Combination of jaw and tongue movement training influences neuroplasticity of corticomotor pathways in humans.

Since humans in daily life perform multiple motor behaviors that often involve the simultaneous activation of both jaw and tongue muscles, it is essential to understand the effects of combined orofacial sensorimotor tasks on plasticity in corticomotor pathways. Moreover, to establish novel rehabilitation programs for patients, it is important to clarify the possible interrelationships in corticomotor excitability between jaw and tongue motor control. The aim of this study was to examine the effect of a combination of a repetitive tooth bite task (TBT) and a repetitive tongue lift task (TLT) on corticomotor excitability of the tongue and jaw muscles as assessed by transcranial magnetic stimulation (TMS). Sixteen healthy individuals participated in three kinds of training tasks consisting of 41-min TBT, 41-min TLT, and 82-min TBT + TLT. Motor-evoked potentials (MEPs) from the tongue muscle, masseter muscle, and first dorsal interosseous muscle were measured before and after the training tasks. The amplitude of tongue MEPs after training with TLT and TLT + TBT, and masseter MEPs after training with TBT and TLT + TBT, were significantly higher than before training (P < 0.05). Tongue MEPs and masseter MEPs were significantly higher after TLT + TBT than after TBT or TLT (P < 0.05). The present results suggest that a task combining both jaw and tongue movement training is associated with a greater degree of neuroplasticity in the corticomotor control of jaw and tongue muscles than either task alone.

Comparing the occlusal contact area of individual teeth during low-level clenching.

The aim of this study was to investigate the occlusal contact area (OCA) in individual teeth during low-level tooth clenching in 24 healthy participants. Before measurements were made, the 100% maximum voluntary contraction (MVC) was determined. At baseline, all subjects were instructed to close their mouth and touch the opposing teeth with minimal force. Occlusal contact was recorded during three jaw motor tasks (baseline, 20% MVC, and 40% MVC) using a blue silicone material. OCA thickness was determined from images and defined on five levels: level 1 (0-149 µm), level 2 (0-89 µm), level 3 (0-49 µm), level 4 (0-29 µm), and level 5 (0-4 µm). Premolar and molar OCAs increased significantly from baseline to 20% MVC and 40% MVC. The OCA of each anterior tooth did not change significantly with increasing clenching intensity at all levels. Our findings suggest that premolar and molar OCAs may be altered by low-intensity clenching, affecting the teeth and periodontal tissues.

Effect of a repeated jaw motor task on masseter muscle performance.

OBJECTIVE: The aim of this study was to investigate the effects of 5-days repeated jaw-motor tasks on masseter muscle accuracy performance. DESIGN: Sixteen healthy participants performed a tooth-clenching task (TCT) of 58-minutes on five consecutive days. During measurements, electromyography (EMG) of both masseter muscles was recorded. Each day, the 100% maximum voluntary contraction (MVC) level was determined before the TCT. In the first and third TCT series, participants were instructed to target force levels without visual feedback. During the second TCT series, visual feedback on muscle activity level was displayed. One series consisted of three force levels (10%, 20%, and 40% MVC). In the series, participants alternated between a 30-s rest-block and a 30-s task-block for 360 s. In the task-block, participants alternated between a 5-s rest-block and a 5-s task-block. EMG activity during epochs of 5-s was quantified by calculation of the root-mean-square (RMS) values. To evaluate the accuracy of the performance, the coefficient of determination (CD) of the target force level-EMG curve was calculated from all series. RESULTS: No significant day-to-day differences in EMG RMS amplitudes were observed during MVC. CDs differed significantly between the five days (P<0.001). CDs in the first series on day 1 were significantly lower than CDs in the first series on days 4 and 5 (P<0.05). CONCLUSIONS: The findings suggest that a rigorous training paradigm may improve the performance of masseter muscles in terms of accuracy but not MVC. This might have implications for understanding the rehabilitation of patients with functional disorders in the stomatognathic system.

Rheological properties of elastomeric impression materials for selective pressure impression technique.

