A new method for assessment of marginal sealability of dental restorations

A new testing system was developed for assessment of marginal sealability of dental restorations. It was constructed with a thermocycle loading line and a leakage detecting line partitioned from each other by the restoration under evaluation. The hot and cold solutions of rhodamine B as a tracer were alternately circulated in the thermocycle loading line by the switching of six solenoid valves. Marginal leakage of the tracer into distilled water circulating in the detecting line was periodically monitored by a spectrometer while thermocycling the restoration. The leakage data could be analyzed on the basis of an empirical formula. In application of this method, the durability of a complete seal against thermal changes was estimated on three teeth each of which was restored with an inlay, an amalgam or a glass ionomer.

Adsorption of 2-hydroxyethyl methacrylate on dentin from aqueous solution

The adsorption of 2-hydroxyethyl methacrylate (HEMA) on bovine dentin from aqueous solution was examined to clarify the priming effects of HEMA on dentin bonding. HEMA adsorption was characterized by: (1) slow attainment of equilibrium at higher concentrations (after 72 h); (2) a linear isotherm with a maximum possible adsorption, where an abrupt horizontal plateau occurred; (3) the large adsorption of ca. 2.5% by weight at the plateau; and (4) a vertical initial slope of the isotherm. The morphological difference between dentin powder surfaces before and after adsorption could not be determined. After heating, however, dentin powder which adsorbed HEMA was more resistant to demineralization with 6N HCl than the powder which did not adsorb. SEM examinations demonstrated that there was a demineralization-resistant dentin layer in tooth which adsorbed HEMA. The results indicated that HEMA infiltrated into intertubular dentin during adsorption.

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

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.

[Effects of tooth surface treatment with the acid solutions containing metal chloride on the bond strength of a self-curing resin]

We evaluated the use of aqueous solutions containing an acid and metal chloride as a tooth surface pretreating agent. Nineteen solutions containing 10% acetic (AA), citric (CA), lactic (LA) or phosphoric acid (PA) and/or 0.11 M aluminum (Al), cupric (Cu) or ferric chloride (Fe) were prepared. The tensile bond strengths of the TBB-O initiated MENTA resin to tooth surfaces treated with the respective aqueous solutions were measured. The amount of dissolved Ca by each of the solutions was determined. The demineralization of tooth surface was mainly dependent on the pH of the aqueous solutions. There was no specific relationship between the bond strength and amount of dissolved Ca. This suggests that a high bond strength can be obtained without serious demineralization. Pretreatment with the CA-Cu, CA-Fe or LA-Cu solution markedly enhanced the bond strengths to both enamel and dentin, although these solutions demineralized tooth surface less than the solutions containing 10% PA.

[Effects of monomer compositions on the bond strength of experimental light-cured 3-MBA resins to tooth substrates]

An adhesion promoting monomer of 3-methacryloyloxybenzoic acid (3-MBA) was utilized as an ingredient of experimental dental photocurable bonding resins. Effects of the monomer compositions of the resins on the bond strength to tooth substrates were examined. The bond strength of the light-cured resin was enhanced by the addition of 3-MBA. When 3-MBA was incorporated with Bis-MPEPP2.6E, TEGDMA or their binary comonomers, the bond strengths obtained were ca. 130 kgf/cm2 for the enamel treated with an aqueous solution of 10% citric acid and 3% FeCl3 and ca. 50 kgf/cm2 for the dentin. SEM examination revealed that the acid resistant dentin layer was formed by the infiltration and polymerization in situ of the monomer of the 3-MBA bonding resin. However, bond strengths of less than 50 kgf/cm2 obtained on treated dentin could be explained by the extent of the tag formation in dentinal tubules.

Initial dimensional change of composites in dry and wet conditions

A study of the expansion of restorative composite resins associated with water sorption was conducted. The results show that this expansion depends upon the mass of water absorbed by the composite restorative. Although polymerization shrinkage can be compensated by expansion due to water sorption, the length of time involved in achieving this compensation is approximately seven d.

