Impact velocities of the teeth after a sudden unloading at various initial bite forces, degrees of mouth opening, and distances of travel.

A potentially dangerous situation arises when an individual bites on hard and brittle food which suddenly breaks, since the impact velocity of the lower teeth onto the upper teeth after the food is broken can be high and may cause dental damage. The present experiments were designed to study the magnitude of the impact velocity after a sudden unloading at various initial bite forces, degrees of mouth opening, and distances of travel. Subjects were asked to perform a static biting task during which the resistance to the bite was suddenly removed. The upward mandible movement was arrested after a certain distance. The velocity of the lower teeth at impact was calculated just before the mandible came to a standstill in combinations of 4 different bite forces (100, 80, 60, and 40 N), 4 different initial degrees of mouth opening (33.5, 30.5, 27.5, and 24.5 mm), and 3 different distances of travel of the mandible (4.5, 3.0, and 1.5 mm). We found that the bite force rapidly declined after the unloading, resulting in a small impact velocity of the lower front teeth. This impact velocity largely depended on the magnitude of the initial bite force and the distance traveled; it was barely sensitive to variations in degree of initial mouth opening. The maximal velocity of the lower teeth was 0.43 m/s (at an initial bite force of 100 N). This maximum was reached after a distance of travel of about 4 mm in 12 ms. The data suggest that the rapid decline in bite force coupled with a limitation of impact velocity is due to the force-velocity properties of the active jaw muscles and is not caused by neural control.

Influence of visual feedback on human isometric bite-force tremor.

In contrast to recent reports, during an isometric short forceful bite, visual feedback had a significant influence on the force tremor spectrum. The value of a 'half-value frequency', being the frequency f1/2 at which, with increasing frequency, the amplitude of the spectrum for the first time drops to half its initial value, was used as an indicator for the spectral behavior. Under visual feedback, the amplitude contribution to the force spectrum in the 3-5 Hz frequency range was much larger than after deprivation of visual feedback. Elevations in the frequency range between 3 and 5 Hz in the force spectrum are interpreted as an expression of a visual feedback loop with a tau between 100 and 200 ms. This is supported by the visual/oral reaction times recorded, which were between 110 and 190 ms.

An electromyographic study of whether the digastric muscles are controlled by jaw-closing proprioceptors in man.

Whether in the oral system the digastric muscles (which lack muscle spindles) are under the control of proprioceptive information from the masseter muscles (which contain muscle spindles) was investigated by analysing whether and how the masseters and digastrics showed coordinated behaviour during a static, forceful bite. Subjects were asked to maintain a 100-N force for 15 s with and without visual guidance; bite force exerted, and masseter and digastric electromyograms (EMGs) were recorded. Under visual guidance all subjects co-contracted their digastric muscles during the isometric bite. They held the force for a short time, followed by periods with fluctuations (peak-to-peak force amplitude about 15-20 N). Fluctuations in bite force correlated with the masseter EMGs, the maximum in the correlogram occurring at about -50 ms with the force lagging the masseter. In 75% of the subjects a significant periodic component in the masseter and in the force spectra was found at about 4 Hz. This was also seen in the amplitude spectra of the forces, which showed in 80% of the subjects a significant elevation between 7-10 Hz as well. No correlation between the digastric EMGs and the bite forces, and between the EMGs of masseter and digastric could be detected. Spectra of digastric EMGs showed no prominent maxima. When subjects were deprived of visual feedback, maxima at -50 ms in the cross-correlation functions of the masseters and the forces were reduced considerably; periodicities of +/- 250 ms disappeared.(ABSTRACT TRUNCATED AT 250 WORDS)

Digastric muscle response as a function of knowledge of the task to be performed.

