Showing papers in "Journal of the Japanese Society for Dental Materials and Devices in 1990"

Effect of adhesion promoting monomers on adhesion to ground dentin

Abstract: Effect of adhesion promoting monomers dissolved in photocurable bonding agents on adhesion to ground dentin was investigated. They were MDP, Phosmer-M and 4-MET. The effect of those monomers was compared with that of Phenyl-P. The bonding agents contained campherquinone (CQ) as a photosensitizer, N-phenylglycine (NPG) as a reducing agent and TEGDMA as a base monomer. Phenyl-P was the best adhesion promoting monomer among those studied monomers. MDP could not polymerize well enough to give good bond strength. Phosmer-M could permeate through the smeared layer but could not make a stable resin reinforced dentin. 4-MET did not permeate through the smeared layer and did not adhere to ground dentin.

[Stable adhesion to dentin. Combination of EDTA 3-2 (NH4/Fe) pretreatment and 2% 4-META/MMA-TBB resin].

Abstract: An excellent durability of adhesion was obtained in the adhesion of 2% 4-META/MMA-TBB resin to dentin pretreated with EDTA 3-2 (NH4/Fe). Such high bond strength as 15 MPa did not change for up to one year even when they were immersed in water at 37 degrees C. The resin reinforced dentin, a hybrid, was identified between dentin and the cured resin by TEM. The collagen and hydroxyapatite were encapsulated with the copolymer and the crystals were not removed by the demineralization with either HCl or electronic staining. Tensile fracture between the hybrid and dentin like the adhered samples to 10-3 treated dentin did not occur and the cohesive failure in the resin was observed here after the storage in water for a year. EDTA 3-2 (NH4/Fe) could not completely demineralize the hydroxyapatite especially at the deeper portion and the width of the demineralized dentin became thinner than 1 micron. The encapsulated crystals with the resin could improve the resistance of collagen against deterioration in water and such good bonding durability could be obtained.

Grinding of titanium. 2. Commercial vitrified wheels made of alumina abrasives

Abstract: Cast titanium was ground with commercial vitrified wheels made of alumina abrasives, and their grinding performance was investigated. For cutting, the appropriate circumferential speed of the alumina wheels was about 700 m/min. A speed lower or higher than this yielded unfavorable grinding results, which were attributed to wheel loading or chemical attrition of the abrasive, respectively. The hard wheel made of the A abrasive was suitable for grinding of titanium, and moreover, the wheel of the WA abrasive was more suitable than that made of the A abrasive. Generally, the cutting rate of the alumina wheels was inferior to that of the silicon carbide ones investigated previously. Depression of the wheel against the work yielded unfavorable grinding results; the manner in which the wheel was moved over the work during grinding was very important, compared with the silicon carbide wheels. Although the wheel was moved over the work, the high circumferential speed of the wheel resulted in chemical attrition of the abrasive and discoloration of the work surface, or grinding burn. The grinding burn layer mainly consisted of a few microns-thick titanium oxide.

Grinding of titanium. 1. Commercial and experimental wheels made of silicon carbide abrasives

Abstract: Cast titanium was ground with commercial and experimental wheels made of silicon carbide abrasives, and their grinding performance was investigated. With the vitrified wheels made of the GC abrasive, at a higher the wheel circumferential speed and heavier the grinding pressure, the cutting rate was greater, accompanied by violent wear of the wheel. Being independent of the wheel speed, the grinding ratio reached about 1 under pressure heavier than 100 gf. The MgO-MgCl2-bonded wheels of the C abrasive exhibited a similar tendency. The manner in which the wheel was moved over the work during grinding proved to be very important, compared with the Ni-Cr alloy as reported previously. Only depression of the wheel against the work resulted in chemical attrition of the abrasive and discoloration of the work surface, or grinding burn, due to oxidation of titanium. Even when the wheel was moved over the work, chip-formation process of the cutting edge was far from ideal, and the work surface was contaminated due to reaction of titanium with the abrasive. At a higher wheel circumferential speed, more chips were loaded or built-up in the wheel and strongly rubbed the work surface, resulting in violent wear of the wheel; loading and dislodging of such chips were repeated.

[Durability of bonding between 4-META/MMA-TBB resin to dentin pretreated with 10-3. The effect of 10-3 pretreating period and subsequent glutaraldehyde treatment].

