Hideo Onose

Effect of light exposure on fracture toughness and flexural strength of light-cured composites

OBJECTIVES This study was conducted to investigate the curing characteristics of light-cured composites and their related mechanical properties. METHODS Single-edge notch specimens [25 mm x 2.5 mm x 5 mm with a 5 mm notch (a/W = 0.5)] were prepared for fracture toughness measurements. For flexural strength testing, a stainless steel mold (25 mm x 2 mm x 2 mm) was used. Light-cured composites were condensed into the mold, and the middle third of the specimen was first activated for 30 s with 400 mW/cm2, for 60 s with 200 mW/cm2, or for 120 s with 100 mW/cm2. Then the remaining thirds were activated at the same intensity and curing time as the middle third. After 24 h storage in 37 degrees C water, three-point bending tests were performed with a span length of 20 mm at a crosshead speed of 0.5 mm/min. A one-way ANOVA, followed by a Newman-Keuls test (p < 0.05), were used to compare the data obtained from each group to test the effect of the curing conditions. RESULTS Fracture toughness, flexural strength, and flexural modulus varied with resin composites. Among the three curing conditions for each material, there were no significant differences in fracture toughness, flexural strength, or flexural modulus. SIGNIFICANCE The fracture toughness and the flexural strength were the same when irradiations with the same amount of energy (light intensity multiplied by curing time) were used. It was found that, at lower light intensity, longer curing was required to provide comparable mechanical properties. An accumulated irradiation energy obtained through a product of the light intensity and curing time may serve as a guideline to produce samples exhibiting equivalent fracture toughness as well as flexural strengths.

Influence of filler addition to bonding agents on shear bond strength to bovine dentin

OBJECTIVES The purpose of this study was to investigate the influence of adding filler particles to a bonding agent on dentin bond strength and of the temperature change during curing in order to determine the optimum filler level for an experimental bonding agent. METHODS Experimental light-cured bonding agents with microfiller (average size: 0.05 micrometers) content of 0, 10, 20, 30, 40, 50, 60, and 70 wt% were used with the Imperva Bond / Lite-Fil II A (Shofu) restorative material. Bovine incisors were mounted in self-cured resin, and the facial surfaces were prepared with 600-grit SiC paper. After dentin surface pretreatment with dentin primer, experimental bonding agents were applied to the dentin surface and bonded with resin composite. Ten samples per test group were stored in 37 degrees C water for 24 h, then shear tested at 1.0 mm/min. The temperature change of the bonding agent was monitored during the exthothermic polymerization reaction according to the method of ISO standard #4049. The peak temperature and the time required to reach peak temperature were recorded. RESULTS Bond strength to dentin and the temperature change were greatly affected by the filler level. Maximum dentin bond strength (14.3 +/- 2.3 MPa) was obtained with a filler level of 10 wt% and decreased with filler level higher than 30 wt% (10.4 +/- 1.7 MPa - 5.3 +/- 2.6 MPa). Peak temperature decreased and the time required to reach peak temperature increased with the higher filler levels. There were strong correlations between the bond strength and temperature change of experimental bonding agents. SIGNIFICANCE The initial setting behavior of bonding agents containing filler particles may be one of the important factors influencing dentin bond strength. When bonding agents with filler particles are used, it is important to determine if optimum filler levels exist in order to optimize the dentin bond strength.

Analysis of the dentin–resin interface by use of laser Raman spectroscopy

OBJECTIVES Adhesion of resin-bonding agents to dentin is currently believed to result from impregnation of adhesive resin into superficially demineralized dentin. The purpose of this study was to investigate the chemical composition of the resin-impregnated dentin (hybrid) layer using a micro-Raman spectroscopy. METHODS Resin composites were bonded to bovine dentin with the two-step bonding systems, and specimens were sectioned parallel to dentinal tubules. These surfaces were then polished down to 1 microm diamond pastes. Raman spectra were successively recorded along a line perpendicular to the dentin-adhesive interface by steps of 0.2 microm on a computer controlled X-Y stage. The relative amounts of hydroxyapatite (960 cm(-1), P-O), adhesive resin (640 cm(-1), aromatic ring), and organic substrate (1450 cm(-1), C-H) in the dentin-adhesive bonding area were calculated. RESULTS From the Raman spectroscopy results, the hybrid layer represents a gradual transition in the relative amount of adhesive from the resin side to dentin side. Evidence of poor saturation of the adhesive resin in the demineralized dentin with the one-bottle adhesive system was detected. SIGNIFICANCE From the results of this study, inhomogeneity of the hybrid layer composition was detected, and the degree of resin impregnation was found to be different between the bonding systems tested.

Influence of self-etching primer drying time on enamel bond strength of resin composites.

