Shu Fen Chuang

Effect of fluoride containing bleaching agents on enamel surface properties

OBJECTIVES To evaluate the effects of fluoridated bleaching agents and post-bleaching fluoridation treatment on the whitening efficiency and microhardness of bovine enamel. METHODS Twenty five freshly extracted bovine incisors were cut into halves, embedded and then divided into the following five groups: Group 1, untreated controls; Group 2, treatment with 10% carbamide peroxide (CP) bleaching agent; Group 3, treatment with 10% CP followed by a 0.9% sodium fluoride gel application, Group 4, treatment with 10% CP containing 0.11% fluoride; Group 5, treatment with an experimental bleaching agent consisting of 10% CP and 0.37% fluoride. Groups 2-5 were treated 8h per day for 14 days then immersed in saliva for 2 weeks. Enamel morphology changes were evaluated under SEM on Day 14. Changes in enamel color and microhardness were evaluated on Days 7 and 14, and compared with the baseline data. Additionally, microhardness was determined on post-bleaching Days 21 and 28. RESULTS After 2 weeks, an erosion pattern was noted on the specimens in Groups 2 and 3. Groups 4 and 5 showed a milder demineralized pattern. All the bleached enamel specimens revealed increased whiteness and overall color value. Groups 2 and 3 showed significantly decreased enamel microhardness compared to their baseline data. The specimens treated with fluoridated bleaching agents showed relatively less reduction in enamel microhardness than those treated with nonfluoridated agents during the bleaching treatment. CONCLUSIONS The fluoridated bleaching agents produced less demineralization of surface morphology and microhardness. The addition of fluoride did not impede the whitening effect.

Influence of post material and length on endodontically treated incisors: An in vitro and finite element study

STATEMENT OF PROBLEM Cast posts require sufficient length for prosthesis retention and root strength. For prefabricated metal and fiber posts, the effects of different post lengths on the strength and internal stress of the surrounding root need evaluation. PURPOSE The purpose of this study was to examine, using both experimental and finite element (FE) approaches, the influence of post material and length on the mechanical response of endodontically treated teeth. MATERIAL AND METHODS Sixty extracted incisors were endodontically treated and then restored with 1 of 3 prefabricated posts: stainless steel (SS), carbon fiber (CF), and glass fiber (GF), with intraradicular lengths of either 5 or 10 mm (n=10). After composite resin core and crown restorations, these teeth were thermal cycled and then loaded to fracture in an oblique direction. Statistical analysis was performed for the effects of post material and length on failure loads using 2-way ANOVA (α=.05). In addition, corresponding FE models of an incisor restored with a post were developed to examine mechanical responses. The simulated tooth was loaded with a 100-N oblique force to analyze the stress in the root dentin. RESULTS The SS/5 mm and all fiber post groups presented no statistical differences, with mean (SD) fracture loads of 1247 to 1339 (53 to 121) N. The SS/10 mm group exhibited a lower fracture load, 973 (115) N, and a higher incidence of unfavorable root fracture (P<.05). The FE analysis showed high stress around the apical end of the long SS post, while stress was concentrated around the crown margins in the fiber post groups. CONCLUSIONS Both long and short fiber posts provided root fracture resistance comparable to that of SS posts. For metal posts, extending the post length does not effectively prevent root fracture in restored teeth.

Spatially resolved assessments of composite shrinkage in MOD restorations using a digital-image-correlation technique

OBJECTIVES To analyze the influence of cavity geometry and lining materials in MOD composite restorations by characterizing the polymerization shrinkage and cusp deflection. METHODS Eighty intact molars with similar sizes were collected and randomly divided into eight groups. MOD cavities with various widths and depths were prepared on these teeth: Group I, 2((W)) × 2((D)) mm; Group II, 4((W)) × 2((D)) mm; Groups III, IIIf, IIIg, 2((W)) × 4((D)) mm; and Groups IV, IVf, IVg, 4((W)) × 4((D)) mm. In Groups IIIf and IVf, flowable composite liner was placed prior to composite restoration, while glass-ionomer liner was used in Groups IIIg and IVg. Deformations of restorations resulted from composite shrinkage were recorded for 30 min following light irradiation using a digital-image-correlation (DIC) method to subpixel level. The displacements at the boundaries of the restorations were analyzed using one-way ANOVA and the post hoc test at a 5% significance level. The correlation between the geometric factors and the displacements was also analyzed. RESULTS The inward displacements on free surfaces were greater than those on the bonded surfaces. Groups with flowable composite linings showed greater amount of displacements on free and bonded surfaces compared to the unlined and glass ionomer lining groups. The correlation analysis showed that the free surface shrinkage was related with the cavity width and C-factor, while cusp deflections were correlated with the cavity depth and the cusp compliance. SIGNIFICANCE The DIC technique measures composite shrinkage on different boundaries of restorations to facilitate the investigation of polymerization kinetics. Using flowable composite lining and increased cusp depth may aggravate the cusp flexure.

