Filler Mixed Into Adhesives Does Not Necessarily Improve Their Mechanical Properties.

OBJECTIVES: To investigate the influence of filler type/loading on the micro-tensile fracture strength (µTFS) of adhesive resins, as measured 'immediately' upon preparation and after 1-week water storage ('water-stored'). METHODS: The morphology and particle-size distribution of three filler particles, referred to as 'Glass-S' (Esschem Europe), 'BioUnion' (GC), and 'CPC_Mont', were correlatively characterized by SEM, TEM, and particle-size analysis. These filler particles were incorporated into an unfilled adhesive resin ('BZF-29unfilled', GC) in different concentrations to measure the 'immediate' µTFS. After 1-week water storage, the 'water-stored' µTFS of the experimental particle-filled adhesive resins with the most optimum filler loading, specific for each filler type, was measured. In addition, the immediate and water-stored µTFS of the adhesive resins of three experimental two-step universal adhesives based on the same resin matrix but varying for filler type/loading, coded as 'BZF-21' (containing silica and bioglass), 'BZF-29' (containing solely silica), and 'BZF-29_hv' (highly viscous with a higher silica loading than BZF-29), and of the adhesive resins of the gold-standard adhesives OptiBond FL ('Opti-FL', Kerr) and Clearfil SE Bond 2 ('C-SE2', Kuraray Noritake) was measured along with that of BZF-29unfilled (GC) serving as control/reference. Statistics involved one-way and two-way ANOVA followed by post-hoc multiple comparisons (α<0.05). RESULTS: Glass-S, BioUnion, and CPC_Mont represent irregular fillers with an average particle size of 8.5-9.9 µm. Adding filler to BZF-29unfilled decreased µTFS regardless of filler type/loading. One-week water storage reduced µTFS of all adhesive resins except BZF-21, with the largest reduction in µTFS recorded for BZF-29unfilled. Among the three filler types, the µTFS of the 30 wt% Glass-S and 20 wt% BioUnion filled adhesive resin was not significantly different from the µTFS of BZF-29unfilled upon water storage. CONCLUSIONS: Adding filler particles into adhesive resin did not enhance its micro-tensile fracture strength but appeared to render it less sensitive to water storage as compared to the unfilled adhesive resin investigated.

Influence of airborne particle abrasion on dentin bonding effectiveness of a 2-step universal adhesive.

OBJECTIVE: To determine the effect of airborne particle abrasion (APA) on micro-tensile bond strength (µTBS) to dentin using different air-abrasion/polishing powders. METHODS: The bonding effectiveness of G2 Bond Universal (G2B), used in etch-and-rinse (E&R) and self-etch mode (SE), was tested on bur-cut dentin and dentin air abraded/polished using six different powders (aluminum oxide 29 µm (AO29) and 53 µm (AO53), aluminum trihydroxide (AT), sodium bicarbonate (SB), sodium bicarbonate soft (SBsoft) and bioactive glass (BG); Velopex). Adhesive-composite resin specimens were immersed in distilled water at 37 °C for one week and cut into microspecimens. Half of the specimens were subjected to 50,000 thermocycles (aged). Immediate and aged µTBS to dentin were measured. Statistical analysis was performed using linear mixed-effects (LME) modeling (p < 0.05). RESULTS: Comparing the aged bond strengths to air-abraded/polished dentin with bur-cut dentin, pretreatment with SB and SBsoft in combination with G2B used in E&R mode, and BG air polishing in combination with both application modes (E&R, SE), resulted in a significantly higher bond strength. Dentin bond strength was only significantly lower when air abraded with AO29 and using G2B in SE mode. Aging did not significantly influence bond strength for both application modes (E&R, SE), except for AO29 and AT-treated dentin, where bond strengths decrea sed significantly using G2B in SE mode. In general, G2B reached significantly higher bond strengths on air-abraded/polished dentin in E&R mode than in SE mode. CONCLUSION: Air-abrasion/polishing did not impair dentin bond strength using G2B, except when dentin was air abraded with AO29 and using G2B in SE mode. Air polishing positively influenced the bond strength to dentin in specific groups. CLINICAL SIGNIFICANCE: APA is safe concerning bonding to dentin. The E&R application mode is preferred using G2B as adhesive on air-abraded/polished dentin. Air polishing with BG positively influenced dentin bond strength for both application methods.

Bonding performance of experimental HEMA-free two-step universal adhesives to low C-factor flat dentin.

