The effects of cooling shrinkage and reheating of impressions on the reproducibility of edentulous working models were evaluated.The models were prepared with a single-mix impression technique or putty-wash relining technique, using hydrophilic vinyl silicone rubber impression materials, at room temperature or oral temperature during setting.The models were measured using a three-dimensional measuring system.In addition, impressions made at oral temperature during setting were reheated by 4methods, and the effects of reheating on the reproducibility of models were evaluated.1.In both the single-mix impression technique and putty-wash relining technique, reheating the impressions improved the reproducibility of models.2.In the putty-wash relining technique, the effects of reheating impressions on the reproducibility of models were similar irrespective of the method of reheating.However, in the single-mix impression technique, the effects differed with the reheating method used.

View full abstract

This study evaluated the mechanical properties and bond strengths of four commercially available adhesive resin cements, Bistite Resin Cement(BR), Imperva Dual(ID), Panavia 21(PT)and Super-Bond C & B(SB), to bovine tooth substrates and two types of precious dental alloys.The effect of adhesive primer for metal on the shear bond strengths of resin cements to silver-palladium-copper-gold and type IV gold alloys was investigated.Three resin cements except for SB showed almost the same compressive and transverse strengths after one day's immersion in 37℃ water.The transverse strength of BR immersed in water for six months decreased to one-third compared with that immersed for one day.The transverse strength and modulus of elasticity of SB were about a half of those of other resin cements.The tensile bond strengths of four resin cements to bovine enamel were approximately 13MPa.For dentin, SB, PT, BR and ID showed a bond strength of approximately 14, 11, 6 and 5MPa, respectively.Adhesive luting agents showed excellent shear bond strength of approximately 30〜40MPa to both precious alloys after 50, 000 thermocycles consisting of alternately immersing the specimen in 4℃ and 60℃ water for one minute each.The application of adhesive primer for metal on sandblasted precious dental alloys was effective in greatly improving the bond strengths of resin cements compared with unprimed alloys.

View full abstract

Durability of adhesion between acrylic resin and precious metals treated with alkanedithiols HS-(CH₂)_n-SH(n=2, 3, 4, 6, 8, 10)and a thiophosphoric methacrylate(MEPS)was evaluated using PMMA/MMA-TBB resin and experimental fluoropolymer/MMA-TBB resin.Changes in tensile bond strength were measured after thermal cycling in water.The dithiols showed that gold and its alloys were similar to MEPS in effectiveness.Durability against thermal stress depended on the type of resin.Adhesion with the fluoropolymer/MMA resin was more durable than that with the PMMA/MMA resin.

View full abstract

We prepared and evaluated new experimental soft denture lining materials composed of fluoropolymer(vinylidene fluoride/hexafluoropropylene copolymer)and methyl methacrylate(MMA)by modifying three kinds of cross-linking agents.Sorption of water or oil, hardness, water contact angle, elution of MMA, and viscoelastic property were measured.By the addition of cross-linking agents, water sorption, hardness, and viscosity decreased, but the values of water contact angle, and the amount of MMA eluted remained almost unchanged.Oil sorption decreased with time, suggesting the need for further research and improvement.

View full abstract

CP titanium(Ti)specimens were prepared with sandblasting, wire electric discharge machining(Wire EDM), and anodic oxidation.Two commercial composite resin materials for veneering, Dentacolor and New Metacolor, were adhered on the titanium specimens with each particular bonding system, Silicoater MD and 4-META bonding respectively.Shear bond tests were performed on samples stored in air at 23℃ for 24 hours, in water at 37℃ for a month, and in water for 10⁴ thermal cycling with a cycle of 37, 4, 37 and 60℃.Shear bond strengths of all Ti specimens with Dentacolor and New Metacolor stored in air for 24 hours were similar to those of Ni-Cr alloy as a control.Shear bond strengths of Ti and Ni-Cr specimens with Dentacolor did not decrease after storing in water for a month or even after thermal cycling.However, shear bond strengths with New Metacolor decreased significantly after thermal cycling.Ti specimens treated with Wire EDM showed sufficient shear bond strength with both Dentacolor and New Metacolor.However, Ti specimens treated with anodic oxidation in this study did not show sufficient strength especially with New Metacolor.Therefore, surface treatment of titanium was important to achieve excellent adhesion with composite resin.