Abstract: Adhesion durability between dentin pretreated with 10-3 and 4-META/MMA-TBB resin was studied. Reduction of etching periods with 10-3 was not so effective as expected. The weakening of bond strength during immersion in water at 37 degrees C to the dentin pretreated for 1 sec occurred faster than those for either 5 sec or 10 sec. The strength decreased from 12 MPa at 1 day to 9 MPa at 3 months, 3 MPa at 6 months and finally 2 MPa at 1 year in the case of 1 sec pretreated dentin. On the other hand, the strength became half after the storage in water for 1 year in the cases of 5 and 10 sec pretreated dentins. Combination of 10-3 pretreatment and subsequent glutaraldehyde treatment could stabilize the decrease but not completely. SEM and TEM examinations suggested that dentinal collagen exposed by the etching but not entangled and impregnated by poly (4-META-co-MMA) easily deteriorated by water during the longer immersion. Collagen modified with 10-3 and then with glutaraldehyde was also changed by the longer immersion.

[Corrosion of dental titanium alloys binary system of Ti-Al, Ti-Cu, Ti-Ni].

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-5 at % Al to Ti-50 at % 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 10 at %. In the case of the Ti-5 at % Cu to Ti-70 at % 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 50 at %. In the case of Ti-5 at % Ni to Ti-70 at % Ni system alloys, 50 at % Ni was estimated to be the maximum content to keep corrosion resistance of the titanium alloys.

Influences of composition on brush wear of composite resins. Influences of particle size and content of filler

Abstract: The influences of the composition on abrasion resistance of composite resins were examined using various experimental composite resins which had various matrix resin, filler size and content. The abrasion test was conducted by the experimental toothbrush abrasion testing machine developed in our laboratory. Three series of heat-curing composite resins were tested. One series was made from a Bis-MPEPP or UDMA monomer, and a silica filler with an average particle size of 0.04, 1.9, 3.8, 4.3, 7.5, 13.8 and 14.1 microns. The filler content of this series was constant at 45 wt%. The second series contained a silica filler of 4.3 microns in a content ranging from 35 to 75 wt%. The third series contained a microfiller (0.04 microns) and macrofiller (4.3 microns) in total content of 45 wt%. In this series, the microfiller was gradually replaced by 5, 15, 25 and 45 wt% of the macrofiller. The results obtained for these three series indicated that the abrasion resistance of composite resins was controlled by the inorganic filler, mainly filler size and content. The abrasion loss did not vary with the difference of matrix resin. When the particle size of the filler was below about 5 microns, the abrasion resistance decreased markedly with the decrease in filler size. The composite resin which contained a 0.04 or 1.9 micron filler was less resistant to toothbrush wear than the unfilled matrix resin. However, the microfiller also contributed to abrasion resistance when used in combination with the macrofiller, although abrasion resistance decreased with the increase in the microfiller concentration. The increase of filler content clearly improved the abrasion resistance when used the macrofiller. The analysis of these results and SEM observations of the brushed surfaces of samples suggested that the toothbrush abrasion was three-body abrasion caused by the abrasive in the toothpaste, and affected by the difference in the particle size between abrasive and filler, and between the abrasive size and the interparticle distance of the filler.

Effect of various pretreatments for dentin bonding on dentin permeability

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.

Casting accuracy and shrinkage of titanium castings made with zirconia investments

Abstract: To obtain inlay, crown and bridge types of titanium castings with sufficient size accuracy, zirconia investments with 10 mol % calcia were studied for their setting expansion, thermal expansion, total expansion, casting shrinkage compensation deficiency on fabrication method, particle size of zirconia, calcia and mixing liquid. Addition of calcia caused the change from contraction to expansion in setting and the additional thermal expansion at 900-1,200 degrees C. The choice of calcia and mixing liquid makes great influence on setting expansion, which is grouped into saturation type and delayed expansion type. By the use of the saturation type, total expansion could be controlled in the range of -2-(+)3%. Titanium castings of inlay, crown, bridge types with the values -2-(+)2% of casting shrinkage compensation deficiency could be successfully obtained. The casting shrinkage of titanium was estimated as 1.8-2%. The castings have the surface with a metallic brilliance as cast.

