The influence of pontic distribution on the marginal and internal gaps of CAD/CAM five-unit anterior zirconia framework.

Background/purpose: Nowadays, zirconia-based framework has been used for longspan or full-arch fixed dental prostheses (FDPs). This study aimed to evaluate the effect of pontic distribution on marginal and internal gaps of five-unit anterior zirconiabased DPs. Materials and methods: Right maxillary central incisor and second premolar were selected as terminal abutments and three different edentulous conditions with one nonterminal abutment were simulated. Marginal and internal gaps in each zirconia-based samples(n = 10) were examined by computer-aided replica technique. Five regions, including marginal gaps at mesial or distal finishing line, internal gaps at the mesial or distal axial wall, and occlusal surface, were statistically analyzed (α = .05). Results: Most of marginal gaps and internal gaps at axial wall were clinically acceptable, but larger at occlusal surface. For the three experimental groups, clinically accepted percentage with qualified gaps were less than 30%.There were statistical differences at axial wall over pontic side and marginal gaps over non-pontic side between groups (P<0.05). For sum of gaps of all abutments in each group, statistical differences were found at marginal and axial wall (P < 0.05). As for those on terminal and non-terminal abutments, statistical differences were found on second premolar (P < 0.05). Conclusion: Except for occlusal surface, the overall marginal gaps and internal gaps at axial wall of five-unit anterior zirconia-based FDPs with different pontic distribution were clinically acceptable. However, the percentage with qualified gaps were low (<30%). Greater gaps were noted when adjacent pontic existed. Different pontic size and distribution with curvature had an influence on the gaps.

Assessments of polymerization shrinkage by optical coherence tomography-based digital image correlation analysis-Part II: Effects of restorative composites.

OBJECTIVES: To examine the polymerization shrinkage of different resin-based composite (RBC) restorations using optical coherence tomography (OCT) image-based digital image correlation (DIC) analysis. METHODS: The refractive index (RI) of three RBCs, Filtek Z350XT (Z350), Z350Flowable (Z350F), and BulkFill Posterior (Bulkfill), was measured before and after polymerization to calibrate their axial dimensions under OCT. Class I cavities were prepared in bovine incisors and individually filled with these RBCs under nonbonded and bonded conditions. A series of OCT images of these restorations were captured during 20-s light polymerization and then input into DIC software to analyze their shrinkage behaviors. The interfacial adaptation was also examined using these OCT images. RESULTS: The RI of the three composites ranged from 1.52 to 1.53, and photopolymerization caused neglectable increases in the RI values. For nonbonded restorations, Z350F showed maximal vertical displacements on the top surfaces (-16.75 µm), followed by Bulkfill (-8.81 µm) and Z350 (-5.97 µm). In their bonded conditions, all showed increased displacements. High variations were observed in displacement measurements on the bottom surfaces. In the temporal analysis, the shrinkage of nonbonded Z350F and Bulkfill decelerated after 6-10 s. However, Z350 showed a rebounding upward displacement after 8.2 s. Significant interfacial gaps were found in nonbonded Z350 and Z350F restorations. SIGNIFICANCE: The novel OCT image-based DIC analysis provided a comprehensive examination of the shrinkage behaviors and debonding of the composite restorations throughout the polymerization process. The flowable composite showed the highest shrinkage displacements. Changes in the shrinkage direction may occur in nonbonded conventional composite restorations.

Assessments of polymerization shrinkage by optical coherence tomography-based digital image correlation analysis-Part I: Parameter identification.