View full abstract

Dynamic viscoelastic properties of commercial resins for crown and bridge were determined using a torsion pendulum apparatus at temperatures ranging fron room temperature(23℃)to 200℃.Dynamic torsional shear moduli(G's)of the materials measured at individual temperatures of 37 and 80℃ were in the ranges of 1.27〜3.85 and 0.72〜3.24GPa, respectively.The G's of each material decreased relative to the increase in temperature, and the reduction in G's(80℃)was in the range of 15.8〜54.9% compared with G's(37℃).When a second run was repeated on the same specimen, G's(37℃)were approximately 1.07〜3.13 times higher than that in the first run.This was due to the decrease in the amount of unreacted substances, as each specimen was heated.The correlation between the Knoop hardnesses 24h from the start of irradiation and G's(37℃)of each material was relatively high(r=0.89).However, there was not an adequate correlation(r=0.41)between the inorganic filler contents(wt%)and G's(37℃).

View full abstract

The effect of the powder-liquid ratio in light-activated glass ionomers for base/liner on marginal gaps either in dentinal or teflon cavity, and shear bond strength to dentin was observed.Specimens were tested immediately after the light-activation.After increasing the amount of powder, the marginal gap did not decrease(p>0.10)in the dentinal cavity, but decreased(p<0.001)in the teflon cavity.Shear bond strength to human dentin increased(p<0.05)following the increased powder.The marginal gap in the dentinal cavity was not influenced by that in the teflon cavity and shear bond strength to dentin.

View full abstract

Bis-GMA is one of the important dimethacrylates for dental composite resins.Some impurities were often detected in Bis-GMA contained in composites.Cytotoxicities of two raw materials of Bis-GMA with different purities were determined by the cell recovery test using L-929 and HEp-2 cells to screen the effects of the impurities.The purities of tested Bis-GMAs were 96 and 64% by weight.64% Bis-GMA showed higher cytotoxicity than 96% Bis-GMA due to water-soluble impurities.Fortunately, it was assumed that Bis-GMA employed in available composites contained fewer impurities than 64% Bis-GMA.Although TEGDMA monomer showed relatively strong cytotoxicity, it enhanced polymerization conversion of self-and photo-curable composite matrices.

View full abstract

A glass of diopside composition(CaO・MgO・2SiO₂)melts at 1, 500℃.However, its softening temperature is much lower than that and it shows good viscous flowability at 800〜900℃.High flowability more than 80% was confirmed at 830℃ under a pressure of 0.1MPa.The glass ingot can be pressed into a dental crown shape under a pressure of 1.0MPa.After crystallization by firing at 850℃ for 25min, its flexural strength was about 400MPa and fracture toughness was 2.7MPa・m^1/2. This glass ceramics crystal phase look similar to natural teeth in color and translucency without any deformation.Such a press-forming method should be superior to casting method for the glass ceramic materials, because it is free from casting defects and it can be done at significantly lower temperatures than their melting points.

View full abstract

The variation of polymerization shrinkage stress with time for 26 experimental composite resins applied on one surface of a thin glass beam has been determined.In addition, the amount of polymerization shrinkage of these materials during setting was investigated using a small glass cell and an electric strain transducer(UL-50).From these results, the effects of filler shape, filler particle size and filler content on shrinkage stress during setting were discussed.The increase of filler content in the material enlarged the deflection(shrinkage stress)of the glass beam and reduced the amount of linear shrinkage during setting.Furthermore, this shrinkage stress also decreased with the use of a larger filler in an average particle size.In cases using the same filler content materials, the effects of filler shape(irregular or spherical filler)on shrinkage stress was not recognized in this experiment.

View full abstract

The function of 2-hydroxy-3-(2-naphthoxy)propyl methacrylate(HNPM)in bonding of HNPN/MMA-TBB resins to dentin conditioned with 10-3 was studied.HNPM does not have a reactive group with dentinal components but it has hydrophobic and hydrophilic groups similar to 4-META which facilitates the formation of a hybrid layer by promoting diffusion of monomers into the conditioned substrate.The highest bond strength, 13MPa, was attained with the 3% HNPA/MMA-TBB resin.Creation of the hybrid layer by HNPM/MMA-TBB resins was identified by SEM and TEM.The hybrid layer of 3% HNPM/MMA-TBB resin showed the strongest resistance against degradation with NaOCl.Thickness of the hybrid layer and the resin content were decreased by increasing the concentration above 3% as the rate of polymerization measured with the differential scanning calorimetry(DSC)was increased.TEM examination revealed that encapsulated crystals of hydroxyapatite existed at the bottom of the hybrid layer.It was concluded that a chemical reaction is not essential to create bonding but the hybrid layer formation is the bonding mechanism to dentin.

View full abstract