Physiological tremor in human jaw-muscle system

Differential contributions of the two human cerebral hemispheres to action timing

Rhythmic actions benefit from synchronization with external events. Auditory-paced finger tapping studies indicate the two cerebral hemispheres preferentially control different rhythms. It is unclear whether left-lateralized processing of faster rhythms and right-lateralized processing of slower rhythms bases upon hemispheric timing differences that arise in the motor or sensory system or whether asymmetry results from lateralized sensorimotor interactions. We measured fMRI and MEG during symmetric finger tapping, in which fast tapping was defined as auditory-motor synchronization at 2.5 Hz. Slow tapping corresponded to tapping to every fourth auditory beat (0.625 Hz). We demonstrate that the left auditory cortex preferentially represents the relative fast rhythm in an amplitude modulation of low beta oscillations while the right auditory cortex additionally represents the internally generated slower rhythm. We show coupling of auditory-motor beta oscillations supports building a metric structure. Our findings reveal a strong contribution of sensory cortices to hemispheric specialization in action control.

Periodontal mechanoreceptors and bruxism at low bite forces

OBJECTIVE

In this study, we examined if 6-9 Hz jaw tremor, an indirect indicator of Periodontal Mechanoreceptor (PMR) activity, is different in bruxists compared to healthy participants during production of a low-level constant bite force.

METHODS

Bite force and surface EMG from the masseter muscle were recorded simultaneously as participants (13 patients, 15 controls) held a force transducer between the upper and lower incisors very gently.

RESULTS

Tremor in 6-9 Hz band for bruxists was greater on average compared to controls, but the difference was not significant, both for force recordings and EMG activity.

CONCLUSIONS

The low effect sizes measured with the current protocol contrast highly with those of our previous study, where larger, dynamic bite forces were used, and where jaw tremor was markedly different in bruxists compared with controls.

SIGNIFICANCE

We have now gained important insight into the conditions under which abnormal jaw tremor can be elicited in bruxism. From a scientific standpoint, this is critical for understanding the 'abnormality' of PMR feedback in bruxism. From a clinical perspective, our results represent progress towards the development of an optimal protocol in which jaw tremor can serve as a biological marker of bruxism.

Jaw tremor as a physiological biomarker of bruxism

OBJECTIVE

To determine if sleep bruxism is associated with abnormal physiological tremor of the jaw during a visually-guided bite force control task.

METHODS

Healthy participants and patients with sleep bruxism were given visual feedback of their bite force and asked to trace triangular target trajectories (duration=20s, peak force <35% maximum voluntary force). Bite force control was quantified in terms of the power spectra of force fluctuations, masseter EMG activity, and force-to-EMG coherence.

RESULTS

Patients had greater jaw force tremor at ∼8 Hz relative to controls, along with increased masseter EMG activity and force-to-EMG coherence in the same frequency range. Patients also showed lower force-to-EMG coherence at low frequencies (<3 Hz), but greater coherence at high frequencies (20-40 Hz). Finally, patients had greater 6-10 Hz force tremor during periods of descending vs. ascending force, while controls showed no difference in tremor with respect to force dynamics.

CONCLUSION

Patients with bruxism have abnormal jaw tremor when engaged in a visually-guided bite force task.

SIGNIFICANCE

Measurement of jaw tremor may aid in the detection/evaluation of bruxism. In light of previous literature, our results also suggest that bruxism is marked by abnormal or mishandled peripheral feedback from the teeth.

Task-dependent control of the jaw during food splitting in humans

Although splitting of food items between the incisors often requires high bite forces, rarely do the teeth harmfully collide when the jaw quickly closes after split. Previous studies indicate that the force-velocity relationship of the jaw closing muscles principally explains the prompt dissipation of jaw closing force. Here, we asked whether people could regulate the dissipation of jaw closing force during food splitting. We hypothesized that such regulation might be implemented via differential recruitment of masseter muscle portions situated along the anteroposterior axis because these portions will experience a different shortening velocity during jaw closure. Study participants performed two different tasks when holding a peanut-half stacked on a chocolate piece between their incisors. In one task, they were asked to split the peanut-half only (single-split trials) and, in the other, to split both the peanut and the chocolate in one action (double-split trials). In double-split trials following the peanut split, the intensity of the tooth impact on the chocolate piece was on average 2.5 times greater than in single-split trials, indicating a substantially greater loss of jaw closing force in the single-split trials. We conclude that control of jaw closing force dissipation following food splitting depends on task demands. Consistent with our hypothesis, converging neurophysiological and morphometric data indicated that this control involved a differential activation of the jaw closing masseter muscle along the anteroposterior axis. These latter findings suggest that the regulation of jaw closing force after sudden unloading of the jaw exploits masseter muscle compartmentalization.

