Calcium hydrogen phosphate(CHP), calcium hydrogen phosphate dihydrate(CHPD)and calcium sulfate dihydrate(gypsum)crystals were added to calcium sulfate hemihydrate as gypsum model materials for improvement of the mechanical properties.The compressive and the bending strength of the hardened mixture were improved by the addition of CHPD, while the addition of CHP and gypsum crystals had no effect.The setting time was markedly reduced by the addition of CHPD, but remarkably increased in the case of CHP crystals.On the other hand, the setting expansion was decreased by the addition of CHP and CHPD crystals.Based on the observation of the surface profile of the calcium sulfate hemihydrate set in contact with glass, and scanning electron microscopy(SEM)of the fractured surface of the hardened mixture, the surface roughness was markedly decreased by the addition of CHPD.When the crystals were added to the metastable solution of calcium sulfate supersaturated with respect to gypsum, the presence of gypsum crystals deposited on the CHPD surface was confirmed by atomic absorption analysis, fourier transform infrared spectroscopy, SEM and electron X-ray microanalysis.Therefore, epitaxial growth of gypsum occurred on the CHPD surface and the addition of CHPD improved the mechanical properties of the gypsum materials.

View full abstract

To improve the mechanical characteristics of hydroxyapatite(HAP)ceramics, a metalceramic composite formed by a solid state direct diffusion bonding system was studied.The joining treatment was carried out of a high vacuum and high temperature, for the bioactive ceramics(HAP)and the following biometals;platinum, gold-platinum alloy, titanium and titanium alloys, zirconium, niobium and aluminium alloy.The effects of the variations of thermal expansion mismatch and the interactive reactions at the interface were investigated by fractographic observation(SEM), X-ray diffraction method and EPMA analysis.On some of these joining combinations, the bonding strength had the same bonding strength as the adhesive materials.The results of interface observations showed that the bonding strength is affected by the interface reactions and the diffusion phenomena.

View full abstract

Cell recovery from damage induced by four types of commercial denture adhesives(three powder types, three rubber types, two cream types and one tape type)was studied in vitro, using 3 tissue culture cell lines(L-929 cells, HEp- cellells and Gin-1 cells).After cultivation with a denture adhesive for two hours, the cells were transferred to a normal culture environment, and their growth was evaluated.Two cytotoxicity tests, i.e., agar overlay test and extraction method, were also carried out.The pH value of the extract was moderately acidic in one powder type(Faston).By contrast, the extracts of the other types were in the neutral range.Cell recovery hardly occurred in the rubber type adhesives and, moderate in the powder types.Slight recovery was noted with the cream types and the tape type.The results of the agar overlay test and the extraction method differed from those of the cell recovery test.The powder type showed marked inhibition of cell growth.The effects of other specimens decreased in the tape type, cream types and rubber types.In the extraction method, the extracts obtained from each specimen did not affect cell growth.Among the cells used, Gin-1 cells showed a slightly higher rate of cell recovery.The discrepancy among the results obtained by the different methods was considered to be due to the state of the specimen and differences in testing conditions.This indicates the importance of selecting an appropriate testing method.

View full abstract

The feeling of hardness, stickiness and the rate of cutting during the preparation of a tooth were compared using an extracted tooth, melamine resin and crystalline glass ceramics model tooth(Bioram M)which are used in dental education and practice.Diamond burs were used with an air-turbine handpiece in this experiment.The results indicated that Bioram M had the desirable characteristics of a model tooth and there was a strong resemblance feeling of cutting between Bioram M and a natural vital tooth.

View full abstract

To assess the effects of H₂O on the fracture stress of sintered hydroxyapatite(HAP)ceramics prepared by procedures similar to those used for HAP coated on the metal substrate, four-point bending tests were carried out at 0.5mm/min either in air(20℃, R.H.73%)or in distilled water(37℃).Then, strength data obtained were analyzed using two-parameter weibull statistics.In each condition, the bending strength data gave a good fit to single-mode weibull distribution.Weibull analysis of the data gave weibull parameter m=7.8, σ₀=26.2MPa in air and m=8.1, σ₀=18.5MPa in distilled water, which were calculated assuming a surface flaw model.The mean value of bending strength was 27.3MPa in air and 18.2MPa in distilled water.These results indicated that a corrosive environment such as H₂O affects fracture stress when measured at a constant stress rate.This effect is considered to be caused by slow crack growth of cracks, occurring at a loading level lower than those at which specimens will fail.Using the weibull distribution function estimated in this study, the effects of both dimension of specimen and stress distribution in the specimen on the mean fracture stress have been predicted.