Thermal properties of human teeth and dental cements

Abstract: The thermal properties (thermal diffusivity and thermal conductivity) of human teeth and some dental cements were measured. The temperature change at the pulp cavity was also measured before and after the restoration with the full metal crown. The thermal insulating efficiency of dental cement was discussed. The thermal diffusivity of human dentin, enamel and dental cements was 2.04 x 10(-3), 3.22 x 10(-3), and 0.99-1.87 x 10(-3) cm2/sec, respectively. The thermal conductivity was 1.36 x 10(-3), 1.84 x 10(-3), and 0.50-1.51 x 10(-3) cal/cm.sec.degrees C, respectively. The dental cement beneath the crown insulated the thermal shock to dental pulp and the thermal insulating efficiency was increased with the decrease in the thermal diffusivity and thermal conductivity of dental cement.

Influence of types and surface treatment of dental alloy and film thickness of cements on bond strength of dental luting cements

Abstract: The goal of this study was to test the influence of the type and oxidation treatment of dental casting alloys on the tensile bond strength of luting cements. Also, the influence of film thickness of luting cements on the tensile bond strength of different dental casting alloys was examined. Four different luting cements (zinc phosphate, polycarboxylate, glass ionomer and adhesive resin cements) and four different dental casting alloys (Au-Ag-Cu, Ag-Pd, hardened Ag-Pd and Ni-Cr alloys) were used. Cylindrical alloy rods for the tensile bond strength test were casted, and then, top surfaces of the rods were cemented with each luting cement to the bottom surfaces of other rods, using the film thickness adjustment apparatus. The film thickness of luting cement was adjusted to 20, 30, 50, 75 or 100 microns. The tensile bond strengths of each cement to different casting alloys at each film thickness were measured one day after the rods had been cemented. The tensile bond strength of the zinc phosphate cement could not be determined in this study due to the separation of the alloy rods cemented with the zinc phosphate cement in water before the tensile test. The tensile bond strength to the adhesive resin cement to any alloy showed the greatest strength; however, that of the glass ionomer cement to any alloy was the lowest strength among the cements examined. The Ni-Cr alloy had the highest bond strength of any luting cement, compared to other alloys. The tensile bond strengths of luting cements significantly decreased with the increase in film thickness of cement layer. The adhesive resin cement had the greatest bond strength, and the glass ionomer cement was the lowest bond strength at any film thickness. The oxidation treatment significantly increased the bond strength of the adhesive resin cement to both Au-Ag-Cu and Ag-Pd alloys. The tensile bond strength of the adhesive resin cement was most dependent upon the film thickness of cement layer, and that of the polycarboxylate cement was least dependent upon the film thickness of cement layer among the cements examined. In addition, the oxidation treatment for precious alloys could be a factor contributing to the increase in the bond strength of the adhesive resin cement.

[Pre-treatment of porcelain surface for adhesive restorations].

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 degrees C) 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 450-520 kgf/cm2, was obtained even after a 24-hour immersion in 37 degrees C water, when with the etching the porcelain surface was pretreated with a 30% HF solution (60 sec) or heating treatment (150 degrees C, 10 min) when applying the silane coupling agent (2% gamma-MPTS). Then the adhesive durability was examined by a thermal cycling test (4-60 degrees C, 60 sec). After the thermal cycling test (20,000 times), the adhesive strength to the porcelain surface polished by #600 sand-paper was strong and only when the 2% gamma-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 degrees C, 10 min) when applying the 2% gamma-MPTS.

Study of pure titanium electrolytic polishing

Abstract: This study attempted to polish pure titanium test pieces electrolytically to mirror surface at the size of cast denture frames. Electrolytic polishing of pure titanium could be done on an area of 30 cm2 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 30 V for 6 min at 25 degrees C using a solution consisting of 70 ml ethyl alcohol, 30 ml iso-propyl alcohol, 6 g aluminum chloride, and 25 g zinc chloride. The solution was safe and had less restriction of frequency of use.

Cytotoxic effects of restorative materials on early passage cultured cells derived from human gingiva (in vitro)

Abstract: This study evaluated the cytotoxic effects of three common restorative materials on early passage cultured cells derived from human gingiva (HG cells). Other early passage cultured cells derived from human dental pulp tissues (HP cells) and L cells established obtained from mouse subcutaneous tissues, were also examined. Cytotoxicity was assessed according to the uptake of neutral red and dye exclusion with nigrosin. The cytotoxicity was in the order of glass ionomer cement (highest), light cured composite resin and dental amalgam. HG cells were the least sensitive to the materials tested. Both filtration of extracts and addition of serum into the extract affected the cytotoxicity. Glass ionomer cement increased the acidity of the extract, but the two other materials did not. Dissolution of metal ions, aluminium, tin, copper, mercury and zinc occurred from the materials. The extract of the composite resin had a different absorbance. This study disclosed differences in the cellular reaction to the restorative materials. Thus, utilization of early passage cultured cells is necessary for biological evaluation of the dental materials in vitro.