Objective assessment of mandibular motor control using a 'reach-and-hold' task

Mandibular motor function is well known to be impaired in the presence of temporomandibular disorders. However, while a vast literature is available concerning accuracy of motor control in limbs, quantitative and objective assessment of mandibular motor control has been seldom performed, also because of the lack of adequate investigative tools. Aim of this work is to present a technique for reliable evaluation of the motor performance of the mandible based on a kinesiography-monitored reach-and-hold task. Nineteen healthy subjects were engaged in a task in which they had to drive a cursor on a screen by corresponding movements of the mandible in the frontal plane and reach 30 random targets sequentially displayed on the screen. The whole task was repeated three times per session in two different days. The individual performance was assessed by different indices evaluating precision and steadiness of target matching. The performance progressively improved in the three trials of the first session, further improved and stabilised in the second session, with an average positioning error of 0·59 ± 038 mm and was slightly correlated with the horizontal dimension of the mandible border movement (r = 0·55). Intraclass correlation coefficient ranged between 0·76 and 0·94 for the different indices indicating good repeatability. The kinesiographic technique allowed for objective and reliable assessment of the voluntary control of the mandible position. Its potential applications include support to the characterisation of temporomandibular disorders and to motor training and progress monitoring in rehabilitation treatments.

The daily habitual in vivo strain history of a non-weight-bearing bone

Daily mechanical loading strongly influences the architecture and composition of bone tissue. Throughout the day, the amplitudes, rates, frequencies, and the dispersion over time of these loads vary. Nevertheless, most experimental and descriptive studies on the aforementioned relationship consider only cyclic loading and, in addition, focus on weight-bearing bones. A more complete assessment of the daily loading of bone might lead to a better understanding of the natural everyday stimulus for bone maintenance or adaptive responses. In the present study, we measured the daily habitual strain history of the non-weight-bearing mandible bone in the rabbit. Long-term continuous strain recordings were made using an implantable telemetry device able to read out bone-bonded strain gauges. The lateral surface of the rabbit mandibular corpus was chosen as the bone surface of interest. During the recordings, which lasted up to 33 h, the rabbits (N=7) were able to move unrestrictedly in their cages, performing their habitual behaviours. Analysis of the recordings revealed that the measured bone surface was subjected to 2.9 (+/-1.4)x10(3) strain events per hour of which 1.8 (+/-1.0)x10(3) had amplitudes < or =10 microstrains (muvarepsilon). Larger strain amplitudes occurred less often and principal strains fell within the range of -517 (+/-118) muvarepsilon to 298 (+/-81) muvarepsilon. Strain rates never exceeded 10,000 muvarepsilon/s and only 8.9% (+/-7.2%) of the habitual strain rates were higher than 1000 muvarepsilon/s. Strain frequency spectra displayed clear peaks at 4-5 and 9 Hz. The wirelessly recorded daily strain history of the rabbit mandible featured peak strain amplitudes resembling those of other mammalian mandibles, but much smaller than those found in many long-bone strain measurements.

Mandibular physiological tremor is reduced by increasing-force ramp contractions and periodontal anaesthesia

We have previously shown that the application of anaesthesia to periodontal mechanoreceptors (PMRs) dramatically reduces the 6-12 Hz physiological tremor (PT) in the human mandible during constant isometric contractions where visual feedback is provided. This current study shows that during a ramp contraction where force is slowly increased, the amplitude of mandibular PT is almost five times smaller on average than when the same force ramp is performed in reverse, i.e. force is slowly decreased. This smaller tremor is associated with a higher mean firing rate of motor units (MUs) as measured by the sub-30 Hz peak in the multi-unit power spectrum. The decrease in the amplitude of PT following PMR anaesthetisation is associated in some instances with a similar increase in the overall firing rate; however this change does not match the diminution of tremor. The authors postulate that the decrease in mandibular PT during increasing force ramps may be due to a change in the mean firing rate of the MUs. The change in tremor seen during PMR anaesthetisation may in part be due to a similar mechanism; however other factors must also contribute to this.

