Preventive and restorative alternatives to caries by irradiation: a scoping review.

PURPOSE: This study is to conduct a comprehensive scoping review to map scientific evidence and clarify concepts regarding the commonly recommended preventive and restorative dental treatments for patients diagnosed with head and neck cancer (HNC) and subjected to radiotherapy. MATERIAL AND METHODS: This systematic scoping review was performed under the PRISMA-ScR guidelines. The study's experimental design was registered in the Open Science Framework. In vitro studies that evaluated preventive and restorative dental treatment over 50 Gy radiation doses were included. The search was conducted in November 2023 in five electronic databases (PubMed, Scopus, Web of Science, Cochrane Library, and Embase) without language or date restriction. A search strategy was applied based on keywords, MeSh terms, or synonyms. A descriptive analysis was conducted. RESULTS: A total of 49 studies, out of 3679 original articles identified, were included and reviewed. Of the included studies, three evaluated saliva stimulants and 35 evaluated fluoride-based preventive materials: gel (n = 18) toothpaste (n = 11) mouth rinse (n = 8) and varnish (n = 5) while 14 evaluated restorative materials: resin composite (n = 12) glass ionomer cement (n = 6) and amalgam (n = 1) Of those studies, 36 were clinical trials and 13 were in vitro studies. CONCLUSION: Fluoride gel was the most frequently recommended preventive material for preventing radiation caries with supportive clinical evidence. Resin composite and glass ionomer were the most frequently used restorative materials, respectively. However, there is not yet clinical evidence to support the use of resin composite in irradiated teeth.

Effects of simulation-based flipped classroom in developing head and neck examination skills in dental students.

OBJECTIVES: To evaluate the effectiveness of a simulation-based flipped classroom in developing dental students' head and neck examination skills. METHODS: Second-year dental students (n = 118) participated in a simulation-based flipped classroom experience. A pre-class recorded lecture and video were posted online one week before the simulation date. In class, students completed a pre-quiz assessing pre-class coursework understanding of head and neck examination content. Then, students attended a question-and-answer session before the patient simulation. A formative assessment of head and neck examination simulation on a patient in the clinic was completed. Lastly, students completed an in-class post-quiz assessing understanding of head and neck examination content after the simulation, classroom simulation discussion, and open-response survey. RESULTS: Student post-quiz scores were significantly higher than pre-quiz scores, with an average improvement of 4.8 ± 7.2 percentage points. There was no difference between male and female improvement in scores. Student survey responses indicated that students liked combining simulation and flipped classroom methodology, flexibility with their schedules, learning at their pace, and opportunities for a deeper level of learning. Students reported wanting more practice sessions for their technical skills and more accessibility to faculty. CONCLUSIONS: According to this study's results, students had significantly higher post-quiz scores than pre-quiz scores and reported positive perceptions of this combined teaching technique. A simulation-based flipped classroom might be an effective teaching approach for developing head and neck examination skills that can be applied at other dental facilities with potentially similar results.

A survey of amalgam use to guide dental education curriculums.

OBJECTIVES: To identify behavioral, preferential, and professional factors influencing the use of amalgam in private practices; and to compare the incidence of the placement of amalgam versus composite resin restorations in the province of Ontario and its pedagogical implications on dental curricula. METHODS: Participants responded anonymously to a 23-question online survey about their current use of dental amalgam and composite resins as well as their opinions regarding both dental materials. The explanatory variables were associated bivariately with the outcome variables, and the most significant predictors were identified using the multivariate analysis. RESULTS: Higher percentages of amalgam use were reported among clinicians who trained in Canada only (P = .009), who graduated before 1980 (p = <.001) and who work outside private practice (p = <.001). Familiarity with amalgam was higher among clinicians who are female (p = <.001), older (p = <.001), trained only in Canada (p = .017), who graduated prior to 2000 (p = <.001), and who work in locations with populations over 100,000 (p = .042). Familiarity with composite resin was higher among clinicians who graduated more recently (p = .002). A higher percentage of females (p = <.001), younger clinicians (p = <.001), recent graduates (p = <.001), and clinicians who work in private practice (p = .043) suggested that over 50% of dental student training time be allocated to amalgam. CONCLUSIONS: Decreased amalgam use was reported by later dental graduates and private practitioners; this may be impacted by familiarity with dental amalgam. As amalgam remains a safe and effective dental material, its removal may not be prudent. Dental educators play a crucial role in the future of amalgam opinion and use.