OBJECTIVES: Experimental two-step universal adhesives (2-UAs) providing a particle-filled hydrophobic adhesive resin with a significant film thickness to hydrophobically seal the adhesive interface were designed and synthesized. This study aimed to characterize their interfacial interaction with dentin, to determine whether the 2-UA formulations achieve durable bonding to low C-factor flat dentin and to measure their water sorption. METHODS: Bonding effectiveness of 2-UAs that combine a 10-MDP-based primer with hydrophobic adhesive resins differing only for filler (BZF-21, BZF-29, and BZF-29_hv) were comparatively investigated with the commercial adhesive Clearfil SE Bond 2 (C-SE2, Kuraray Noritake). Adhesive-dentin interfaces were characterized with TEM. Adhesive-resin disks were immersed in distilled water at 37 °C for 1 week, 6 months and 1 year to measure water sorption and solubility. 'Immediate' and 'aged' micro-tensile bond strength (µTBS) of the adhesives applied in etch-and-rinse (E&R) and self-etch (SE) bonding mode to low C-factor flat dentin were measured. Statistical analyses involved linear mixed-effects (LME) modelling and Kruskal-Wallis testing (p < 0.05). RESULTS: TEM revealed that E&R hybrid layers were more sensitive to aging than SE hybrid layers. Lower water sorption was recorded for all UAs compared with C-SE2. The immediate µTBS of BZF-21 and BZF-29 was not significantly different from that of C-SE2. The 1-year aged µTBS of all 2-UAs was significantly lower than that of C-SE2, except for BZF-29 applied in E&R mode. A significant reduction in µTBS upon 1-year aging was recorded for BZF-21 and BZF-29 applied in E&R mode. A significant difference in µTBS between E&R and SE bonding modes was recorded for all adhesives except BZF-21. SIGNIFICANCE: Experimental 2-UAs with a hydrophobic adhesive-resin design produced± 20-µm thick adhesive-resin layers, absorbed less water and resulted in bonding performance that was more aging-resistant when applied in SE than in E&R bonding mode. The silica-filled BZF-29 2-UA revealed the most comparable bonding performance with C-SE2 in a low C-factor condition (flat dentin).

Acrylamide monomers in universal adhesives.

OBJECTIVES: The mono-functional monomer 2-hydroxyethyl methacrylate (HEMA) is often added to universal adhesives (UAs) to improve surface wetting and prevent phase separation. Nevertheless, HEMA promotes water sorption and hydrolysis at adhesive interfaces, hereby affecting long-term bonding to dentin. This study investigated if two acrylamide monomers could replace HEMA in an UA formulation applied in etch-and-rinse (2E&R) and self-etch (1SE) bonding mode. METHODS: Four experimental UAs were bonded to bur-cut dentin. In addition to 12 wt% 10-MDP, 25 wt% Bis-GMA and 10 wt% TEGDMA as common monomer composition, 20 %wt ethanol and 15 %wt water as solvent, and 3 wt% polymerization-related additives, the four formulations solely differed for either the acrylamide cross-linker monomer 'FAM-201' as TEGDMA alternative and HEMA replacement, the hydroxyethyl acrylamide monomer 'HEAA' as HEMA alternative, HEMA ('HEMA+'), or extra TEGDMA in a HEMA-free control ('HEMA-'), all added in a 15 wt% concentration. The split-tooth study design involved application in 2E&R mode on one tooth half versus 1SE mode on the corresponding half. Micro-tensile bond strength of half of the micro-specimens was measured upon 1-week distilled water storage ('immediate' 1w µTBS), with the other half measured after additional 6-month storage ('aged' 6 m µTBS). Statistics involved linear mixed-effects (LME) modelling (p < .05). Additionally, interfacial TEM characterization, thin-film (TF) XRD surface analysis, LogP determination, and a cytotoxicity assay were carried out. RESULTS: FAM-201 revealed significantly higher µTBS than HEMA+ at 1w and 6 m when applied both in E&R and SE bonding modes. HEAA's µTBS was significantly lower than that of HEMA+ at 1w when applied in SE mode. TF-XRD and TEM revealed similar chemical and ultrastructural interfacial characterization, including stable 10-MDP_Ca salt nano-layering. FAM-201 was least cytotoxic and presented with an intermediary LogP, while HEAA presented with the highest LogP, indicating high hydrophilicity and water-sorption sensitivity. SIGNIFICANCE: The acrylamide co-monomer FAM-201 could replace HEMA in an UA formulation, while HEAA not.

Touch-Cure Polymerization at the Composite Cement-Dentin Interface.