OBJECTIVE: To investigate the feasibility of optical coherence tomography (OCT)-based digital image correlation (DIC) analysis and to identify the experimental parameters for measurements of polymerization shrinkage. METHODS: Class I cavities were prepared on bovine incisors and filled with Filtek Z350XT Flowable (Z350F). One OCT image of the polymerized restoration was processed to generate virtually displaced images. In addition, the tooth specimen was physically moved under OCT scanning. A DIC software analyzed these virtual and physical transformation sets and assessed the effects of subset sizes on accuracy. The refractive index of unpolymerized and polymerized Z350F was measured via OCT images. Finally, different particles (70-80 µm glass beads, 150-212 µm glass beads, and 75-150 µm zirconia powder) were added to Z350F to inspect the analyzing quality. RESULTS: The analyses revealed a high correlation (>99.99%) for virtual movements within 131 pixels (639 µm) and low errors (<5.21%) within a 10-µm physical movement. A subset size of 51 × 51 pixels demonstrated the convergence of correlation coefficients and calculation time. The refractive index of Z350F did not change significantly after polymerization. Adding glass beads or zirconia particles caused light reflection or shielding in OCT images, whereas blank Z350F produced the best DIC analysis results. SIGNIFICANCE: The OCT-based DIC analysis with the experimental conditions is feasible in measuring polymerization shrinkage of RBC restorations. The subset size in the DIC analysis should be identified to optimize the analysis conditions and results. Uses of hyper- or hypo-reflective particles is not recommended in this method.

Simulating the shrinkage-induced interfacial damage around Class I composite resin restorations with damage mechanics.

OBJECTIVES: To investigate the shrinkage-induced damage at the composite-tooth interface by finite element analysis (FEA) using the cohesive zone model (CZM). METHODS: Axisymmetric models of Class I restorations were created to illustrate the interfacial damage around composite resin restorations of different dimensions, with polymerization shrinkage modeled analogously to thermal shrinkage. The damage to the adhesive interface was determined using a CZM based on the fracture strength and fracture energy. To show the effects of damage, conventional models with perfectly bonded composite resin restorations were created as controls. RESULTS: The results indicated interfacial damage at the butt-joint cavosurface margin, dentinoenamel junction, and internal line angle. The percentage of damaged interfacial area was found to increase with decreasing diameter for restorations of the same height. For a given diameter, the damage was more severe for restorations of greater depth. The effects of the damage were further illustrated in the model with a restoration of 2-mm diameter and height. The interfacial damage occurred primarily at the internal line angle (83.3 % of all the damaged interfacial area), leading to local stress relief (from 18.3 MPa to 12.8 MPa), but also higher stress at the damage fronts. Greater local shrinkage was found in composites adjacent to the damage. SIGNIFICANCE: The damage mechanics-based CZM is an essential refinement of the FEA to predict interfacial damage and its implications. The extent of damage was found to be greater around restorations with smaller diameters and greater depths. The entire simulation is available via an open-source platform to facilitate further applications in adhesive dentistry.

Dentin Sialoprotein/Phosphophoryn (DSP/PP) as Bio-Inductive Materials for Direct Pulp Capping.

Conventional direct pulp capping, such as calcium hydroxide (Ca(OH)2) or silicate products, usually induces an inflammatory reaction to provoke pulp regeneration. Phosphophoryn (PP) and dentin sialoprotein (DSP), the two most abundant non-collagenous proteins in the dentin matrix, are responsible for dentin mineralization, pulp cell migration, and differentiation. Here we examined the PP and combined DSP/PP as bio-inductive pulp capping materials by in vitro and in vivo tests. Firstly, the effects of the PP dose on pulp cell migration and matrix protein expression were examined by an agarose bead test. Secondly, the role of recombinant DSP (recDSP) and recDSP/PP on stimulating DSP-PP transcript expression was examined by RT-PCR. DSPP mRNA was also knocked down by RNA interference (RNAi) to examine their functions on dentin matrix mineralization. Finally, we used ferret animal models to test PP and recDSP/PP acting as capping agents on in vivo pulp responses and reparative dentin formation. The result showed that intermediate-dose PP was the most effective to enhance cell migration and differentiation. RecDSP/PP strongly enhanced the DSP-PP transcript expression, while inhibition of DSPP mRNA expression by siRNAs partially or completely affected dental pulp cell mineralization. The in vivo results showed that intermediate-dose PP and recDSP/PP proteins induced less pulp inflammation and promoted reparative dentin formation. Contrarily, conventional calcium hydroxide induced severe pulp inflammation. With these findings, DSP and PP could serve as capping agents for pulp capping therapy.