OBJECTIVES In clinical situations, a dentine primer is usually applied to enamel, in addition to dentine. This study was carried out to determine the influence of the air drying time of self-etching primers on their bond strength to enamel. METHODS Three self-etching primer systems, Fluoro Bond (FB, Shofu Inc.), Liner Bond II (LB, Kuraray Co.) and Mac Bond 2 (MB, Tokuyama Co.) were employed. Bovine mandibular incisors were mounted in self-curing resin and the enamel surfaces were wet ground with number 600 SiC paper. Following primer application, the enamel surface was dried with compressed air for 0, 2, 5, 10, 20, and 30 s from 10 cm above the enamel surface using a three-way syringe. Adhesives were applied and composites were condensed into a Teflon mold (4 x 2 mm) placed on the enamel and light activated. Ten samples per test group were stored in 37 degrees C water for 24 h, then shear tested at a crosshead speed of 1.0 mm min-1. One-way ANOVAs followed by Duncan tests (p < 0.05) were done. RESULTS The enamel bond strengths varied with the different air drying times and ranged from 8.2 +/- 3.2-17.4 +/- 3.0 MPa for LB, 10.9 +/- 2.8-16.3 +/- 3.3 MPa for FB, and 17.5 +/- 2.5-19.4 +/- 2.2 MPa for MB. For LB and FB, there appeared to be a minimum drying time of 10 s. LB was the most affected by drying time, while MB did not change. CONCLUSIONS The data suggests that the enamel bond strengths of these bonding systems can be influenced by the drying time of the primer applied prior to bond agent application and the tendency was different among the systems studied.

Effect of filler content of light-cured composites on bond strength to bovine dentine.

The bonding of light-cured composites to tooth tissues is known to be disturbed by polymerization shrinkage, and polymerization shrinkage is affected by the filler content of composites. This in vitro research examined the relationship between the filler content and bond strength to dentine of light-cured composites. Experimental light-cured composite systems with filler contents of 45, 55, 65 and 75 per cent by volume were used in both bond strength to dentine and shrinkage tests. The surfaces selected as substrates were a flat surface and a box-shaped cavity prepared in bovine dentine. The lowest bond strength was obtained with the 45 per cent filler content composite in the box-shaped cavity. Bond strength increased with increasing filler content. Volumetric polymerization shrinkage decreased with increasing filler content. The polymerization shrinkage at 120 s after light curing was 5.24 per cent for the 45 per cent filler content system, 4.77 per cent for the 55 per cent, 2.14 per cent for the 65 per cent and 1.68 per cent for the 75 per cent. The correlation between bond strength and shrinkage was greater for the cavity than it was for the flat surface. This implies that bond strength in the box-shaped cavity may have been affected more by polymerization shrinkage than with the flat surface. The findings lend support to the view that filler content is one of the important factors influencing the physical properties of composites.

Dentin Bond Strength of Light-cured Glass-ionomer Cements

The purpose of this study was to investigate the influence of surface treatments and irradiation conditions on the bond strength of light-cured glass-ionomer cements to dentin. The light-cured glass-ionomer cements used in this study were Vitrabond, XR Ionomer, and Fuji Lining LC. Three experiments were designed to study the influence of the following factors on bond strength to dentin: (1) effect of the surface treatment of the dentin, (2) effect of the irradiation time, (3) effect of an increase in the interval between mixing of the cement and irradiation. Samples were stored in water for 24 hours, after which shear bond testing was performed at a cross-head speed of 1 mm/min. For Vitrabond, the Scotchprep and Gluma 2 treatments gave the greatest shear bond strengths. For XR Ionomer and Fuji Lining LC, the Scotchprep treatment gave the greatest shear bond strengths. The bond strengths for all cements increased with prolonged irradiation time. Bond strengths decreased with a longer elapsed time between mixing and light-curing. This means that light-curing should be done soon after the cement is placed. The failure mode was found to be cohesive in the ionomer.

Selected curing characteristics of light-activated composite resins.

Abstract The purpose of this study was to clarify some curing characteristics of light-activated composite resins. Three visible light-activated and 1 UV light-activated resin were cured with each generator recommended by the manufacturer. The relative degree of cure was determined using Knoop hardness measurements on the irradiated and longitudinally sectioned surfaces of cylindrical specimens. The maximum surface hardness numbers were obtained from the area just under the top of the specimen (0.5 to 1.0 mm below the irradiated surface) and the hardness gradually decreased with the depth. In the specimens cured in 100% N 2 gas environment, the degree of cure on the irradiated surface was improved. Coloring agents in composites have an effect on decreasing the degree of cure. Re-irradiation on the longitudinally sectioned surface of the specimen increased the degree of deep cure. The degree of cure at each depth was improved by prolonged irradiation time.

