Voluntary Tooth Clenching Research Articles

Interrelationships between dental occlusion and plantar arch

The aim of this study was to evaluate the influence of different jaw relationships on the plantar arch during gait. 168 subjects, participating in this study, were distributed into two groups: a control (32 males and 52 females, ranging from 18 to 36 years of age) and a Temporomandibular joint disorders group (28 males and 56 females, ranging from 19 to 42 years of age). Five baropodometric variables were evaluated using a baropodometric platform: the mean load pressure on the plantar surface, the total surface of feet, forefoot vs rearfoot loading, forefoot vs rearfoot surface, and the percentage of body weight on each limb. The tests were performed in three dental occlusion conditions: mandibular rest position (REST); voluntary teeth clenching (VTC); and cotton rolls placed between the upper and the lower dental arches without clenching (CR). The variables were analyzed through repeated measures ANOVA. The Mann-Whitney test was used to compare the postural parameters of the two groups. The level of significance was p<0.05. As to the intra-group analysis of TMD group, all posturographic parameters in both lower limbs showed a significant difference between REST vs CR (P<0.001) and between VTC vs CR (p<0.001), except for the percentage of body weight on each limb. The control group showed a significant difference between REST vs VTC, REST vs CR and VTC vs CR (p<0.001) in the mean load pressure on the plantar arch, forefoot surface, rearfoot surface and total surface of feet. The mean load pressure on the plantar arch in VTC, and the forefoot and total surfaces of feet in CR (p<0.05) were significantly higher in the TMD group in both limbs. The results of this study indicate that there are differences in the plantar arch between the TMD group and control group and that, in each group, the condition of voluntary tooth clenching determines a load reduction and an increase in surface on both feet, while the inverse situation occurs with cotton rolls. The results also suggest that a change in the load distribution between forefoot and backfoot when cotton rolls were placed between the dental arches can be considered as a possible indicator of a pathological condition of the stomatognathic system (SS) which could influence posture. Therefore the use of posture monitoring systems during the treatment of stomatognathic system is justified.

The effects of chewing-side preference on human brain activity during tooth clenching: an fMRI study

Chewing-side preference (CSP) may be associated with dominant cerebral hemispheric organisation. However, little information exists regarding whether CSP is reflected by preferential activity in the opposite (to the CSP) cerebral hemisphere. The aim of this study was to investigate the effects of CSP on cerebral cortex response to bilateral tooth clenching. Sixteen right-handed participants with left (two men: 29·0±8·4 years old, six women: 32·3±4·8 years old) or right (four men: 31·0±6·1 years old, four women: 30·8±4·7 years old) CSP were scanned by functional magnetic resonance imaging during moderate levels of voluntary tooth clenching. The on-off sequence of scanning was 30 s of clenching (on) and 30 s of rest (off) a total of five times. The results showed that blood oxygen level-dependent signals in the contralateral (to the CSP) primary sensorimotor cortex increased more than in the ipsilateral primary sensorimotor cortex in participants with both left and right CSP (P≤0·001). The supplementary motor area was activated in participants with left (P≤0·001) but not right CSP. Activation of the inferior frontal gyrus and inferior parietal lobule was greater in participants with right versus left CSP (P≤0·001). Significant (P≤0·001) activation was observed in the parahippocampal gyrus in five of eight participants with left CSP, whereas no activation was observed in those with right CSP. These findings suggest a relationship between hemispheric dominance and CSP in the primary sensorimotor cortex responsible for bilateral tooth clenching.

Excitability changes in human hand motor area induced by voluntary teeth clenching are dependent on muscle properties

To investigate whether the early effects of voluntary teeth clenching (VTC) among the first dorsal interosseous (FDI), abductor digiti minimi (ADM), and abductor pollicis brevis (APB) muscles are differently modulated depending on their muscle properties, we examined the responses of motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation with selected current directions and by brainstem magnetic stimulation (BMS). Although MEP responses with anterior-medially current direction (preferentially elicited I1-waves) were facilitated in all three muscles, those responses with posterior-laterally current direction (preferentially elicited I3-waves) were different among FDI, ADM, and APB muscles. That is, MEP responses in FDI and APB muscles were significantly reduced, whereas those responses in ADM muscle were not significantly reduced. Further, inhibitory effects of VTC in FDI muscle were more potent than those in ADM or APB muscles. On the other hand, the responses to BMS were unchanged by VTC in all three muscles, suggesting that the modulations of MEP were attributed to the cortical origin. On the basis of our previous findings that the inhibitory connections in FDI muscle are more potent than those in ADM muscle (Takahashi et al. in Clin Neurophysiol 116:2757-2764, 2005), the cortical effects of VTC among three hand muscles are differently modulated, depending on muscle properties, presumably the extents of inhibitory connections to corticospinal tract neurons. Considering that the functional capacity in FDI muscle is higher than that in ADM or APB muscles, the cortical inhibitory effect of VTC might contribute to the sophisticated regulation of the motor outputs even during VTC.