Interaction of silane with 10-MDP on affecting surface chemistry and resin bonding of zirconia.

OBJECTIVE: To investigate the effect of silane contents on their chemical interaction with 10-methacryloyloxydecyl-dihydrogen phosphate (MDP), and affecting the bonding of MDP to zirconia by time-of-flight secondary ion mass spectrometry (TOF-SIMS) and solid-state nuclear magnetic resonance (SSNMR) spectroscopy. METHODS: Zirconia (Cercon ht, Dentsply) slabs were prepared and fully sintered. Experimental primers SE-5 and SE-10 were formulated by adding 5 wt% and 10 wt% γ-methacryloxypropyltrimethoxysilane to an MDP-based primer SE BOND (SE), respectively. SE, SE-5, and SE-10 were applied on the assigned zirconia slabs. The chemical compositions on the surface and adhesive interfaces were examined by TOF-SIMS in a depth-profiling mode. Hydrophilicity and resin affinity of treated zirconia were analyzed. The bond strengths to resin cylinder were examined either after 24-h storage or thermocycles. In addition, zirconia powders treated with three primers were assessed by SSNMR spectrometry for the adsorption of MDP. RESULTS: TOF-SIMS analysis showed that SE treatment generated the greatest amount of P-O-Zr related ions, which reduced in SE-5 and SE-10 groups. The 3D ion-images illustrated the generation of ZrO2(OH)- ions with silane contents. The SSNMR analysis revealed that the chemical bonding was mainly P-O-Zr ionic bonds in SE but shifted to P-OH-Zr hydrogen bonds in SE-5 and SE-10. SE-5 and SE-10 treated zirconia presented higher hydrophilicity and affinity to resin compared to Zr did. SE showed the highest initial bond strength which significantly decreased after thermocycling. SIGNIFICANCE: MDP adsorption onto zirconia via P-O-Zr ionic bond promotes bonding with resin. The silane enhances the hydroxylation of zirconia and impairs the adsorption of MDP, but does not adversely affect the bond durability.

Investigations of silane-MDP interaction in universal adhesives: A ToF-SIMS analysis.

OBJECTIVES: The purposes of this study were to investigate whether the presence of silane in universal adhesives affects the functions of 10-methacryloyloxydecyl dihydrogen phosphate (MDP) and adhesion to zirconia. METHODS: Two silane-containing universal adhesives (Scotchbond Universal (SBU) and Clearfil Universal-Bond (CUB)) and two silane-free adhesives (All-Bond Universal (ABU) and SE-Bond primer (SE)) were individually applied on zirconia disks. Time-of-flight secondary-ion-mass-spectrometry (ToF-SIMS) examined the distributions of MDP- and silane-related ions, as well as evidence of zirconium phosphate (ZrP) compounds, on the surface and interfacial regions using a depth profiling mode. The hydrophilicity and resin wettability of the treated zirconia were examined using a contact angle test. For the shear bond strength (SBS) test, the zirconia disks were air-blasted, treated with the assigned adhesives, and bonded with pre-cured composite cylinders using a resin cement. These resin-zirconia assemblies received a bond test after 24-h storage. RESULTS: Both SBU and CUB exhibited silane-related ions and ZrO2(OH)-, but fewer PO- ions in the interfacial regions. CUB had more siloxane-related ions. SE-treated zirconia had abundant PO- ions and particularly high PO3-- and ZrP- related ions in the interfacial regions. The silane-free adhesives exhibited a higher affinity to both water and adhesive liquids. SE showed significantly higher SBSs compared to ABU, while SBU and CUB were not statistically different. SIGNIFICANCE: The silane content may cause hydroxylation of zirconia and affect MDP adsorption. An acidic pH accelerated the condensation of silanol. The bond performance of the MDP-based adhesive could be influenced by the silane content and other components.