Reinforcement of acrylic resins for provisional fixed restorations

In Part I, it was found that (i) 2 vol.% admixture of reinforcing elements in PMMA (Jet) resin matrix had a significant beneficial effects on the mechanical properties, and (ii) among these, zirconia exhibited the greatest improvements in modulus of elasticity, transverse strength, toughness, and hardness number. Using the best combination (i.e., PMMA resin matrix and 2 vol.% ZrO2), exothermic temperature raise and polymerization shrinkage were further investigated. Deterioration in mechanical properties due to prolonged water sorption were also studied for 5 weeks. The following can be concluded: (1) By increasing liquid/powder ratio for PMMA control samples, the peak temperature occurrence was retarded by 3 min and raised by 8 degrees C. (2) The effect of admixed oxide particles to PMMA resin matrix or the heat generated during polymerization was not significant. (3) The polymerization volumetric shrinkage was influenced by the a mixture of particles, with increases as large as 0.9% (or 0.3% in linear shrinkage). (4) PMMA resin admixed with 2 vol.% of zirconia particles showed a continuous weight gain due to water sorption, mechanical properties appears to be increasing up to 1-week sorption, followed by rapid drop of all properties. (5) Autopolymerizing acrylic resins are a resin-resin composite material of pre-polymerized beads embedded in a newly formed acrylic matrix. The main fracture modality appears to occur through the matrix and at the interface, although some trans-beads fractures were identified. (6) It was suggested that incorporating certain type of oxide particles into the pre-polymerized beads would provide stronger resin matrix.

Influence of light irradiation of dentine primers on dentine-resin bond

OBJECTIVE The purpose of this study was to examine the influence of light irradiation of dentine primers that contain camphoroquinone (CQ) on the shear bond strength to dentine and their contact angle. METHOD Three dentine bonding systems which contain CQ, Imperva Bond, OptiBond and XR-Bond; and Light Bond, which does not contain CQ, were employed. Labial surfaces of freshly extracted lower bovine incisors were ground with no. 600 grit SiC paper. Dentine primers were applied to the dentine surfaces in two groups, irradiated and non-irradiated. A shear bond strength test was performed and the direct contact angle was measured. RESULTS Statistical analysis (Newman-Keuls multiple comparison P < 0.05) of the data indicated that light irradiation of the dentine primer for systems containing CQ resulted in increased bond strength and decreased contact angle. CONCLUSION This study indicates that for these dentine bonding systems containing CQ in their primers, light irradiation of the dentine primer is effective in improving wettability and increasing the bond strength to dentine.

Effect of Electrical Potential on the Adherence of Streptococcus sanguis to Gold Plate

The formation of plaque on dental restorative materials may be of importance because of possible harmful effects of the plaque both on the gingivae and on the materials themselves (Onose et al., J Dent Res 56:1336, 1977). Key factors in the adherence of bacteria to enamel are thought to be bacterial extracellular polysaccharides and certain salivary constituents (Gibbons & Van Houte, J Periodontol 44:347, 1973). Presumably those factors also contribute to the binding of bacteria to dental materials. A less explored factor of possible importance in the adherence of bacteria to solid surfaces is the electrical potential of the surface. The authors decided to examine the effect of this variable using pure gold plates as the test restorative material and S. sanguis ATCC strain 10558 as the test organism. Pure gold was used because of the availability of its electrochemical information. A polarization circuit was employed which permitted the application of any desired potential to the gold plate during exposure of the plate to a suspension of S. sanguis cells. The cell concentration of suspension was adjusted to the optical density 0.6 at 550 nm. The potential applied to the gold plate was limited to ±0.7 V (all potential reported in this paper refers to a saturated calomel electrode) to avoid the possibility of electrolytic evolution of hydrogen or oxygen gas at the golds surface. After exposure of the gold plates to the bacterial suspensions, the bacterial cells adhering to the surface of the gold were counted by the improved method of 9rstavik et al. (Infect & Immun 9:794, 1974), using an ultraviolet incident light microscope. At potentials between -0.7 and +0.7 V, a small number of bacteria adhered to the gold in the first ten min. At potentials between -0.7 and 0.0 V, few additional bacteria adhered during the rest of the two-hour experiment, and a moderate but variable increase occurred during the second hour only at 0.0 V. At potentials between +0.3 and +0.7 V, however, adherence after the first ten min was markedly and consistently increased, with maximal adherence at +0.5 V (Fig. 1). In supplementary experiments, the potential of the gold plates during the first hour was set at either +0.5 or -0.5 V. Then, for the second hour, the potential was switched to the opposite sign, at either -0.5 or +0.5 V. When the potential was switched from negative to positive, the adherence of S. sanguis subsequently proceeded exactly as if a positive potential had been applied from the beginning. On the other hand, when the potential was switched from positive to negative, the number of adherent cells decreased markedly. Clearly, the results indicate that the electrical potential applied to a gold plate has a marked effect on the number of S. sanguis cells adhering to the plate. The reversibility of the bacterial adherence caused by switching the potential from positive to negative or vice versa indicates both that the mechanism of adherence is simple and is dominated by the electrical potential of the gold. The observation that the adherence was maximal at 0.5 V was in agreement with the results of organic substances adsorption on gold observed by Dahms & Green (J Electrochem Soc 110:1075, 1963), who interpreted their results with zero point of charge and water adsorption on gold. The asymmetry of the shape of the adherence maximum that drops off quite abruptly at negative polentials (. 0.4 V) may be due to the fact that S. sanguis cells have a small net negative charge (Olsson et aL, Arch Oral Biol 21:605, 1976). It follows that this small net negative charge may be responsible for the greater adsorption at positive gold potentials.