Hiroshi Yagasaki

Matsumoto Dental University, Shiojiri, Nagano-ken, Japan

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Publications (6)10.14 Total impact

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    ABSTRACT: With use of dental implants on the rise, there is also a tandem increase in the number of implant fracture reports. To the end of investigating the stress occurring in implants, elasticity and plasticity analyses were performed using the finite element method. The following results were obtained: (1) With one-piece type of implants of 3.3 mm diameter, elasticity analysis showed that after applying 500 N in a 45-degree direction, stress exceeding 500 MPa which is the proof stress of grade 4 pure titanium - occurred. This suggested the possibility of fatigue destruction due to abnormal occlusal force, such as during bruxism. (2) With two-piece type of implants that can tolerate vertical loading of 5,000 N, plasticity analysis suggested the possibility of screw area fracture after applying 500 N in a 45-degree direction. (3) On the combined use of an abutment and a fixture from different manufacturers, fracture destruction of even Ti-6Al-4V, which has a high degree of strength, was predicted.
    Dental Materials Journal 08/2008; 27(4):633-9. · 0.81 Impact Factor
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    ABSTRACT: This study investigated the effects of H(2)O(2)-treatment of titanium surfaces on cement shear bond strengths, and characterized H(2)O(2)-treated titanium surfaces. Using 34.5% hydrogen peroxide solution, cp Ti plates (10 mm x 10 mm x 1 mm) were treated by (1) an immersion method, and (2) halogen irradiation while immersed in H(2)O(2) for varying times. A cylindrical block (6 mm diameter, 4 mm height) of four different cements was bonded onto H(2)O(2)-treated surfaces. The cement bond strengths were evaluated under shear mode. Treated surfaces were also characterized for color change, wettability, AC impedance, and transmission electron diffraction of stripped oxide film. The cement shear bond strength of cp Ti treated with H(2)O(2) and halogen for 160 s was the highest and was approximately 14 times higher than the un-treated control cp Ti plates. Bond strengths are correlated linearly to wettability. The more surface wetted with the cement material, the higher the resultant shear bond strength value. These results suggest that a combined treatment of hydrogen peroxide and halogen light irradiation provides an effective surface condition with appropriate oxide film thickness to enhance the cement bond strength.
    Dental Materials 01/2006; 21(12):1087-97. · 3.77 Impact Factor
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    ABSTRACT: The thermal expansion rate, coefficient of thermal expansion, and high temperature strength of two types of commercially available alloy for metal-bond porcelain, KIK-HII (KIK) and Degubond-J2 (J2), were evaluated up to the liquidus point temperature using a thermo-mechanical analyzer. Furthermore, microstructure in the solid-liquid coexisting region was observed for evaluation. Our results revealed the following findings: 1. For KIK, solidus point was 1,209.3 +/- 3.2 degrees C, liquidus point was 1,308.3 +/- 7.10 degrees C, and melting expansion rate was 0.41+/- 0.16%. 2. For J2, solidus point was 1,198.3 +/- 0.6 degrees C, liquidus point was 1,253.0 +/- 4.4 degrees C, and melting expansion rate was 4.50 +/- 0.80%. 3. At high temperature, the mechanical characteristics of KIK greatly differed from those of J2. The risk of causing deformation during porcelain baking was suggested for KIK. Removal of segregation during casting was considered difficult in J2.
    Dental Materials Journal 04/2005; 24(1):140-8. · 0.81 Impact Factor
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    ABSTRACT: Dolomite, a mineral composed of calcium magnesium carbonate (CaMg (CO3)2), is used as a food supplement that supplies calcium and magnesium. However, the effect of magnesium supplementation on bone metabolism in patients with osteoporosis is a matter of controversy. We examined the effects of daily supplementation with dolomite on calcium metabolism in ovariectomized (OVX) rats. Dolomite was administered daily to OVX rats for 9 weeks. The same amount of magnesium chloride as that supplied by the dolomite was given to OVX rats as a positive control. Histological examination revealed that ovariectomy decreased trabecular bone and increased adipose tissues in the femoral metaphysis. Dolomite or magnesium supplementation