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.     

Factors influencing the position of artificial upper anterior teeth

Some of the factors which must be considered in the positioning of upper artificial anterior teeth have been considered. In particular, the effects of abnormal morphology of the lips and tongue are discussed, and the difficulties which they present are described. The appreciation of these difficulties allows dentists to assess and treat their patients effectively.     

Factors influencing the excitatory-masseteric reflex evoked by gingival electrical stimulation in man

An excitatory-masseteric reflex response was evoked by electrical gingival stimulation in five subjects; the reflex had an onset latency of 7.6 ms with a SD of 0.7 ms. The response occurred just prior to the silent or inhibitory period. There was consistent post-inhibitory synchronization of the EMG activity record in all subjects. Several methods were used to demonstrate this short-latency reflex. The first was to average non-rectified EMG activity. The second was to increase the number of receptors which are simultaneously activated by using multiple electrodes so that a greater area of gingiva could be stimulated. A third was to increase the level of excitability of the motoneurons in the masseteric pool by increasing voluntary bite force. Fourthly, the excitatory reflex was evoked during increased-excitatory drive to the pool, with the additional feature that the reflex appeared at a time when no ongoing EMG activity was present in the masseter muscle. Little doubt should now remain concerning the existence of a short latency excitatory reflex from intra-oral receptors to masseteric motoneurons in man.     

Changes in horizontal jaw position and intraoral pressure

OBJECTIVE: To determine the effect of an imbalance in buccolingual pressure that may be involved in molar dental compensation in the mandible and asymmetry of the dental arch in subjects with facial asymmetry. MATERIALS AND METHODS: We performed simultaneous measurement of the buccolingual pressure on the mandibular right first molar when subjects without facial asymmetry experimentally shifted the mandible laterally. Buccolingual pressures in the rest position (RP), right-shifted position (RS), and left-shifted position (LS) were compared. Moreover, T1-weighted magnetic resonance images were obtained in RP, RS, and LS. RESULTS: Tongue pressure tended to decrease in the order LS > RP > RS, while cheek pressure tended to increase in the order LS < RP < RS. The tongue/cheek pressure ratio tended to decrease in the order LS > RP > RS. There were significant positive (in RS) and negative (in LS) correlations between displacement of the tongue and tongue pressure. CONCLUSIONS: This imbalance in buccolingual pressures in the laterally-shifted mandibular position may partly explain molar dental compensation in the mandible and asymmetry of the dental arch in subjects with facial asymmetry.     

Sensorimotor discrimination during free jaw movements in man

Subjects carried out voluntary, unrestrained movements of the jaw to estimate position in space, a task involving coordinated sensory and motor functions. Incisal separation was derived from three-dimensional coordinates of movement by a magnetometer-based transducer system. Two experimental paradigms were used. One, based upon information theory, was designed to measure discrimination in terms of the number of reliably attained levels when 10 were attempted over two different ranges in both the opening and closing directions. The other method measured the variation of mean step size when subjects moved their jaws in small, equally-spaced incremental steps, opening and closing at different speeds. Using perceptual references at the intercuspal and maximum jaw opening positions, 6.5 separate levels were distinguished with accuracy, irrespective of the direction of movement. The group performance was significantly less (4.5 levels) over the smaller range between intercuspal and half-open positions. Discrimination was always best near the intercuspal position and at maximum jaw opening. No significant differences were evident in mean step size for the group in either direction of movement or speed of performance but there were marked individual differences in performance. In some, variations in incremental step size were smaller in closing than in opening phases. The presence of a zone of heightened sensorimotor acuity near the intercuspal position during free jaw movement indicates a specific adaptation of the nervous system.     

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.     

Tooth pulpal reflexes in jaw musculature in man.

Electrical stimulation of the tooth pulp elicited a short-latency, transient bilateral suppression of the voluntary masseter electromyogram. The threshold of this pulp reflex was somewhat higher than the pulpal sensory threshold, which coincided with the occurrence of a slight but distinct painful pricking sensation. When tested following pulp stimulation, the myotatic masseter reflex exhibited a phase of depression that had a much longer duration in resting conditions than during voluntary jaw closure. It is concluded that nociceptive afferents in the oro-facial area, most probably belonging to the A-delta group, have oligosynaptic inhibitory reflex connections with jaw-closing motoneurons.     

The regulation of the jaw bite force in man.

Experiments were carried out on 12 adult subjects to measure their ability to grade forces between their anterior teeth when using different conditions of perceptual reference. The subjects placed their anterior teeth across a force transducer to produce isometric forces corresponding to a series of integers called by the experimenter in random order every two seconds. The value zero was taken to be zero force and the largest integer as their maximum comfortable bite. The results, which were analyzed by a technique based upon information theory, demonstrated that the mean performance of the subjects as a group was 2.5 bits, S.D. ± 0.3 (approximately 5 discrete levels) when 10 levels were attempted. When reinforcing references were regularly interspersed between the test integers, the mean performance was raised to 2.8 bits, S.D. ± 0.2 (approximately 6 levels). The use of 20 input levels raised the performance of 4 selected subjects to 3 bits (approximately 8 levels). The results suggest that bite force discriminatory ability is much better when an immediately preceding reference is used, than when an overall conceptual reference is employed.     

