Misao Kawara

Comparison of cerebral activity during teeth clenching and fist clenching: a functional magnetic resonance imaging study

Using functional magnetic resonance imaging (fMRI), we compared the cerebral activity during bilateral light fist-clenching and light-teeth clenching to provide more information on the central processing mechanisms underlying awake bruxism. Fourteen subjects participated in our study. Statistical comparisons were used to identify brain regions with significant activation in the subtraction of light fist clenching and light teeth clenching activity minus baseline. Participants also evaluated the perceived effort of clenching for each task, using a visual analogue scale of 0-100, after fMRI was performed. Bilateral light fist-clenching significantly activated the bilateral sensorimotor cortex, while light teeth-clenching was significantly associated with activation of the bilateral sensorimotor cortex, supplementary motor area, dorsolateral prefrontal cortex, and posterior parietal cortex. The VAS scores for fist clenching and teeth clenching were not significantly different. As light teeth-clenching activates a more extensive cortical network compared with light fist-clenching, we suggest that the teeth clenching may induce a more complex cerebral activity compared with the performance of a hand motor task. The clinical significance of these findings remains unknown but could perhaps be related to the propensity to trigger awake bruxism.

Repeated clenching causes plasticity in corticomotor control of jaw muscles

This study tested the effect of short-term tooth-clenching on corticomotor excitability of the masseter muscle using transcranial magnetic stimulation (TMS). Fifteen subjects with normal stomatognathic function participated. All subjects performed a tooth-clenching task (TCT) on five consecutive days. The TCT consisted of 10, 20, and 40% of maximum voluntary contraction in a randomized order within 1xa0h. All subjects underwent TMS in four sessions: pretask day 1 (baseline), post-task day 1, pretask day 5, and post-task day 5. Motor-evoked potentials (MEPs) from the masseter and the first dorsal interosseous (FDI) muscles were obtained using TMS in four sessions. Motor thresholds decreased, after the TCT, for the masseter muscle MEPs. Masseter muscle MEPs were dependent on stimulus intensity and on session, whereas FDI muscle MEPs were only dependent on stimulus intensity. Post-hoc Tukey tests demonstrated significantly higher masseter muscle MEPs post-task on day 5 with 80 and 90% stimulus intensity and above when compared with pre- and post-task day 1 values. Our results suggest that the performance of repeated TCTs can trigger neuroplastic changes in the corticomotor control of the jaw-closing muscles and that such neuroplastic changes may contribute to the mechanism underlying the clinical manifestations of tooth clenching.

Influence of visual feedback on force–EMG curves from spinally innervated versus trigeminally innervated muscles

OBJECTIVEnWe compared the influence of visual feedback between spinal and trigeminal muscle activity.nnnDESIGNnTwelve subjects participated in two tasks: a finger pinch and a tooth clench task and performed a series of muscle activations with visual feedback as a training task and two series without visual feedback as pre- and post-training tasks. Five target force levels at 10%, 20%, 40%, 60% and 80% maximal voluntary contraction were performed in each series for both tasks. During all series electromyographic (EMG) activity and force were recorded. Target force-actual force and target force-EMG curves were compared with and without visual feedback for both tasks. The variability in each series was determined as the coefficient of variation (CV) from the EMG and force recordings.nnnRESULTSnAlthough positive linear relationships were found between the target force level and the actual force value, and target force level and root mean square (RMS)-EMG amplitude from the tooth clenching task, the curves calculated from the finger pinch task were not completely linear. CVs of the actual force value and CVs of the RMS-EMG amplitude in both masseter and temporalis muscles were significantly influenced by visual feedback (P<0.001). However, the CVs of the RMS-EMG amplitude in right abductor pollicisbrevis and right first dorsal interosseous muscles were not significantly influenced by the three series.nnnCONCLUSIONSnThe influence of visual feedback was different between spinally innervated and trigeminally innervated muscles, and the specific mechanism of force execution from muscle activity appears to be different between the tasks.

Clinical management of implant prostheses in patients with bruxism.

There is general agreement that excessive stress to the bone-implant interface may result in implant overload and failure. Early failure of the implant due to excessive loading occurs shortly after uncovering the implant. Excess load on a final restoration after successful implant integration can result in physical failure of the implant structure. Many clinicians believe that overload of dental implants is a risk factor for vertical peri-implant bone loss and/or may be detrimental for the suprastructure in implant prostheses. It has been documented that occlusal parafunction, such as, bruxism (tooth grinding and clenching) affects the outcome of implant prostheses, but there is no evidence for a causal relation between the failures and overload of dental implants. In spite of this lack of evidence, often metal restorations are preferred instead of porcelain for patients in whom bruxism is presumed on the basis of tooth wear. The purpose of this paper is to discuss the importance of the occlusal scheme used in implant restorations for implant longevity and to suggest a clinical approach and occlusal materials for implant prostheses in order to prevent complications related to bruxism.

Influence of age and gender on trigeminal sensory function and magnetically evoked masseteric exteroceptive suppression reflex

OBJECTIVEnThe present study explored the effect of age and gender on trigeminal sensory function and masseteric exteroceptive suppression (ES) reflex responses.nnnMETHODSnYoung healthy men (n = 12) and women (n = 12) (age: 23.5 ± 3.0 years) and older healthy men (n = 12) and women (n = 12) (age: 58.5 ± 5.2 years) participated. Sensory function was assessed on the skin overlying the mental foramen using mechanical stimuli. Surface EMG was recorded from the left masseter muscle to assess ES reflex responses evoked by a magnetic stimulus applied to the skin above the left mental nerve.nnnRESULTSnThe older group had significantly higher tactile detection thresholds. Early ES1 was present in all subjects. Onset latency of ES1 was significantly delayed in older subjects. ES2 was present in all young subjects, but only in 5 of the 12 men and 8 of the 12 women in the older group. Significant gender differences were found for sensory and pain thresholds to mechanical stimuli as well as for duration of ES.nnnCONCLUSIONSnAgeing affects tactile detection thresholds, onset latency of ES1 responses, and appearance of ES2. The present results indicate that trigeminal sensory function and brainstem reflex responses differ between genders and age groups. These findings may have implications for assessment of craniofacial pain conditions.