Novel Polymerization of Dental Composites Using Near-Infrared-Induced Internal Upconversion Blue Luminescence.

Blue light (BL) curing on dental resin composites results in gradient polymerization. By incorporating upconversion phosphors (UP) in resin composites, near-infrared (NIR) irradiation may activate internal blue emission and a polymerization reaction. This study was aimed to evaluate the competency of the NIR-to-BL upconversion luminance in polymerizing dental composites and to assess the appropriate UP content and curing protocol. NaYF4 (Yb3+/Tm3+ co-doped) powder exhibiting 476-nm blue emission under 980-nm NIR was adapted and ball-milled for 4-8 h to obtain different particles. The bare particles were assessed for their emission intensities, and also added into a base composite Z100 (3M EPSE) to evaluate their ability in enhancing polymerization under NIR irradiation. Experimental composites were prepared by dispensing the selected powder and Z100 at different ratios (0, 5, 10 wt% UP). These composites were irradiated under different protocols (BL, NIR, or their combinations), and the microhardness at the irradiated surface and different depths were determined. The results showed that unground UP (d50 = 1.9 µm) exhibited the highest luminescence, while the incorporation of 0.4-µm particles obtained the highest microhardness. The combined 20-s BL and 20-120-s NIR significantly increased the microhardness on the surface and internal depths compared to BL correspondents. The 5% UP effectively enhanced the microhardness under 80-s NIR irradiation but was surpassed by 10% UP with longer NIR irradiation. The combined BL-NIR curing could be an effective approach to polymerize dental composites, while the intensity of upconversion luminescence was related to specific UP particle size and content. Incorporation of 5-10% UP facilitates NIR upconversion polymerization on dental composites.

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.

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 Restorative Materials on Dental Pulp Stem Cell Properties.

INTRODUCTION: Dental pulp stem cells (DPSCs) are multipotent progenitors for biotechnological practices, but the influences of existing restorations on their viability and differentiation are not well-known. This study was aimed to investigate in vivo and in vitro responses of DPSCs to restorative materials. METHODS: Class I cavities were prepared on molars scheduled to be extracted and then restored with a resin-based composite (RBC), a glass ionomer cement, or zinc oxide eugenol. Intact teeth were used as controls. Twelve molars in each group were extracted on day 7 or day 30 after restorations to assess the early or intermediate pulp responses and were then cut in half. One half was processed for histopathological analysis, and the other was used to isolate DPSCs for a colony-forming unit assay and real-time polymerase chain reaction for NANOG, OCT4, and CD44 expression. RESULTS: All restored teeth showed pulp damage at various levels, whereas mild to moderate inflammation persisted in the RBC group until day 30. The existence of DPSCs in the pulp cores of all groups was revealed based on CD44 immunoreactivity. Glass ionomer cement and zinc oxide eugenol did not affect the relative percentages of DPSCs in either early or intermediate stages, whereas RBCs reduced the percentage. The colony-forming units in all restoration groups were comparable with those in the control. Nevertheless, the restorations significantly enhanced OCT4 expression, especially in RBC/day 30. CONCLUSIONS: Dental restorations cause mild pulp damage but do not affect DPSC viability. RBC decreases DPSC densities but might increase the stemness of surviving DPSCs through an inflammation-stimulation process.

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.

Promoting porcelain-zirconia bonding using different atmospheric pressure gas plasmas.