Excitability changes of hand motor area dependent on afferent inputs induced by different facilitation methods

Motor imagery (MI) and voluntary teeth clenching (VTC) induce excitability changes of hand motor area in primary motor cortex (M1). Additionally, M1 receives thalamocortical and corticocortical afferent inputs. However, relationships between afferent inputs and excitability changes induced by above-mentioned facilitation methods are not clear. Therefore, we investigated whether different effects of MI and VTC occur on early and late I-wave components using transcranial magnetic stimulation (TMS). TMS was applied to the left M1 by antero-medially directed current (to preferentially elicited early I-wave components) and by postero-laterally directed current (to preferentially elicited late ones). Motor evoked potentials (MEPs) were recorded from the right relaxed first dorsal interosseous (FDI) muscle. During MI, MEP amplitudes of late components were significantly larger than those of early ones, although both MEP amplitudes were enhanced. Surprisingly, during VTC, MEP amplitudes of early components were significantly enhanced, but those of late ones were rather depressed. That is, afferent inputs to M1 induced by MI enhanced both early and late I-wave components, but those by VTC have separate influences. Thus, it is assumed that early and late components of I-waves could be differently recruited depending on different afferent input pathways to M1.

Remote effects of voluntary teeth clenching on excitability changes of the human hand motor area

The aim of the present study was to investigate effects of voluntary teeth clenching (VTC) on motor evoked potentials (MEPs) from the first dorsal interosseous (FDI) muscle to transcranial magnetic stimulation (TMS) by different oriented currents (anterior-medially (AM), posterior-laterally (PL)) of the human motor cortex. In active FDI, VTC enhanced MEP responses induced by AM directed current but reduced these responses induced by PL. In relaxed FDI, VTC enhanced MEP responses by AM but had no significant effects on those by PL. Thus, the results suggest that any components produced by AM directed current were enhanced, whereas those by PL directed currents were not affected or reduced. The present evidence indicates that I-waves recorded at the same latency were not completely the same between those produced by AM and PL directed currents. Because VTC had no influence on responses to brainstem electrical stimulation (BES) or F-waves just after the onset of teeth clenching [T. Furubayashi, K. Sugawara, T. Kasai, A. Hayashi, R. Hanajima, Y. Shiio, N.K. Iwara, Y. Ugawa, Remote effects of self-paced teeth clenching on the excitability of hand motor area, Exp. Brain Res., 148 (2003) 261–265], these modulatory effects on MEPs to both AM and PL directed currents must be due to changes of the motor cortical excitability. Thus, we conclude that VTC affects the motor cortical circuits activated by PL and AM directed currents differentially; it facilitates the one and inhibits the other. This is the first demonstration of opposite effects of the same maneuver on MEP responses elicited by AM and PL directed currents.

Facilitation of motor evoked potentials and H-reflexes of flexor carpi radialis muscle induced by voluntary teeth clenching

In order to examine the effects of remote facilitation on cortical and spinal sites, we recorded motor evoked potentials (MEPs) and H-reflexes from the flexor carpi radialis muscle of 13 healthy subjects. The H-reflex was used to assess excitability changes at the spinal level, while the MEP following transcranial magnetic stimulation was used to study excitability changes at the cortical level. We induced remote facilitation by means of voluntary teeth clenching (VTC), the so-called Jendrassik maneuver, because this procedure is known to be effective and reliable. Although the facilitation induced by VTC was observed in both evoked potentials (i.e., H-reflex and MEP), which is consistent with previous reports, MEP onset latencies were shortened by VTC in proportion to an increased MEP amplitude, whereas the latencies of the H-reflex were not. Furthermore, statistically significant relationships between MEP latencies and amplitudes were observed in all subjects, whereas no such relationships were observed for the H-reflex. On the basis of these results, two neural pathways are presumed: one involving a release of pre-synaptic inhibition at the spinal level and the other involving an unmasking of lateral excitatory projections at the cortical level.