Ceramic restorations are often adhesively luted onto the tooth prep. The so-called touch-cure concept was developed to yield optimum polymerization of composite cement at the restoration-cement-tooth interface for immediate bond stabilization. Although this touch cure is theorized to initiate polymerization at the interface when the accelerator in the primer makes contact with the cement, this process has not yet been proven. This study aimed to elucidate the mechanism of touch cure by measuring the degree of conversion (DC) of composite cement applied with or without an accelerator-containing tooth primer (TP) versus an accelerator-free primer using real-time Fourier-transform infrared spectroscopy (RT-FTIR) and attenuated total reflection (ATR)-FTIR. Interfacial bond strength was measured in shear mode, the accelerator composition confirmed by X-ray fluorescence analysis (XRF), and the interfacial interaction of TP and composite cement with dentin investigated by X-ray diffraction (XRD), focused-ion-beam scanning electron microscopy (FIB-SEM) with 3-dimensional interface reconstruction, and transmission electron microscopy (TEM). RT/ATR-FTIR revealed the significantly highest DC when the composite cement was applied with the accelerator-containing primer. XRF disclosed a vanadium compound as a novel chemical accelerator within TP, instead of a classic chemical curing initiator system, to set off touch cure as soon the cement contacts the previously applied primer. Although the TP contains the acidic functional monomer 10-MDP for adhesion to tooth tissue, touch cure using the accelerator-containing TP combined the fastest/highest DC with the highest bond strength. FIB-SEM and TEM confirmed the tight interfacial interaction at dentin with submicron hybridization along with stable 10-MDP also Ca-salt nanolayering.

Multiparameter evaluation of acrylamide HEMA alternative monomers in 2-step adhesives.

OBJECTIVE: As frequently added to adhesives, the mono-functional monomer 2-hydroxyethyl methacrylate (HEMA) acts as co-solvent and improves surface wetting. Nevertheless, HEMA promotes watersorption and hydrolysis at adhesive interfaces, affecting bond durability to dentin. This study investigated if two acrylamide co-monomer alternatives could replace HEMA in experimental adhesive-resin formulations as part of 3/2-step universal adhesives applied, respectively, in etch-and-rinse (E&R) and self-etch (SE) bonding modes. METHODS: Upon priming dentin with the 10-MDP-based Clearfil SE Bond 2' primer ('C-SE2p'; Kuraray Noritake), three experimental adhesive resins, consisting of 50 wt.% Bis-GMA, 15 wt.% TEGDMA, and either 35 wt.% diethyl acrylamide ('DEAA'), hydroxyethyl acrylamide ('HEAA') or HEMA ('HEMA+'), were applied. The control HEMA-free adhesive resin contained 60 wt.% Bis-GMA and 40 wt.% TEGDMA ('HEMA-'). All adhesives were evaluated for 'immediate' and 'aged' micro-tensile bond strength (µTBS) to dentin upon, respectively, 1-week (1w) and 6-month (6m) water storage, TEM adhesive-dentin interfacial interaction, 24-h and 6m three-point bending, contact-angle wetting, viscosity and watersorption. RESULTS: Linear mixed-effects model statistics revealed significantly better bonding performance of the adhesives applied in E&R than SE mode, except for DEAA_1w, with the highest µTBSs recorded for DEAA and HEMA- applied in SE mode. In E&R mode, aging did not significantly reduce DEAA's µTBS. Best wetting on primed dentin was recorded for HEMA+, significantly better than DEAA, further HEAA and HEMA-, these directly related to their viscosity. HEAA absorbed significantly more water than all other adhesive-resin formulations. HEMA->DEAA>HEAA>HEMA+ was the significant order for 6m bending strength. CONCLUSIONS: The acrylamide co-monomer DEAA could replace HEMA, while HEAA not.

Nano-Layering Adds Strength to the Adhesive Interface.

X-ray diffraction (XRD) surface analysis and ultrastructural interfacial characterization using transmission electron microscopy (TEM) confirmed that the functional monomer 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP) self-assembles into nano-layers at adhesive-tooth interfaces. Self-assembled nano-layering is thought to contribute to the durability of bonding to tooth dentin, although this has not been proven yet. In order to disclose this potential bond-durability contribution of nano-layering, we observed the 3-dimensional (3D) spreading of nano-layering by a series of focused-ion-beam (FIB) milled cross sections by scanning electron microscopy (FIB-SEM) and examined the mechanical properties of self-assembled nano-layering using scanning probe microscopy (SPM). A commercial 10-MDP-containing 3-step self-etch adhesive partially demineralized dentin up to submicron depth, forming a submicron hydroxyapatite-rich hybrid layer. TEM chemically and ultrastructurally confirmed the formation of interfacial nano-layering. FIB-SEM 3D reconstructions disclosed a 3D network of self-assembled nano-layering extending from the hybrid layer up to within the adjacent adhesive-resin layer. SPM revealed that nano-layering within the adhesive-resin layer possessed a higher elastic modulus than that of the surrounding adhesive resin, hereby suggesting that nano-layering contributes to the mechanical strength of adhesives like filler particles do. Nano-layering's 3D expanded structure is expected to strengthen the surrounding resin, as well to better interconnect the adhesive-resin layer to the hybrid layer. In conclusion, this exploratory study demonstrated that nano-layering constitutes a strong phase at the adhesive interface, which may contribute to the clinical longevity of the 10-MDP-based bond to dentin.

Effect of Immediate Dentin Sealing and Surface Conditioning on the Microtensile Bond Strength of Resin-based Composite to Dentin.

