In the past two decades, a number of methods and products have been developed in an attempt to reduce the microbial burden on denture surfaces. In addition, a number of denture lining materials have been commercialized as aids in alleviating denture discomfort, as through their viscoelastic properties, they act as shock absorbers and reduce and distribute the stress on denture-bearing tissues. However, some of these products are susceptible to deterioration due to denture hygiene/control methods, in which care they can act as reservoirs of infection.
In addition, the proportion of the elderly in the community, particularly those on medications, has been increasing with advances in medical diagnosis, treatment, and care systems, especially in developed countries. As distinguished above, a number of factors should be considered in the management of denture plaque that accumulates on prostheses. These considerations include 1) the ecology of microbial flora, particularly Candida, in plaque, 2) predisposing factors for infection (Table 1), 3) safety of hygiene products, 4) damage to materials caused by the hygiene method, and 5) the efficacy of plaque control methods.
Thus a large pool of patients in the community are in need of dentures that require hygienic maintenance once provided.

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Purpose: To clarify the time course of the coordination pattern in the jaw and hyoid muscle activities during swallowing following mastication.
Methods: Nine healthy dentate subjects were directed to chew a test food, consisting of sphere-shaped gelatin, on the preferred chewing side, and to swallow it. The test food was standardized in diameter, hardness and viscosity. Bilateral EMG activities of masseter, anterior temporal, suprahyoid, and infrahyoid muscles were recorded simultaneously with the mandibular movement, detected by a mandibular kinesiograph, and the laryngeal movement was detected by a handmade device with a highly sensitive force transducer.
Results: After rhythmical chewing strokes, while masseter and anterior temporal muscles were pausing, suprahyoid and infrahyoid muscle activities were seen to coincide with jaw opening and closing strokes, different from chewing strokes. The onset of infrahyoid muscle activities was significantly delayed from that of suprahyoid muscle activities. These activities finished at the same time. Subsequently, masseter and anterior temporal muscles were activated, while the mandible moved to the jaw-closing position. The suprahyoid and infrahyoid muscles were then activated again. The onset of suprahyoid and infrahyoid muscle activities was significantly delayed from that of masseter and anterior temporal muscle activities. The offset of infrahyoid muscle activities was significantly delayed from that of masseter, anterior temporal, and suprahyoid muscle activities. Rapid laryngeal movement occurred during this stage. The first activities of the suprahyoid and infrahyoid muscles may be equivalent to the transport stroke. The masseter and anterior temporal muscle activities, and the second activities of suprahyoid and infrahyoid muscles, may have a role in stabilizing the mandible during oral propulsive and pharyngeal stages of swallowing.
Conclusions: These results clarify the time course of the coordination pattern of jaw and hyoid muscle activities during swallowing following mastication.

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Purpose: To find the most suitable hardness of gumi-jelly for analyzing masticatory movement, movement of the mandibular incisal point was analyzed for chewing of four types of gumi-jelly having different hardness, containing, respectively 5, 6.5, 8 and 9.5% gelatin.
Methods: Twenty healthy subjects were asked to chew four types of gumi-jelly. For ten cycles, beginning with the fifth cycle, of mastication, the SD/OD (standard deviation/opening distance) of lateral components of opening and closing, and of the vertical component, and the CV (coefficient of variation) of opening, closing, occluding, and cycle time, were calculated and compared among the four types of gumi-jelly. Also, subjective investigation after chewing was performed.
Results: The feel of mastication was excellent for all four types of gumi-jelly. The indicators increased in the order of 8, 6.5, 9.5, and 5% for the lateral component of opening; 6.5, 8, 9.5, and 5% for the lateral component of closing; 8, 6.5, 5, and 9.5% for the vertical component; and, 8, 6.5, 9.5, and 5% for the opening and cycle time; 8, 6.5, 5, and 9.5% for the closing time, and 6.5, 8, 9.5, and 5%, for the occluding time, respectively. There were significant differences in almost all pairwise comparisons, and the 6.5 or 8% gumi-jelly had significantly smaller values than the 5 or 9.5% gumi-jelly. In other words, the masticatory movement was most stable for gumi-jelly containing 8% gelatin, and it was the second best for 6.5%.
Conclusions: It was concluded that the best gumi-jelly was the one containing 8% gelatin, and 6.5% was the second best.