Decreased Salivary Flow Rate as a Dipsogenic Factor in Hemodialysis Patients: Evidence from an Observational Study and a Pilocarpine Clinical Trial

Decreased salivary flow rate causes xerostomia (symptoms of oral dryness) in patients who undergo hemodialysis (HD); however, whether it thus contributes to thirst and excess interdialytic weight gain (IDWG) remains undetermined. In the observational study, 3 mo of data of 90 stable HD patients were collected, and sensations of thirst and xerostomia were assessed by 100-mm visual analog scales (VAS). Multivariate analyses revealed that the VAS oral dryness score was an independent determinant for thirst, daily IDWG, and IDWG%. Unstimulated whole salivary flow rate (UWS) was measured in 45 participants and was negatively correlated with VAS oral dryness score (r = -0.690, P <or= 0.001), daily IDWG (r = -0.361, P = 0.016), and daily IDWG% (r = -0.302, P = 0.045). In the interventional trial, the test drug was 5 mg of oral pilocarpine solution or placebo. Sixty hyperdipsic HD patients (IDWG% > 2%/d) were randomly assigned to either the sequence pilocarpine (2 wk)-washout (3 wk)-placebo (2 wk)-washout (2 mo)-placebo (3 mo) or placebo (2 wk)-washout (3 wk)-pilocarpine (2 wk)-washout (2 mo)-pilocarpine (3 mo) with 35 participants completing the trial. During the 2-wk crossover period (the first to seventh weeks), pilocarpine increased UWS and decreased xerostomia and thirst. The IDWG(2d) decreased (by approximately 0.2 kg; P = 0.013) but not IDWG(3d). During the 3-mo interventional period, pilocarpine increased UWS but decreased both IDWG(2d) (by 0.76 kg; P = 0.021) and IDWG(3d) (by 1.07 kg; P = 0.007). It also modestly increased serum albumin and decreased mean BP. Pilocarpine-related adverse effects were generally mild. In conclusion, decreased salivary flow is a dipsogenic factor in HD patients, and pilocarpine can alleviate it.

Contraction behaviors of dental composite restorations — Finite element investigation with DIC validation

The objective of this study was to examine the effects of cavity configuration on the polymerization shrinkage and stress of light-cured composite restorations by combining local strain measurement and a finite element analysis (FEA). Dental mesio-occluso-distal cavities of various widths and depths (each for 2 vs. 4 mm), representing different configuration factors, were prepared on extracted molars. The displacements of the bonded tooth cusps and cavity floors, caused by polymerization shrinkage of composite restorations, were assessed utilizing a digital-image-correlation (DIC) technique. The cervical marginal microleakage was investigated by examining the resin replicas of these restorations under SEM. The local material properties of the polymerized composite along the curing depth were defined by the nanoindentation test and applied in the subsequent FEA. In the FEA, four models were generated to correspond with the experimental restorations. In the DIC measurement results, the 4(w)×4(D) mm cavity presented the greatest values of inward displacements at the cusp and floor. The cavity depth, rather than the cavity width, was found to significantly correlate to the floor deformation, the location of shrinkage centers, and also the cervical microleakage ratio. The FEA simulation results showed that the 2(w)×4(D) mm cavity presented the maximal von Mises and principal stress located respectively on the cervical margins and cavity floor. Additional safety factor analysis showed a high risk of bond failure over the cavity floor in the 4-mm deep cavity. With the experimental validation, the simulation revealed that the cavity depth was significant to the formation of contraction stress and the incidence of interfacial debonding.

Riboflavin-ultraviolet-A-induced collagen cross-linking treatments in improving dentin bonding.

OBJECTIVES To evaluate the collagen cross-linkers, riboflavin-ultraviolet-A (RF/UVA) and glutaraldehyde, with regard to their efficacy in cross-linking the dentinal collagen and improving dentin bonding. METHODS Glutaraldehyde and different RF/UVA protocols (0.1%RF/1-minUV, 0.1%RF/2-minUV, and 1%RF/1-minUV) were first evaluated by gel electrophoresis to determine their abilities of collagen cross-linking. The mechanical properties of acid-etched dentin receiving these cross-linking treatments were examined in either dry or wet condition by a nanoindentation test. Fifteen teeth with exposed occlusal dentin received the microtensile bond strength (μTBS) test. The teeth were primed either with RF/UVA or glutaraldehyde, followed by adhesive treatment and composite restorations, and then cut into resin-dentin microbeams. Half of the microbeams received the μTBS test after 24h, and the other half received test after 5000 thermocycles. Nanoleakage at the bond interface was examined under TEM. The alignments of collagen fibrils in the hybrid layers were also defined by an image analysis. RESULTS Gel electrophoresis showed that glutaraldehyde induced strong collagen gelation, while RF/UVA generated milder collagen cross-linking. Glutaraldehyde, 0.1%RF/2-min-UVA, and 1%RF/1-minUV showed higher stiffness compared to untreated and 0.1%RF/1-minUV in wet condition. All the crosslinking treatments improved early μTBS, but 0.1%RF/2-minUVA treatment maintained high μTBS after theromocycles. Under TEM, glutaraldehyde-treated dentin showed dense and enclosed collagen network on the adhesive interface. 0.1%RF/2-minUVA showed the least nanoleakage, and this could be associated with the suspended collagen fibrils in the hybrid layer. SIGNIFICANCE 0.1%RF/2-minUVA treatment enhanced resin-dentin bond possibly through enhancing the stiffness and maintaining the expanding collagen matrix in the hybrid layer.