This study evaluated the microtensile bond strength (µTBS) of resin-based composite (RBC) to dentin after different immediate dentin sealing (IDS) strategies and surface-conditioning (SC) methods and on two water storage times. Human molars (n=48) were randomly divided into eight experimental groups involving four different IDS strategies-IDS-1L with one layer of adhesive, IDS-2L with two layers of adhesive, IDS-F with one layer of adhesive and one layer of flowable RBC, and DDS (delayed dentin sealing) with no layer of adhesive (control)-and two different SC methods-SC-P with pumice rubbing and SC-PC with pumice rubbing followed by tribochemical silica coating. The µTBS test was performed after one week and after six months of water storage, being recorded as the "immediate" and "aged" µTBS, respectively. Composite-adhesive-dentin microspecimens (0.9×0.9×8-9 mm) were stressed in tension until failure to determine the µTBS. Failure mode and location of failure were categorized. Two-way analysis of variance was applied to analyze the data for statistically significant differences between the experimental groups (p<0.05). Two-way analysis of variance revealed no significant differences between the one-week µTBS specimens for IDS strategy (p=0.087) and SC methods (p=0.806). However, the interaction of IDS strategy and SC methods appeared statistically significant (p=0.016). The six-month specimen evaluation showed no significant difference in µTBS for SC (p=0.297) and SC/IDS interaction (p=0.055), but the µTBS of the IDS strategies differed significantly among them (p=0.003). For tribochemical silica-coated IDS, no significant effect of aging on µTBS was recorded (p=0.465), but there was a highly significant difference in µTBS depending on the IDS strategy (p<0.001). In addition, the interaction of IDS and aging was borderline statistically significant (p=0.045). The specimens failed mainly at the adhesive-dentin interface for all experimental groups. Dentin exposure during clinical procedures for indirect restorations benefits from the application of IDS, which was shown to result in higher bond strength. No significant differences were found between cleaning with solely pumice or pumice followed by tribochemical silica coating.

Benefits of Nonthermal Atmospheric Plasma Treatment on Dentin Adhesion.

OBJECTIVES:: This study aimed to evaluate the influence of two nonthermal atmospheric plasma (NTAP) application times and two storage times on the microtensile bond strength (µTBS) to dentin. The influence of NTAP on the mechanical properties of the dentin-resin interface was studied by analyzing nanohardness (NH) and Young's modulus (YM). Water contact angles of pretreated dentin and hydroxyapatite blocks were also measured to assess possible alterations in the surface hydrophilicity upon NTAP. METHODS AND MATERIALS:: Forty-eight human molars were used in a split-tooth design (n=8). Midcoronal exposed dentin was flattened by a 600-grit SiC paper. One-half of each dentin surface received phosphoric acid conditioning, while the other half was covered with a metallic barrier and remained unetched. Afterward, NTAP was applied on the entire dentin surface (etched or not) for 10 or 30 seconds. The control groups did not receive NTAP treatment. Scotchbond Universal (SBU; 3M ESPE) and a resin-based composite were applied to dentin following the manufacturer's instructions. After 24 hours of water storage at 37°C, the specimens were sectioned perpendicular to the interface to obtain approximately six specimens or bonded beams (approximately 0.9 mm2 in cross-sectional area) representing the etch-and-rinse (ER) approach and another six specimens representing the self-etch (SE) approach. Half of the µTBS specimens were immediately loaded until failure, while the other half were first stored in deionized water for two years. Three other bonded teeth were selected from each group (n=3) for NH and YM evaluation. Water contact-angle analysis was conducted using a CAM200 (KSV Nima) goniometer. Droplet images of dentin and hydroxyapatite surfaces with or without 10 or 30 seconds of plasma treatment were captured at different water-deposition times (5 to 55 seconds). RESULTS:: Two-way analysis of variance revealed significant differences in µTBS of SBU to dentin after two years of water storage in the SE approach, without differences among treatments. After two years of water aging, the ER control and ER NTAP 10-second groups showed lower µTBS means compared with the ER NTAP 30-second treated group. Nonthermal atmospheric plasma resulted in higher NH and YM for the hybrid layer. The influence of plasma treatment in hydrophilicity was more evident in the hydroxyapatite samples. Dentin hydrophilicity increased slightly after 10 seconds of NTAP, but the difference was higher when the plasma was used for 30 seconds. CONCLUSIONS:: Dentin NTAP treatment for 30 seconds contributed to higher µTBS after two years of water storage in the ER approach, while no difference was observed among treatments in the SE evaluation. This result might be correlated to the increase in nanohardness and Young's modulus of the hybrid layer and to better adhesive infiltration, since dentin hydrophilicity was also improved. Although some effects were observed using NTAP for 10 seconds, the results suggest that 30 seconds is the most indicated treatment time.

Etching Efficacy of Self-Etching Functional Monomers.