Preheating of dental composite resins: A scoping review.

OBJECTIVE: The aim of this scoping review was to analyze the available evidence on the preheating process and its effects on physicochemical properties of composite resins. MATERIALS AND METHODS: A systematic search was performed in August 2021 using PubMed/Medline, Embase, Scopus, and ISI Web of Science databases and gray literature without language or date restriction. Inclusion criteria were in vitro studies that assessed the physicochemical properties and marginal adaptation of preheated composite. Exclusion criteria were studies with experimental composite, literature reviews, clinical studies. Data from selected studies were qualitatively analyzed. RESULTS: In total, 104 studies were found from which 39 were included. The most used composite resin was Filtek Z350 XT (3 M/ESPE), preheated at 68°C in a Calset device (AdDent Inc., Danbury, CT, USA) for 5 or 15 min. Most studies showed decreased viscosity, increased conversion degree and microhardness of composite resins, and better marginal adaptation of direct and indirect restorations. Also, flexural strength was not affected, and data about bond strength were inconclusive due to heterogeneity among studies. CONCLUSION: The parameters used for preheating composite resins are heterogeneous. Preheating decreased viscosity, increased the conversion degree and microhardness of composite resins, and improved the marginal adaptation of direct and indirect restorations. CLINICAL SIGNIFICANCE: The analysis of evidence showed a high heterogeneity among preheating protocols. Preheating may benefit the handling and physicochemical properties of composite resins.

Perception of COVID-19 pandemic restrictions on dental researchers.

BACKGROUND/OBJECTIVES: Historical evidence shows a gender-based disproportionate effect of pandemics across different populations. In 2020, the coronavirus disease 2019 (COVID-19) pandemic began spreading its devastating effects worldwide. The goal of the present study was to investigate the effect of the COVID-19 pandemic on research productivity, work-life arrangements, and mental health of dental professionals worldwide with focus on gender differences. METHODS: A 38-item survey, concerning demographics, career stage, employer support, family structure, mental health, and relationships, was distributed to 7692 active members of the International Association for Dental Research. Bivariate associations between independent variables and the primary outcome variable were tested using Spearman's correlation test. A logistic regression model was used to assess the simultaneous, independent associations between each variable and researcher productivity. RESULTS: A total of 722 responses were obtained, indicating a 9.4% response rate. Higher productivity was reported by male respondents (p = 0.021), and by those in senior career stages (p = 0.001). Institutional support was associated with higher productivity (p < 0.0001). Lower productivity was reported by younger researchers (p = 0.003). Remote work negatively affected productivity (p < 0.0001) and female respondents reported working more hours, regardless of work location (p = 0.004). Poor mental health was associated with low productivity (p < 0.0001). CONCLUSIONS: Our results showed that the COVID-19 pandemic significantly affected dental professionals' perceived productivity and mental health around the globe. Younger individuals and women were disproportionally affected, and institutional support had a significant influence to mitigate effects of the pandemic for dental researchers.

Room-temperature atomic layer deposition of SiO2 on microcracked ZrO2 layers.