Predicting foodstuff from jaw dynamics during masticatory crushing in man

In previous in vitro experiments using an Instron instrument, each test food was found to have characteristic textural properties. In vivo experiments were now made (1) to determine the degree to which variations in the vertical jaw movements during the crushing phase of mastication can be explained by the inherent properties of the foodstuff being chewed, and (2) to establish the degree to which the foodstuff being chewed can be identified by certain features of the jaw dynamics. Five adults were used for chewing tasks with standardized pieces of beef, carrot or peanut. Each subject made two trials with each foodstuff. The movement of the lower incisal point was monitored; features of movement associated with jaw closing in the first chewing cycle were considered. Five of these features were not suitable to categorize the various test foods. Each of the remaining 4, however, was able to distinguish either one food from the 2 others (2 cases), or one from another (2 cases). Pattern recognition techniques based upon principal component analysis could differentiate jaw closing patterns associated with chewing beef from those involving peanut or carrot. The extent to which peanut could be distinguished from carrot was not as predictable as the categorization of peanut or carrot versus beef. Cross-correlation of in vitro force-time breakage characteristics and the jaw movement data showed that on average 52% of the variation in the vertical jaw movement during crushing of food could be explained by the inherent properties of the food.(ABSTRACT TRUNCATED AT 250 WORDS)     

Progressive jaw muscle fatigue of experimental tooth clenching in man

To determine the onset of subjective fatigue and the subjective endurance of fatigue and pains in the masseter muscle, ten adult male subjects exercised maximal voluntary tooth clenching. Concomitantly, the electrical activity in the masseter muscle was recorded by bipolar surface electrodes and integrated. Onset of subjective fatigue occurred on average 21 s after onset of muscle hyperactivity and the isometric endurance time of the masseter muscle, during which period severe muscle pains were present, was on average 82 s. The electrical muscle activity of the threshold and endurance tests showed a curvilinear configuration (y = a+b logx), and it is suggested that progressive physiological, or peripheral, muscle fatigue accompanied the psychological, or central, endurance of tooth clenching.     

The effect of prior jaw motion on the plot of electromyographic amplitude versus jaw position

Fabrication of interocclusal splint at a thickness determined by the vertical dimension at which the jaw muscle EMG amplitude is minimum has been recommended. However, the effect of prior jaw motion and the effect of the recording site on the EMG amplitudes and on the vertical dimension of minimum EMG activity have not been documented. IEMG amplitudes at various static jaw positions achieved during opening and during closing were analyzed in nine subjects. Surface IEMGs were recorded over the left anterior temporal muscle, left masseter and left suprahyoids muscles, and by nonspecific EMG recording as described by Rugh and Drago. The jaw position was recorded in 5 mm increments by a kinesiograph. After 30 seconds of relaxation, 10 successive IEMG reading at 4-second integration times were obtained at each recording site. These 10 recordings at each requested jaw position were averaged and analyzed. The IEMG activity changed with different jaw position. As the jaw opened from centric occlusion, the IEMG from jaw closing muscles decreased to a minimum and then increased with further opening. Moreover, the IEMG for a particular jaw position differed depending on the history of the jaw movement, that is, whether the position was achieved after an opening step or after a closing step. Two factors, the amount of jaw opening and the history of jaw movement to reach that position, seemed to influence the IEMG differently in each of the recorded muscles.     

In vivo cross-sectional area of human jaw muscles varies with section location and jaw position

Muscle cross-sectional area (CSA) is used as a measure for maximum muscle force. This CSA is commonly determined at one location within the muscle and for one jaw position. The purpose of this study was to establish a method to standardize the analysis of the CSA of the masticatory muscles in vivo, and to compare the CSAs along their entire length for two different jaw positions (opened and closed). The CSAs in the planes perpendicular to the long axes of the masseter, medial, and lateral pterygoid muscles were measured in ten normal young adult subjects by magnetic resonance imaging. Our results showed large differences among the muscles and a non-uniform change in CSA after jaw-opening. The method enables the CSA measurement to be standardized in vivo, and allows for a correct comparison of CSAs in different skull morphologies.     

口腔矫形颌位研究进展与临床应用

提要：口腔矫形颌位通常是用来描述髁突位于舒适的、可重复的、重建牙尖交错牙合时必须依赖的颌位,是患者义齿修复或正畸等口腔治疗后,下颌的生理功能运动颌位。本文从临床操作的角度,及垂直向的关系和髁突位置确定,总结目前争论的几种口腔矫形颌位观点,并认为口腔矫形颌位不是某个单独的点,而是一个可接受的范围,只要能够形成和谐的颌位与牙尖交错牙合关系,牙合能在生理位行使功能,就可以认为矫形颌位的定位是正确的。     

Soluble proteins in fluid from non-keratinizing jaw cysts in man.

Fluids from non-keratinizing cysts of the jaws contain the main proteins found in plasma. The low relative concentration of macromolecular non-immunoglobulin proteins shows that there is no free passage of plasma proteins into the cyst fluid. Sufficient evidence was found to conclude that the immunoglobulins in cyst fluid are partly produced locally and partly derived from plasma. Cyst fluid immunoglobulins have antibody activity to rabbit erythrocytes. Accumulation of cyst fluid is primarily due to inadequate lymphatic drainage of the cyst cavity. Differential diagnosis between various types of non-keratinizing cysts cannot be based on the plasma protein patterns of their fluids.     

Factors influencing the bioavailability of fluoride from calcium-rich, health-food products and CaF2 in man

After single, oral doses, 8 h profiles of fluoride (F) concentrations in plasma were determined in healthy human volunteers. Bioavailability of F from bone-meal, calcium and CaF2 tablets was evaluated in relation to that of NaF. Tablets were administered either whole or as a finely-ground powder, either to fasting subjects or with breakfast. Availability was lowest from whole tablets taken by fasting subjects, and highest from powdered substances given with breakfast. Mean F availabilities ranged between 7.2 and 39 per cent with bone-meal tablets, between 20 and 59 per cent with Ca tablets, and between 0 and 47 per cent with CaF2 tablets.