failed to improve these bone histological features. Calcium content in the femora was decreased in OVX rats. Neither calcium nor magnesium content in the femora in OVX rats was significantly increased by dolomite or magnesium administration. Urinary deoxypyridinoline excretion was significantly increased in OVX rats, and was not affected by the magnesium supplementation. Serum concentrations of magnesium were increased, and those of calcium were decreased, in OVX rats supplemented with dolomite or magnesium. However, there was a tendency toward decreased parathyroid hormone secretion and increased calcitonin secretion in OVX rats supplemented with dolomite or magnesium. Serum 1,25-dihydroxyvitamin D(3) and osteocalcin levels were significantly increased in the supplemented OVX rats. These results suggest that increased magnesium intake improves calcium metabolism in favor of increasing bone formation, through the modulation of calcium-regulating hormone secretion.
    Journal of Bone and Mineral Metabolism 02/2005; 23(2):140-6. · 2.22 Impact Factor
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    ABSTRACT: Previously, high temperature properties of the silver-palladium-copper-gold alloy were investigated. In this study, the thermal expansion percentage and coefficient, and high temperature strengths of ADAS Type 3 gold alloy were investigated up to the liquidus temperature. Furthermore, microstructural and compositional changes in the solid/liquid dual phase were studied. The following conclusions were obtained. (1) The solidus point of the Type 3 gold alloy was 899.3+/-11.7 degrees C, and the liquidus point was 962.3+/-2.4 degrees C. (2) The thermal expansion percentage at the solidus point was 1.636+/-0.046%, while it was 4.853+/-0.213% for the liquidus point. The thermal expansion percentage of the melt was 3.217+/-0.257%. (3) The melt expansion was observed even under the measuring pressure of 373.75 HPa, which was quite different from the fact that the melt expansion disappeared at the pressure of 20.87 HPa for the silver-palladium-copper-gold alloy. (4) The morphology of solid phase in the solid/liquid dual zone of this alloy was quite different from those observed with the silver-palladium-copper-gold alloy.
    Dental Materials Journal 07/2004; 23(2):81-8. · 0.81 Impact Factor
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    ABSTRACT: Dental casting accuracy is influenced by the setting expansion of investment materials. Although setting expansion can help compensate for casting shrinkage, it cannot be fully realized under a confined wax pattern. Exactly how soft a wax pattern should be to ensure optimum setting expansion has not been determined. In this study, the relationship between wax characteristics and the casting accuracy of a three-quarter crown was investigated. Four different wax materials were used: paraffin 135 with a softening temperature of 37.5 degrees C (P38), paraffin 1080 with a softening temperature of 63.5 degrees C, Shofu Red with a softening temperature of 41.5 degrees C, and Shofu Hard with a softening temperature of 51 degrees C. Two mixtures of phosphate-bonded investment were prepared: one with 100% special liquid and another with 75% special liquid plus 25% distilled water. For both, the liquid/powder ratio was 16:100. A type IV gold alloy was cast into a three-quarter crown mold. The discrepancy at 6 locations (1 lingual, 1 mesial, 1 distal, and 3 facial) was measured with a traveling microscope. Five readings were collected. Means and standard deviations were calculated for all data. A 2-way analysis of variance followed by the Student-Newman-Keuls test for multiple comparisons was used to identify significant differences between groups at the 95% confidence level. For the gingival measurement sites (lingual, mesial, and distal), there was no significant difference in cast adaptation when Shofu Hard and paraffin 1080 waxes were used. However, the results with these 2 waxes were different than with Shofu Red and P38. For the 3 facial measurement sites, significantly different measurements were found for each wax; P38 demonstrated the best results. Casting shrinkage was smaller with the use of 100% special liquid. Within the limitations of this study, casting shrinkage was affected by the type of wax used and was sensitive to the site at which dimensional measurements were performed. The higher the softening temperature, the larger the casting shrinkage.
    Journal of Prosthetic Dentistry 02/2002; 87(1):57-61. · 1.72 Impact Factor