Our previous studies demonstrated that thermal and mechanical load deteriorated the marginal integrity at the incisal enamel margins of cervical wedge-shaped cavities when they were restored with self-etching primer systems. However, our other study revealed that enamel bevel preparation significantly improved sealing durability at the incisal enamel margins of cervical cavities. The present study analyzed the stress distribution along the cavity walls of the cervical resin composite restoration induced by thermal change. In addition, the effect of enamel bevel on stress distribution under thermal and mechanical load was investigated. A model for the two-dimensional finite element method (FEM), which simulated the longitudinally sectioned surface through the center of the specimen used in the previous studies, was prepared using a FEM software package (Ansys 7.0). Meshing was carried out by eight-node quadrangle elements (4,044 nodes and 1,228 elements). Enamel, dentin and resin-based composite were assigned to Young's moduli of 41.4, 18.6 and 10.0 GPa, respectively. Poisson's ratios were 0.3, 0.3 and 0.3, respectively. The coefficients of thermal expansion for enamel, dentin and a resin-based composite were 1.2, 0.8 and 2.5, respectively. The node of root apex was fixed in both the X and Y directions. Deformation by thermal changes was assumed to be isotropic and homogeneous for each substance. Thermal changes (+37℃ and -18℃) were applied uniformly to the model. Moreover, two finite element models with the enamel bevel were prepared. For the analysis of thermal stress, a beveled cavity model consisted of 4,335 nodes and 1,323 elements. The root apex node was fixed in both the X and Y directions. Thermal change (-18℃) was applied uniformly to the models. For the analysis of mechanical stress, a beveled cavity model consisted of 3,087 nodes and 920 elements. All the nodes in the bottom row (2mm below the apical margin) of the model were fixed in both the X and Y directions. A labio-lingual displacement of 1mm was applied to the incisal edge of the tooth. Cooling produced tensile and shear stresses along the adhesive interface, whereas heating generated compressive and shear stresses. There were no significant differences in the magnitude of stresses (von Mises) between the incisal and the apical walls. The enamel bevel relieved the thermal and mechanical stresses at the superficial portion of the incisal wall, especially at the cavity margin. In addition the bevel mainly reduced normal (tensile) stress.

View full abstract

Pulpitis mainly results from the invasion of dental caries-related bacteria into dental pulp. It is known that various proinflammatory cytokines and adhesion molecules play important roles in the exacerbation of pulpitis but detailed expression mechanisms have not been fully elucidated. The purpose of this study was to determine the effect of bacteria-derived factors on proinflammatory cytokines and adhesion molecules expression in cultured dental pulp fibroblasts. The expressions of interleukin-8 (IL-8) and vascular endothelial growth factor (VEGF) in pulp fibroblasts exposed to lipopolysaccharide (LPS) or peptidoglycan (PGN) were examined by reverse transcriptase-polymerase chain reaction (RT-PCR) and enzyme-linked immunosorbent assay (ELISA). In addition, adhesion molecules (ICAM-1 and VCAM-1) expression on pulp fibroblasts was analyzed by flow cytometry. Both LPS and PGN increased not only the expression of IL-8 and VEGF but also the expression of ICAM-1 and VCAM-1 in pulp fibroblasts. These results suggest that pulp fibroblasts play an important role in pathogenesis of pulpitis via expression of proinflammatory cytokines and adhesion molecules in response to bacteria-derived factors.