[Study on adhesion mechanism of MMA-TBBO resin to dentin. A model experiment using decalcified dentin].

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 lowest molecular weight in the presence of an appropriate amount of ferric or cupric ion. These results suggested that ferric compounds adsorbed to decalcified dentin are involved in the polymerization of MMA and thus will influence the bond strength of the MMA-TBBO resin to dentin.

Studies on titanium casting. (1) Influence of the mold temperature on titanium castings

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. However, 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.

[The surface degradation of various light-cured composite resins by thermal cycling].

Abstract: The durability of four commercially available light-cured composite resins was investigated by thermal cycling, GR containing inorganic fillers treated with the graft polymerization of acryl ester, LF inorganic fillers treated with a silane coupling agent, PC silanized inorganic fillers and organic composite fillers, and the MFR-type SI containing the organic composite fillers. These materials were given 10,000, 30,000 and 50,000 thermal cycles (4 degrees C-60 degrees C) and the deterioration of materials by thermal cycling was evaluated by the measurement of the mechanical properties and the SEM observations of the surface of the thermocycled materials. Compressive strength and bending elastic moduli for all materials did not change greatly by thermal cycling. However, bending strength, toothbrush abrasion resistance and surface hardness decreased with increasing number of thermal cycles between 0 and 30,000, and changed little after 30,000 cycles. The percentage of bending strength after 50,000 thermal cycles to that of the non-thermocycled sample was 75% for GR, 60% for LF, 50% for PC and 65% for SI, respectively. Deterioration of materials was observed as cracks on the surface, which generated at the interface of the filler and matrix. The cracks generated relatively earlier during thermal cycling for SI and PC which contained the organic composite filler, later for LF which contained the silanized inorganic fillers, and the number of cracks on LF were fewer than SI and PC. On the other hand, for GR, no cracks were observed even after 50,000 thermal cycles. From these results, it can be presumed that the pre-treatment of filler by the graft polymerization is more effective to improve the durability of composite resin.

[Study on visible-light curing soft resins consisting of fluoropolymers. 2. Effect of fluoroalkyl alpha-fluoroacrylate monomers].

Abstract: New experimental visible-light cured soft resins were prepared by combining a vinylidene fluoride/hexafluoropropylene copolymer (2-6 F), or a vinylidene fluoride/tetrafluoroethylene/hexafluoropropylene copolymer (2-4-6 F) with 2,2,3,3-tetrafluoropropyl alpha-fluoroacrylate (4 FFA), 2,2,3,3,3-pentafluoropropyl alpha-fluoroacrylate (5 FFA), 2,2,3,3,4,4,5,5-octafluoropentyl alpha-fluoroacrylate (8 FFA), or 2,2,3,3,4,4,5,5,6,6,7,7-dodecafluoroheptyl alpha-fluoracrylate (12 FFA). Some properties of the resins prepared were measured to determine whether they would be candidate materials for the soft denture liner. Water contact angles increased with the increase in the number of fluorine atoms contained in the monomers: 85-90 degrees in 4 FFA, 90-93 degrees in 8 FFA, and 92-93 degrees in 12 FFA. Softer resins were obtained with 2-4-6 F, especially by the combination with 8 FFA giving the softest resin with a hardness of 43-46 (JIS A). Water sorption after 10 weeks was considerably low: 0.4-2%: 0.4-0.9% in 4 FFA/2-6 F and about 2% in 8 FFA/2-4-6 F. Solubility was 0.2-0.4% in the resins with 4 FFA and 1.0-1.4% with 8 FFA or 12 FFA. Amount of residual monomer was low, and the maximum was 0.4% in the resin with 12 FFA. Thus hardness and water sorption of the resins prepared were superior to those of the commercial soft materials.

[Experimental development of a chitosan-bonded hydroxyapatite bone filling paste].