OBJECTIVES: To evaluate the effects of different atmospheric-pressure plasma (APP) on the physicochemical properties of yttria-stabilized zirconia, and promoting the adhesion of veneering porcelain. METHODS: Cercon base zirconia disks were prepared to receive different treatments: as-polished, three APPs (oxygen, OP; argon, AP; and CF4, CP), and grit-blasted (GB). Their surface roughness and hydrophilicity were measured, and surface morphology was examined either after treatments, after simulated porcelain firing, or additional thermal etching. X-ray photoelectron spectroscopy (XPS) analysis characterized the surface chemical compositions. Shear bond strength (SBS) tests examined the adhesion between veneering porcelain and zirconia either before or after thermocycling. The layered ceramic disks were also sectioned to inspect the porcelain-zirconia interfaces. Statistical analysis was performed with one-way ANOVA and post hoc Duncan's test. RESULTS: Grit-blasting caused surface damage and increased roughness. All APP-treated disks exhibited deeper grain boundaries and enlarged grain sizes after thermal etching, while CP disks revealed additional particle dispersions. Three APPs rendered the zirconia surface superhydrophilic. XPS spectra of three APP groups revealed increased hydroxyl groups and reduced C-C contents, and CP group especially showed the existence of Z-F bonds. CP exhibited the highest SBS both before and after thermocycling, while AP and GB also showed improved SBSs compared to the as-polished. OP presented reduced SBS, and its cross-sections showed increased microporosities in the veneering porcelain. SIGNIFICANCE: APP did not change surface morphology but enhanced wettability. CP and AP improved porcelain-zirconia SBSs, primarily through surface hydroxylation. OP induced the microporosities in porcelain and adversely affected the adhesion.

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.

Influence of enamel wetness on resin composite restorations using various dentine bonding agents: part I-effects on marginal quality and enamel microcrack formation.

OBJECTIVES: To investigate the influence of enamel wetness on marginal quality and enamel microcrack formation using various dentine bonding adhesives; and to determine the changes of marginal quality and enamel microcracks over time. METHODS: Forty extracted molars were each prepared with a cylindrical cavity and divided into five groups. Groups 1-4 were acid-etched and treated separately with either an ethanol-based adhesive (Single Bond) or an acetone-based adhesive (Prime&Bond NT) on either dry or wet enamel. Teeth in Group 5 were treated with a self-etching adhesive (Clearfil SE Bond). Epoxy replicas of different stages were taken after cavity preparation, after restoration, after 24 h storage, and after thermocycling test. These replicas were examined under a scanning electron microscope for their restorative marginal quality and enamel microcrack. Quantitative measurement was performed to measure the length ratio of different margin patterns, and enamel microcracks. RESULTS: There was no difference in the marginal quality when ethanol-base adhesive was applied on dry or wet dentin. The group using acetone-based adhesive on the dry enamel presented higher intact margin ratio than the group on wet enamel did only after restoration. Using self-etching primer led to higher open margin ratio at all stages. Enamel microcracks were found in all five groups and were higher in groups using acetone-based adhesives. CONCLUSION: Enamel wetness did not show a significantly adverse effect on the marginal quality of the restorations using either acetone- or ethanol-based adhesive. Using self-etching adhesive may lead to high incidence of margin deterioration.

Influence of enamel wetness on composite restorations using various dentine bonding agents: part II-effects on shear bond strength.

