This study examined in detail the effect of the sintering process on hydroxyapatite (HAp) particles. The electron spin resonance (ESR) signal intensity of Mn²⁺ was measured and were made scanning electron microscopy (SEM) observations after the partial dissolution of HAp particles by lacttic acid.
The ESR signal intensity of Mn²⁺ decreased rapidly as the HAp particles fired at low temperature were dissolved with lactic acid solution, but that of the HAp fired at high temperature decreased only slightly. From these results, it was clear that the thermal reaction by firing at a low temperature took place only in the surface layer, and proceeded uniformly into the inner layer of the particles as the firing temperature increased. SEM observation of HAp particles before and after dissolution showed that the dentification of HAp particles occurred only from the surface layer and/or interface particles at lower firing temperature.
The sintering mechanism of HAp can be shown in a schematic representation, and is similar to the sintering theory of ceramics. ESR measurement is also applicable to elucidating the sintering mechanism of other ceramics as well as HAp.

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The quantity of the remaining double bonds (RDB) in two polymerized binary system visible light-cured resins using BisGMA has been measured. As monomers, BisGMA [2, 2-Bis{4-(3-methacryloxy-2-hydroxypropoxy)phenyl}propane], BMPEPP [2, 2-Bis(4-(methacryloxypolyethoxy)phenyl}propane] and TriEDMA (triethyleneglycol dimethacrylate) were prepared for the visible light-cured resins by changing the mixing ratio of BisGMA and diluent monomers, from 20 to 80 wt% and 80 to 20 wt%. Trial resins were examined by means of a fourier transform infrared spectrophotometer by the liquid film method. The infrared absorbance measurements were made before irradiation and chronologically repeated after the resins had been polymerized by irradiation for 90 sec.
The quantity of RDB in all trial resins decreased linearly with the logarithmic time after the start of irradiation. The quantity of the binary system resins decreased with the increase in the mixed amount of BMPEPP or TriEDMA, and revealed a minimum at 80 wt%. The conversion in binary system resins showed a higher value than the additive mean of the conversion in component monomers. The viscosity of binary system resins decreased monotonously with the increase in the mixed amount of BMPEPP or TriEDMA.TriEDMA decreased the viscosity more effectively than BMPEPP.The conversion in two binary system resins showed similar values in the range of 20 to 80wt%.The conversion of the binary system resin may be affected mainly by the conversion of the component monomer and by the change in viscosity.

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Hydrocolloid impression materials are often used in clinical practice. However, the materials may undergo chronological dimensional changes caused by loss of water because of their fluid nature.
Therefore, as an experimental material we used, reversible hydrocolloid impression materials which are often used when taking precise impressions of inlay and crown-bridge, and conducted the measurement with a scanning laser microscope to examine the chronological microdimensional changes, reproduced on a 20 μm detail line on the surface of the impression.
Thanks to the scanning laser microscope, microdimensional changes which appeared immediately after removal of the impression could be measured without touch, which had been considered impossible by previous methods of measurements.
The reversible hydrocolloid impression materials shrank chronically. The shrinkage tendency differed with the kind of impression material. Some impression materials shrank slowly and ohers shrank markedly immediately after the removal of the impression.
All the tested materials showed a similar rate of shrinkage, 26 minutes after the removal of the impression.

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4-MET in the 4-META resin system was determined by NMR spectroscopy. 4-META was easily hydrolyzed to 4-MET. To investigate the biological activity of 4-MET, we examined the interaction of 4-MET with phosphatidylcholine and phosphatidyl ethanolamine liposomes at various pHs using NMR spectroscopy. Chnages in the chemical shift of the liposomes induced by 4-MET were small at pH7.0, but very large at pH2.5, indicating that 4-MET interacts markedly with phospholipids at a low pH. Since dentin is a highly solid buffer, it was concluded that the effect of 4-MET to damage the pulp would be small at pH7.0.

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To improve the hydrolytic stability of the silane at the interface, di-methacrylate silane was designed, and its hydrolytic stability was examined.
Before hydrolytic degradation, the di-methacrylate silane gave a higher bond strength of the resin against the silane-treated silica surface than γ-methacryloxypropyltrimethoxysilane. However, when the specimen was immersed in hot water, the bond strength decreased markedly. The failure mechanism changed from cohesional failure to interfacial failure. The hydrolytic stability of the di-methacrylate silane was much worse than γ-methacryloxypropyltrimethoxysilane because of strict hindrance of the organic group in the silane molecule. The bond strength was measured, after the specimens were immersed in hot water and dried up at room temperature. The bond strength increased and failure mechanism changed from interfacial failure to cohesional failure of resin.

