Purpose: This study aimed to investigate whether clenching aids performance in a task requiring mental concentration in healthy participants.
Materials and methods: Twenty healthy participants (an average of 26.8 ± 2.0 years, nine females and 11 males) were recruited from our department. Participants performed four sets of hundred-square calculations as a mental arithmetic task. Among these sets, the third and the fourth sets were randomly divided into two conditions: one condition where participants made an effort to not allow their teeth to touch (‘no tooth-contact’ condition), and another where they made a conscious effort to rhythmically clench their teeth (‘clenching’ condition) during the task. Surface EMG was recorded from the left masseter muscle. Arithmetic performance was compared between the tooth-contact and clenching conditions using paired t-tests. In addition, we computed Pearson product–moment coefficients of the correlations between the difference in EMG activity and arithmetic performance in the two conditions.
Results: No significant change in arithmetic score was found between the no tooth-contact (68.8 ± 12.2) and clenching conditions (66.5 ± 12.7; P = 0.27). No significant correlation was found between the difference in EMG activity and changes in arithmetic performance (r = 0.32, P = 0.17).
Conclusion: In healthy participants, conscious clenching was not associated with any benefit in arithmetic performance. Although our study was limited by only measuring the short-term effects of clenching on a simple arithmetic task, the results indicate that it may not be harmful for clinicians to encourage their patients to refrain from clenching their teeth.

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Purpose: The aim of this study is to evaluate a newly developed polymethyl methacrylate (PMMA) powder.
Methods: The particle size distribution, surface area, and particle shape of both new and traditional powders were compared. The shear bond strength of the resin cement with the new powder to a silver–palladium–copper–gold alloy was determined and compared to that for a cement with the traditional powder. Also, the weight of mixture held by the brush at one time of both powders was also calculated and compared as an index of mixability.
Results: The surface area of the new powder was smaller than that of the traditional powder, while the particles size distributions were similar. The new powder included various-sized spherical particles as well as irregular particles, while the traditional powder consisted of only irregular particles. The new powder showed significantly higher mixability, although its bond strength was not significantly different from that of traditional powder.
Conclusion: The results of this study show that the interminglement of spherical and irregular particles cannot influence the bond strength to the alloy but is helpful to improve the working properties.

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Purpose: To evaluate the clinical performance of the Crystaleye Spectrophotometer^®, a dental color analysis system.
Methods: Three color-measuring devices (Crystaleye Spectrophotometer^®, CAS-ID1, MSC-2000) were tested and the differences in color measurements among them were evaluated using Scheffe's F-test. Color measurements with the Crystaleye Spectrophotometer^® were repeated 10 times by the same operator. The color difference (ΔE) between the first and tenth measurements was calculated. The Crystaleye Spectrophotometer^® was used to measure the color of the maxillary left central incisor under two conditions (light and dark) and the effect of exterior lighting was analyzed to assess the accuracy of measurements. Furthermore, five different operators performed color measurements, and ΔE among the three devices was calculated. The ΔE between the target tooth and the crown of a single maxillary central incisor crown fabricated using data from the Crystaleye Spectrophotmeter^® was calculated. Color differences between prebleaching and postbleaching were also analyzed with the Crystaleye Spectrophotometer^® using the parameters ΔE, ΔL*, Δa*, and Δb*.
Results: The data from the three spectrophotometers were not significantly different. The ΔE during repeated color measurements by the same operator was 0.6. The ΔE between light and dark conditions was 0.9. The data from the five operators were not significantly different. The mean ΔE value between the target tooth and the fabricated crown was 1.2 ± 0.4, and the mean ΔE value between prebleaching and postbleaching was 3.7 ± 1.0.
Conclusions: The Crystaleye Spectrophotometer^® is an easy-to-use color analysis system producing accurate color measurements under clinical conditions.

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Purpose: The present study was carried out to identify how gum chewing with and without occlusal support by implant prostheses affects brain function as well as chewing function.
Methods: Twenty-four subjects rehabilitated with implant-supported fixed prostheses were evaluated. An electroencephalograph (EEG) (ESA-Pro) and mandibular kinesiograph (Bio PAK^®) wear used to measure brain function and chewing function, respectively, before and after gum chewing with and without an implant superstructure. Based on brain function estimated by the Dα values derived from measurement data, the subjects were divided into the normal region group (including the sub-normal region group) (n = 15; Dα ≥ 0.952) and the impaired region group (n = 9; Dα < 0.952). All the data were statistically analyzed using the Wilcoxon test (α = 0.05).
Results: Brain function in the normal region group showed no change after gum chewing, whether or not an implant superstructure was in place (p > 0.05). However, brain function in the impaired region group showed significant improvement after gum chewing (p < 0.05). Seven of 9 subjects using an implant superstructure in impaired region group indicated an increase or no change in brain function compared to the results without an implant superstructure. In the impaired region group, there was a high positive correlation between brain function and masticatory movement (γ = 0.75).
Conclusions: Subjects in the impaired region group revealed a strong positive correlation between brain function and masticatory movement, indicating that occlusal support by implant-supported fixed prostheses has the potential to enhance brain function.