Whether the motor programme executed by the digastric muscles during a forceful bite is modified according to a subject's expectation that the resistance between the teeth will change was investigated. There were two experimental conditions: (1) tracking a ramp (drawn on an oscilloscope screen) by biting (isometrically) on a force transducer and holding it at 120 N, and (2) tracking the same ramp with a sudden unloading at 100 N. There were two groups of experiments: (1) control experiments in which subjects underwent a sudden and unexpected unloading of the jaw, and (2) experiments in which subjects were previously informed whether or not there was to be an unloading. In all experiments the subjects co-contracted their digastric muscles during the bite as compared to the state at rest. The subjects' responses fell into the three different types: (i) those who varied the level of tonic digastric activity only as a function of the experimental condition, (ii) those who co-contracted the digastric muscles at the same time as the masseter muscles, and (iii) those who changed the contraction pattern of the digastric muscles as a function of the experimental condition. If modulation of the digastric muscles occurred this is a 'feedforward' strategy mainly based on immediate past performance.

Bite-force endurance in patients with temporomandibular joint osteoarthrosis and internal derangement.

The aim of this study was to investigate the potential clinical relevance of testing bite force endurance in patients with articular temporomandibular disorders. The endurance of a 50 N bite force was measured in 51 patients with painful temporomandibular joint disorders. The results were compared to those of a control group of 20 subjects. The force exerted was sustained until this task could not be continued because of intolerable pain or fatigue. The endurance test was repeated following therapy. Testing bite force endurance could be reliably carried out (paired t-test not significant, product-moment correlation coefficient 0.87). The mean endurance time in the patient group was significantly different from that of the control group (t = 7.43, df = 69, P < 0.01). The 95% confidence intervals for patients and controls did not show any overlap. No difference in endurance time between diagnostic subgroups could be detected (F = 1.30, df = 4,46, P < 0.28). Following treatment, all patients showed a significant increase of endurance time (t = 8.09, df = 50, P < 0.01) and reported a decrease in post-test pain. The mean difference between pre- and post-treatment endurance was 60s. Subjects of the control group stopped the biting effort predominantly because of muscle fatigue. By contrast, the main reason of the patients to cease the effort was TMJ pain. The results of this study indicate that the discriminatory power of the test is sufficient to justify its utility as a complementary tool in assessing the functional capacity of the masticatory system.

Perception of forces exerted by jaw and thumb.

By comparing the results of force matching between the jaw and the thumbs, whether subjects have any knowledge about the magnitude of exerted forces irrespective of the motor system was studied. Subjects were asked to match isometric forces of their own choice exerted by flexion of one of the thumbs with the jaw, and the other way around. The results were compared with control experiments in which subjects matched forces exerted by flexion of one of the thumbs with the other thumb and vice versa. None of the subjects was able to match correctly in all experimental conditions. All subjects displayed inconsistent matching behaviour, showing a mixture of correct matching and mismatching. This holds both for absolute matches (in N), and for matches relative to the maximal forces. The results show that knowledge about the magnitude of exerted forces is different for the jaw and the thumbs. Sensations about isometric forces exerted by the jaw or the thumbs are different within each subject and from subject to subject.

Clinical significance of bite force reproduction ability.

The objective of this study was to evaluate the clinical relevance of measurement of bite force reproduction ability. This parameter was measured at reference force levels of 2, 10, and 50 N in a group of patients with articular and nonarticular temporomandibular disorders and in a control group. The ability to reproduce the reference forces was measured at four equidistant occasions. All subjects poorly and imprecisely reproduced the reference force levels. A trend in the matches or in their imprecision could not be found (P greater than 0.05). Bite force reproduction ability did not differ between the patient group and the control group (P greater than 0.05). It was concluded that measurement of bite force reproduction ability does not provide a useful clinical assessment tool.

Psychophysical investigations into the tenderness of meat.

Tenderness of five veal meat cuts was determined by two groups of subjects: a panel of skilled butchers and a consumers' panel. The butchers estimated tenderness with and without assistance of visual information. For the consumers a procedure to measure the oral sensation by a forced choice method of successive comparison was developed. It was shown that: imprecision of the opinions of the consumers was greater than that of the butchers; use of additional visual information did not affect the butchers' precision; and the butchers' views of tenderness had a poor relationship to the consumers' oral perception of this quality.