An evaluation of temporomandibular joints and jaw muscles after orthodontic treatment involving premolar extractions

Experimental subjects (n = 29) were patients who had undergone orthodontic treatment in combination with extraction of maxillary or mandibular premolar teeth, or both. Control subjects (n = 29) were healthy dental students with no orthodontic or extraction experience. Sagittal (corrected axis) tomograms of the TMJs were used to determine the narrowest linear distances between the anterior and posterior outlines of the TMJ condyle and the TMJ fossa, expressed as the joint space ratio. There were no significant (p greater than 0.05) differences between the control and experimental ratios. Bipolar surface electromyograms of the masseter and anterior temporalis muscles were used to determine the isometric contraction velocities of these muscles until 50% and 100% voluntary isometric contraction effort (teeth clenching) was achieved. There were no significant (p greater than 0.05) differences between the control and experimental subjects. Electromyograms were also used to determine the relative contribution of the masseter and anterior temporalis muscles to the bite force developed during brief maximum voluntary tooth clenching, expressed as the activity index. There were no significant (p greater than 0.05) differences between the control and experimental subjects.

Single-wire recording of regional activity in the human masseter muscle

A new approach is presented for recording regional muscle responses by means of a single intramuscular wire electrode. The technique exploits differences in impedance due to the locations of the recording and indifferent electrodes and also relies upon bandpass filtering. The method is demonstrated by recording the activity of the right masseter muscle at three different sites during voluntary tooth clenching. The results indicate that satisfactory separation of regional muscle activity can be obtained and that the recorded signals can be sampled digitally for quantitative purposes. As this method for recording jaw muscle activity avoids the problem of variable interelectrode distances commonly associated with conventional bipolar wire electrodes, it is advantageous when recording from human muscles in the facial and intraoral regions.

Aging and the accuracy of jaw muscle control.

This study was designed to investigate the effect of ageing upon the accuracy with which subjects control jaw muscle activity during voluntary tooth clenching. The experiment involved the subjects clenching on a force transducer to achieve and maintain a predetermined target force displayed using a biofeedback light bar. The subjects were seventeen healthy, dentally fit males, ranging in age from 22 to 77 years, each of whom performed the experiment on one occasion. Target forces of 0.5, 1 and 2 Kg were demanded, and were maintained for two minutes duration with an equivalent rest period between each task. Subject performance was a measure of the difference between the force achieved and the target force, and was obtained electrically using an integrator controlled by a timing device. The results showed that performance deteriorated both at higher force levels and with increasing age, although there was a wide variation in ability between different individuals.

Cumulative electromyography of the human masseter muscle during fatiguing isometric contractions.

Six adult males performed maximum voluntary tooth clenching (MVC) for 10, 20, 30, 40 s and, after 15 min, for 40, 30, 20, 10 s. During the isometric exercises the electrical currents of the masseter muscle were sampled by integrated and cumulative surface electromyography. Subjective masseter fatigue was present after 30 and 40 s of MVC clenching, accompanied by changes in myoelectrical activity. Strength testing of the masseter muscle, before and after endurance testing, showed that the strength increased by a significant 16% following two endurance tests. This observation was explained by a post-tetanic potentiation and/or a differentiated use of motor units in the fatigued muscle. It is concluded that brief MVC isometric activity, or strength testing, is not a reliable measure of fatigue in the masseter muscle when cumulative electromyography is used. Prolonged MVC isometric activity, or progressive endurance testing, monitors reliably the onset and progression of masseter fatigue.

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 log x), and it is suggested that progressive physiological, or peripheral, muscle fatigue accompanied the psychological, or central, endurance of tooth clenching.

Jaw muscle fatigue and pains induced by experimental tooth clenching: a review.

Maximal voluntary isometric contractions of the elevator jaw muscles, with the mandible in the position of maximal intercuspation of the natural teeth, can induce fatigue and weak as well as severe pains in the jaw muscles of children and adults. A subjective sensation of muscle fatigue appears after about 0.5 min of clenching, and it seems as if the psychological parameter of fatigue onset to a large, but unknown, extent is determined by the physiological parameter of the amount of isometric contractile activity, and perhaps also the original strength of the muscle; however, also individual psychological factors seem to play a role. Pains in the elevator jaw muscles appear after about 1 min of clenching. Although isometric contractions are necessary to cause onset of jaw muscle pains, it is conceivable that perception of the moment of onset of pains to a large, but unknown, extent depends on psychological factors, perhaps the intensity of pains; however, also physiological factors may play a role. The severe muscle pains and gradually increasing subjective exhaustion of the elevator jaw muscles can be endured for about 2 min of maximal voluntary tooth clenching. It is suggested that the psychological parameter of muscle endurance to a large, but unknown, extent depends on the physiological parameter of isometric contractile activity but also individual psychological factors seem to play a role.