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Purpose: We have developed a new type of laminated mouthguard, the Hard & Space mouthguard, which incorporates a hard material insert and a space to prevent contact between the mouthguard and the buccal surfaces of the teeth. The purpose of this study was to investigate the effect of this new design on shock absorption.
Methods: Three types of mouthguard (a conventional laminated EVA mouthguard and two 3-layer type “Hard & Space” mouthguards made of 1.8-mm or 3.0-mm thick pre-laminated material (Konbiplast) and EVA with a 1.0-mm space) were impacted. Shock absorption was measured by means of a pendulum type steel ball impact testing machine at impact distances of 10, 20, and 30 cm and a dental study model with strain gauges attached to the lingual surfaces and an accelerometer fixed to the maxilla.
Results: Distortion of the impacted tooth and acceleration of the model were significantly reduced by all types of mouthguard at all 3 impact distances. The effect of the mouthguard was remarkable in terms of tooth distortion: both thicknesses of Hard & Space mouthguard showed more than 90% shock absorption, compared with only approximately 55–78% with an EVA mouthguard. Furthermore, shock absorption with the thicker 3.0-mm Hard & Space mouthguard reached more than 95% at the highest impact power.
Conclusion: Within the limitations of this laboratory study, Hard & Space mouthguards showed significantly greater buffer capacity than a conventional EVA mouthguard in terms of tooth distortion at the 3 impact powers tested.

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Objectives: Based on a maxillary premolar restored with laminate veneer and using the 3-D finite element analysis (FEA) and μCT data, the aim of this study was to evaluate the influence of different types of buccal cusp reduction on the stress distribution in the porcelain laminate veneer and in the resin luting cement layer.
Methods: Two 3-D FEA models (M) of a maxillary premolar were built from μCT data. The buccal cusp reduction followed two configurations: Mt – buccal cusp completely covered by porcelain laminate veneer; and Mp – buccal cusp partially covered by porcelain laminate veneer. The loading (150 N in 45°) was performed on the top of the buccal cusp. The finite element software (Ansys Workbench 10.0) was used to obtain the maximum shear stress (τ_max) and maximum principal stress (σ_max).
Results: The Mp showed reduced the stress (σ_max) in porcelain laminate veneer (from −2.3 to 24.5 MPa) in comparison with Mt (from −5.3 to 27.4 MPa). The difference between the peak and lower stress values of σ_max in Mp (−6.8 to 26.7 MPa) and Mt (−5.3 to 27.4 MPa) was similar for the resin luting cement layer. The structures not exceeded the ultimate tensile strength or the shear bond strength.
Conclusions: Cusp reduction did not affect significant increase in σ_max and τ_max. The Mt showed better stress distribution (τ_max) than Mp.

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Purpose: There are two aspects of human balance: static balance and dynamic balance. However, to the author's knowledge, no studies have investigated how changes in the stomatognathic system influence dynamic balance. This aim of this study was to determine the effect of horizontal mandibular deviation on stability of upright posture on an unstable platform in order to clarify the relationship between the stomatognathic system and the dynamic balance.
Methods: Fifteen healthy adult participants were selected. To determine the effect of changes in the stomatognathic system on dynamic balance, three experimental conditions were established: a resting mandibular position, a position in which experimental horizontal mandibular deviation was maintained by a splint, and a mandibular rest position maintained by a splint. Each participant was instructed to stand in a natural upright posture on an unstable board, and resulting variation in angle of the board was then measured. Measurements were taken 15 times under each condition.
Results: Variation in angle of inclination of the board as dynamic balance was the largest in the splint-maintained deviated mandibular position.
Conclusion: Horizontal deviation in mandibular position interfered with stability of upright posture on an unstable platform, suggesting that changes in the stomatognathic system affect dynamic balance.

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Purpose: The purpose of this study was to investigate the influence of the major connector in a maxillary denture.
Methods: Eight patients with sound dentition were recruited in this study. The experimental connectors were fabricated with Co–Cr. Two positional designs were used: one was the middle type (M), and the other was the anterior and posterior type (AP). The six sounds, /shi/, /hi/, /chi/, /ki/, /ri/, and /ni/, were chosen as the test sounds. Using a speech recognition system, the intelligibility, change and duration of the test sounds were evaluated under three conditions: without connector (WO·C), with the M-type connector (W·M), and with the AP-type connector (W·AP).
Results: The intelligibility in WO·C and W·AP was 86.9 ± 17.4% and 57.1 ± 30.1% for /shi/, 79.2 ± 19.7% and 70.5 ± 27.0% for /hi/, 74.7 ± 14.6% and 43.6 ± 24.3% for /chi/, 54.4 ± 16.5% and 23.2 ± 17.8% for /ki/, 85.2 ± 15.9% and 55.1 ± 30.6% for /ri/, and 76.9 ± 26.9% and 50.2 ± 31.3% for /ni/. Significant differences were recognized between WO·C and W·AP in /shi/, /chi/, /ki/ and /ri/(P < 0.01). The intelligibility in W·M was higher than that in W·AP in all test sounds. As for incorrect labels, significant differences were recognized in the appearance ratio of /hi/ as the incorrect label in the test sound /shi/ (P < 0.05). There were significant differences in some incorrect labels of /hi/ and /chi/. As for duration of consonant part, there was no significant difference in each test sound.
Conclusion: It was indicated that the middle type of the major connector would be recommended from a viewpoint of phonetic function.