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Several properties of capsulated light-cured glass polyalkenoate restorative cement were examined in vitro in comparison with the conventional non-capsulated cement. A clinical trial was conducted.
Mechanical mixing of the cement with an mechanical mixer decreased the area ratio of the porosity in a cement mix, and slightly increased the translucency of the set cement and core/matrix area ratio. Capsulation and mechanical mixing of light-cured glass polyalkenoate cement required some modification in shade in order to obtain a better color matching because of decreased opacity due to decrease in porosity rate in the set cement.

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Polishability and durability of six kinds of newly designed diamond disc abrasives for dental restorations were precisely determined from the surface roughness and gloss of polished resin composite surfaces.
These discs were superior in polishability and durability to the conventional ones.

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The Vickers hardness number (VHN) of a Ni-Cr-Mo-Si alloy (NCMS No. 8), prepared by addition of 12%Mo and 2%Si to the 80Ni-20Cr mother alloy was 203, but the tensile strength of this alloy was 40.5 kgf/mm², which was a marked low value when compared with the one-third value (68) of the VHN. To improve the mechanical properties of this alloy, Ni-Cr-Mo-Si-B alloys were prepared by addition of 0.2%-0.6% boron to the NCMS No. 8 alloy. The tensile strength of the cast specimens of the alloy containing 0.4% boron was increased by 68%, to 68 kgf/mm². Similarly, addition of 0.6% boron to Ni-Cr-12Mo-6Si alloy (NCMS No. 16) increased the tensile strength by 32%, to 73.8 kgf/mm². By XMA analysis, the Mo-B lamella structure was observed in the inter-dendritic arm, and the X-ray diffraction pattern caused by chromium boride was observed.

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Effects of processing conditions (exposure time and time elapsed after mixture) on the amount of monomers eluted from a light-activated relining material were investigated. The types and amount of eluted monomers were analyzed by high-performance liquid chromatography. The amount of eluted monomers decreased with increasing light exposure time. The time elapsed after the mixture significantly affected the amount of eluted monomers. These findings suggested that prompt manipulation and prolonged light exposure are required to reduce the amount of residual monomers.

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The influence of cement mixture exposure period and the surface roughness of dental casting alloy on both contact angle and tensile bond strength of different luting cements were examined.
Contact angle and tensile bond strength of three different luting cements (zinc phosphate, polycarboxylate and glass ionomer cements) on four different dental casting alloys (Au-Ag-Cu, Ag-Pd, Ni-Cr alloys and Ti) were measured according to the methods described in the previous study. The cement mixture was exposed for 15, 30 and 60 seconds after each cement was mixed and each dental casting alloy was polished with #120, #400 and #600 SiC paper.
The contact angles of each luting cement on all substrates significantly increased with the increase in exposure time. The tensile bond strengths of polycarboxylate and glass ionomer cements to the substrates were not changed with the increase in exposure time.
The contact angles of zinc phosphate and glass ionomer cement on the substrates increased with the increase in the surface roughness of dental casting alloy; however, the polycarboxylate showed no change with the increase in the surface roughness of dental casting alloy. The polycarboxylate and glass ionomer cements showed the greatest tensile bond strength to the materials polished with #400 SiC paper.
These findings suggested that the casting restorations should be cemented from 15 to 60 seconds after the cement was mixed.

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The mechanical properties of titanium castings have been examined by using a tensile test. However previous studies have not evaluated the elasticity (apparent elastic modulus and proof stress) of titanium castings.
The objective of this study was to examine elasticity of titanium castings. The apparent elastic modulus and proof stresses were calculated from the results of a three-point bending test, their usefulness was assessed by a hardness test and observation of the microstructure.
The elastic modulus of titanium castings was similar to that of As cast and aging of ADA type IV gold alloy. The proportional limit, 0.03% proof stress and 0.2% proof stress of titanium castings were similar to those of a solid-solution-treated ADA type IV gold alloy. The superficial hardness of titanium castings varied with the casting condition. This variation in the hardness correlated with the elastic modulus. Thus, the elastic modulus of titanium castings was found to be markedly affected by the superficial hard layers of titanium castings. These findings suggest that the three-point bending test is useful in assessing the apparent elastic modulus of titanium castings.

