The effects of cell density, exposure time and assay method on evaluation of cytotoxicity of chemicals were investigated using an in vitro test with extraction.L-929 cells, seeded at 4×10^3cells/ml, 2×10^4cells/ml and 1×10^5cells/ml were exposed for 1, 3 and 5 days to 8 metal salts.The 50% cell inhibitory concentration values(IC_<50>)increased with cell density but decreased with exposure time.Of the four assay methods examined, the neutral red and MTT methods were found to be useful in evaluation of cytotoxicity of chemicals.The gradient obtained from IC_<20> and IC_<80> values of the concentration-response curves was dependent upon the kinds of metal ions, cell density, exposure time and assay methods.These findings are applicable to the evaluation of the cytotoxicity of dental materials.

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Three methods were employed to improve the adaptability of composite resin inlays to the MOD-type cavity.In the first method, the convergence angle of the axial walls in the cavity, was increased.In the second method, a separating agent was applied two times.In the third method, a spacer was used.A Clearfil CR Inlay and a Lite-fil CR Inlay, light-cured types, and an Inlay/Onlay, a heat-cured type, were used as materials.The adaptability of the inlays was improved with the first and the second methods.Never-theless, the combination of the two methods could not shorten the distance less than 50μm from the inlays to the gingival wall.The use of silicone rubber impression materials as a spacer with a high flow improved greatly the adaptability of the light-cured types.However, the double bond conversion of the light-cured types was interfered by the impression materials tested.Therefore, a spacer which does not interfere with the double bond conversion of the composite resin must be developed.In the case of the heat-cured type, none of the three methods improved greatly the adaptability.

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The properties of the conventional dental cyanoacrylate cement"FH"(Nissin Dental Product Inc.)were reported in the previous number of this publication.Attempts were mode to improve the bond strength by changing the base component and hastening the setting time.The bond strength durability was greatly improved by the change to isobutyl-cyanoacrylate from ethyl-cyanoacrylate and addition of methylmethacrylate polymer to the base element.Water absorption was reduced, but compressive strength was decreased.The effect of hastening setting time was smaller than that of element change.The addition of barium sulfate for X-ray opacity as lining cement use, did not decrease the bond strength for dentin, but it decreased the compressive strength by about half that of conventional cement.

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Tensile strength properties and acoustic emission(AE)characteristics of commercial denture base resins during tensile test were examined for the specimens stored in a distilled water at 37℃ for 7 days.Among the heat-cure type PMMA-based denture base resins, conventional products(Acron and Urban)and modified products(L-Resin and Luxon)showed a significant difference in Young's modulus and elongation at maximum load.In particular, Luxon had a lower Young's modulus and a higher elongation, compared with conventional products, from the incorporation of about 1μm rubber particles.Two thermoplastic denture base resins, reinforced polycarbonate and polysulfone showed a definite yield behaviour and a higher AE activity and elongation, compared with conventional heat-cure type PMMA-based materials.Reinforced polycarbonate provided high Young's modulus from the incorporation of chopped glass fibers.

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The fracture toughness of commercial denture base materials was examined using compact tension(CT)and single-edge notched(SEN)specimens.Deformation and fracture behaviour during the fracture toughness test were monitored from the acoustic emission characteristics.The fracture toughness obtained was discussed from ASTM E399.Fracture toughness obtained from maximum load of thermoplastic denture base products(reinforced polycarbonate and polysulfone)and rubber-toughened material(Luxon)showed a higher value than that of conventional PMMA-based materials(Acron, L-Resin and Urban).The fracture toughness of the conventional materials using CT specimen was accepted as a plane-strain fracture toughness K_<IC> according to ASTM E399.

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The quantity of the remaining double bonds(RDB)in polymerized three binary system visible light-cured resins have been measured.As monomers, the synthesized cyclophosphazene monomer P_4N_4(CF_3CH_2O)_2[CH_2=C(CH_3)COOCH_2CH_2O]_6[4PN-(TF)_2-(EMA)_6], and commercial UDMA, Tri-EDMA and BMPEPP were prepared for the visible light-cured resins by changing the mixing ratio of 4PN-(TF)_2-(EMA)_6 and commercial monomers, from 20 to 80wt% and 80 to 20wt%.Trial resins were investigated 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 90sec.The quantity of RDB of all trial resins decreased linearly with the logarithmic time after the start of irradiation.The quantity of the binary system resins decreased monotonously with the increase of the mixed amount of commercial monomers.Mixture of UDMA or BMPEPP with 4PN-(TF)_2-(EMA)_6, increased viscosity and decreased the quantity of RDB.