View full abstract

Lifetime and survival probability of brittle materials under given conditions of stress distribution, environment and component size can be predicted using an SPT(Strenght-Probability-Time)diagram.The SPT diagram for sintered hydroxyapatite(HAP)coated on the metal substrate via the fused glass was drawn and the safe working stress in 37℃ distilled water was estimated.The dynamic fatigue test carried out in 37℃ distilled water gave the fatigue parameter n=19 for sintered HAP.This parameter is constant except that it depends on material and environment, and the greater the value of n, the greater the resistance to fatigue failure.For sintered HAP, therefore, the resistance to fatigue failure under this wet condition is low.To draw the SPT diagram, anchor points were taken from the fracture stress and time-to-failure data measured at 1mm/min.The SPT diagram obtained suggested that a lifetime of 20years with survival probability of 99.9% required the safe working stress of less than 3.2MPa.The effects of accuracy of fatigue parameter on the uncertainty in SPT diagram were estimated.

View full abstract

The initial movement of the tooth was investigated by the finite element method.The mechanical properties of the periodontal membrane were estimated from the results of the simulation.The analysis was made by using an 8-point isoparametric finite element method.The upper central incisor with an axial directional load was simulated.The concept of the apparent elastic modulus was introduced to the stress-strain relation of the soft tissue, that is, soft tissue such as the periodontal membrane and the gingiva was considered to have stress dependent elastic moduli.At the first stage of the simulation, the stress and strain distributions in the paradentium were calculated by the conventional method.From the next stage, the apparent elastic moduli for the soft tissue were introduced to the calculation, that is, the elastic moduli for the soft tissue were computed from the calculated equivalent stresses of the previous stage.The calculations were repeated until the displacement values converged.The initial displacement of the tooth was not proportional to the applied load but showed a non-linear relationship.The relationship between load and displacement was approxiated by the n-th power equation.These results were in agreement with the clinical experimental results.The periodontal membrane is built up of different elastic moduli.The elastic moduli were low at the alveolar edge and the root apex.The equivalent stresses of the periodontal membrane were marked low in the alveolar edge and the root apex.In conclusion, the concept of the apparent elastic modulus is useful to investigate the initial movement of the tooth and estimate the mechanical properties of the periodontal membrane.

View full abstract

The polished surfaces of three set dental cements for luting(zinc phosphate cement, polycarboxylate cement, and glass ionomer cement)were observed by cryo-SEM at a specimen temperature of -160℃ to prevent damage of the cement specimens and also the specimens were analyzed by EDX.Furthermore, the SEM composition images of the polished cement surface were transferred to an image analyzer to obtain the core/matrix area ratio of the set cements.
1.The polished surface of set dental cement could be clearly observed by cryo-SEM without damaging the cement specimens.
2.The image analyzer showed that the core/matrix area ratio of the zinc phosphate cement and the glass ionomer cement was approximately 2 to 8, whereas that of the polycarboxylate cement was approximately 3 to 7.
3.The elements detected in the zinc phosphate cement were Ca, Zn, Mg, Al, and P, in the polycarboxylate cement were Ca, Zn, Mg, Si, and Sr, and in the glass ionomer cement were Al and Si.

View full abstract

According to the classification proposed by Hosodo, six core resins could be divided into two categories on the basis of the elemental composition and size distribution of filler particles by SEM observation and EDX analysis.Furthermore, several mechanical properties of the resins were determined.The following facts were found:
Bell Feel Core, Clearfil Core, Clearfil PhotoCore, Core Max, and Core Max II resins were classified as a semihybrid resin, and Microrest Core resin as a hybrid type resin.
The elements detected in the resins by the EDX were Si, Zr, Al, Ba and La.
The mechanical properties of the resins were shown to be highly stable at one day or one week after curing.
The mechanical properties of the resins suggest that the subsequent crown preparation and impression taking should be postponed until the next appointment.