Mandibular tremor during isometric contractions

The purpose of this short review is to consider the various hypotheses that are attributed to genesis of physiological tremor seen in the human jaw during isometric contractions.

Postural control of the human mandible

This article reviews recent experimental evidence explaining the mechanisms that support the mandible in its rest or postural position when the head is stationary and during locomotion. At rest, and during slow jaw movements, there is alternating activation of the jaw-opening and jaw-closing muscles which arises from a central pattern generator. However, this cannot account for the rest position of the mandible even when the head is stationary. Jaw movements and masticatory muscle activity were measured in subjects who stood, walked and ran on a treadmill. Even during walking, there are no bursts of masseter EMG time-locked to heel-landing. However, when subjects ran, the downward movement of the mandible in each step evokes a burst of EMG in the masseters. This is a stretch reflex in the jaw-closing muscles, which acts to limit the downward movement of the mandible relative to the maxilla during locomotion, and to restore the mandibular position towards its rest position. Thus, when the head is stationary, the low-level activity in the jaw-opening and jaw-closing muscles does not contribute to the rest position. Instead, the mandible is supported by passive viscoelastic forces in perioral soft tissues which limit vertical jaw movements even when the head moves gently up and down during walking. When the head moves more vigorously up and down, stretch reflexes in the jaw-closing muscles limit the movement of the mandible. That is, both passive forces and active reflex responses maintain jaw posture within narrow limits during brisk head movements.

Periodontal anaesthesia reduces common 8 Hz input to masseters during isometric biting

During isometric contractions of the jaw muscles, oscillations in the rectified masseteric EMG record that are coherent with the mandibular force output are evident at ~8 Hz. We have investigated the load dependence of these oscillations under both force and EMG feedback conditions and the extent to which these oscillations are coupled bilaterally in the jaw muscles. We further investigated the extent to which afferent information arising from the periodontium during biting influenced the extent of ~8 Hz EMG tremor and the bilateral coupling between masseters at this frequency. Using coherence analysis we have shown that a significant load-independent coupling of EMG between the closing muscles of the jaw occurs at ~8 Hz as a result of common ~8 Hz input to the masseters. This common input is significantly reduced when afferent information from the periodontium is blocked. These results suggest that afferent information arising from the periodontium enhances the expression of peripheral tremulous activity, which may be important for optimising the response of the jaw to changes in forces occurring between the teeth.

Methods of time and frequency domain examination of physiological tremor in the human jaw

This paper discusses, using the human jaw as a model, some of the commonly used techniques for examining physiological tremor. The EMG component driving mandibular physiological tremor approximately 7Hz can be revealed in the time domain manifestation of EMG by demodulation. The co-occurrence of approximately 7Hz physiological tremor (PT) in force and EMG can also be seen in the frequency domain representations of these signals and coherence analysis provides a method by which the degree of co-occurrence can be statistically investigated. Additionally, estimation of time lags between the signals by phase and cumulant density analysis provides evidence of the direction of dependence. Data presented herein using these techniques illustrates that for the human jaw, PT arises from a rhythmic component of EMG. This component is frequency and amplitude invariant across a range of bite forces indicating that it is not due to interaction between the stretch reflex and the mechanical resonance of the system.

Stability of the Implant/Abutment Joint in a Single-Tooth External-Hexagon Implant System: Clinical and Mechanical Review

Rigorous efforts have recently been made to reduce the recurrence of implant/abutment joint failure in single-tooth implant restorations. However, the current knowledge about the stability of implant/abutment joints in an external hexagon implant system is incomplete. We reviewed clinical data regarding single-tooth implant treatment with Brånemark implants, specifically the CeraOne abutment system (Nobel Biocare AB, Göteborg, Sweden). In vitro studies on joint stability were systematically assessed. Bending overload and the presence of misfit at the implant/abutment joint interface are the critical mechanical conditions that can make the joint unstable. Appropriate joint fitness and proper alignment of the implant should be assessed, and occlusal adjustment by narrowing the restoration width and flattening cuspal inclination should be applied to avoid bending moments caused by the lateral component of occlusal forces. Sufficient clinical reports of longer duration that evaluate and verify longer-term success of the newly manufactured joint components were unavailable.