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We examined the cellular adhesive strength of L-929 cells to specimens made of four kinds of polymers using a cone and disk type viscometer which produces a uniform shear stress of 0.74 Pa. The relative cell adhesion rates for the three kinds of polymers, polyether sulfone (PES), polysulfone (PSF) and polycarbonate (PC), but not polymethyl methacrylate (PMMA), were similar to those for glass used as a control. Longer application of shear stress resulted in a lower percent of adherent cells, particularly on PMMA and PC. The percent of adherent cells was considerably low in the absence of serum. Cells cultured for 4 hours were more easily detached from the specimen surface than those cultured for 24 hours. Thus, the present method is considered to be effective in evaluating the cellular adhesive strength necessary to obtain cell compatibility with biomaterials.

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Density, specific surface area and particle size distribution of surface-treated silica were examined. As silica, Aerosil 130 (specific surface area: ca 150 m²/g) was selected. γ-Methacryloxypropyltrimethoxysilane (γ-MPTS), Vinyltrimethoxysilane (VTMS) and Vinyltrichlorosilane (VCLS) were used to coat the surface of silica by varying the concentration in the wet process. Density was decreased in accordance with the increase in the concentration of the coupling agent. The density of silica treated with VCLS showed a distinct turning point at 11 mg/ml. The specific surface area of the silica treated with γ-MPTS was quickly decreased to 22 mg/ml, and that of the silica treated with VTMS and VCLS was decreased to 5.5 mg/ml. The median diameter of silica showed a tendency to increase in accordance with the increase in the concentration of the coupling agent.

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The synthesis of new dental resin monomers and the properties of monomers and bulk polymers were examined. Octachlorocyclotetraphosphazene (P₄N₄Cl₈) was used as material, 4 kinds of cyclophosphazene linear monomers were synthesized as follows: 2 chlorines (Cl) were replaced by diol (HOROH), and the residual Cl, were replaced by 2-hydroxyethyl methacrylate (HEMA). These monomers were analyzed by IR, NMR and elemental analysis and the physical properties of bulk polymers were examined.
The molecular weight of the synthesized monomers was 2500〜3000. Refractive index and viscosity were 1.4978〜1.4995 and 42〜73 poise, respectively. Among the 4 kinds of bulk polymers, [P₄N₄-(O(CH₂)₆O)₁(EMA)₆] _nshowed preferable mechanical properties.

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The working and setting times, viscosity, elastic recovery, elastic modulus, hardness and dimensional change of ten denture base soft lining materials were measured. The endurance of these materials was examined using a thermal cycling test. Moreover, effects of voids caught in the material on these physical and mechanical properties and endurance were investigated.
The working time was 2.5 to 17.0 min, the setting time 3.8 to 14.3 min. For two silicones, the viscosity was markedly increased after the start of measurement. Six materials showed a more than 90% elastic recovery at 2-9 min after the start of mixture. The elastic modulus for acrylates and silicones was 3.57×10⁶ to 9.01×10⁶ dyne/cm². These values were nearly equal or lower than that for oral soft tissue(0.7-4.4×10⁷ dyne/cm²). In dimensional change, four acrylates showed expansion. The materials with no voids had a higher elastic modulus and hardness, and lower dimensional change than those of the materials with a large number of voids. After the thermal cycling test, the surface roughness, weight change and gel strength change of acrylates and silicones with a large number of voids were larger than those of materials with no voids. These findings show that the presence of voids affects the endurance of materials.

