The purpose of this study was to investigate the mechanism of the crystal bonding, which had been suggested as a new surface treatment.
　Calcium sulfate dihydrate (gypsum) was precipitated on an enamel surface that was treated with two different aqueous solutions, acrylic acid-ammonium sulfate and citric acid-ammonium sulfate.
　The crystal was found to combined with the enamel surface by an adhesion test, X-ray diffraction, scanning electron microscope and X-ray microanalysis.
　Based on the misfit of atomic arrangement of gypsum and hydroxyapatite, it was suggested that gypsum was deposited on the enamel surface by epitaxial growth.

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2'-Acryloxyethyl 2-methacyloxyethylcarbamate (UAMA) was prepared to improve the mechanical properties of light cured resin. Light cured UAMA was superior to light cured bis-2-methacryloxyethyl (2, 2, 4-trimethylhexamethylene) dicarbamate (UDMA). UAMA cured with TMPT filler and glass filler by two processes was adequate for crown and bridge resin. However, UDMA cured with the same filler by light irradiation had lower Brinell hardness and compressive strength than those prepared by heat curing.

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　The relationship between the load-displacement curve and the microfeatures of the fracture surface of Ag-alloys was investigated using the precracked test pieces.
　When the distribution density of I-type dimples was low, the local plastic deformation that was initiated at the precrack tip field showed large values. The mechanical strength was related to the slip bands in the I-type dimples.
　The needle-like phases in the matrix of Ag-Cu alloys became fine or were decreased by adding iridium or rhodium; II-type dimples also decreased. The ductility of the alloys was moderately improved.

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　Immersing 15 commercial chromium-nickel casting alloys in 1% lactic acid, 0.05% HCl, and Ringer's solution for 7 days at 37℃, released metallic elements, the composition of these alloys were determined by inductively coupled plasma emission spectrometry.
　The results indicated that the release of metallic elements in chromium-nickel alloys containing an excess of 14% chromium was decreased. The released metallic element concentration of only Ni in the tested chromium-nickel alloys was less than 1.0 mg/cm². Among the 3 immersion solutions, the corrosiveness of 1% lactic acid was large, whereas that of Ringer's solution was below the limit of detection.

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　To compare the tensile test with the compressive test, factors that influence compressive properties of dental casting alloys, wrought metals and annealed metals were investigated.
　Compressive yield strength was affected by the height/diameter ratio of the specimen and the compressing rate.
　The upper and lower yield points of normalized steel were clearly observed in the compressive test.
　Compressive yield strength agreed with tensile yield strength within experimental errors for wrought metals and annealed metals, but they differed greatly in casting alloys. The tensile test was affected significantly by casting defects.

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　This study, investigated the factors that influence compressive properties of dental amalgam, such as proof stress, crushing strength and compression.
　Compressive proof stress was affected significantly by the compression rate whereas crushing strength was affected by various factors. The stress relaxation and the creep of amalgam had large effects on the compressive properties. Compressive proof stress was more useful for evaluations the strength of materials than by crushing strength because proof stress values showed less deviation than those of crushing strength.

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　This study examined the mechanical properties and the clinical applications of a newly produced O-ring elastic made from thermosetting polyurethane. This O-ring elastic was compared with 5 other elastics, all of which had the same shape, in terms of strength, elongation and clinical usefulness.
　None of the 6 types of elastics showed significant changes in weight after storage in water (37℃) for 30 days. Clinical expansion was possible with all types.
　The properties of O-ring elastics were superior to the other 5 types in the S-S curve test. O-ring elastic showed the least deformity among the types tested. Due to its superior elasticity, O-ring elastics may allow constant force to be applied to the teeth. O-ring was easily, manipulated.

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Residual stress in the tapered plates produced by injection molding at the various gate positions was photoelastically analyzed. Residual stress and monomer crack of experimental upper complete dentures produced by injection molding under the resulting conditions were also investigated. Stress in the specimens that were injected at the thin part of the tapered plate was concentrated remarkably near the boundary between the thin part and the tapered part. On the other hand, in specimens that were injected at this boundary, the principal stress difference curves were smooth and the smallest stress concentration developed at the boundary. The stress concentration or monomer crack of the upper complete dentures that were injected at the incisive papilla portion of the denture were less than those of the dentures that were injected at the thinner palate region or the posterior ridge portion. These results suggest that the stress concentration, monmer crack or sink can be decreasad in the injection molded resin base denture.