View full abstract

This study investigated the effects of a bleaching agent with titanium dioxide and low-concentration hydrogen peroxide on tooth color, and the influences of various light sources on the performance of the bleaching agent. Pyrenees® (Mitsubishi Gas Chemical Company) was used as the bleaching agent. The tooth color was measured with an SE2000 spectral color-difference meter (Nippon Denshoku) and evaluated with the CIE L*a*b* color system. The light source experiment was conducted using hematoporphyrin-dyed paper and three different light sources: Hyperlightel (Kuraray Medical), Optilux 501 (Demetron, USA), and Pencure (Morita). The largest color change in the light source experiment was obtained in the samples irradiated by Hyperlightel. The smallest color change was obtained in the samples irradiated by Pencure. We next examined the effect of the bleaching on extracted human teeth using irradiation by Hyperlightel. Color measurements were taken before the bleaching treatment (L*₀, a*₀, b*₀), and at 1 minute (L*₁, a*₁, b*₁), 5 minutes (L*₅, a*₅, b*₅), and 10 minutes (L*₁₀, a*₁₀, b*₁₀) after the treatment. The L*₅ and L*₁₀ levels were significantly higher than the L*₀ level. The a* levels gradually increased, but did not rise significantly above the pre-treatment levels. The b*₁, b*₅, and b*₁₀ levels were significantly lower than the b*₀ level. On average, ΔE*ab₁,ΔE*ab₅, and ΔE*ab₁₀ were 3.57 (SD±1.45), 5.44 (SD±1.60), and 7.50 (SD±1.88), respectively. These levels confirm the very high effects of the bleaching. Based on these results, we concluded that Hyperlightel had a higher bleaching effect than the other light sources, when used in combination with the bleaching agent Pyrenees®. Pyrenees® was also found to have a high potential as bleach.

View full abstract

Root canal obturation was performed by the lateral condensation method on straight root canal models with taper of 6/100 under the pre-condition that preparation had been conducted by using an engine-driven rotary instrument. The spreaders used included: a type of stainless steel spreader (DentalEZ: Star Dental D11T (S-D11T)), and 6 types of Ni-Ti spreader (Roeko: NiTi #15 (R-15), NiTi #25 (R-25), NiTi #35 (R-35), NiTi D11T (R-D11T)); and Brasseler: Naviflex NT D11T (B-D11T) and Naviflex NT 4SP (B-4SP). Hermetic seal was evaluated according to 3 evaluation methods, i.e. the percentage of canal wall area coated with sealers of <5μm in thickness (GP pressure welding ratio), the percentage of gutta-percha-obturated canal area determined on micro focus X-ray CT images (GP/CT condensing ratio), and the mean percentage of gutta-percha-obturated canal area in all canal cross-sections and each position from the apex (GP total condensing ratio and GP local condensing ratio, respectively). The following conclusions were obtained: 1.GP pressure welding ratio by the remaining sealer was the highest in B-D11T, being 92.1%, followed by R-D11T, S-D11T, R-15, R-25 and R-35 in this order. B-4SP showed the lowest value of 57.9%. 2. In GP/CT condensing ratio by the micro focus X-ray CT device, S-D11T showed the highest value of 96.6%, followed by B-D11T, R-D11T, R-25, R-35 and R-15 in this order. B-4SP showed the lowest value of 88.9%. 3. In GP total condensing ratio on the cutting surface of the model, S-D11T showed the highest value of 97.9%, followed by B-D11T, R-D11T, R-25, B-4SP and R-15 in this order. R-35 showed the lowest value of 90.4%. 4. In GP local condensing ratio at the cutting position on the cutting surface of the models, S-D11T exhibited a value as high as 96.9% or more at each cutting position. On the other hand, B-D11T showed a lower condition in pressure welding compared with S-D11T. A tendency was noticed that the condensing ratio decreased at the root canal tip. Also, the condensing ratio tended to decrease at the middle of the root canal in R-D11T, R-25, R-15, B-4SR and R-35 in this order. 5. Statistical analysis was performed on the difference of the evaluation methods on GP/CT condensing ratio and GP total condensing ratio on the cutting surface by using the micro focus X-ray CT device. As a result, no statistically significant difference was noted between the two. These results reveal that the evaluation method by using the micro focus X-ray CT device has high reliability and usefulness.