Yttria-stabilized zirconia (Y-SZ) has become a reliable material option to restore severely compromised teeth. Y-SZ materials are prone to low-temperature degradation (LTD), which generates a tetragonal-to-monoclinic (t-m) transformed, porous layer. We suggest that room-temperature atomic layer deposition (RT-ALD) could be used for the infiltration and deposition of nanoscale SiO2 film over this layer, creating a protective hybrid surface against further degradation by LTD. This study investigated the potential of developing a Y-SZ transformed layer under controlled conditions for the infiltration of silica using RT-ALD, aiming to develop a hybrid zirconia-silica interface, and to investigate the effect of silica deposition/infiltration via RT-ALD on the surface roughness and wettability of zirconia-based materials. Sintered specimens (14 mm × 4 mm x 2 mm) were prepared from four different Y-SZ materials (n = 40): low translucency 3 mol % Y-SZ (3Y-LT; Ceramill ZI, Amann Girrbach); high translucency 4 mol % Y-SZ (4Y-HT; Ceramill Zolid); and two high translucency 5 mol % Y-SZ (5Y-HT - Lava Esthetic, 3M; 5Y-SHT - Ceramill Zolid, FX white). Specimens were exposed to hydrothermal treatment (HTT) to develop similar depths of crystalline changes. RT-ALD was used to deposit a thin film of silica (SiO2). Surface roughness and wettability analyses were performed to investigate the effect of treatment (HTT and RT-ALD) and material on Y-SZ surface properties, and data was analyzed by two-way ANOVA and Tukey HSD (p < 0.05). RT-ALD and HTT-RT-ALD treated specimens of 3Y-LT and 5Y-HT materials were exposed to further hydrothermal aging (HA) and the surface was characterized by time-of-flight secondary ion mass spectrometry (ToF-SIMS). There was a significant interaction effect of material and treatment (HTT and RT-ALD) on roughness (p = 0.02), and surface wettability (p < 0.001). Silica deposition via RT-ALD resulted in a significant increase in surface roughness of all materials tested, while surface wettability was either improved or not changed based on the material type and HTT exposure. Nanofilms of SiO2 were successfully deposited on Y-SZ materials and infiltrated 3Y-LT zirconia.

Surface and bulk properties of zirconia as a function of composition and aging.

Yttria-stabilized zirconia (Y-SZ) materials with different levels of translucency have been used for indirect dental restorations. Y-SZ composition and microstructure are modified to improve translucency, and it is not clear how these materials respond to aging. This study evaluated the effect of hydrothermal aging (HA) performed in an autoclave on the properties of four dental Y-SZ materials with different compositions. Sintered bar-shaped specimens (14 x 4 x 2 mm) were prepared from four different zirconia-based materials (n = 40): low translucency 3 mol % Y-SZ (3Y-LT; Ceramill ZI, Amann Girrbach); high translucency 4 mol % Y-SZ (4Y-HT; Ceramill Zolid); and two high translucency 5 mol % Y-SZ (5Y-HT - Lava Esthetic, 3M; 5Y-SHT - Ceramill Zolid FX). Fully sintered specimens were exposed to HA for different times (control - 0 h, 5 h, 10 h, or 15 h at 134 °C, 2 bar pressure) and characterized for surface roughness, flexural strength (three-point bending), hardness and elastic modulus (nanoindentation), surface wettability (sessile drop technique) and crystalline content (x-ray diffraction, XRD). Data was analyzed by two-way ANOVA and Tukey HSD (p < 0.05). Zirconia composition significantly affected roughness (p = 0.016). Zirconia*aging interaction affected flexural strength (p = 0.012), surface wettability (p < 0.001), and hardness (p = 0.002). Zirconia composition (p = 0.011) and aging (p = 0.001) affected elastic modulus, while the interaction effect was not significant (p = 0.94). HA affects zirconia-based materials in different degrees. For 3Y-LT and 4Y-HT, surface and bulk properties were affected by aging to a similar extent. However, surface and bulk properties may change during clinical use as a result of prolonged degradation of Y-SZ.

Irradiation therapy and chewing simulation: effect on zirconia and human enamel.

PURPOSE: Ionizing radiation therapy (RT) is the main option for head and neck cancer treatment, but it is associated with multiple side effects. This study aimed to evaluate the effect of RT associated with chewing simulation on the surface of human enamel and Yttria-partially stabilized zirconia (Y-TZP). METHODS: Maxillary premolar cusps and Y-TZP slabs were divided in 7 experimental groups: CO: no RT (control); EZ groups had irradiation applied to both, enamel and zirconia samples (simulating restoration prior to RT); E groups had irradiation applied to enamel only (simulating restoration after RT). RT doses were either 30, 50 or 70 Gray (Gy). Enamel cusps were abraded against zirconia slabs in a chewing simulator (CS - one million cycles/ 80 N/ 60 mm/min, 2 mm horizontal path, artificial saliva, 37˚ C). Zirconia hardness was evaluated before CS; zirconia roughness and enamel volume (wear) were evaluated before and after CS. Hardness and wear data were analyzed by one-way Analysis of Variance and Tukey post hoc test. Roughness was analyzed by Repeated Measures test and Bonferroni test (p=0.05). RESULTS: There was no significant effect of enamel or zirconia irradiation on enamel cusp wear (p=0.226), regardless of the irradiation dose used - up to 70 Gy. Irradiation also did not affect Y-TZP surface roughness (p=0.127) and hardness (p=0.964). CONCLUSIONS: RT does not promote significant changes to the surface characteristics of zirconia. Irradiated enamel abraded against zirconia does not show higher wear volume when compared to non-irradiated enamel.