OBJECTIVES: To investigate the bond strength and fracture characteristics of various dentine bonding agents (DBAs) on wet or dry enamel. METHODS: Forty molar halves with enamel flattened were divided into 5 groups. Groups 1-4, teeth were acid-etched and treated separately with either an ethanol-based adhesive (Single Bond) or an acetone-based adhesive (Prime & Bond NT) on dry or wet enamel. Group 5 was treated with a self-etching adhesive (Clearfil SE Bond). The treated enamel surfaces were bonded with Z 250 composite with metal rings. The composite-ring assemblies were subjected to shear bond test until failure. Failure patterns on the debonded surfaces were inspected under a scanning electron microscope. The bonded enamel thickness was measured on sectioned specimens to investigate their statistical evidence. Results of open margin and enamel microcrack ratios from Part I of this study were compared with these fracture characteristics. Data was analyzed statistically. RESULTS: There was no difference in the bond strength between paired groups using the same DBA on wet or dry enamel. The teeth restored with self-etching adhesive exhibited lower bond strength and higher adhesive-enamel failure rate than the other groups. No correlation between enamel thickness, microcrack, bond strength, and failure patterns was revealed. CONCLUSION: The self-etching adhesive had a lower enamel bond strength than the other bonding systems. The difference in the failure patterns of groups with ethanol- and acetone-based DBAs on wet or dry enamel was indistinct. The frequently observed enamel microcrack cannot be directly correlated with the bond strength of bonding systems.

Shrinkage behaviors of dental composite restorations-The experimental-numerical hybrid analysis.

OBJECTIVES: To investigate the effects of light curing protocols on the shrinkage behaviors, contraction stress, and microleakage in composite restorations by an experimental-numerical hybrid analysis. METHODS: Three groups of human molars were collected to receive different light-curing protocols: vertical or oblique curing at regular intensity, and vertical curing at reduced intensity. For each tooth, the composite fillings were consecutively placed under unbonded and bonded states, and their shrinkage behaviors were examined with a digital image correlation (DIC) technique. The strains of the unbonded restorations were input into two finite element analysis (FEA) models with settings of the composite as either homogeneous or hardened along polymerization gradients. The preliminary solutions were verified by their individual deformations in the bonded restorations. The interfacial microleakage of restorations was also determined by micro-CT scanning and compared with the FEA results. RESULTS: The bonded restorations showed centripetal shrinkage patterns with greater downward displacements than their unbonded restorations. Vertical curing at regular intensity caused the greatest shrinkage strain, contraction stress, and microleakage among the three protocols. Low-intensity curing reduced overall shrinkage strain and displacements at cervical margin, but did not prevent the formation of microleakage. Oblique curing caused asymmetric shrinkage with the tooth-shielded side revealing less deformation. Setting the polymerization-dependent elastic moduli of the composite enhanced the reliability of FEA. SIGNIFICANCE: This hybrid analysis comprehensively examined the polymerization shrinkage behaviors. Both the light intensity and direction affect the shrinkages and contraction stress. Oblique curing decreases shrinkage due to the attenuated irradiation by tooth-shielding rather than modulations of shrinkage direction.

Characterization of the elastic and viscoelastic properties of dentin by a nanoindentation creep test.

Dentin is the main supporting structure of teeth, but its mechanical properties may be adversely affected by pathological demineralization. The purposes of this study were to develop a quantitative approach to characterize the viscoelastic properties of dentin after de- and re-mineralization, and to examine the elastic properties using a nanoindentation creep test. Dentin specimens were prepared to receive both micro- and nano-indentation tests at wet and dry states. These tests were repeatedly performed after demineralization (1% citric acid for 3 days) and remineralization (artificial saliva immersion for 28 days). The nanoindentation test was executed in a creep mode, and the resulting displacement-time responses were disintegrated into primary (transient) and secondary (viscous) creep. The structural changes and mineral densities of dentin were also examined under SEM and microCT, respectively. The results showed that demineralization removed superficial minerals of dentin to the depth of 400 µm, and affected its micro- and nano-hardness, especially in the hydrate state. Remineralization only repaired the minerals at the surface layer, and partially recovered the nanohardness. Both the primary the secondary creep increased in the demineralized dentin, while the hydration further enhanced creep deformation of untreated and remineralized dentin. Remineralization reduced the primary creep of dentin, but did not effectively increase the viscosity. In conclusion, water plasticization increases the transient and viscous creep strains of demineralized dentin and reduces load sustainability. The nanoindentation creep test is capable of analyzing the elastic and viscoelastic properties of dentin, and reveals crucial information about creep responses.