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　We investigated the relationship between bond strength deterioration and some characteristics of adherends by the thermal cycling method, decided in previous report.
　The bond strength deterioration of resin adhesives appeared to be independent of the strain resulting from the difference of the thermal expansion coefficients between the adherends and adhesives. This suggest that deterioration of bond strength should be prevented by methods such as surface pretreatment of adherends.
　Deterioration of bond strength for teeth are small compared to that for alloys. Therefore, the detachment of metallic restoration appeares to occur at the boundary of the metal and adhesive, and not at that of the tooth and adhesive, in long-term in vivo use.
　The structure of oxide film was related to bond strength deterioration. The passive state of nonprecious alloys was better than that for the precious alloys with thick oxide film, which was produced by heating. The film of in the passive state was thin and strong.
　Higher stiffness of adherend was associate with reduced bond strength deterioration by thermal cycling. But, the effects is relatively small compared to the deterioration in long term use.

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　The addition of dicalcium phosphate dihydrate (DCPD) crystals to calcium sulfate dihydrate solution to improve mechanical properties of gypsum materials was studied.
　The structure of DCPD crystals resembled that of gypsum. When the crystals were added to metastable solution of calcium sulfate supersaturated with respect to gypsum, laminiferous gypsum deposits were observed on the DCPD surface by the atomic absorption analysis, fourier transform infrared spectroscopy, scanning electron microscopy and electron X-ray microanalysis.
　Therefore, DCPD is a remarkably useful crystalline nucleous for making gypsum crystals. It is suggested that the addition of DCPD improved the mechanical properties of gypsum materials.

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The application of zirconia (ZrO₂), a highly refractory material, to investment material was studied. Various zirconia powders and binders, organic and inorganic, were tested with respect to the conditions for higher temperature casting such as adequately high strength, non-reactivity and good castability. Pure zirconia and zirconia stabilzed with calcia, magnesia, yttria were tried as refractory materials. Zirconia sol, phosphoric acid, phosphates, surfactants, water-soluble acrylic bond and methyl methacrylate (MMA) chemically polymerized at an ambient temperature were used as bonding agents. The properties of investment material such as water/powder ratio, fluidity of slurry, setting time, setting expansion, thermal expansion, thermal conductivity, green and fired compressive strength, surface reactivity and defects contained inside castings were measured. Zirconia was an excellent investment material for the higher temperature casting, which produced titanium castings with a shiny surface without reaction and oxidation as cast, and with fewer casting defects inside.

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　A commercial light-cured composite resin filled in Class II cavities that were prepared in extracted human upper premolars was cured by using 5 different irradiation methods in combination with conventional and new matrix techniques.The degree of marginal leakage at the cervical areas was examined by dye penetration. Also, Knoop hardness of the proximal resin was determined in the various portions.
　In the case of the matrix bond technique using a sectionally transparent band, the lateral irradiation method was very effective to reduce the microleakage at the gingival margins, because the band allowed focusing the light toward the given areas. Minute enamel cracks were often observed at the gingival marginal areas without bevelling. Accordingly, bevelling the margin is recommended. Marginal microleakage could not be prevented at the gingival wall of dentin only, with any techniques in tested.
　From the distribution of Knoop hardness of the proximal resin, the gingival margin portions showed good polymerization, when the lateral irradiation method was applied in combination with the new matrix technique.

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　The durability of posterior composites under repeated stress was studied using a dynamic testing machine. Cylindrical specimens (2.5 mm in diameter, 5 mm in height) made from two types of composites (one chemically and one light cured resin) were stored in distilled water at 37℃ for one week. Ten specimens in each group were used for the compressive tests. Other spesimens were tested for repeated stress using a dynamic testing machine and each specimen was repeatedly subjected to stress under a given load. Repetition numbers of intermittent load were counted until the specimens broke. The specimens that did not break after 6, 000, 000 cycles were tested to determine the compressive strength. The fracture surfaces of broken specimens were observed by SEM.
　Durability of the composites under repeated stress seems to depend upon the compressive strength. There was no difference in the compressive strength between the specimens with and without being repeatedly stressed. Fine striped cracks were observed on macrofillers of the specimens that were repeatedly stressed.
　Air bubbles in the specimens played an important role in the poor durability of composites under repeated stress.

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