Frictional wear tests were carried out on variously heat-treated dental Ag-Pd-Cu-Au (12 mass%Au) type alloys in various corrosive environments. The effects of microstructure and corrosive environment on friction wear property of dental Ag-Pd-Cu-Au type are discussed. In every environment, the weight loss of specimen subjected to aging treatment, which gives a large amount of precipitated ft phase, is greater than other heat treated specimens by wear. However, the weight loss of mate material is significantly small compared with the heat treated specimens. Friction wear tests of this alloy showed adhesive wear. The total weight loss (sum of the weight loss of specimen and mate material) correlates with the surface roughness of the contact surface. The reason for the surface roughness change was considered to be due to the change of hardness near the contact surface.

View full abstract

In recent years, tooth decay has been found to be caused by the activity of bacteria and foods on the surface of teeth. In order to clarify the cause of tooth decay, contact angles on the tooth surface of oleic acid and linoleic acid, which are main components of lipids in foods, were measured, in addition to that of nicotine. The surface of enamel used was prepared to receive a calcium hydroxyapatite product (ASAHI PRECISION). Each solution was prepared by disolving lipids in artificial saliva preparations of sodium carboxymethyl cellulose (cmc-Na) or hydropropyl cellulose (HPC) dissolved in pure water. The nicotine solution was prepared in a similar manner. The contact angles increased as the concentration of lipids increased. From these findings, the maximum contact angles in cmc-Na and HPC were determined to be 71 degrees and 37 degrees, respectively. In the case of nicotine, the contact angle was measured below 20 degrees over a wide range of concentrations.

View full abstract

The purpose of this study was to evaluate the adhesive efficiencies of four one-step bonding systems (AQ Bond, One-up Bond F, Reactmer Bond and Xeno CF II Bond), and one wet-bonding system (Excite) as a control, by observation of the resin-dentin adhesive interfaces using a scanning electron microscope. To evaluate the compatibility of resin to dentin and the hybrid layer formation, the longitudinally sectioned surfaces of adhesive samples were polished or then treated with 1N-HCl for 40 sec followed by 2.5% NaClO for 10 min, and observed. In addition, the resin sides of adhesive samples, which were completely dissolved dentin minerals in 6N-HCl for 2 days followed by dissolving of organic components of dentin in 5% NaClO for 10 min, were also observed to evaluate resin tag formation. The compatibility of resin to dentin was good in all systems. For Excite, which utilizes phosphoric acid-etching, the formation of a hybrid layer of 5〜8μm and thick and long resin tags were observed. For one-step bonding systems, on the other hand, no clear hybrid layer was detected by the method used in this study. The resin tags formed with one-step systems were thin and short, except with Xeno CF II Bond.

View full abstract

The objective of this study was to examine whether the recharging of fluoride releasing restorative materials with fluoride affects their strength. Three different types of glass ionomer materials were tested. Disk-shaped specimens were prepared for each material. One hour after mixing or curing, the specimens were immersed in ion exchanged water at 37℃ for up to 21 weeks. Fluoride recharging was done at 3, 6, 9, 12, 15 and 18 weeks after the start of immersion using an acidulated fluoride phosphate gel. The diametral tensile strength of specimens was determined one day after the start of immersion (baseline) and then 3 weeks after recharging. For all materials, the recharged specimens did not show significant differences (p>0.05) in strength after either the first or sixth recharging compared to the specimens without recharging. Under the experimental conditions tested, fluoride recharging of fluoride releasing restorative materials did not appear to deteriorate the strength of the materials.

View full abstract

The linear polymerization shrinkage coefficient of experimental photo-activated composite resins, consisting of two different filler sizes and content, was determined using a balanced plastometer when a high-intensity xenon lamp was used to irradiate at different light intensities, irradiation time, and specimen thickness. The depth of cure was also determined according to JIS. The linear polymerization shrinkage coefficient of the resins at the same irradiation time increased with increase of the light intensity, decrease of the specimen thickness, increase of filler particle size, and decrease of filler content. However, it decreased with increase of the light intensity at the same irradiation energy in the low energy range. The depth of cure at the same irradiation time increased with increase of the light intensity, increase of the filler particle size, and decrease of the filler content. However, it decreased with increase of the light intensity at the same irradiation energy. These results suggest that shorter irradiation of light at high intensity resulted in insufficient polymerization in comparison to long irradiation of light at low intensity at the same irradiation energy.

View full abstract

In this study, a new method for examining vertical root fracture was innovated and it was employed to analyze the effects of master apical file size, root canal obturated length and amount of gutta-percha deformation. Root canal-obturated teeth were embedded in acrylic resin and load was applied using a Co〜Cr rod to obturated teeth in the root canal. Fracture mode and load were analyzed using the ranked logistic analysis and 3-way ANOVA, respectively. Vertical root fractures were observed in 88.8% of the specimens, thus this experimental set-up was confirmed to be suitable for vertical root fracture evaluation. Regarding fracture mode, only the interaction between master apical file size and root canal obturated length was significant (p<0.05). Moreover, the greater size of master apical files significantly showed smaller load fracture.

View full abstract

The aim of this study was to compare the mechanical properties of various pressable diopside glass crowns (CaCO₃-MgO-SiO₂-TiO₂-Al₂O₃). Four crystallization temperatures (890, 900, 910, 920℃) and durations (1, 10, 20, 30 min) were used to evaluate the effects on the compressive strength, bending strength and diametral tensile strength. Thermal expansion ratio, X-ray analysis and crystal structure were also examined. The compressive strength, bending strength, and diametral tensile strength were 635-1483 MPa, 179-422 MPa and 19-58 MPa, respectively. The results varied by crystallization temperatures and durations. No crystal was observed in the glass ingot, 1-2μm crystals were formed at 890℃ for 1 min, 3-4μm crystals were formed at 900℃ for 30 min, over 10μm crystals were formed at 920℃ for 30 min. According to the results obtained, the crystallization temperature and durations have statistically significant effects on the particle size and number of crystals.

View full abstract

The tensile strength of casting ring liners was measured and the relationships between bulk density, rate of vacant space and compression strain were evaluated. Strength was determined for fourteen ceramic liners (A : rockwool type, B : ceramic fiber low temperature type, C : ceramic fiber standard type, D : ceramic fiber low temperature-standard type, E : kaolin type). Dumbbell type test specimens were prepared, and the width of the narrow part was 10 mm, and the length was 20mm (n=5). The tensile strength of E group showed a remarkably large value of 12.73 MPa, and in the other 13 products, it ranged from 0.27 to 3.01 MPa. There was a positive correlation between tensile strength and bulk density, and negative correlations between tensile strength and both rate of vacant space and compression strain.

View full abstract