Differential modulation of tremor and pulsatile control of human jaw and finger by experimental muscle pain

Resting tremor is seen in both the limbs and in the trigeminal motor system. These rhythmical perturbations are the result of alternating activation of antagonistic muscles, and these increase in amplitude during slow, voluntary movements. In the present study, we examined the effect of experimental muscle pain on finger and jaw tremor. The tremor in the mandible and in the middle finger was measured on separate occasions, at rest and during two constant-velocity movements. Pain was then induced by the infusion of hypertonic saline into a jaw-closing muscle (masseter) or into a finger extensor muscle (extensor digitorum longus, EDL). During masseter pain, the power at the peak tremor frequency of the mandible decreased significantly both when the jaw was at rest and during voluntary jaw movements at two velocities. In contrast, pain in EDL resulted in a significant increase in the power of finger tremor only during the two speeds of voluntary movement. No change in the peak tremor frequency was seen in either the finger or the jaw during pain. The most likely explanation for these data is that muscle pain tonically modulates the amplitude of the outputs from the central "pulsatile control" generators that drive the alternating activation of antagonistic muscles which produce tremor at rest and during movements. This modulation is in the opposite direction for systems controlling jaw and hand, suggesting a specific interaction of the nociceptive afferents with separate central oscillators.

Pulsatile control of the human masticatory muscles

Spectral analysis of jaw acceleration confirmed that the human mandible 'trembles' at a peak frequency around 6 Hz when held in its rest position and at other stationary jaw openings. The 6 Hz tremor increased during very slow movements of the mandible, but other lower-frequency peaks became prominent during more rapid jaw movements. These lower-frequency peaks are likely to be the result of asymmetries in the underlying, voluntarily produced, 'saw-tooth' movements. In comparison, finger tremor at rest and during slow voluntary movements had a mean peak frequency of about 8 Hz: this frequency did not change during rhythmical finger flexion and extension movements, but the power of the tremor increased non-linearly with the speed of the movement. The resting jaw tremor was weakly coherent with the activity of the masseter and digastric muscles at the tremor frequency in about half the subjects, but was more strongly coherent during voluntary movements in all subjects. The masseter activity was at least 150 deg out of phase with the digastric activity at the tremor frequency (and at all frequencies from 2.5-15 Hz). The alternating pattern of activity in antagonistic muscles at rest and during slow voluntary movements supports the idea that the masticatory system is subject to pulsatile control in a manner analogous to that seen in the finger.

Effect of an antirotation resistance form on screw loosening for single implant-supported crowns

STATEMENT OF PROBLEM

The gold screw of the single tooth implant-supported restoration has frequently been reported to exhibit the problem of screw loosening.

PURPOSE

This in vitro study considered an antirotation resistance form with an increased moment arm length as a strategy to increase the net effect of the retaining screw preload.

MATERIAL AND METHODS

Three groups examined included (a) standard abutments (3 mm), (b) modified standard abutments (3 mm) with 4 milled (1 x 1 mm) notches placed equidistant around the periphery, and (c) Estheticone abutments (1 mm). Crown castings of 2% gold-palladium-based alloy were made using plastic and resin patterns molded to fit inside a quarter-inch socket wrench. For testing, each specimen consisted of a 15-mm endosseous threaded implant analog, its abutment (20 N force) and crown casting (10 N force) screwed together and securely clamped in a vise. A clockwise shear (rotational) force was applied using a manual torque driver and the torque recorded to failure of the gold screw or loosening of the abutment screw.

RESULTS

Analysis revealed that the dislodging force for groups (b) and (c) was similar (69.6 and 71.0 N.cm, respectively), whereas group (a) (standard implant external hex) demonstrated that the smallest moment arm required a significantly smaller dislodging force (21.3 N.cm).

CONCLUSION

These findings suggest that the addition of an antirotation resistance form increased the length of the moment arm, thereby increasing the effect of preload and reducing the problem of screw loosening.