View full abstract

The healing of periapical infections is achieved by physiologically sterilizing the root canal up to the apical foramen. However, for intractable periapical infections that are treated in the usual manner but do not manage to heal properly, surgical endodontic treatment may be chosen. In previous studies, we found that, for intractable periapical infections, by cleaning the periapical lesion through the root canal with 5% sodium hypochlorite solution, necrosed tissue and bacteria of the periapical lesion could be resolved and removed, thus resulting in early healing. Meanwhile, in studies on lesion cleaning using sodium hypochlorite solution, none of the research either focused on the bone matrix proteins or analyzed their prevalence through biochemical investigation. Therefore, in this study, a periapical lesion was produced in rats, sodium hypochlorite solution was used for cleaning the lesion, the bone matrix proteins around the lesion were extracted, collagenous proteins and non-collagenous proteins were fractionated according to their extraction properties into an EDTA soluble fraction and a guanidine soluble fraction, and their respective quantitative and qualitative changes were observed. In this study, by the treatment, quantitative changes in the collagenous proteins were thus found to increase in the short term. In addition, in a search for collagen molecular species using the electrophoretic method, types I, III and V collagens were detected but the rate of type III collagen substantially decreased in comparison to normal tissue. As for non-collagenous proteins, in an electrophoretic search, differences in the component ratio at the peak of the specific molecular weight were found in both the EDTA fraction and guanidine fraction. In a search for glycoproteins using a lectin blot analysis, differences were found in the protein component ratio, at around 50kD for the EDTA fraction, but such differences were hardly observed for the guanidine fraction. Based on the above findings, it was found that, by cleaning sodium periapical lesions with hypochlorite solution, the quantity of collagenous proteins, the EDTA fraction, namely the quantity of mineral-associated proteins, and the electrophoretic behavior changed during the healing process. Further analyses on the molecular weight of the proteins that were observed to have especially and substantially changed in this experiment are now underway in order to help elucidate the process of periapical lesion healing.

View full abstract

In the previous reports on the removal of cemented postcores by means of ultrasonic vibration, we demonstrated that the best result was achieved when lateral vibration was simultaneously induced by two removal tips using two ultrasonic generators. In this study, in order to increase the efficiency of removing a cemented postcore attached to molar teeth, we measured the time taken to remove cast posts that had been divided into two parts and those that had not been divided into two parts. In this research, after removing a full veneer crown and applying ultrasonic vibration to the attached cast post, we measured the elapsed time until removing the cast post from the root canal by means of gripping the post with pliers. In addition, the cast posts were also divided into two for maxillary molar teeth in the mesiodistal direction and for mandibular molar teeth in the buccolingual direction. Ultrasonic vibration was applied to the cemented postcore, using two ultrasonic generators, by setting them at an angle of 90° between the tooth axis and the removal tip, setting the two sites for applying the tips in opposite directions, and with a power setting of 10. Consequently, the time taken to apply ultrasonic vibration and remove the cast post was 28.9±24.6 seconds for divided posts and 100.3±49.5 seconds (mean±SD) for undivided posts. As a result, such cast posts attached to molar teeth could be removed in a short time by dividing them into two and applying ultrasonic vibration from two different directions. However, a problem that remains for future consideration is to both evaluate and develop improved methods for dividing and removing the post safely without excessively cutting the pulp floor at the time of dividing the cast post.

View full abstract

It is well known that titanium reacts with oxygen and other elements, resulting in the formation of a surface reacted layer. This can cause deterioration of the mechanical properties and corrosion resistance of titanium castings. It has been reported that coating the wax mold with boron nitride (BN) before the investment casting process can improve the surface condition of gold-silver-palladium and cobalt-chromium alloy dental casting by reducing the tendency of the investment to cling to the alloy, improving ease of mold release and cleaning, and decreasing surface roughness. In this study, the effect of a coating of BN on the wax mold of titanium castings was investigated by observation of the surface condition of the castings and measurement of micro-Vickers hardness, surface roughness and elemental distribution. Wax patterns with a BN coating (BN(+)group) and without a coating (BN(-)group) were invested using two types of investment: an experimental magnesia-based alumina-cement-bonded investment for titanium crowns (HK-7F), and a phosphate-bonded investment for non-precious alloys used in the porcelain-fused-to-metal technique (Hi-Temp). The micro-Vickers hardness (n=4), elemental distribution (measured by electron probe micro-analysis, EPMA) and surface roughness (measured using a surface roughness tester, n=5) of the resulting titanium castings were obtained. The average and standard deviation for each group were statistically analyzed using one-tail repeated-measures ANOVA (α=0.05) and one-tail Mann Whitney U-test (α=0.05). The clinging properties of the investment were examined using Hi-Temp rather than HK-7F. For both investments, no significant difference was observed in the micro-Vickers hardness values of the BN(+) and BN(-) groups. For HK-7F, when the surface oxygen content was raised gradually from 75μm below the surface of the titanium castings, the curve representing surface oxygen content was steeper for the BN(-) group than for BN(+). In terms of elemental distribution (Si, P, O, S, Mg, Ca), for Hi-Temp, there was no significant difference between the BN(+) and BN(-) groups, and in terms of surface roughness, for HK-7F, there was no significant difference between the two groups. Exfoliation of the BN coating was observed at the mold; this may have been due to a lack of adhesive strength between the BN coating and the inner wall of the mold after burnout. In order to improve the physical properties of castings, it may be necessary to improve the adhesive strength at the interface, or to examine experimental investments with various BN percentages.