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Purpose: Recently, it was said that mastication has various influences not only on the stomatognathic system but also on other organs in the whole body. However, little consideration has so far been given to the change of masticatory ability due to the effect of aging, loss of teeth, or both, in the humans. In this study, we consider the effect of age-related changes in masticatory ability, and the effect of molar teeth loss and longitudinal molarless condition, on masticatory ability, using the SAMP 8 (Senescence-Accelerated Mouse P 8).
Methods: SAMP 8 shows that the age-accelerated characteristic in a short life span and median survival time is 12 months. Sixty male SAM P 8, aged 2 months, were divided into 6 groups. In three groups of 10 mice as controls, each group was kept until 3 (young adult), 5 (middle age), and 8 (old age) months of age (normal condition). The remaining three groups, of 10 mice each, had their maxillary molar teeth extracted, and they were kept until 3, 5, and 8 months of age (molarless condition). The test meal (almond) was fed after the mice had been fasted for 24 hours. The masticatory ability in SAMP 8 evaluated the particle size distribution of masticated bolus removed from their stomach.
Results: SAMP 8 with molar teeth showed significant age-related changes in impairment of masticatory ability between 3 and 5 months of age. SAMP 8 without maxillary molar teeth had significant impaired masticatory ability compared with SAMP 8 with molar teeth. And when the term of teeth loss was extended, impairment of masticatory ability continued.
Conclusions: In SAMP 8, it became clear that impairment of masticatory ability was caused by aging and molar teeth loss. It was suggested that this phenomenon is similar to that seen in humans.

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Purpose: Film thickness of dental luting cement is an important property that influences in success of fixed prosthodontics. Conventionally, it has been considered that the film thickness depends on the maximum particle size of cement powder and the viscosity of cement mixtures. However, the detailed mechanism remains uncertain. This study aimed to measure basic properties, consistency, film thickness, distribution of powder particles and viscosity change after mixing, and the factors given to the film thickness were investigated.
Methods: Four commercial dental luting cements were used. Basic properties were measured following the standard of JIS T-6602 (Japanese Industrial Standard, corresponding to ISO 9917: Dental Water-based cements), and in addition, cement powder distributions were measured. Apparent viscosity changes after mixing were measured by a Cone-Plate-type Rotary Viscometer, and film thickness was measured at each viscosity of cements.
Results: Consistency rose with the time course, and film thickness was not always increased. No relation could be recognized between the maximum particle size of cement powder and film thickness. As a result of the measurement, the viscosity increased, when it grew on doing the film thickness. Both relations were almost rectilinear. The tendency was different according to the type of cement.
Conclusions: For film thickness of the cement, an effect of particle size could not be observed. It was indicated that the viscosity change after mixing, and the particle size distribution of cement powder, influenced film thickness.

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Purpose: Fracture of an all-ceramics crown is a serious problem in fixed prosthodontics restoration. The purpose of this study was to clarify the stress distribution at a fissure caused by the difference in occlusal surface forms of molar all-ceramic crowns.
Methods: The distal half area of the mandibular right first molar was generated in this analysis model. The three-dimensional finite element method was used for the analysis. The model was changed factors of the tooth form such as fissure radius, the thickness of ceramics, and the cusp angle for computing. The load condition was a circular loading area assumed to be taking foods. The load was applied on the central fossa. Analyzing model at fissures concentrated stress. Therefore their areas meshed in detail for exact calculation of stress. For the material of ceramics crown and abutment tooth authors assumed Olympus Castable Ceramics and dentin. The above matters were analyzed by ANSYS 5.5 (ANSYS Inc.).
Results: The results were that great tensile stress appeared at the fissure and when compression load was applied. This stress increased according to a decreasing fissure radius and an expanding cusp angle. Tensile stress was found at the boundary of the ceramics crown and the abutment tooth. This stress increased with decreasing occlusal thickness of the crown.
Conclusions: Reducing the risk of a ceramics fracture is effective for increasing the fissure radius and equalizing the thickness of ceramics

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Purpose: We evaluated the effect of surface treatments on bending bond strength in repair restoration of high-filler-content prosthetic composite resins.
Methods: Prosthetic composite resins (2×2×12.5mm) were made of Estenia enamel (EE) and Estenia dentin (ED). Specimens (2×2×25mm) for bending bond strength test were made of both prosthetic composite resins and two types of repair resins, such as Cleafil AP-X for the direct method (D), and EE for the indirect method (I). Estenia and Cleafil AP-X were polymerized according to the manufacturer's instructions. Surface treatments to the bonding surface were classified into the following four groups: no treatment (NT), silane surface treatment (S), bonding treatment (B), and silane+bonding treatment (SB). Bending bond strength was determined by the three-point bending test after 24 hours' storage in distilled water (37°C).
Results: Bending bond strength showed a large value in the order of D-S=D-SB>D-B>D-NT (one-way ANOVA, p<0.05) and I-SB>I-S>IB>I-NT. SEM observation of the adhesion surface after the bending bond test showed adhesive failure in NT, and cohesive failure in B, S, and SB.
Conclusions: SB and S were effective surface treatments in both direct and indirect repair restorations.

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