Mechanical properties of aged yttria-stabilized tetragonal zirconia polycrystal after abrasion with different aluminum oxide particles.

STATEMENT OF PROBLEM: A consensus on the benefits of airborne-particle abrasion of zirconia with alumina particles of different sizes is still lacking. Larger particle size may improve micromechanical retention but may generate deep microcracks on the zirconia surface. PURPOSE: The purpose of this in vitro study was to evaluate the effect of different size of Al2O3 particles used for surface abrasion on the mechanical properties of yttria-stabilized tetragonal zirconia polycrystal (Y-TZP). MATERIAL AND METHODS: Sixty Y-TZP specimens were divided into 6 groups according to the treatment: control (without treatment) or airborne-particle abrasion with Al2O3 particles (45 µm or 150 µm). Half the specimens were stored for 24 hours in water while the other half was exposed to 1.5×106 mechanical cycles before flexural strength analysis at 1 mm/min crosshead speed. Specimens were also characterized by micro-Raman spectroscopy and X-ray diffraction (XRD) to evaluate the crystalline composition. The data were subjected to 2-way ANOVA and Tukey HSD test (α=.05). RESULTS: Airborne-particle abrasion with alumina (P=.030) and mechanical fatigue (P<.001) had a significant effect on flexural strength. Specimens abraded with 45-µm Al2O3 particles (847 ±204 MPa) presented higher flexural strength than those of the control group (670 ±210 MPa). The size of the alumina particles was not significant for flexural strength. Flexural resistance (664 MPa) significantly decreased after mechanical fatigue. All groups showed only the tetragonal phase on the micro-Raman spectra, which was confirmed by XRD. CONCLUSIONS: Airborne-particle abrasion with smaller Al2O3 particles increased the flexural strength on Y-TZP without causing phase transformation. However, flexural strength was decreased after mechanical fatigue.

Effect of ionizing radiation on mechanical properties and translucency of monolithic zirconia.

This study aimed to evaluate the effect of radiation therapy (RT) on mechanical properties and translucency of monolithic zirconia. Yttria- stabilized zirconia (Y-PSZ) materials (14 × 4.0 × 1.5 mm) were divided in four experimental groups (n = 30): High-translucency/control (HT/C), high-translucency/irradiated (HT/I), low-translucency/control (LT/C), low-translucency/irradiated (LT/I). Irradiated specimens were submitted to a single dose irradiation of 70 Gray. Flexural strength (n = 10) (FS-3-point bending test), fatigue limits (n = 15) at 100,000 cycles (FLs-staircase approach), and translucency (n = 5) (TP-dental spectrometer) were analyzed. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the materials. FS and TP data were analyzed by one-way Analysis of Variance (ANOVA) and Tukey HSD. FLs were analyzed using Dixon and Mood method, and Kaplan-Meier survival analysis. RT affected FS of LT zirconia (p = .032) but not of HT zirconia (p = .86). FLs and TP of both materials were not affected by RT (p > .05). Higher cubic content after RT was observed. In conclusion, RT may affect flexural strength and crystalline content of zirconia-based materials, but this effect was not observed under fatigue. Translucency of Y-PSZ restorations is also not affected by RT.

Ultrashort-pulse laser as a surface treatment for bonding between zirconia and resin cement.