View full abstract

This study attempted to polish pure titanium testpieces electrolytically to mirror surface at the size of cast denture frames.Electrolytic polishing of pure titanium could be done on an area of 30cm² with a non-aqueous electrolyte.
Small pure titanium plates could be polished electrolytically, but a uniformly smooth surface could not be obtained easily with large testpiece.
The optimal electrolytic conditions were 30V for 6 min at 25℃ using a solution consisting of 70ml ethyl alcohol, 30ml iso-propyl alcohol, 6g aluminum chloride, and 25g zinc chloride,
The solution was safe and had less restriction of frequency of use.

View full abstract

Pretreatment of the dentinal surface with acidic solution is now considered to be necessary to obtain strong bonding between dentin and resin materials.Effects of various surface treatments for dentin bonding on dentinal smear removal have been reported by SEM observation.In this study, changes of dentin permeability after the surface treatments were measured using Pashley's technique.
Forty-second treatments with either phosphoric acid gel, aqueous phosphoric acid, 10-3 solution or aqueous 10-20 Ca Solution(10% citric acid containing 20% CaCl₂)produced greater increases in dentin permeability compared with the treatments using either 10-20 Ca gel, Tenure Conditioner, Scotchprep or Gluma 2 for 40 seconds.
Treatments with primers including adhesive monomers such as Scotchprep and Mirage Conditioner also increased the dentin permeability by removal of the smear layers and smear plugs.
Application of a bonding agent(Clearfil Photobond)for 1 minute did not remove the smear plugs and the dentin permeability remained the same as before the treatment.
The changes in dentin permeability were not predictable by SEM observation in some cases.

View full abstract

The effects of the Tannin-Fluoride preparation, called the HY-agent, on dentin substrate were examined by immersion of the dentin previously treated with HY-agent into the calcifying fluid at 37℃.
The changes in the nature of the dentin surface and penetration of elements in the dentin were observed.
CaF₂ was formed on the dentin as a result of a reaction between F, an element of the HY-agent, and Ca which precipitated from the calcifying fluid.This observation was confirmed by means of the X-ray diffraction method.
Penetration of F, Zn, Sr and Ca into dentin was confirmed by the use of an electron probe X-ray microanalyser(EPMA).
Application of the HY-agent to the dentin for sevendays increased the hardness of the dentin about 10%.
Application of the HY-agent to the dentin for sevendays increased the contact angle about 3%.These results suggested that the organic and inorganic matter in dentin can be stabilized effectively both chemically and physically by treatment with an HY-agent.

View full abstract

New experimental visible-light cured soft resins were prepared by combining 4kinds of fluoroalkyl methacrylate monomers with a vinylidene fluoride/hexafluoropropylene copolymer, or a vinylidene fluoride/tetrafluoroethylene/hexafluoropropylene copolymer.Some properties of the resins prepared were examined to determine their value as a soft denture liner material.Water contact angles were more than 90 degrees in all cases and increased with the increase in the number of fluorine atoms in the fluoroalkyl groups.Hardness（JIS A）was less than 50 for the softest resin.Water sorption after 10 weeks was 3〜4.5wt%.Solubility 0.1〜0.3wt% and amount of residual monomer 0.2wt% were very low.The properties of these new experimental resins were comparable or superior to the commercial soft materials available.

View full abstract

Mechanical properties and cure depth of visible light-cured composite resins based on six types of UDMA(IP-HEMA, IP-HPMA, XY-HPMA, MC-HPMA, UEDMA and UPDMA)monomers were investigated.Under wet condiotions, the mechanical properties of the composite resins based on aliphatic UDMA(UEDMA and UPDMA)monomers were inferior to those based on the other UDMA monomers containing aromatic or cyclohexane rings in their chemical structures.The cure depth for these UDMA-based composite resins increased with increasing irradiation time.The composite resin based on the XY-HPMA monomer showed a cure depth and transmission coefficient superior to the other composite resins.

View full abstract

The mechanical behavior of the Ti-6Al-4V alloy denture base made by superplastic forming was studied using various bending tests.The bending strength of the Ti-6Al-4V alloy plate made by superplastic forming was as strong as that of the casting Co-Cr alloy plate with the same thickness.The bending strength of the superplastic forming plate with channels was higher than that of the plain plate.Increase in the arch curvature of the plate decreased the strength, because increase in the arch curvature of the plate decreased the thickness of the superplastic forming plate.Similar results were obtained for the clinical models.The appropriate original thickness for superplastic forming denture base was derived empirically from the experimental values for depth of palate.