The control of bite-force in relation to jaw position and load in man.

Whether bite-force is controlled independently of mouth opening, and jaw position is controlled independently of the imposed load of the mandible, was examined. Subjects were asked to match, at 15 and 30 mm inter-incisor distance, reference forces of 2, 10 and 50 N exerted at 15 and 30 mm mouth opening. They were able to grade bite-force, as matches made of the different forces hardly overlapped. However, at a 5 per cent level of significance in 90 per cent of the cases, the matches were dependent on jaw position. The imprecision of the 2 N matches was about 40 per cent of the reference force; those of the 10 and 50 N matches were about 25 per cent. Thus a mechanism controlling bite-force with any degree of precision is absent in the jaw system.

A hierarchy of neural control of mastication in the rat.

In rats anaesthetized with ketamine, rhythmic jaw-opening and jaw-closing movements were induced by palatal stimulation. The two masseter muscles (jaw-closing) and the four digastric muscles (jaw-opening) were fitted with electrodes, which could be used either for electrical stimulation or for recording electromyographic responses. Electrical stimulation of the masseters in the phase when the digastrics were the contracting muscles, caused responses in the digastrics. The amplitude of these responses was dependent on whether the stimulated masseters were active or not. The responses in digastric persisted when contraction of the masseters during stimulation was prevented by dantrolene sodium but they disappeared when the masseteric nerves were blocked with xylocaine. The responses in digastric are thus reflexes from stimulating afferent fibres in the masseteric nerves. Likewise, electrical stimulation of the four digastrics in the phase when the masseters were contracting, caused responses in the masseters. The amplitude of these responses, however, was independent of the state of activity of the stimulated digastrics. Furthermore, the responses in masseter disappeared when contraction of the digastrics was prevented by dantrolene sodium; but they persisted when the digastric nerves were blocked with xylocaine, provided the digastrics continued to twitch to the electric stimuli. The responses in masseter are thus reflexes in masseter caused by mechanical stretch transmitted from the digastric twitches. In the rhythmic preparation, prevention of contraction of the masseters of digastrics by dantrolene sodium or xylocaine leaves the overall frequency and amplitude of the evoked rhythmic activity unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)

Peripheral influences on the central pattern-rhythm generator for tongue movements in the rat.

Rhythmic activity in the tongue and other oral muscles was evoked by mechanical stimulation of the hard palate in ketamine-anaesthetized rats. The relation between neural discharges of single hypoglossal motoneurones and activity in the masseter and anterior digastric muscle, and stimulus parameters was analysed. By varying the stimulus parameters, hypoglossal-motoneurone activity was modulated from phasic reflex activity into rhythmic activity. The tongue and other recorded muscles are likely to be controlled by the same pattern-rhythm generator, when rhythmic activity is induced by palatal stimulation. Exteroceptive oral stimuli can activate the tongue pattern-rhythm generator independently of proprioceptive feedback. Proprioceptive input from oral structures influenced the burst duration of rhythmic hypoglossal motoneurone activity. The cycle duration and the number of discharges/burst, however, was not affected by proprioceptive input. The hypoglossal-motoneurone pool may be influenced by more than one pattern-rhythm generator because the burst characteristics of rhythmically-firing hypoglossal motoneurones depend upon stimulus conditions. These hypoglossal pattern-rhythm generators have possible hierarchic relations because a shift from one burst pattern into another can be evoked by changing stimulus conditions. The pattern-rhythm generators for oral movements may be composed of a neural-pattern generator and a neural-rhythm generator, which can be modulated separately by peripheral inputs.

Relationship of the preferred vertical dimension of occlusion to the height of the complete dentures in use.

The dependence of the preferred vertical dimension of occlusion (PVDO) of edentulous subjects on the height of the complete dentures the subjects were wearing was investigated. Two experiments were carried out. Experiment I tested whether PVDO is influenced by a change in the vertical dimension of occlusion of the dentures. Experiment II compared PVDO values before and after insertion of dentures that had been constructed with an occlusal vertical dimension equal to PVDO. It is concluded that PVDO is independent of the height of the complete dentures the subject is wearing. Clinical implications are discussed.