OBJECTIVES: To evaluate ultrashort-pulse laser (UPL) as a surface treatment for improved bond strength to Yttria-tetragonal zirconia polycrystalline (Y-TZP). METHODS: Fully-sintered Y-TZP samples received either no treatment (CTL), or were treated by alumina blasting (ALB), tribochemical silica coating (SIL), or one of two UPL patterns: multiple pulses laser surface dots with 2.5µm spacing (8mJ, 10kHz)(LSD); or single pulse laser surface lines with 2.5µm spacing (4mJ, 6.7kHz)(LSL). Surface roughness, wettability (contact angle), and quantification of crystalline phases were evaluated for each group (n=3/group). Y-TZP treated slabs were cemented to resin composite slabs using silane and 10-methacryloyloxydecyl dihydrogen phosphate (MDP)-containing adhesive. Beams from the Y-TZP/resin blocks were microtensile tested (n=5/group) after 48h water incubation (37°C) with or without subsequent thermocycling (5-55°C, 5000 cycles). RESULTS: All surface treatments increased surface roughness values versus control (P<0.001). Contact angles were lowest for SIL (6.57±2.37°) and highest for control (50.97±6.30°). LSL and LSD were the only treatments that did not increase the relative monoclinic phase. All surface treatments significantly increased microtensile bond strengths (µTBS) compared with the control group (P<0.001), with highest values for UPL (LSD: 35.40±4.53MPa>LSL: 31.84±8.46MPa>SIL: 19.95±3.99MPa=ALB: 19.51±2.55MPa>CTL: 14.51±2.23MPa). Thermocycling significantly reduced bond strength for all treatments in a surface treatment-dependent manner. SIGNIFICANCE: The ability of UPL to alter Y-TZP surface morphology, increase wettability and µTBS without increasing the monoclinic content suggests its potential to improve bonding to the underlying resin cement and tooth without compromising the strength of the restoration.

Microstructural and Mechanical Characterization of CAD/CAM Materials for Monolithic Dental Restorations.

PURPOSE: To determine and compare the microstructure, flexural strength, flexural modulus, fracture strength, and microhardness of four types of computer-aided design/computer-aided manufacturing (CAD/CAM) materials for monolithic dental restorations. MATERIALS AND METHODS: A lithium disilicate (LD; IPS e.max CAD), a zirconia-reinforced lithium silicate (ZLS; VITA Suprinity), a hybrid high-performance polymer (HPP) composite resin (GC Cerasmart), and a hybrid polymer-infiltrated ceramic network (PICN) material (VITA Enamic) were used to manufacture monolithic ceramic posterior crowns (n = 10) that were adhesively cemented on resin-based composite dies and loaded until fracture. In addition, 40 rectangular bars (n = 10) were milled and polished for three-point flexural strength testing. Microhardness (Vickers indentation), as well as quantitative (energy dispersive spectroscopy) and qualitative (scanning electron microscopy) structural analysis were conducted on fracture surfaces. Data were analyzed by one-way ANOVA and Tukey HSD post-hoc test (p = 0.05). RESULTS: Mechanical testing results showed that the material type has a significant effect on the fracture strength (p < 0.0001) of the monolithic crowns with ZLS and LD presenting significantly higher fracture strength than the PICN and HPP hybrid materials. LD showed the highest flexural strength (p < 0.0001) followed by ZLS, HPP, and PICN, respectively. The lowest flexural modulus and hardness were presented by HPP whereas ZLS had the highest flexural modulus and hardness. The LD presented the highest modulus of resilience and the PICN the lowest. CONCLUSIONS: All CAD/CAM crown materials exhibited high values of fracture and flexural resistance, making them suitable materials for posterior full-crown restorations. Glass-ceramics suffered more from catastrophic and nonreparable fracture patterns, whereas minimal chipping and type II fracture patterns were more common in hybrid materials. The combination of more flexibility, less stiffness, and increased softness with satisfactory flexural and fracture strength values observed in PICN and HPP makes these two hybrid materials suitable choices for chairside monolithic crown fabrication.

Influence of residual thermal stresses on the edge chipping resistance of PFM and veneered zirconia structures: Experimental and FEA study.