[Studies of dental methacrylic resin. (Part 8) Flexural and impact adhesive strength of self-curing methacrylic resin to cross-linked polymethyl methacrylate resins (author's transl)]

Heat-curing methacylic resins cross-linked with three kinds of dimethacrylates, i.e. EDMA, tri-EDMA and nona-EDMA, were prepared, and the flexural and the impact adhesive strength of the dough moulding type and the pour type self-curing methacrylic resins to them were examined. The results obtained were as follows. (1) The flexural and the impact adhesive strength of the pour type self-curing resin were higher and more stable than those of the dough moulding type one. (2) The flexural and the impact adhesive strength of self-curing resins to cross-linked adherent resins rised in order of nona-EDMA greater than tri-EDMA greater than EDMA in the range of higher concentration of cross-linking agents. This tendency was marked for the pour type resin. (3) When the dough moulding type resin was used as adhesive resin, to adherent resin cross-linked with EDMA or tri-EDMA which had relatively short chain, the respective adhesive strength of that decreased with increasing the concentration of cross-linking agent. While, to adherent resin cross-linked with nona-EDMA, which had relatively long and more flexible chain, the respective adhesive strength of the dough moulding type resin increased with increasing the concentration of cross-linking agent. (4) When the pour type resin was used as adhesive resin, to adherent resin cross-linked with EDMA or tri-EDMA, the respective adhesive strength of that showed the maximum at the concentration near 2 to 5 mole% and decreased with increasing the concentration. While, to adherent resin cross-linked with nona-EDMA, the flexural adhesive strength of the pour type resin was approximately constant without the effect of the concentration, but the impact adhesive strength of that increased with increasing the concentration. (5) The flexural adhesive strength of the dough moulding and the pour type resin under the wet condition decreased about 30 to 60% compared with under the dry condition, but the percentage of the falling for the impact adhesive strength was about 15 to 30% and relatively small. (6) The flexural adhesive strength showed a similar adhesive tendency to the tensile adhesive strength, but the impact adhesive strength showed a different adhesive tendency from those.

[Dimensional stabilities of microfilled composite resins (author's transl)]

Thermal expansion, expansion by water sorption, water sorption and solubility to water, which will influence to the marginal closure of cavities restored, were evaluated in seven microfilled composite resins, comparing with two conventional composite resins. The following results were obtained. 1) Coefficients of linear thermal expansion of microfilled composite resins were 41 to 84 x 10(-6)/degrees C in the dry condition and 57 to 93 x 10(-6)/degrees C in the wet condition. Those values were about 2 to 3 times those of conventional composite resin, and about 6 to 8 times that of teeth. 2) Linear expansion by water sorption was highly correlated to water sorption. Microfilled composite resins had higher values of liner expansion by water sorption compared to conventional ones. 3) Generally, water sorption of microfilled composite resins were higher than those of conventional ones. After 7 days, they were about 0.4 to 1.9 mg/cm2. Values of microfilled ones which contained larger amount of filler or was composed of more hydrophobic matrix resin, however, were lower than others. 4) Solubilities to water of microfilled composite resins also were higher than those of conventional ones. Particularly, microfilled ones which were used organic composite filler and UDMA monomer as base material and photopolymerized had relatively higher values.

[Mechanical properties of microfilled composite resins (author's transl)]

The various mechanical properties of seven microfilled composite resins were evaluated, comparing with two conventional composite resins. The following results were obtained. 1) Though microfilled composite resins were more excellent than conventional ones on surface roughness after polishing or brushing, but they were inferior to conventional ones on the resistance to brushing abrasion. 2) Tensile, compressive and bending strength were dependent on inorganic filler content and properties of matrix resin. Particularly, their mechanical properties in the wet condition were strongly influenced by whether matrix resin was hydrophilic or hydrophobic. Compared with conventional composite resins, microfilled ones contained 30 to 40 wt% of filler had lower values of tensile and bending strength, and near values of compressive strength. While, microfilled ones contained 50 to 60 wt% of filler had higher values of compressive strength, and near values of tensile and bending strength, compared to conventional ones. 3) Modulus of elasticity and hardness were mainly dependent on inorganic filler content. Moduli of elasticity of microfilled composite resins were about 25 to 33% of them of conventional ones, and hardnesses of microfilled ones were approximately 50% of them of conventional ones except some materials. Hardnesses of microfilled composite resins with 50 to 60 wt% of filler were similar to them of conventional ones.