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Superplastic forming of the Ti-6Al-4V alloy is a quite attractive method of fabricating a full denture base.However, it is not easy to make a partial denture frame by superplastic forming because a retainer like clasp can not be fabricated with it.The thickness of the superplastic denture base can not be controlled to change partially as in casting.Diffusion bonding, which makes it possible to join the retainer or titanium plate to the denture during the superplastic forming of Ti-6Al-4V alloy denture base was attempted.Pure titanium, Ti-6Al-4V, Ti-20Cr-0.2Si successfully joined the Ti-6Al-4V alloy denture base with intermediate metal foils such as palladium and 14K-20K gold alloy solders by diffusion bonding during the superplastic forming.Co-Cr alloys which were coated in advance with 14K-20K gold alloy solders were also joined to the superplastic forming Ti-6Al-4V alloy denture base.

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Modeling of 7 and 6 molars by the cutting method was attempted by means of a personal computer controlled modeling machine CAMM-3 using three dimensional shape data measured previously.A program for cutting using the flat end mill as a tool was made taking the tool offset and tool interference into consideration.The sign of the tool offset was determined according to the inclination of the date, and the z values between tool end and the data were always compared so as to avoid tool interference.7 abutment tooth and 7 molar for data base were cut from the wax block by successive carving using the flat end mill with a diameter of 3.0mm, 1.0mm and 0.6mm.The time required for total cutting was about one hour.Consequently, products which were nearly identical with the stone model measured were obtained.The inside cutting of 7 abutment tooth was also accomplished by the program which took the tool offset and interference for inside cutting into consideration.The 6 molar with an undercut could be cut by recognizing the height of contour and the excess cuttings was avoided.Thus, the inner and outer cuttings of crown were accomplished according to the measurement data.

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We have measured the spreading areas of seven commercial soldering fluxes and silver solder on 18% Cr-8% Ni stainless steel plates at 750℃, 800℃ in air.We have also tested the tensile strengths of the joints of the same steel bars bonded with the same fluxes and solder and calculated the soldered area fractions of the total cross-section of the soldered bars.Significant differences were found between these seven fluxes in the spreading of the flux and the solder, the joint strength, and the soldered area fraction(p<0.01).The spreading of the solder did not necessarily increase with the increased spreading of the flux.The flux which enhanced the spreading of the solder had a tendency to increase the joint strength and the soldered area fraction.These find suggested that each of the fluxes were different in surface tension, viscosity, ability of dissolving the oxides and lowering of the surface tension of the melted solders.We concluded that the fluxes tested were acceptable clinically because of adequate joint strengths.

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The influence of Ti-Ni shape memory alloy post for a post hole was investigated.Prefabricated straight-slit type posts(Smooth surface, 1.6mm diameter)caused stress because of lack of homogeneous contact.The stress on the post hole caused by a new trial post(Theraded surface, 1.5mm diameter)above the transformation point(martensite structure austenite structure)was two times as large as that below that point.At a temperature of 35℃, the shape memory alloy's post made of martensite structure caused less stress on the shape memory alloy's post made of austenite structure.Stress of new trial posts made of a martensite structure for the post hole was approximately 1.5MPa.The thickness of a post had no influence on the thickness of the cement line;the average thickness was 13.4μm.The thickness of the cement line did not change with the change of the structure of shape memory alloy.A new trial post(No.1-4)was always cemented rightly in the center of the post hole.

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Electron spin resonance(ESR)spectroscopy was employed to elucidate the setting process of the glass-ionomer cement by use of manganese ions(Mn^<2+>)as a spin probe.The effects of powder liquid ratio, temperature, mixing time, additives of tannin fluoride and tartaric acid, and particle size of the powder on setting time were investigated.The glass-ionomer cement powder was mixed with cement liquid containing Mn^<2+>.The peak height of the highest field component of hyperfine splittings due to nuclear spin of Mn(I=5/2), which was proportional to the concentration of free Mn^<2+> was measured at 30-second intervals beginning 2.5 minutes after mixing.The setting time decreased with the increase in the powder liquid ratio, increase in the enviromental temperature, decrease in the mixing time, and decrease in the powder particle size.Although tannin fluoride had no essential effect on either setting process or setting time, the presence of tartaric acid increased the setting time.However, the setting time thus estimated by ESR without any load, was slightly longer than that estimated with a load of 454g according to the ADA No.9 specification.The findings obtained by use of 2, 2, 6, 6-tetramethylpiperidine(TEMPO)as a spin probe instead of Mn^<2+> were also reported.

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