OBJECTIVE: Chipping fractures of the veneering porcelain are frequently reported for veneered all-ceramic crowns. In the present study, the edge chipping test is used to measure the toughness and the edge chipping resistance of veneered zirconia and porcelain-fused-to-metal (PFM). The aim is to describe an edge chipping method developed with the use of a universal testing machine and to verify the accuracy of this method to determine the influence of residual thermal stresses on the chipping fracture resistance of veneering porcelain. A finite element analysis (FEA) was used to study the residual stress profiles within the veneering porcelain. METHODS: Veneered zirconia and PFM bar specimens were subjected to either a fast or a slow cooling protocol. The chipping resistances were measured using the edge chipping method. The load was applied in two different directions, in which the Vickers indenter was placed in the veneering porcelain either parallel or perpendicular to the veneer/framework interface. The mean edge chipping resistance (ReA) and fracture toughness (KC) values were analysed. ReA was calculated by dividing the critical force to cause the chip by the edge distance. KC was given by a fracture analysis that correlates the critical chipping load (FC) regarding edge distance (d) and material toughness via KC=FC/(ßd1.5). RESULTS: The ReA revealed similar values (p>0.005) of chipping resistance for loads applied in the parallel direction regardless of framework material and cooling protocol. For loads applied in the perpendicular direction to the veneer/framework interface, the most chip resistant materials were slow cooled veneered zirconia (251.0N/mm) and the PFM fast cooled (190.1N/mm). KC values are similar to that for monolithic porcelain (0.9MPa.√m), with slightly higher values (1.2MPa.√m) for thermally stressed PFM fast cooled and veneered zirconia slow cooled groups. SIGNIFICANCE: The developed and reported edge chipping method allows for the precise alignment of the indenter in any predetermined distance from the edge. The edge chipping method could be useful in determining the different states of residual thermal stresses on the veneering porcelain.

Characterisation of a new plasma-enhanced film to improve shear bond strength between zirconia and veneering ceramic.

The aim of this study was to develop and characterise a new plasma-enhanced chemical vapor deposition (PECVD) film for improving shear bond strength (SBS) between yttria-stabilised tetragonal zirconia (Y-TZP) and veneering ceramic. In total, 192 Y-TZP samples (13 × 5.4 × 5 mm) were divided into 6 groups: control - no treatment (C), airborne-particle abrasion with 27 µm aluminum oxide particles (Al27), 110 µm aluminum oxide particles (Al110), and 250 µm aluminum oxide particles (Al250), application of liner for zirconia (L) and the PECVD film application (P). The Y-TZP surface was characterised by means of Scanning Electronic Microscopy (SEM), Energy-dispersive Spectroscopy (EDS), atomic force microscopy (AFM), surface profilometry and surface-free energy (SFE). SBS between Y-TZP and veneering ceramic was tested before and after thermocycling (20,000 cycles of 5 and 55 °C), and failure mode was also evaluated. Data were analysed by ANOVA and Tukey's HSD test (α = 0.05). Data analysis showed that PECVD film had no effect on surface roughness of Y-TZP (p > 0.05 vs control), whilst the other groups presented higher roughness values (p < 0.05). All treatments increased SFE, except the Al27 group. The highest SBS was presented by the P group (p < 0.05), and values were similar to those of the Al27 group (p = 0.107). Mixed failures were prevalent in all groups, and premature failures were found only in Al groups after thermocycling. Whilst PECVD treatment did not affect Y-TZP surface roughness, high SBS between Y-TZP and the veneering layer was observed. Therefore, PECVD treatment is a promising alternative to improve the performance of bi-layer zirconia-based restorations.

Effect of Training Method on Dental Students' Light-Curing Performance.

The aims of this study were to evaluate 1) the effect of different light-curing training methods on first-year dental students' light-curing performance immediately after and one year after training and 2) the effect of dental student year (first vs. third year) and different light-curing training methods on students' light-curing efficacy. The study was organized into two parts, conducted in 2013 and 2014. In the first part, all students (N=66) in the first-year restorative course at a Canadian dental school were invited to participate in a light-curing activity immediately after training; after 12 months, those students (now in their second year) were asked to again perform the light-curing activity. In the second part, all the subsequent first-year students (N=75) and third-year students (N=95) were invited to perform the same light-curing activity after training. Each class was divided into three groups: the Control group received a lecture; the Tips group received the same lecture plus tips on "7 Steps to Ensure Better Light Curing"; and the Simulator group received the lecture, tips, and hands-on training using the patient simulator. In the first part of the study, all 66 students participated in the first year; 39 of those participated a year later. In the second part, all students in both years participated. For the first part, time had a significant effect on light-curing performance, with significantly higher results in the second year. When the first-year students' results were isolated, training method had a significant effect on mean and maximum irradiance values. The second part found a significant effect of training on mean (p=0.003) and maximum (p=0.021) irradiance. The interaction DDS*training was significant (p=0.037) when the after-training values were analyzed. These results point to the need for further research to determine the most effective light-curing training methods.

Effect of tooth whitening strips on fatigue resistance and flexural strength of bovine dentin in vitro.