Besides chemically interacting with hard tooth tissue, acidic functional monomers of self-etch adhesives should etch the prepared tooth surface to dissolve the smear layer and to provide surface micro-retention. Although the etching efficacy of functional monomers is commonly determined in terms of pH, the pH of adhesives cannot accurately be measured. Better is to measure the hydroxyapatite (HAp)-dissolving capacity, also considering that functional monomers may form monomer-Ca salts. Here, the etching efficacy of 6 functional monomers (GPDM, phenyl-P, MTEGP, 4-META, 6-MHP and 10-MDP) was investigated. Solutions containing 15 wt% monomer, 45 wt% ethanol, and 40 wt% water were prepared. Initially, we observed enamel surfaces exposed to monomer solution by scanning electron microscopy (SEM). X-ray diffraction (XRD) was employed to detect monomer-Ca salt formation. Phenyl-P exhibited a strong etching effect, while 10-MDP-treated enamel showed substance deposition, which was identified by XRD as 10-MDP-Ca salt. To confirm these SEM/XRD findings, we determined the etching efficacy of functional monomers by measuring both the concentration of Ca released from HAp using inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and the amount of monomer-Ca salt formation using 31P magic-angle spinning (MAS) nuclear magnetic resonance (NMR). ICP-AES revealed that the highest Ca concentration was produced by phenyl-P and the lowest Ca concentration, almost equally, by 4-META and 10-MDP. Only 10-MDP formed 10-MDP-Ca salts, indicating that 10-MDP released more Ca from HAp than was measured by ICP-AES. Part of the released Ca was consumed to form 10-MDP-Ca salts. It is concluded that the repeatedly reported higher bonding effectiveness of 10-MDP-based adhesives must not only be attributed to the more intense chemical bonding of 10-MDP but also to its higher etching potential, a combination the other functional monomers investigated lack.

Grain-Boundary Engineering for Aging and Slow-Crack-Growth Resistant Zirconia.

Ceramic materials are prone to slow crack growth, resulting in strength degradation over time. Although yttria-stabilized zirconia (Y-TZP) ceramics have higher crack resistance than other dental ceramics, their aging susceptibility threatens their long-term performance in aqueous environments such as the oral cavity. Unfortunately, increasing the aging resistance of Y-TZP ceramics normally reduces their crack resistance. Our recently conducted systematic study of doping 3Y-TZP with various trivalent cations revealed that lanthanum oxide (La2O3) and aluminum oxide (Al2O3) have the most potent effect to retard the aging kinetics of 3Y-TZP. In this study, the crack-propagation behavior of La2O3 and Al2O3 co-doped 3Y-TZP ceramics was investigated by double-torsion methods. The grain boundaries were examined using scanning transmission electron microscopy and energy-dispersive spectroscopy (STEM-EDS). Correlating these analytic data with hydrothermal aging studies using different doping systems, a strategy to strongly bind the segregated dopant cations with the oxygen vacancies at the zirconia-grain boundary was found to improve effectively the aging resistance of Y-TZP ceramics without affecting the resistance to crack propagation.

Release of monomers from composite dust.

OBJECTIVES: Dental personnel are more at risk to develop asthmatic disease, but the exact reason is so far unknown. During abrasive procedures, dental personnel are exposed to nano-sized dust particles released from dental composite. The aim of this study was to investigate whether respirable composite dust may also release monomers. METHODS: Respirable (<5µm) composite dust was collected and the release of methacrylate monomers and Bisphenol A (BPA) in water and ethanol was evaluated by liquid chromatography/mass spectroscopy (LC-MS/MS). The dust was ultra-morphologically and chemically analyzed by transmission electron microscopy and energy-dispersive X-ray spectroscopy (TEM-EDS). RESULTS: LC-MS/MS analysis revealed that, irrespective of the type of composite, the respirable fraction of composite dust may release relatively high concentrations of unpolymerized methacrylate monomers, both in water and ethanol. Higher release was observed in ethanol. The endocrine disruptor BPA also emanated from the composite dust particles. TEM showed that most particles were nano-sized, although particle size ranged between 6nm and 5µm with a mode value between 12 and 39nm. Most particles consisted of several filler particles in resin matrix, although single nano-filler particles could also be observed. Elemental analysis by TEM-EDS proved that the particles collected on the filters originated from the dental composites. CONCLUSION: Theoretically, composite dust may function as a vehicle to transport monomers deeply into the respiratory system. The results of this study may shed another light on the increasing incidence of respiratory disease among dental personnel, and more care should be taken to prevent inhalation of composite dust. CLINICAL SIGNIFICANCE: Special care should be taken to prevent inhalation of composite dust, as the dust particles may release methacrylate monomers.

Microtensile Bond Strength of Composite Cement to Novel CAD/CAM Materials as a Function of Surface Treatment and Aging.