Factors contributing to the incompatibility between simplified-step adhesives and self-cured or dual-cured composites. Part II. Single-bottle, total-etch adhesive.

PURPOSE: As adhesives containing hydrophilic/acidic resin components are vulnerable to water movement after polymerization, this study tested the hypothesis that coupling of a single-bottle adhesive (OptiBond Solo Plus) to self/dual-cured composites is compromised by adhesive permeability, even with the adjunctive use of chemical co-initiators. MATERIALS AND METHODS: Two versions of chemical co-initiators (activators) were investigated: the proprietary resin-containing OptiBond Solo Plus Activator (A), and a resin-free solution of 2% benzene sulphinic acid sodium salt in ethanol (B). For microtensile bond testing, hydrated (H) or dehydrated (D) bonded human dentin were coupled to a dual-cured composite (Bis-Core) under light- (L) or self-activation (C) mode. A delayed light-activation mode (DL) was also employed to simulate the slower rate of polymerization of self-cured composites but without the influence from adverse chemical interaction. Nine groups were tested: 1) L-H (control); 2) DL-H; 3) DL-D; 4) C-H; 5) C-D; 6) CA-H; 7) CA-D; 8) CB-H; and 9) CB-D. For transmission electron microscopy, a light-cured and an experimental self-cured composite of the same composition were used for the nine groups. RESULTS: Only the bond strength results of the experimental groups DL-D (Group 3) and CB-D (Group 9) were not significantly different from the control group L-H (p > 0.05). TEM revealed the presence of discrete silver-filled water blisters along the adhesive-composite interface in groups 2, 6, and 8, and within the composite in group 4. Adverse chemical interaction in groups 4 and 5 resulted in the observation of a line of silver deposits along the adhesive composite interface. CONCLUSION: The coupling of composites after prolonged contact with hydrated dentin bonded with OptiBond Solo Plus is affected by the intrinsic permeability of the adhesive. The adjunctive use of the Activator is only slightly effective in improving the coupling of this adhesive with self/dual-cured composites. Although the use of resin-free benzene sulphinic acid sodium salt solution completely eliminates the adverse chemical interaction, the inherent permeability of the polymerized adhesive precludes optimal coupling of self/dual-cured composites to bonded hydrated dentin.

Influence of flowable composite lining thickness on Class II composite restorations.

This in vitro study aimed to investigate the influence of flowable composite lining with different thicknesses on the marginal quality and internal porosity of Class II composite restorations. Thirty-two intact molars, each prepared with two box-only Class II cavities, were randomly divided into four groups: Group 1, P60 filling alone; Group 2, ultrathin flowable composite lining/co-cured with overlaying composite; Group 3, thin lining/pre-cured and Group 4, thick lining/pre-cured. The teeth were then thermocycled for 1500 cycles (between 5 degrees C and 60 degrees C) and dye immersed for 24 hours. Exterior surface replicas of these restorations were fabricated before and after thermocycling and examined by SEM to evaluate percentages of the five marginal patterns. Data was statistically evaluated using one-way ANOVA test. The teeth were subsequently sectioned longitudinally. The interface microleakage of cervical margin was measured as to the extent of dye penetration. Internal voids were separately recorded in the cervical interface and the cervical and occlusal halves of the restorations. Mann-Whitney test was applied to analyze the interface microleakage and internal voids. Results revealed that replicas of Group 4 presented the highest percentage of marginal openings both before and after thermocycling in SEM examination. Group 2 exhibited superior marginal quality in interface microleakage evaluation compared to the other groups, while Group 4 exhibited the worst. The pre-cured groups (Group 3 and 4) showed significant reduction in interface and cervical voids. Despite the reduction in interface voids, a thick lining may impair the marginal sealing, especially after thermocycling. It was concluded that a minimally thin flowable composite lining improved cavity adaptation and marginal sealing.