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In this study, a new pretreatment method which enhances the adhesion between dentin and restorations was examined. The new pretreatment method consisted of: (1) acid etching the dentin adherend surface: (2) removing an exposed dentin collagen with an organic substance dissolving agent; and (3) applying the adhesive directly on the irregularities made of exposed dentin apatite. As primary agents for treating the dentin adherend surface, three types of acid etching agents (40% phosphoric acid, 0.5 M EDTA and 10% citric acid) were used. As secondary agents, two types of organic substance dissolving agents, 10% sodium hypochlorite solution (NaOCl-Solution) and 10% sodium hypochlorite solution containing 8-18 wt% of alumina (NaOCl-Gel), were used. In addition, a commercial bonding agent and another bonding agent having an increased concentration reducing agent (sodium benzenesulfinate) were also used. After carrying out the primary treatment with the phosphoric acid solution and the secondary treatment with the NaOCl-Solution, Panavia EX was applied to dentin through a commercial bonding agent.
The tensile adhesive strength obtained was 11.0 MPa in the horizontal direction. Hypothesizing an extracoronal tooth, the adherend surface was the vertical side and the secondary treatment was carried out with the solution for the same length of time (60 seconds). The resultant adhesive strength was only 3.6 MPa in the vertical direction. When the NaOCl-Gel containing 14 wt% of bodying agent was used for 60 seconds in the secondary treatment process, the initial tensile adhesive strength was 10.6 MPa. SEM observation and FT-IR analysis revealed that the NaOCl-Gel could hold well to the vertical adherend surface and efficiently dissolve an exposed collagen layer. There was a close relationship between the dissolution and removal of the collagen layer and the degree of tensile adhesive strength. When a bonding agent containing 5 wt% of sodium benzenesulfinate was used, an tensile adhesive strength of 8.1 MPa was retained after 20, 000 thermal cyclings and there was also no decrease in adhesion due to broken adherend. Use of our surface treatment method for dentinal abutment teeth produced excellent marginal adaptation compared with no pretreatment and when only primary pretreatment was made. Marginal adaptation was improved by increasing the concentration of sodium benezenesulfinate.
Our pretreatment method is considered instrumental for providing greater tensile adhesive strength and improved marginal adaptation.

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The surface reaction of bi-phase ceramics containing HAP and βTCP or CaO was investigated by immersing the materials into the medium containing 10% bovine serum.
βTCP-HAP bi-phase ceramics, which were stable even at 1200℃, were obtained during sintering of the Mg-containing hydroxyapatite powders with different Ca/P molar ratios.
Samples containing large amounts of βTCP dissolved more quickly after the addition of bovine serum, but, the dissolution rate was 1/3 of that of αTCP-containing HAP ceramics, and Mg dissolution was less than 1 ppm.
βTCP-HAP bi-phase ceramics showed better stability than αTCP-containing HAP ceramics, and HAP fine particles were precipitated on their surface when they were immersed into medium saturated with Ca and P ions.
These findings confirmed that the βTCP-HAP bi-phase ceramics can maintain good biostability after implantation.

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The adaptability of a cobalt-chromium cast plate denture constructed by a controlled polymerization system, that is a DS-system, was investigated in comparison with that constructed by autopolymerization and heat polymerization. The heat polymerization was carried out in a water bath at 70℃ for 24 hours, and at 70℃ for 40 minutes and then at 100℃ for 30 minutes.
The cobalt-chromium cast plate had a bending elastic modulus of 23, 000 kgf/mm². Discrepancy of the dentures constructed by the DS-system to stone cast was about 0.10 mm at the median line and denture border of the dentures. The 0.1 mm discrepancy was equal to one-half or one-third the discrepancy of the dentures constructed by other polymerization methods. Compared with other polymerization methods for adaptability, the DS-system was advantageous. Although the discrepancy after the unilateral occlusion test increased, the DS-system gave the smallest increment of the discrepancy among these polymerization methods.

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The four point bending strength of porcelain-fused-to-metal (PFM) in Ar gas at 37℃ was measured at the strain rate of 8.33×10^-5sec^-1, and the critical stress intensity factor was obtained by SEPB (single edge precracked beam) method. Using the strength data in distilled water at 37℃ reported previously at five strain rates, the effect of water on the crack growth of PFM was investigated. According to Weibull analysis of strength in both Ar gas and distilled water at the same strain rate, the strength coincided at the side of low fracture probability and the Weibull parameters were 15.3 in distilled water and 8.1 in Ar gas. The ratio of crack length just before rupture in distilled water to Ar gas increased with increasing fracture probability and finally became about twice. Besides, supposing that the fracture was initiated from the surface flaw and that the crack growth length in Ar gas was zero, the length of the crack growth by the start of rupture in the distilled water was estimated to be 5〜36 μm and increased with decreasing strain rates. Furthermore, as the strain rate decreased, the crack growth in each specimen became almost equal and did not depend on the initial size of the flaw.

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