Shear Bond Strengths of Plastic Brackets With a Mechanical Base

This study compares the shear bond strengths of plastic brackets with a mechanical base and metal brackets using two different adhesives, and examines the modes of failure using a scanning electron microscope (SEM). Forty extracted human premolars were selected for bonding. Two types of brackets: metal and plastic-Spirit MB, and two orthodontic adhesives: System 1 + and Enlight, were used. After bonding, all samples were put into a 37 degrees C distilled water bath for 24 hours before shear bond strengths were tested. The bond strengths of the plastic brackets were significantly lower than those of the metal brackets (P < .0001). There was a statistically significant difference in bond strengths between System 1 + and Enlight for plastic brackets (P < .05), but not for metal brackets. The modes of failure predominantly occurred at the enamel/adhesive interface in the metal bracket-System 1 + group, within the adhesive in the metal bracket-Enlight and plastic bracket-System 1 + groups, and at the bracket/adhesive interface in the plastic bracket-Enlight group.

Stress-strain analysis for evaluating the effect of the orientation of dentin tubules on their mechanical properties and deformation behavior.

A model whose porosity does not vary with compression depth is developed for evaluating the mechanical properties of dentin tubules with various orientation angles from micro-pillar nanocompression tests. Experimental results for a range of loading rates indicate that the yielding parameters and the elastic modulus are little affected by the creep behavior. For a given compression depth, the hardness, elastic modulus, and yielding strength decrease with increasing orientation angle of dentin. The mechanical properties obtained using the proposed model are consistent with the reported data, and are actually more precise since they consider the orientation angle. The proposed testing method can be applied to materials that yield a negative value of the elastic modulus due to creep behavior.

Effect of fluoridated carbamide peroxide gels on enamel microtensile bond strength

The aim of this study was to examine the resin bond strength on enamel treated with different fluoridated bleaching agents. Forty-eight bovine incisors were divided into four groups to receive bleaching treatments, over a 14-d period, as follows: no treatment; 10% carbamide peroxide (CP) bleaching; 10% CP containing 0.11% fluoride; and 10% CP containing 0.37% fluoride. Immediately, and 7 and 14 d after bleaching, the enamel surfaces were respectively bonded with composite and sectioned to create resin-enamel beams. These beams were subjected to the microtensile bond strength (microTBS) test, then assessed for failure mode under scanning electron microscopy. The results showed that the 0.37% fluoridated group demonstrated a microTBS equivalent to that of the unbleached group at all stages. Non-fluoridated and 0.11% fluoridated groups showed a weaker microTBS after bleaching but regained the bond strength after 14 or 7 d of storage, respectively. In the non-fluoridated group, adhesive failure was the predominant fracture pattern that comprised the enamel prism demineralization change and widely dispersed voids on the resin-enamel interfaces. No evident enamel erosion and fewer microporosities were found in the 0.37% fluoridated group. Accordingly, treatment with 0.37% fluoridated CP maintained the microTBS as effectively as the unbleached enamel. Additional fluoride in the bleaching agents may facilitate subsequent restorative treatment by inhibiting enamel demineralization.

Effects of silane- and MDP-based primers application orders on zirconia–resin adhesion—A ToF-SIMS study

OBJECTIVE To evaluate the 3-methacryloyloxypropyltrimethoxysilane (MPS)- and 10-methacryloyloxydecyl-dihydrogen-phosphate (MDP)-base primers, in their single or sequential applications, with regard to modifying zirconia surfaces and improving resin-zirconia adhesion. METHODS Zirconia disks received different treatments: without primer (Zr), MPS-base primer (S), MDP-base primer (M), MPS/MDP mixture (SMmix), MPS followed by MDP (SM), and MDP followed by MPS (MS). The compositions and chemical interactions of the coatings to zirconia were analyzed using time-of-flight secondary ion mass spectrometry (ToF-SIMS) and reconstructed 3D ion images. Surface wettability of these coatings to water and resin adhesive was assessed. The shear bond strength (SBS) between resin and the treated zirconia was also examined before and after thermocycling. RESULTS Groups S and MS presented substantial OH- ions in the coatings and zirconia substrate. PO2- and PO3- fragments existed in all MDP-treatment groups with various proportions and distributions, while groups M and SM showed higher proportions of PO3- and the zirconium phosphate related ions. In 3D ion images, PO3- in groups M and SM was denser and segregated to the interface, but was dispersed or overlaid above PO2- in SMmix and MS. All the primers increased the surface wettability to water and resin, with M and SM presenting superhydrophilic surfaces. All MDP-treatment groups showed improved SBS before thermocycling, while M and SM retained higher SBS after this. SIGNIFICANCE The MDP-base primer shows a relevant function in facilitating POZr bonding and enhancing resin-zirconia bonding. The co-treated MPS impairs the chemical activity of MDP, especially if it is the final coat.