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Purpose: To clarify the difference in the state of occlusal contact and masticatory function between two patterns of masticatory movement path that differed in the closing path.
Methods: Fifteen healthy subjects with Pattern I (a linear or concave opening path and a convex closing path) and Pattern II (similar opening path to that in Pattern I and a concave closing path) were selected. The state of occlusal contact on the working and balancing sides and the masticatory function (integral value of the masseter muscular activity, gape, masticatory width, cycle time, indicators representing the stability of the path and rhythm, and glucose extraction) were compared between the two patterns.
Results: The occlusal contact on the working side was about the same. For the balancing side, occlusal contact at the molar region was observed for Pattern II in most cases, whereas no occlusal contact was observed for Pattern I. The integral value of the masseter muscular activity and the glucose extraction were greater for Pattern I. The gape was not different between the two patterns. Pattern I had a wide masticatory width and a short cycle time. The values of the indicators representing the stability of the path and rhythm were smaller for Pattern I.
Conclusion: It was suggested that Pattern I with a convex closing path had a functional difference and a superior masticatory function from Pattern II with a concave closing path, and the difference in the occlusal contact on the balancing side was related.

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Patients: A 45-year-old female patient came to the institute complaining of reduced salivation, pain and food lodgment in multiple teeth, and difficulty in eating. The systemic examination revealed dry eyes, dry mouth, cracking of corners of mouth and lack of appetite. The diagnostic tests were conclusive of Sjogren's syndrome, which is associated with xerostomia, ocular dryness and connective tissue disorders. Major oral problems in such patients include high caries rate, burning of oral mucosa, early tooth loss, increased tooth wear, poor tolerance for dentures and repeated failure of dental restorations.
Discussion: Prosthodontic therapy for this unique patient group is challenging and neglected, due to limited choice of abutments, loss of vertical dimension and poor occlusion. Two-year follow up of a patient of Sjogren's syndrome who was rehabilitated by a combination of fixed and removable prostheses, with a simplified palatal salivary reservoir is presented.
Conclusion: Though the patient felt an improvement in quality of life due to the prosthesis, slurred speech and frequent reservoir refilling remained problems.

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Purpose: The purpose of this study was to evaluate the vibratory properties of maxillary dentition in maxillectomy patients wearing metal framework obturator prostheses with three different material types of metal frameworks and to determine the optimal type by applying in vivo modal analysis.
Methods: The three metal framework obturator prostheses using gold alloy, cobalt–chromium alloy and titanium alloy were fabricated for four maxillectomy patients, whose shapes were almost the same for each patient. All patients were classified into Aramany's Class II. The transfer functions of the maxillary dentition with and without obturator prostheses were measured to obtain the natural frequency, modal shape and damping ratio. A forced response simulation was carried out, in which an impact assuming occlusal force was applied to the lateral segment teeth on the non-defect side, and the maximum displacements of the measurement points were calculated.
Results: As for the natural frequency, wearing the obturator prostheses caused reduction of the natural frequency of the maxillary dentition. The modal shapes of titanium alloy obturator prostheses showed the strongest splint-like effect and were concluded to be the most preferable. The damping ratio of titanium alloy obturator prostheses was significantly higher than cobalt–chromium obturator prostheses. The maximum displacement of gold alloy obturator prostheses was significantly the highest of all.
Conclusion: From the viewpoint of preserving abutments and remaining teeth, we conclude that titanium alloy is a more preferable material than gold alloy and cobalt–chromium alloy for the metal frameworks of metal framework obturator prostheses.

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Purpose: The aim of this study was to develop and apply a new method for easy intraoperative adjustment of a provisional fixed full-arch restoration, in order to allow immediate implant loading following computer-guided surgery, regardless of any implant positioning errors compared to the virtual planning.
Methods: In accordance with the NobelGuide™ protocol, a provisional restoration for immediate loading of six maxillary implants was prepared prior to surgery. Because small shifts between the planned and the actual implant positions were to be expected, the provisional restoration was not fabricated directly on temporary cylinders as a conventional one-piece superstructure, but was divided into two portions: six custom made abutments and a long span fixed restoration which were left unconnected. After implantation, the custom abutments were attached to the six implants to be immediately loaded, and the superstructure was cemented simultaneously to all abutments using dual cure resin cement. After the excess cement was cleaned and polished, the superstructure was then reseated. Passive fit was achieved between implants and the superstructure.
Conclusion: The superstructure described in this article can be easily seated and adjusted to accommodate any possible shifts in implant positioning occurring during computer-guided surgery. Through this method uneventful immediate implant loading can be achieved in a reasonable operative time.

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The first names of all authors of this article should have been spelled out. Their full names are shown correctly above.

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