Abstract: Chitosan, a naturally occurring high molecule or weight polymer which is stable in vivo like collagen, has proved a useful biomaterial in applications such as suture thread and artificial skin. A hydroxyapatite bone-filling material was developed with chitosan sol as a binder. Measurements were made of the solubility of chitosan, and of the setting time, compressive strength, and pH value of the material. X-ray microanalysis and diffraction analysis of the material were also conducted. Chitosan is freely soluble in malic acid and succinic acid, but insoluble in citric acid and oxalic acid. The setting time of the bone-filling material tended to decrease with increase in the amounts of its CaO and ZnO components. The setting time increased with increasing ratio of sol to other (powdered) ingredients. The pH value decreased with increasing ratios of chitosan sol to the powdered ingredients. High pH values resulted from increases in both CaO and ZnO. The time between the preparation of the chitosan sol and the onset of kneading significantly affected the compressive strength of the material when set. The strength measured roughly 50% more for times between 90 and 150 minutes than for times shorter or longer than that range. Greater compressive strength was generally observed for increases in both CaO and ZnO. The results of the X-ray microanalysis of the hardened paste showed calcium crystals other than hydroxyapatite.

[Transmittance of light-cured composite resins. 1. Measurement of transmittance with time].

Abstract: The factors that influence the change of the transmittance of light-cured composite resins during polymerization were examined. Six experimental composite resins such as used organic composite filler and fourteen commercial composite resins were investigated using the experimental device for measurement of the transmittance. The transmittance of the experimental composite resins were from 25 to 73%. The transmittance of the experimental composite resins changed with time; a group with increasing transmittance (G1), a group giving a maximum transmittance (G2), and a group with decreasing transmittance (G3). These changes of transmittance were influenced by the difference in the refractive indices between matrix resin and filler, but the refractive index that indicated a maximum transmittance of matrix resin during polymerization could not be observed when the refractive index of the matrix resin and filler was same. The light transmitted through the commercial composite resins was less than half of the irradiation light. This change of transmittance could also be classified into the three groups mentioned above. The commercial composite resins belonged to the group with increasing transmittance (G1).

[The application of fluorinated aromatic dimethacrylates and diacrylates to light-cured composite resins. 1. Improvement of transmittance of composite].

Abstract: To improve the transmittance of light-cured composite resins, the matching of the refractive indices of the matrix monomer and filler were examined. Eight fluorinated aromatic dimethacrylates and diacrylates with a low refractive index were synthesized. Three of these monomers had a poor solubility to TEGDMA. The depths of cure and the other physical properties of eight experimental composites prepared from one of five fluorinated monomers, TEGDMA and UDMA were determined compared with those of the control composites prepared from Bis-GMA or Bis-MEPP/ and TEGDMA in the same ratio. These composites contained 70 wt% of silanized silica filler. The depths of cure of the experimental composites except for one were significantly greater than those of the control composites. The mechanical properties of the experimental composites were comparable to those of the control composites with a few exceptions, but the difference in the mechanical properties between these exceptional composites and the control was small. The water sorption of the experimental composites was markedly lower than that of the composite containing Bis-GMA and comparable to that of the composite containing Bis-MEPP. These five fluorinated monomers are considered useful as the matrix monomer of light-cured composite resins.

Bond strength of dental luting cements to titanium castings

Abstract: This study compared the tensile bond strength of dental luting cements to the titanium castings with gold alloy (Type IV), 12% Au-Ag-Pd alloy and Ni-Cr alloy in dental clinical practice. In this experimental condition, the zinc phosphate cement did not adhere to any dental casting alloy tested. The tensile bond strength of polycarboxylate cement to the titanium castings was 96 kg/cm2, about two times as high as that gold alloy (Type IV). The tensile bond strength of the glass ionomer cement to titanium castings was 32 kg/cm2, about two times as high as that of dental precious alloys. The tensile bond strength of the adhesive resin cement to titanium castings was 220 kg/cm2, about three times as high as that of dental precious alloys.

Fracture toughness of human enamel irradiated by CO2 laser

Abstract: The fracture toughness (KIC) of human enamel irradiated by CO2 laser was examined by means of indentation microfracture method (IM method). The irradiation of laser from 1.5 watt (16 J/cm2) to 12.3 watt (133 J/cm2) was applied to enamel for 0.2 s, and the diamond cone of micro vickers hardness tester was pressed down by the 2.45 N (250 gf) under ordinary circumstances. At the irradiated spot, KIC was 0.24-0.86 MPam1/2 and Hv was 3.6 x 10(3)-4.3 x 10(3)MPa (370-440). At the non-irradiated spot, KIC was 0.83-0.9 MPam1/2 and Hv was 3.9 x 10(3)-4.3 x 10(3)MPa (400-440). A decreasing tendency of KIC was seen in all irradiated spots. The marked decrease of KIC started at the irradiation of 4.4 watt (47 J/cm2) and the rate of decrease was 22%, and 54% when exposed to the irradiation of 5.7 watt (61 J/cm2). The rate of the lowering of the KIC of 0.05 seconds' irradiation x 4 times was less than that of 0.2 s x 1 time, while cumulative total irradiation energy was equal. The measurement of KIC by the low force of 0.98 N (100 gf) seemed to be possible when KIC was 0.41 MPam1/2 and less.