View full abstract

Pure titanium and titanium alloys have excellent biocompatibility, excellent corrosion tarnish resistance, a quarter of the specific gravity of gold base alloys and suitable mechanical properties for dental clinical use.Howevre, titanium has some undesirable characteristics.For example, it has a high melting temperature and chemical reactivity at high temperatures.The investigate influence of the mold temperature on castability and mechanical properties of the titanium castings was study.The castability was found to be good at various mold temperatures, but the higher the mold temperature during casting, the greater the adhesive phenomenon between titanium surface and the investment.From the EPMA observation of the surface layer on the titanium castings, an Si-rich layer whose thickness increased considerably with increasing temperature was observed.The Vickers hardness in the surface layer was greater than that in the inner part.At a higher mold temperature during casting the thickness of the high Vickers hardness layer was increased.There was a relationship between the Vickers hardness value and the thickness of Si rich layer.In the tensile strength test of titanium castings, elongation values decrease and tensile strength values increased with the rise in mold temperature.

View full abstract

A wear test method based on a new concept was developed for predicting the wear rate of posterior composite restorations in vivo.The concept based on the conclusions derived from our clinical tests on posterior composites that composite restorations would be deteriorated chemically and by repeated compressive stress as well as by abrasion by food passing during mastication.New wear tests were perfomed on several posterior restorative materials under various conditions to compare the effect of abrasion using the tooth-brush.The results of wear rate using the present test on specimens stored in a 0.1N-NaOH solution(37℃)for 6 days were parallel to those measured in clinical tests.However, the abrasion test results obtained using a tooth-brush on deteriorated specimens did not match the clinical data.These findings suggested that degradation or aging of specimens and repeated compressive stress are necessary for the wear test to simulate the wear of posterior composites in vivo.

View full abstract

A laser displacement meter equipped with two PSD(Position Sensitive Device)sensors located 45° symmetrical to the incident beam was developed to measure the three dimensional shape of a tooth with a higher efficiency and accuracy.This displacement meter enabled compensation of the data at the shaded points without the rotational measurement which could not be done by the former single sensor displacement meter.The output voltages on the declined plane measured by the two sensors were different on the complicatedly declined plane.in such a case, the output measured by the sensor which made a larger angle with the declined plane was adopted since it was affected less by the secondary scattering.All regions of the 6molar were measured without errors, and the cusps and fissures of the occlusal plane were measured as precisely as by the single sensor displacement meter.The time required for the measurement was about 10 min, which was one third that required for the former single sensor displacement meter.For measurement of a crown abutment tooth with steeply inclined slope and margin, by the single sensor displacement meter, the tooth area had to divided into four parts by tilting and rotation on the gonio stage, and required about one and half hours.However, with the present displacement meter, only the measurements with tilting were required and the time was reduced to about 40 min.Besides, the data connection for CG was accomplished in about 5 min, and the complete data without errors and overlap could be saved.This data will be able to give the inner shape of a crown for machining by CAM technique.As mentioned, the newly developed double sensor laser displacement meter greatly simplified the procedure to measure the shape of a tooth and enabled measurements with high precision.

View full abstract

The effects of pre-treatment of porcelain surfaces, such as etching and treatment with a silane coupling agent, for obtaining firm adhesion between the adhesive resin, Panavia EX, and the porcelain were examined.The effects of heating treatment(100, 150 and 200℃) in applying the silane coupling agents to porcelain surfaces were examined.Infrared spectra and energy dispersive X-ray spectra were obtained for the porcelain surface, on which a silane coupling agent was applied. A stable shearing adhesive strength, such as 450〜520kgf/cm², was obtained even after a 24 hour immersion in 37℃ water, when with the etching the porcelain surface was pretreated with a 30% HF solution(60 sec)or heating treatment(150℃, 10 min) when applying the silane coupling agent(2% γ-MPTS). Then the adhesive durability was examined by a thermal cycling test(4〜60℃, 60 sec). After the thermal cycling test(20,000times), the adhesive strength to the porcelain surface polished by #600 sand-paper was strong and only when the 2%γ -MPTS was used did detachment occur.However, the adhesive strength was maintained at about 70〜80% of the early strength with the etching treatment by the pre-treatment with 30% HF for 60 sec and the heating treatment(150℃, 10 min)when applying the 2%γ -MPTS.