Masseter, digastric and omohyoidal responses from weak mechanical oral stimuli in the chewing rat.

In rats anesthetized with ketamine, effects of jaw jerks (rise time 5 ms, strength 70 mN) applied in the open direction were studied both in the quiescent preparation and during rhythmic jaw-opening and jaw-closing movements (induced by mechanical stimulation of the palate). The left masseter, posterior and anterior digastric and omohyoid muscles were fitted with electrodes for recording electromyographic responses. In the quiescent state a jaw jerk causes responses in all recorded muscles. In the rhythmic preparation masseter responses persisted but digastric and omohyoidal responses were suppressed. This suppression was most prominent when jerks were applied during closing. However, responses in digastrics and omohyoid persisted when masseter nerves and periodontal afferents of the lower incisors were blocked with xylocaine. The suppression of digastric and omohyoidal responses therefore originates--at least partly - from masseter and periodontal afferents. When the periodontal afferents alone were blocked with xylocaine only the response in the opening phase persisted. The suppression of digastric and omohyoidal responses during opening are therefore reflexes from periodontal receptors. These observations support the concept that oral activity is controlled by two relatively independent systems. The first program-oriented system generates the basic movements whereas the second, environment-oriented, system has access to the program for correcting perturbations. There is evidence that muscles poorly supplied with muscle spindles (digastrics and omohyoid) act as pure effectors. Modulation of their activity is prompted by exteroceptors and muscle receptors in the masticatory muscles.

Factors influencing jaw position sense in man.

The relative contribution of muscle sensors, joint sensors and trigeminal exterosensors to the position sense of the mandible was investigated. Subjects compared the sizes of gauges placed between the teeth with the size of a previously explored standard gauge of 4.75 mm. The experimental conditions were either anaesthesia of both mandibular joints and the oral exterosensors, or load on the jaw-closing muscles, or adaptation to an extreme jaw position. The results were compared with control experiments under normal conditions. (1) All subjects matched to standard fairly well in control conditions, the imprecision of the match being approximately 0.5 mm. Under anaesthesia, the matches were 1-1.5 mm greater than the controls, the imprecision being almost unaffected. Anaesthetizing the joints and oral exterosensors before or after exploring the standard gauge did not affect the match. Imposing various loads (less than or equal to 3.4 kg) did not change the match to the standard. Adaptation to a wide open position introduced a considerable mismatch of the standard, leaving the imprecision of the match unaffected. (2) The experiments provide evidence that muscle afferents are important sources in sensing jaw position and fulfil the criteria for an independent position servo in the trigeminal system. It is suggested that the engram of the standard is built up mainly from muscle sensor information.

On the self-perception of jaw positions in man.

A longitudinal study shows that, in the short-term, dentate subjects are able to match fairly well the magnitude of jaw separations to any of three types of memorized standards (imagined, verbally imposed and physically imposed) of various dimensions. All subjects showed instability of the comparison mechanism or long-term changes in the engram. About 66 per cent of the matches were performed with the same precision. The relative precision of the matches is almost independent of the size of the standard. The absolute precision of matching of dentate subjects is similar to that of edentulous subjects. In half of the experiments, the subjects showed a match that equals the magnitude of the verbal standard or the real standard. The subjects tend to give more accurate matches when they refer to a verbal standard than when they refer to a real standard, but the long-term instability of matching is much greater when the subjects refer to a verbal standard than when the subjects refer to a real standard.

The precision of measurement and the stability of the preferred vertical dimension of occlusion in man.

A longitudinal study on the preferred vertical dimension of occlusion (PVDO) established with edentulous subjects shows that these subjects can be divided into two groups, one in which there is significant instability of PVDO, and a second where the instability of PVDO is not greater than can be expected from the imprecision of the PVDO matches. It is concluded that the subjects in the latter group must have referred to a reasonably fixed standard during their PVDO determinations.