lb), and 2) during mastication of food the load (3-4 lb) at the condyle is larger than for posterior chewing. It was demonstrated during the calibration of the transducer that as the condyle translates forward over the articulating eminence during opening, a load is produced on it which is as large as the load present during incisal biting. Since recordings were only successfully taken on one animal, the above observations must be confirmed with further experiments.

Animals↗
PubMed1982

Effect of increasing the bulk content of the diet on the rat parotid gland and saliva.

The present study, along with others, shows that when the requirement of a diet for mastication is increased, enlargement of the parotid gland occurs. This study demonstrated for the first time that the magnitude of the gland enlargement is directly related to the amount of nonnutritive bulk incorporated into the diet, and that the concentration of protein is significantly increased in the parotid saliva of rats fed such a diet. The findings show that, although the concentration of protein is increased in the parotid saliva, the composition of proteins is unchanged, leading to the conclusion that all secretory proteins are increased in parallel. A comparison of the gland enlargement observed in response to the bulk diet with that observed in response to chronic isoproterenol treatment reveals many similarities. However, the fact that the protein composition of the parotid saliva is not altered when rats are fed the bulk diet is in marked contrast to the striking changes observed following chronic isoproterenol treatment. The reason for the dissimilar response is not known. However, an investigation into the differences that exist in the gland response to these two means of inducing gland enlargement may lead to a better understanding of the factors that regulate the size of the rat parotid gland and the complement of proteins in its secretion.

Animals↗
PubMed1984

Frequency-dependent modulation of rhythmic human jaw movements.

Modulation of jaw movement patterns by alteration of the chewing rhythm was analyzed in nine subjects. Jaw movements were monitored by an electronic transducer system. The subjects were asked to chew gum either at voluntarily determined rates or at rates guided by a light flash that varied from 2 to 7 Hz at 1-Hz intervals. The results can be summarized as follows: Chewing was generally conducted at rates below 3 Hz when the rhythm was voluntarily determined. When the rhythm was guided, however, the maximum chewing rate could occasionally be raised up to 6 Hz; the duration of the chewing cycle was more closely related to changes in the duration of the opening and occlusion phases than to that of the closing phase; at chewing rates below 3 Hz, the maximum gape did not appreciably change with the chewing rhythm, while it decreased linearly at rates above 3 Hz; and at chewing rates below 3 Hz, the jaw opening velocity increased with speed of chewing, whereas it decreased or sometimes fluctuated over a wide range at rates above 3 Hz. It is concluded that, during normal mastication at rates below 3 Hz, chewing rhythm is altered by changing both the velocity of jaw opening and the duration of the occlusion phase.

Adult↗
PubMed1986

A cinephotographic study of the role of the canine in limiting lateral jaw movement in Macaca fascicularis.

Three specimens of Macaca fascicularis were examined to test the hypothesis that the canines do not restrict lateral masticatory movements in these animals. The animals were filmed while chewing freely before, shortly after, and seven weeks after canine removal. Facial markers, viewed from the lateral plane, allowed for computer tracings of all mandibular movements. The maximal lateral movement of the mandible was assessed for each chewing cycle, and means were calculated for each test condition and animal. These means were compared intra-animal for statistically significant differences in lateral jaw movement before and after canine removal. No significant long-term increase in lateral movement was found in any animal after the removal of its canine teeth. We conclude that, in accordance with our hypothesis, the lateral movement during mastication did not increase significantly after removal of the canine teeth in three specimens of Macaca fascicularis. Therefore, the use of the macaque as a human model for masticatory studies requires further assessment. This study reveals that the canine teeth are not as important as previously assumed in affecting the form of the masticatory cycle in the macaque.

Animals↗
PubMed1993

Amplitude and timing of EMG activity in the human masseter muscle during selected motor tasks.