[Visible-light initiator system using thiobarbituric acid derivatives with methyl substituent].

Abstract: Seven 2-thiobarbituric acid derivatives with methyl substituents were synthesized. Experimental visible-light curing resins were prepared using these compounds as a reducing agent in the dl-camphorquinone (CQ)-reducing agent initiator system. The effects of methyl substitution for thiobarbituric acid on initiation behavior of CQ-thiobarbituric acid initiator system were examined by analysis of polymerization of triethylene glycol dimethacrylate (3 G) in DSC and depth of cure. The solubility of thiobarbituric acid derivatives in 3 G was markedly changed by the introduction of the methyl group. Although the methyl group substitution at position 1 (3) in thiobarbituric acid improved the solubility of the acid in 3 G, it did not enhance the initiation ability. Maximum exothermic rate, time at maximum exothermic peak, amount of insufficiently polymerized layer and depth of cure were significantly improved by the introduction of one methyl group at the position 5. Thus the initiation ability was enhanced by the substitution. However, the introduction of two methyl groups at position 5 did not promote polymerization, but the inhibitory effect became apparent as the concentration of the reducing agent was increased.

Shrinkage stress developed by polymerization of composite resin

Abstract: The effects of the shrinkage stress on the various cavity forms, were examined The shrinkage stress was obtained from the displacement of the brass mold developed by polymerization of the composite resin The shrinkage stress developed in the cavity wall, was analyzed by the finite element method, and the crack of the cylindrical silica tube filled with composite resin, was observed The shrinkage stress 30 minutes after mixing the composite resin was reduced with the increase in the marginal angle as well as the diameter or the depth of cavity Preparation of the bevel at the cavity margin reduced the shrinkage stress In the case of reverse bevel, reduction of the stress was not observed within 24 hours, but in the other cases, the stress immediately decreased after hardening The analysis by the finite element method reveoled the maximum stress at the margin, with less stress for the round, flat and reverse bevel in this order For the observation of the cylindrical silica tube filled with composite resin, the crack was observed in the cylindrical silica tubes prepared round, flat or without bevel, it was not observed in the tube prepared reverse bevel that the crack was developed in this tube and composite resin was detached from this tube within 24 hours

[Mechanical properties and cure depth of UDMA-based composite resins].

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 conditions, 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.

Three dimensional shape measurement of teeth. (5) On the measurement by the newly developed double sensor laser displacement meter

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 6 molar 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.

Quantitative evaluation by measuring affected area for cytotoxicity of dental materials

Abstract: The agar overlay method was modified and employed in this study to quantitatively evaluate the cytotoxicity of dental materials. The area of fibroblasts affected by the release of toxic substances from tested materials was determined with an image analysis system in order to compare the relative cytotoxicity of amalgams and composite resins. The cytotoxicity of amalgams decreased by 50% or more with aging time. Zinc appeared to be the major contributor to the cytotoxicity of amalgams. The cytotoxicity of chemically cured composite resins was also reduced with hardening time. However, even after 24 hours, the cytotoxicity of all chemically cured composite resins was still apparent. The cytotoxicity of light cured composite resins was dramatically reduced after illumination with the light source. The cytotoxicity of both amalgams and composite resins was strongly dependent upon their hardening process, since aging and curing produced a more stable material which was less likely to release toxic substances into the culture medium. The technique developed in the present study for measuring the area of affected cells to quantitatively evaluate the cytotoxicity of dental materials is simple and reproducible.

[The effect of filler content on properties of polyaddition-type silicone impression material].

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 eta storage modulus G' and tensile stress sigma increased with the amount of filler increase. G' and sigma 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.

[Development of dental quick casting with zircon-phosphoric acid investments].

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 1 MPa, fired strength was 10 MPa, 24-hour setting expansion was -0.04% and the thermal expansion at 1000 degrees C 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 degrees C, 5 minutes), the second is heat shock (900 degrees C, 3 seconds), the third is redrying (220 degrees C, 3 minutes) and the fourth is dewaxing (550 degrees C, 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 adapatability.