View full abstract

Proper operation of nickel titanium rotary instruments is very important to avoid iatrogenic events such as file separation and apical canal transportation. The aim of this study was to investigate physical parameters of proficiency for ProTaper instrumentation by means of real-time measurement of the torque and vertical forces exerted during instrumentation by novice and experienced operators. Seventy simulated canals with a 30-degree curvature in clear resin blocks were divided into two groups. Before the measurement, Group A was instrumented with the ProTaper system to a size SX until passive penetration stopped, and Group B was instrumented to a size F2 until the full working length was reached. Two endodontists (6 and 4 years of experience with ProTaper instrumentation) and five residents (no clinical experience of ProTaper instrumentation) then prepared the groups A and B with S1 and F3, respectively, to the full working length(n=5, each), and the torque and vertical forces were measured with a newly designed torque- and force-measuring device. As physical parameters, the force-time product, torque-time product and working time during the preparation were calculated. Moreover, the lengths of time lag between force and torque peaks were assessed. The results obtained were as follows: (1) Regarding the three physical parameters, significant differences between experienced and novice operators were not always detected due to great inter-operator variations among novice operators. (2) The time lag between force and torque peaks was always seen in experienced operators, whereas the time lag was often non-detectable or reversed in novice operators. (3) In experienced operators, the length of time lag for F3 was significantly longer than that for S1 (p<0.01, t-test). It was concluded that the time lag between force and torque peaks may be an indicator of operator proficiency. The longer time lag for F3 could be related to the separation of this instrument due to the delay in the release from high torque.

View full abstract

Objectives: Vital bleaching is generally classified into at-home and in-office bleaching. With at-home bleaching people can bleach their teeth by themselves, while with in-office bleaching their teeth are bleached by dental practitioners. It has been clinically recognized by many practitioners that the appearance of the enamel surface after bleaching is different between at-home and in-office bleaching, presenting a chalky-white appearance after in-office bleaching and a translucent natural-white after at-home bleaching. We assumed that for in-office bleaching the bleaching effect is limited to the enamel and the outer dentin at deepest, while for at-home bleaching the bleaching effect can reach to deep dentin. The science of color explains that the appearance of objects is perceived through the cross-interaction of their color, transparency, surface gloss, light diffusion and fluorescence. Among these factors, color is assumed to be a dominant factor controlling the appearance of enamel surface after bleaching. In this study, therefore, we examined the color change by calculating the whiteness (W) from the value of L*, a* and b* in the CIE system, from which we compared the bleaching effect of at-home and in-office bleaching. Materials and Methods: An extracted human pre-molar was sectioned lingo-buccally avoiding pulp exposure so that a tooth specimen having a thickness of 2.45±0.05 mm was obtained. Bleaching was performed on either a buccal or a lingual surface of the enamel surface in the specimen. At-home bleaching using 10% carbamide peroxide (Hi-Lite Shade Up, Shofu) was performed in half of the specimens (n=9) where the bleaching was done for 6 hours once a day during which the specimen was kept at 37℃ and 100% RH (relative humidity). At-home bleaching was continued for 2 or 4 weeks. The other half of the specimens were treated by in-office bleaching using 35% hydrogen peroxide (Hi-Lite, Shofu) according to the manufacturer's instructions where the bleaching was repeated three times in a single day that continued for 5 days. Simulating vital teeth the hydrostatic fluid pressure of 34cmH₂O was applied constantly to the pulp chamber of every specimen throughout the experimental period. Colorimetric measurement was done for each cross-section of the specimen at two measuring points in enamel and at four measuring points in dentin. Values of L*, a* and b* in the CIE system were assessed before and after the bleaching, by which W was calculated using an equation. Results were statistically analyzed by two-way ANOVA and Tukey's multiple comparison (p< 0.05). The image of the specimen before and after the bleaching was taken with a scanner of which RGB (red-green-blue) values were transformed to values of L*, a* and b* through a mathematical process. From these values W was calculated, and then a color-contour map before/after the bleaching was developed. Results: Results from the colorimetric measurements and the image-scanning statistically treated by two-way ANOVA demonstrated that there was no interaction between the depth from surface enamel and the bleaching methods. The tendency that the change of A W (difference of W at before and after bleaching) with the depth from the surface enamel to the deep dentin was not different between at-home bleaching for 2 weeks and at-home bleaching for 4 weeks and in-office bleaching. In at-home bleaching for 4 weeks and in-office bleaching, the results from a color-contour map before and after the bleaching were similar regarding the area showing increased W and the value of ΔW. Conclusions: Under the limited conditions employed in this study, it was not concluded that the bleaching effect of in-office bleaching is limited to the enamel and the outer dentin at deepest, while for at-home bleaching the bleaching effect can reach to deep dentin.