OBJECTIVE: To determine the effects of whitening strips on bovine dentin fatigue resistance and flexural strength in vitro. MATERIALS AND METHODS: A total of eighty bovine dentin specimens (2x2x17mm) were treated with either: control glycerine gel on plastic film wrap or whitening strips containing 9.5% hydrogen peroxide. Treatment was applied for 30 minutes, twice a day, for 1- or 4-weeks. After the last treatment, ten specimens per group were randomly selected to undergo fatigue testing (106 cycles, 3Hz, 20N) while the other ten were subjected to flexural strength testing after ten days of storage in artificial saliva. Kaplan-Meier method with a log rank test, Wilcoxon test and Cox regression were used to assess fatigue test results (p<0.05). One-way ANOVA and Tukey's tests were used to compare the flexural strength results (p<0.05). RESULTS: There were significant differences in survival during the fatigue test among the groups (p<0.001). Treatment (control or bleach) was a significant factor for specimen survival (p<0.001, Exp(B) = 33.45). There were significant differences in mean flexural strength (p<0.001). No significant difference was found between "1-wk control" and "4-wk control". The mean flexural strength and fatigue resistance of the "4-wk bleach" were significantly lower than all the other groups. CONCLUSIONS: The use of whitening strips reduced the fatigue resistance and flexural strength of bovine dentin in vitro. Until the effect of whitening strips on mechanical properties of human dentin is fully elucidated, it remains prudent to advise patients to avoid excessive direct use of whitening strips on dentin.

Effect of accelerated aging on dental zirconia-based materials.

OBJECTIVE: To investigate the effect of aging on phase transformation and mechanical properties of yttria-tetragonal zirconia polycrystal (Y-TZP). MATERIALS AND METHODS: Fully-sintered Y-TZP slabs, IPS E-max ZirCAD (ZC - Ivoclar) and Z-5 ceramic (Z5 - C5 Medical Werks), were artificially aged in autoclave for: 0, 30, 60 or 90min. Flexural strength (FS), crystalline changes (X-ray diffraction analysis - XRD) and surface topography were analyzed. 0 and 90min-aged samples were evaluated by nanoindentation to measure hardness and modulus, and results were compared using Wilcoxan Mann Whitney rank sum test (p≤0.05). FS results were compared using two-way ANOVA and Tukey HSD (α=0.05). RESULTS: Material factor had significant effect (p=0.001) on flexural strength (Z5=966.95MPa; ZC=847.82MPa), but aging did not. Nanoindentation showed incidence of typical load/depth curves combined with some exhibiting features compatible with cracking. When typical curves were considered, aging had no effect on the modulus and hardness, but hardness was dependent on material type. A steady increase in the m phase related to aging time was observed for ZC samples. The maximum incidence of m phase was 6.56% for Z5/60min. SIGNIFICANCE: Flexural strength is not affected by surface transformation in dental Y-TZP. Hydrothermal aging has an effect on m content and surface topography of different zirconia brands, but mechanical tests that can precisely characterize surface changes in aged Y-TZP are still missing.

Matrix metalloproteinase inhibitor modulates esterase-catalyzed degradation of resin-dentin interfaces.

OBJECTIVE: Assess the modulating effect of matrix metalloproteinase (MMP) inhibition on simulated human salivary enzyme (SHSE)-catalyzed degradation of interfacial fracture-toughness (FT) of self-etched and total-etched resin-dentin interfaces. METHODS: Miniature short-rod FT specimens (N=10/group) containing a resin composite bonded to human dentin, using a self-etch (Easy Bond, EB) or a total-etch (Scotchbond, SB) adhesives, were prepared with and without application of an MMP inhibitor (galardin). Specimens were non-incubated or incubated in phosphate buffered saline (PBS) or SHSE for 7, 30, 90, or 180-days. FT data were obtained using a universal testing machine. Incubation media were analyzed by high performance liquid chromatography (HPLC) for the presence of a 2,2-bis-[4-2(2-hydroxy-3-methacryloxypropoxy)phenyl]-propane (bisGMA)-derived degradation product, bis-hydroxy-propoxy-phenyl-propane (bisHPPP). Fractographic analysis was performed by scanning electron microscopy and image processing software (ImageJ). Statistical analysis was performed by ANOVA and Tukey's (p<0.05). RESULTS: More bisHPPP was detected in SHSE vs. PBS for both adhesive systems (p<0.05). EB specimens yielded no difference in FT and failed preferentially in the resin after >30-days (p<0.05). SB specimens yielded lower FT values after 180-days with SHSE ±galardin vs. 0-days/no-galardin (p<0.05) and failed preferentially in the hybrid-layer after >30-days (p<0.05). Galardin mildly modulated the change in fracture mode for both systems. SIGNIFICANCE: Esterase-catalyzed degradation of total-etch interfaces is modulated by MMP-inhibition, however, self-etch interfaces possess greater biostability under simulated intra-oral conditions, regardless of MMP inhibition. This could be related to different chemical compositions and/or mode of adhesion.