View full abstract

Anodic polarization, natural electrode potential, polarization resistance, tarnish, weight change and metal component dissolution in artificial saliva, of Ti-Al, Ti-Cu and Ti-Ni binary alloys were investigated to determine the effects of the added elements on the corrosion characteristics.In the case of Ti-5at% Al to Ti-50at% Al system alloys, increase in the aluminum content of the alloys raised the corrosion resistance.However pitting corrosion presumed at an aluminum content of more than 10at%.In the case of the Ti-5at% Cu to Ti-70at% Cu system alloys, increase in the copper content of the alloys decreased the corrosion resistance and the decrease was marked at a copper content of more than 50at%.In the case of Ti-5at% Ni to Ti-70at% Ni system alloys, 50at% Ni was estimated to be the maximum content to keep corrosion resistance of the titanium alloys.

View full abstract

As a model experiment to understand the mechanism of adhesion of the MMA-TBBO resin to dentin, MMA-TBBO was polymerized using a decalcified dentin sheet treated with aqueous citric acid(CA)solutions of copper fluoride(CF), ferric fluoride(FF), or ferric chloride(FC), which are usually used as treating agents in the bonding of the resin to dentin.The curing time for MMA-TBBO resin was considerably reduced on the decalcified dentin sheet treated with FC-CA, CF-CA or FF-CA at an appropriate concentration.Polymerization of MMA-TBBO resin was accelerated in the presence of a fluoride ion.Molecular weight of PMMA depended on the site of polymerization;PMMA polymerization inside the decalcified dentin sheet gave the highest and that outside gave the lowst molecular weight in the presence of an appropriate amount of ferric or cupric ion.These results suggested that ferric compounds absorbed to decalcified dentin are involved in the polymerization of MMA and thus will influence the bond strength of the MMA-TBBO resin to dentin.

View full abstract

The effect of quartz filler content on viscosity, dynamic viscoelasticity and tensile stress of polyaddition-type silicone impression materials were investigated.A hydrogen polydimethylsiloxane(H-silicone)and two vinyl polydimethylsiloxane(V-silicone)silicone prepolymers were prepared.Quartz fillers were mixed to them at contents between 0 and 50% by weight.The viscosity of these rubber compounds and mechanical properties of set materials were measured by a cone and plate viscometer, the dynamic shear test and the tension test, respectively.Viscosity η, storage modulus G' and tensile stress σ increased with the amount of filler increase.G' and σ were decreased by use of a V-silicone prepolymer which had a high average molecular weight and the material was more flexible.This suggests that the characteristics of the silicone prepolymer greatly affect the mechanical properties of the impression materials.Therefore, the appropriate molecular design for silicone prepolymer and reasonable filler content should be considered to make impression materials that have suitable properties for clinical use.

View full abstract

The application of zircon(ZrSiO₄)that has high refractoriness, high thermal conductivity and a low coefficient of thermal expansion, to quick casting investment was studied.Various zircon powders and phosphoric acid solutions were tested with respect to the higher thermal shock resistance.The formulation and properties of zircon-phosphoric acid investment materials such as water/powder ratio, fluidity of slurry, setting time, setting expansion, thermal expansion, thermal analysis, green and fired compressive strength were measured .Formulation of zircon slurry for coating was zircon flower #600 30%, zircon flower #350 10%, and zircon sand CP 60%, and that for sanding was zircon flower #200.The mixing liquid was 15% phosphoric acid and liquid/powder ratio was 0.1.The slurry using phosphoric acid had good fluidity and good workability.The 24-hour green strength was 1MPa, fired strength was 10MPa, 24-hour setting expansion was -0.04% and the thermal expansion at 1000℃ was 0.31%. Immediately after coating with zircon slurry, the coating layer was dried, sintered and dewaxed by thermal shock.The thermal shock consisted of the following four-step manipulations.The first is hot air drying(50℃, 5 minutes), the second is heat shock(900℃, 3 seconds), the third is redrying(220℃, 3 minutes)and the fourth is dewaxing(550℃, 3 minutes). Small casting of pure titanium and K-metal could be done successfully by the quick casting method using the zircon-phosphoric acid investments. It was found that the total expansion of the secondary investments influenced the casting adaptability.

View full abstract