OBJECTIVES: To evaluate the effect of different surface treatments on the bond strength to a composite and a polymer-infiltrated ceramic CAD/CAM block after six-month artificial aging. METHODS AND MATERIALS: Two types of CAD/CAM blocks (Cerasmart, GC; Enamic, Vita Zahnfabrik) were cut in slabs of 4-mm thickness, divided into six groups, and subjected to the following surface treatments: group 1: no treatment; group 2: sandblasting (SB); group 3: SB + silane (Si); group 4: SB + Si + flowable composite (see below); group 5: 5% hydrofluoric acid etching (HF) + Si; and group 6: 37% phosphoric acid etching (H3PO4) + Si. Sections of the same group were luted together (n=3: 3 sandwich specimens/group) using a dual-cure self-adhesive cement for all groups, except for the sections of group 4 that were luted using a light-curing flowable composite. After three weeks of storage in 0.5% chloramine at 37°C, the sandwich specimens were sectioned in rectangular microspecimens and trimmed at the interface to a dumbbell shape (1.1-mm diameter). One half of the specimens was subjected to a microtensile bond strength (µTBS) test, and the other half was tested after six months of water storage (aging). Data were statistically analyzed with a linear mixed-effects model for the factors surface treatment, material type, and aging, together with their first-degree interactions (α=0.05). RESULTS: The lowest bond strengths were obtained in the absence of any surface treatment (group 1), while the highest µTBSs were obtained when the surface was roughened by either SB or HF, this in combination with chemical adhesion through Si. Loss in bond strength was observed after six-month aging when either surface roughening or silanization, or both, were omitted. CONCLUSIONS: Both the composite and polymer-infiltrated ceramic CAD/CAM blocks appeared equally bonding-receptive regardless of the surface treatment used. Creating a microretentive surface by either SB or HF, followed by chemical adhesion using Si, is mandatory to maintain the bond strength after six months.

Interaction of rat alveolar macrophages with dental composite dust.

BACKGROUND: Dental composites have become the standard filling material to restore teeth, but during the placement of these restorations, high amounts of respirable composite dust (<5 µm) including many nano-sized particles may be released in the breathing zone of the patient and dental operator. Here we tested the respirable fraction of several composite particles for their cytotoxic effect using an alveolar macrophage model system. ​METHODS: Composite dust was generated following a clinical protocol, and the dust particles were collected under sterile circumstances. Dust was dispersed in fluid, and 5-µm-filtered to enrich the respirable fractions. Quartz DQ12 and corundum were used as positive and negative control, respectively. Four concentrations (22.5 µg/ml, 45 µg/ml, 90 µg/ml and 180 µg/ml) were applied to NR8383 alveolar macrophages. Light and electron microscopy were used for subcellular localization of particles. Culture supernatants were tested for release of lactate dehydrogenase, glucuronidase, TNF-α, and H2O2. RESULTS: Characterization of the suspended particles revealed numerous nano-sized particles but also many high volume particles, most of which could be removed by filtering. Even at the highest concentration (180 µg/ml), cells completely cleared settled particles from the bottom of the culture vessel. Accordingly, a mixture of nano- and micron-scaled particles was observed inside cells where they were confined to phagolysosomes. The filtered particle fractions elicited largely uniform dose-dependent responses, which were elevated compared to the control only at the highest concentration, which equaled a mean cellular dose of 120 pg/cell. A low inflammatory potential was identified due to dose-dependent release of H2O2 and TNF-α. However, compared to the positive control, the released levels of H2O2 and TNF-α were still moderate, but their release profiles depended on the type of composite. CONCLUSIONS: Alveolar macrophages are able to phagocytize respirable composite dust particle inclusive nanoparticles. Since NR8383 cells tolerate a comparatively high cell burden (60 pg/cell) of each of the five materials with minimal signs of cytotoxicity or inflammation, the toxic potential of respirable composite dust seems to be low. These results are reassuring for dental personnel, but more research is needed to characterize the actual exposure and uptake especially of the pure nano fraction.

Lack of Buffering by Composites Promotes Shift to More Cariogenic Bacteria.

Secondary caries (SC) remains a very important problem with composite restorations. The objectives of this study were to test the acid-buffering ability of several restorative materials and to evaluate whether buffering of the restorative material has an impact on the microbial composition of the biofilm. Disk-shaped specimens of conventional composite, composite with surface prereacted glass-ionomer filler particles (so-called giomer), glass-ionomer cement (GIC), amalgam, and hydroxyapatite (HAp) (control) were exposed to aqueous solutions with pH 4, 5, 6, and 7 and to the medium containing bacteria-produced acids, and pH changes were recorded over several days. Next, material specimens were immersed in bacterial growth medium with pH adjusted to 5. After a 24-h incubation, the extracts were collected and inoculated with a cariogenic (Streptococcus mutans) and a noncariogenic (Streptococcus sanguinis) species. The bacterial growth was monitored both in a single-species model by spectrophotometry and in a dual-species model by viability quantitative polymerase chain reaction. Amalgam and HAp showed the strongest acid-buffering ability, followed by the GIC and the giomer, while the conventional composite did not exhibit any buffering capacity. Furthermore, due to the lack of acid-buffering abilities, composite was not able to increase the pH of the medium (pH 5), which, in the absence of antibacterial properties, allowed the growth of S. mutans, while the growth of S. sanguinis, a less aciduric species, was completely inhibited. A similar effect was observed when bacteria were cultured together: there was a higher percentage of S. mutans and lower percentage of S. sanguinis with the conventional composite than with other materials and HAp. In conclusion, conventional composites lack the ability to increase the local pH, which leads to the outgrowth of more acidogenic/aciduric bacteria and higher cariogenicity of the biofilm. Together with lack of antibacterial properties, lack of buffering may account for the higher susceptibility of composites to SC.