View full abstract

While the typical anatomical form for human maxillary first premolars shows two distinct roots (the buccal and the palatal) with a single canal in each root, there is a small percentage that have one broad root with two canals and a common foramen. Occasionally, three canals in maxillary premolars having two buccal roots and a single palatal root may occur. In this study, artificial maxillary first premolars with the presence of radiolucency related to bifurcated canals at mid-root level and a single apical foramen for pre-clinical endodontics were newly designed in cooperation with Nissin Co., Japan. The teeth were made from DGEBA (Diglycidyl ether of bisphenol A), a glass filler with presence of radiolucency and other materials. The purpose of the study was to investigate the anatomical morphology of the teeth and the root canals. An interstitial tubercle (marginal tubercle) was situated on the mesio-marginal ridge of the tooth crown. A mild bayonet, the so-called "S" shaped curve of the root, was seen at the buccal and lingual aspects of the teeth. The root form in the proximal aspects showed tapering at the root tips. The lengths of the pulp chambers were longer in these teeth. Roentogenograms in the mesiodistal direction revealed the following: 1. a large pulp chamber with broad buccolingual dimension 2. presence of two pulpal horns 3. a single root with two parallel bifurcated canals at mid-root level, and a single apical foramen 4. severe buccal curve of the lingual canal Cross sections at three levels of the root revealed in micro CT findings. Cervical level: the pulp chamber was very wide in the buccolingnal direction. Mid-root level: the canals were slightly ovoid. Apical third level: although the canals were long ovoid, the apical canal tended to continue. In measurements (in mm) of the teeth (average only), the length of teeth was 22.84mm, length of crown 9.42mm, length of root 13.42mm, mesio-distal diameter of crown 8.11mm, and bucco-lingual diameter of crown 10.06mm. The average reading was slightly longer than the average length reported by Fujita et al., but it was nearly the same as Japanese human teeth. In cross sections, the buccal and lingual canals in the mid-root third showed a wide oval shape with fin-like projections buccolingually. The buccal canal diameter was buccolingually 2.05>mesiodistally 0.95mm under the bifurcated area, 1.51>0.74mm at the apical third, and so 0.63>0.36mm at the apical single canal level. The lingual canal diameter was buccolingually 1.70>mesiodistally 0.97mm under the bifurcated area, and so 1.48>0.87mm at the apical third level. The canal walls tended to become thinner in width at the mesial areas with proximal grooves. The diameter ranged from 0.84 to 1.30mm. This tooth model produced the proper radiodensity similar to natural teeth. The results indicated the feasibility of the model as a teaching model for pre-clinical endodontic practice.

View full abstract