Electromyographic (EMG) activity in the human masseter muscle was registered from six different sites, in the anterior, middle, and posterior regions of the superficial and deep layers of the muscle, during static clenching tasks (intercuspal and incisal), selected jaw movements (alternating protrusion/retrusion, right/left latero-deviation, and open/close excursions), and unilateral chewing on right and left sides. Peak-EMG amplitudes and the timing of the peaks were compared. Activity in the regions of the deep masseter was either higher (in mastication and intercuspal open/close excursions) or lower (incisal clenching) than the activities in the superficial masseter. Superficial and deep masseter also differed in their timing of peak EMG: During chewing, peak activity passed from superficial to deep in the balancing-side muscle, and from deep to superficial on the chewing side. During free latero-deviations, peak activity started in the deep masseter, when the jaw moved to the right side (i.e., the side of the muscle), and then passed to the superficial regions, after the jaw movement was reversed to the left side. In addition, within the deep masseter there existed clear anteroposterior differences in activation level (during incisal clenching and open/close excursions) and in timing (during latero-deviation). Such a differentiation of activity was not found in the superficial masseter.

Adult↗
PubMed1994

Pain responses to experimental chewing in myofascial pain patients.

Although patients with myofascial pain of the masticatory muscles often report that chewing exacerbates their pain, this has never been verified experimentally. In this study, pain was assessed before and after chewing in 20 asymptomatic subjects and in 61 patients with muscle pain. First, self-reports of pain were obtained with a checklist and on five-point category scales (CAT) at the screening visit. None of the asymptomatic subjects reported that mastication or other jaw movements caused pain. On the other hand, the majority of patients reported that movements were painful (67.2%, checklist; 78.7%, CAT). Afterward, pain intensity at rest and after chewing on wax for 3 min was reported on 100-mm Visual Analogue Scales (VAS). No asymptomatic subjects had pain before or after the chewing test, while about 50% of the patients reported an increase of pain after chewing. In this subgroup, mean pain intensity increased by 102.6%. However, mean pain intensity after chewing decreased by 56.6% in about 30% of the patient sample. These patients had significantly higher resting pain than the first subgroup. These data show that a short chewing test can exacerbate pain in most myofascial pain patients but has no effect in asymptomatic subjects. Surprisingly, the exercise decreased pain in an important subgroup of patients. These results suggest that two subgroups of myofascial pain patients may exist with opposite reactions to exercise. It remains to be seen if these reactions are due to two different pathologies or to the fact that the pre-exercise pain levels were significantly different in the two groups.

Adolescent↗
PubMed1994

Within-subject comparisons of implant-supported mandibular prostheses: evaluation of masticatory function.

In the past, fixed prostheses were believed to be more efficient implant-supported devices than removable types for edentulous patients. However, this hypothesis was never properly tested. Therefore, a within-subject crossover clinical trial was designed in which 145 completely edentulous subjects were tested wearing implant-supported mandibular fixed prostheses and long-bar overdentures. Eight subjects received the fixed appliance first and seven the removable type. The patients' perceptions of various characteristics of the implant-supported prostheses were measured after a minimum of two months' adaptation. Mandibular movements and jaw muscle electromyographic activity were recorded while the patients chewed five standard-sized test foods: bread, apple, hard cheese, sausage, and raw carrot. The prostheses were then changed, worn for the same period of adaptation, and the procedures repeated. There were three test sessions per prosthesis, and each included five trials per food. The measurements were repeated three times at one-week intervals. Mastication time was found to be shorter for three foods (bread, cheese, and sausage) when subjects wore the long-bar overdenture. The vertical amplitude of the masticatory strokes was significantly less with the overdenture for all foods except carrot. Cycle duration was significantly longer with the overdenture for sausage and carrot. Contrary to what might be expected, the long-bar overdenture appears to be no less efficient than the fixed prosthesis. Furthermore, these data suggest that patients are capable of adapting their masticatory movements to the characteristics of the two prostheses.

Adult↗