[Mechanical and dimensional properties of thermosetting resins for crown (author's transl)]

The various mechanical and dimensional properties of seven thermosetting methacrylic resins for crown and one heat-curing methacrylic resin as the control were investigated. The obtained results were as follows. 1. The water sorption, namely amount of sorption water and linear expansion by water sorption of hydrophobic poly-bis-MEPP resins were 50 to 70% of that of the control. But hydrophilic poly-EDMA resins indicated the water sorption about 1 to 1.5 times as much as the control. And a poly-UDMA resin was also hydrophilic as poly-EDMA resins, indicated about 1.3 times as much as the control. 2. The properties of poly-bis-MEPP resins were more excellent than that of poly-EDMA resins especially in the wet condition, at least were equal. 3. A poly-UDMA resin contained so-called organic composite fillers, indicated more excellent properties than other resins on hardness, abrasion resistance, linear coefficient of thermal expansion, compressive strength and bending strength in the dry condition. But, in the wet condition, some of these properties of a poly-UDMA resin were approximately equal to those of other resins.

[Thermal expansion of restorative composite resins under the dry and wet conditions (author's transl)]

Linear coefficients of thermal expansion (beta) of twelve restorative composite resins, one Bis-GMA based resin and one self-curing acrylic resin were determined in the temperature range of 4 to 60 degrees C under the dry and the wet conditions on a thermomechanical analyzer. The results were as follows. 1) The thermal expansion curves obtained were nonlinear, and the transition appeared at about 40 degrees C. 2) beta reduced with the elapse of stored time and the number of times of measuring run. It appears that these phenomena were attributable to decrease of monomer remained in each case. 3) beta under the wet condition was larger than one under the dry condition, when specimens were stored for 20 days in each condition at 37 degrees C. Values of beta for composites ranged from 16.1 to 35.3 x 10(-6)/degrees C under the dry condition and from 18.1 to 55.4 x 10(-6)/degrees C under the wet condition. 4) beta of composites varied with filler content and kinds of filler and resin used. Value of beta reduced with increase of filler content, and was lower for Bis-GMA based composite resin than for MMA based composite resin.

[Studies of dental methacrylic resin. (Part. 6) Adhesive strength of self-curing methacrylic resin to polymethylmetacrylates with various cross-linking density. (author's transl)]

The adhesive property of dental acrylic resin to resin teeth and denture base resins is an important property, in the case of preperating, repairing and rebasing denture. Then, as models of denture base resin and resin tooth, heat-curing methacrylic resins which were cross-linked with three kinds of polyethylene glycol dimethacrylate, i.e., EDMA, tri-EDMA, nona-EDMA, were prepared, and the tensile adhesive strengths of self-curing methacrylic resin to them were examined. The results were as follows. 1) The tensile adhesive strength under the dry condition was dependent on cross-linking density of adherent resin, and decreased according to the increase of concentration of cross-linking agent added in adherent resin. 2) The greater the number of chain members of cross-linking agent used to adherent resin was, the higher the adhesive strength was. In particular, the adhesive strength to adherent resin added with nona-EDMA in concentration from 16.7 to 30 mole%, agreed with the tensile strength of adherent resin itself indicating a favourable adhesion. 3) The adhesive strengths under the wet condition, that is, when specimens were immersed in water at 37 degrees C for 21 days, decreased from 30 to 50%, compared with that under the dry condition.

[Studies of dental methacrylic resin. (Part 7) Adhesive strength of fluid methacrylic resin to polymethylmethacrylates with various cross-linked density (author's transl)]

Heat-curing methacrylic resins cross-linked with three kinds of dimethacrylates, i.e. EDMA, tri-EDMA and nona-EDMA, were prepared, and the tensile adhesive strength of fluid methacrylic resin to them was examined. The results obtained were as follows. (1) To adherent resin cross-linked with EDMA, the adhesive strength of fluid resin showed the maximum at the concentration near 2 mole% and decreased with increasing the concentration. (2) To adherent resin cross-linked with tri-EDMA or nona-EDMA, the effect of the concentration of cross-linking agent on the adhesive strength was little, except that the adhesive strength to the latter adherent resin showed a steep increase in the range of lower concentration. (3) The adhesive strength did not appreciably change in the range of L/P ratio of fluid resin from 0.57 to 0.71 ml/g. (4) The adhesive strength under the wet condition decreased about 40 to 50% compared under the dry condition. (5) To cross-linked adherent resin, the adhesive strength of fluid resin was somewhat higher than that of self-curing resin under the dry condition. But, under the wet condition this difference was little. (6) Contrarily, to non-cross-linked adherent resin, the adhesive strength of fluid resin was somewhat lower than of self-curing resin. This may be because the cracks, which are occured on the surface of non-cross-linked adherent resin by contact with abundant MMA of fluid resin, work as defects to decrease the adhesive strength.