Dentin Bonding Testing Using a Mini-interfacial Fracture Toughness Approach.

Measurement of interfacial fracture toughness (iFT) is considered a more valid method to assess bonding effectiveness as compared with conventional bond strength testing. Common fracture toughness tests are, however, laborious and require a relatively bulky specimen size. This study aimed to evaluate a new simplified and miniaturized iFT (mini-iFT) test. Four dentin adhesives, representing the main adhesive classes, and 1 glass ionomer cement were applied onto flat dentin. Mini-iFT (1.5 × 2.0 × 16 to 18 mm) and microtensile bond strength (µTBS; 1.5 × 1.5 × 16 to 18 mm) specimens were prepared from the same tooth. For the mini-iFT specimens, a single notch was cut at the adhesive-dentin interface with a 150-µm diamond blade under water cooling; the specimens were loaded until failure in a 4-point bending test setup. Finite element analysis was used to analyze stress distribution during mini-iFT testing. The correlation between the mean mini-iFT and µTBS was examined and found to be significant; a strong positive correlation was found (r(2) = 0.94, P = 0.004). Weibull data analysis suggested the mini-iFT to vary less than the µTBS. Both the mini-iFT and the µTBS revealed the same performance order, with the 3-step etch-and-rinse adhesive outperforming the 2-step self-etch and 2-step etch-and-rinse adhesive, followed by the 1-step SE adhesive and, finally, the glass ionomer cement. Scanning electron microscopy failure analysis revealed the adhesive-dentin interface to fail more at the actual interface with the mini-iFT test, while µTBS specimens failed more within dentin and composite. This finding was corroborated by finite element analysis showing stress to concentrate at the interface during mini-iFT loading and crack propagation. In conclusion, the new mini-iFT test appeared more discriminative and valid than the µTBS to assess bonding effectiveness; the latter test nevertheless remains more versatile. Specimen size and workload were alike, making the mini-iFT test a valid alternative for the popular µTBS test.

Structural and Chemical Analysis of the Zirconia-Veneering Ceramic Interface.

The interfacial interaction of veneering ceramic with zirconia is still not fully understood. This study aimed to characterize morphologically and chemically the zirconia-veneering ceramic interface. Three zirconia-veneering conditions were investigated: 1) zirconia-veneering ceramic fired on sandblasted zirconia, 2) zirconia-veneering ceramic on as-sintered zirconia, and 3) alumina-veneering ceramic (lower coefficient of thermal expansion [CTE]) on as-sintered zirconia. Polished cross-sectioned ceramic-veneered zirconia specimens were examined using field emission gun scanning electron microscopy (Feg-SEM). In addition, argon-ion thinned zirconia-veneering ceramic interface cross sections were examined using scanning transmission electron microscopy (STEM)-energy dispersive X-ray spectrometry (EDS) at high resolution. Finally, the zirconia-veneering ceramic interface was quantitatively analyzed for tetragonal-to-monoclinic phase transformation and residual stress using micro-Raman spectroscopy (µRaman). Feg-SEM revealed tight interfaces for all 3 veneering conditions. High-resolution transmission electron microscopy (HRTEM) disclosed an approximately 1.0-µm transformed zone at sandblasted zirconia, in which distinct zirconia grains were no longer observable. Straight grain boundaries and angular grain corners were detected up to the interface of zirconia- and alumina-veneering ceramic with as-sintered zirconia. EDS mapping disclosed within the zirconia-veneering ceramic a few nanometers thick calcium/aluminum-rich layer, touching the as-sintered zirconia base, with an equally thick silicon-rich/aluminum-poor layer on top. µRaman revealed t-ZrO2-to-m-ZrO2 phase transformation and residual compressive stress at the sandblasted zirconia surface. The difference in CTE between zirconia- and the alumina-veneering ceramic resulted in residual tensile stress within the zirconia immediately adjacent to its interface with the veneering ceramic. The rather minor chemical elemental shifts recorded in the veneering ceramic did not suffice to draw definitive conclusions regarding potential chemical interaction of the veneering ceramic with zirconia. Sandblasting damaged the zirconia surface and induced phase transformation that also resulted in residual compressive stress. Difference in CTE of zirconia versus that of the veneering ceramic resulted in an unfavorable residual tensile stress at the zirconia-veneering ceramic interface.