PURPOSE: The selective impression technique is recommended for removable prosthodontics. However, the viscoelastic rheological properties of impression materials, particularly the time-dependent viscoelastic changes in impression material immediately after tray seating, are unknown. The objective of this study is to clarify the viscoelastic rheological properties of impression materials by focusing on tray seating. METHODS: In this study, eight types of polyvinylsiloxane (PVS), two types of polyether, and two types of alginate impression materials were used. The storage modulus (G'; degree of stiffness) and loss tangent (tanδ; degree of hardening) were determined as functions of time from 0 to 360 s, commencing immediately after the completion of mixing, using a stress control-type rheometer. Thus, G' and tanδ at 0s and 20 s were compared. RESULTS: Stiffness was found to be widely distributed (4.49-0.26 × 10(4)Pa) among PVS-types, even immediately after mixing the impression material. There was also variation among polyethers (1.55-0.5 × 10(4)Pa) and among alginates (0.64-0.21 × 10(4)Pa). The hardening of all impression materials progresses beyond 20 s after the completion of mixing. CONCLUSIONS: The G' values varied with each impression material, even immediately after mixing, and the accurate impression-taking time was determined from the results of tanδ. These results provide unique insight into the selective impression technique.

Repeated tongue lift movement induces neuroplasticity in corticomotor control of tongue and jaw muscles in humans.

This study investigated the effect of repeated tongue lift training (TLT) on the excitability of the corticomotor representation of the human tongue and jaw musculature. Sixteen participants performed three series of TLT for 41 min on each of 5 consecutive days. Each TLT series consisted of two pressure levels (5 kPa and 10 kPa). All participants underwent transcranial magnetic stimulation (TMS) and electromyographic (EMG) recordings of motor evoked potentials (MEPs) in four sessions: (1) before TLT on Day 1 (baseline), (2) after TLT on Day 1, (3) before TLT on Day 5, and (4) after TLT on Day 5. EMG recordings from the left and right tongue dorsum and masseter muscles were made at three pressure levels (5 kPa, 10 kPa, 100% tongue lift), and tongue, masseter, and first dorsal interosseous (FDI) MEPs were measured. There were no significant day-to-day differences in the tongue pressure during maximum voluntary contractions. The amplitudes and thresholds of tongue and masseter MEPs after TLT on Day 5 were respectively higher and lower than before TLT on Day 1 (P<0.005), and there was also a significant increase in tongue and masseter MEP areas; no significant changes occurred in MEP onset latencies. FDI MEP parameters (amplitude, threshold, area, latency) were not significantly different between the four sessions. Our findings suggest that repeated TLT can trigger neuroplasticity reflected in increased excitability of the corticomotor representation of not only the tongue muscles but also the masseter muscles.

Scratch test of thermoplastic denture base resins for non-metal clasp dentures.

PURPOSE: Several thermoplastic denture base resins have been introduced for the fabrication of non-metal clasp dentures. Although the surface of these materials is easily damaged, the surface roughness and characteristics of scratches created have not been evaluated. The purpose of this study was to evaluate the surface roughness of thermoplastic resins using a scratch test for the development of future materials. METHODS: Four thermoplastic (polyamide: Valplast(®); VLP and Lucitone FRS(®); FRS, polyethylene terephthalate: EstheShot(®); ES, and polyester: EstheShot Bright(®); ESB) and two conventional acrylic (Heat-polymerizing: Urban(®); HC, and Pour type auto-polymerizing: Pro-Cast DSP(®); PR) denture base resins were examined. Eight specimens, approximately 10mm×10mm×30mm in size, of each material were fabricated. Scratch test was performed by a scratch tester with a diamond indenter of 10-µm radius and cone angle 90°, applying a constant load of 500mN, and 2-mm-long scratches were made. The scratch marks were studied under 3D laser measuring microscope and cross-section profiles at approximately 0.5mm, 1.0mm, and 1.5mm from the starting point were extracted and measured with analysis software. Data from 24 cross-section profiles in each denture base material were analyzed. RESULTS: The maximum depths of ES, ESB and FRS were greater than VLP, PR and HC, and the scratch widths of ES, ESB, FRS and VPL were greater than PR and HC. CONCLUSIONS: The results showed that the surface of thermoplastic denture base resins was easily damaged compared to polymethyl methacrylate.