Thirteen-year randomized controlled clinical trial of a two-step self-etch adhesive in non-carious cervical lesions.

OBJECTIVE: The objective of this randomized controlled clinical trial was to evaluate the 13-year clinical performance of a mild two-step self-etch adhesive in non-carious cervical lesions with and without prior selective phosphoric acid-etching of the enamel cavity margins. METHODS: A total of 100 non-carious cervical lesions in 29 patients were restored with Clearfil AP-X (Kuraray Noritake). The composite restorations were bonded following two different approaches: (1) application of Clearfil SE Bond (Kuraray Noritake) following a self-etch approach (CSE-NE); (2) selective phosphoric acid-etching of enamel cavity margins before application of Clearfil SE Bond (CSE-E). The restorations were evaluated after 6 months, 1, 2, 3, 5, 8 and 13 years of clinical service regarding retention, marginal integrity and discoloration, caries occurrence, preservation of tooth vitality and post-operative sensitivity. RESULTS: The patient recall rate at 13 years was 62%. Six restorations, 4 of the CSE-NE group and 2 of the CSE-E group, were clinically unacceptable due to loss of retention (1 CSE-NE, 1 CSE-E), a severe marginal defect (2 CSE-NE, 1 CSE-E) and caries occurrence in combination with a severe marginal defect (1 CSE-NE) leading to a clinical success rate of 86% (CSE-NE) and 93% (CSE-E). Ageing of the restorations was characterized by a further increase in the percentage of restorations with a clinically acceptable small marginal defect (CSE-NE: 87%; CSE-E: 83%) and/or superficial marginal discoloration (CSE-NE: 53%; CSE-E: 56%). The presence of small marginal defects (CSE-NE: 86%; CSE-E: 68%) and superficial marginal discoloration (CSE-NE: 41%; CSE-E: 20%) at the incisal enamel side was more frequently noticed in the CSE-NE group than in the CSE-E group. The difference, however, was not statistically significant (McNemar, p>0.05). SIGNIFICANCE: After 13 years, the clinical effectiveness of Clearfil SE Bond in non-carious Class-V lesions remained excellent, with selective acid-etching of the enamel cavity margins only having some minor positive effect on marginal integrity and absence of marginal discoloration.

Clinical effectiveness of contemporary adhesives for the restoration of non-carious cervical lesions. A systematic review.

OBJECTIVES: The aim of this systematic review was to evaluate the clinical effectiveness of contemporary adhesives for the restoration of non-carious cervical lesions (NCCLs) in terms of restoration retention as a function of time. METHODS: Medline Ovid and IADR abstracts were reviewed for NCCLs clinical trials from 1950 to 2013. The reference list of all eligible trials and relevant review articles was checked to find additional studies. The review did not have any language restrictions. Only randomized controlled clinical trials that evaluated at least two adhesives for a follow-up period of at least 18 months were included. Materials with adhesive potential were categorized into 6 main classes: 3-step etch&rinse adhesives (3E&Ra's), 2-step etch&rinse adhesives (2E&Ra's), 2-step self-etch adhesives (2SEa's), 1-step self-etch adhesives (1SEa's), glass-ionomers (GI's) and self-adhesive composites (SAC's). The first four can bond restorative composite to tooth tissue. Both 2SEa and 1SEa were further sub-divided in 'mild' and 'intermediately strong (1/2SEa_m), with a pH≥1.5, and 'strong' (1/2SEa_s), with a pH<1.5. From the restoration retention rates as a function of time the average annual failure rate (AFR) per adhesive and adhesive class was calculated. RESULTS: The lowest AFR scores [mean (SD)] were recorded for GI [2.0 (1.4)] shortly followed by 2SEa_m [2.5 (1.5)], 3E&Ra [3.1 (2)] and 1SEa_m [3.6 (4.3)] (Tukey Contrasts: p>0.05). Significantly higher AFR scores were recorded for 1SEa_s [5.4 (4.8)], 2E&R [5.8 (4.9)], and 2SEa_s [8.4 (7.9)] (p>0.05). In addition, significant differences in AFR were noticed between adhesives of the same class (Kruskal-Wallis sum test: p>0.05), except for GI (p=0.7) and 2SEa_m (p=0.1). Finally, selective enamel etching did not significantly influence the retention rate of SEa (AFR SEa_etch=0.43 (0.49), AFR SEa_non-etch=1.43 (1.77). SIGNIFICANCE: The adhesive approach significantly influences the clinical effectiveness of adhesives in NCCLs. Within each class, except for GI, there was a wide variation in clinical bonding effectiveness.