Surface and Mechanical Characterization of Dental Yttria-Stabilized Tetragonal Zirconia Polycrystals (3Y-TZP) After Different Aging Processes.

Yttria-stabilized tetragonal zirconia polycrystals (3Y-TZP) is a ceramic material used in indirect dental restorations. However, phase transformation at body temperature may compromise the material's mechanical properties, affecting the clinical performance of the restoration. The effect of mastication on 3Y-TZP aging has not been investigated. 3Y-TZP specimens (IPS E-max ZirCAD and Z5) were aged in three different modes (n=13): no aging (control), hydrothermal aging (HA), or chewing simulation (CS). Mechanical properties and surface topography were analyzed. Analysis of variance showed that neither aging protocol (p=0.692) nor material (p=0.283) or the interaction between them (p=0.216) had a significant effect on flexural strength, values ranged from 928.8 MPa (IPSHA) to 1,080.6 MPa (Z5HA). Nanoindentation analysis showed that material, aging protocol, and the interaction between them had a significant effect (p<0.001) on surface hardness and reduced Young's modulus. The compositional analysis revealed similar yttrium content for all the experimental conditions (aging: p=0.997; material: p=0.248; interaction material×aging: p=0.720). Atomic force microscopy showed an effect of aging protocols on phase transformation, with samples submitted to CS exhibiting features compatible with maximized phase transformation, such as increased volume of the material microstructure at the surface leading to an increase in surface roughness.

Different Strategies to Bond Bis-GMA-based Resin Cement to Zirconia.

PURPOSE: To evaluate the effect of different bonding strategies on short- and long-term bis-GMA-based resin cement bond strengths to zirconia. MATERIALS AND METHODS: One hundred twenty samples of fully-sintered zirconia (Prettau Zirconia) were sandblasted and randomly distributed into 5 groups (n = 24): UA: Scotchbond Universal Adhesive; SZP: Signum Zirconia Bond I + II; ZPP: Z-Prime Plus; EXP: MZ experimental primer; CO: no primer application (control). After performing these surface treatments, translucent tubes (0.8 mm diameter and 1.0 mm height) were placed on the zirconia specimens, and bis-GMA-based cement (Duo-Link) was injected into them and light cured. Specimens were tested for microshear bond strength either 24 h or 6 months (m) after water storage (37°C) and surfaces were characterized by SEM and EDX. Two-way ANOVA and Tukey's post-hoc test (p < 0.05) were used to evaluate bond strength results. RESULTS: The mean bond strengths (MPa ± SD) were: UA=14.6 ± 4.7a (24 h); 16.0 ± 4.8a (6 m); SZP = 14.0 ± 5.4ab (24 h); 11.9 ± 2.6ab (6 m); ZPP=8.0 ± 1.8b (24 h); 8.6 ± 3.3b (6 m); EXP = 1.2 ± 0.5c (24 h); 0.6 ± 0.7c (6 m); CO=1.0 ± 1.2c (24 h); 1.3 ± 1.2c (6 m). Bonding strategy significantly influenced bond strength (p = 0.0001), but storage time did not (p = 0.841). Groups UA and SZP showed a homogeneous layer covering the zirconia surface. In these groups, EDX demonstrated the presence of phosphorus. Group ZPP resulted in a nonhomogeneous layer, exposing the zirconia substrate underneath. EXP application resulted in an undetectable layer. CONCLUSION: Water storage did not affect resin cement bond strengths to zirconia irrespective of the surface treatment. Bis-GMA-based resin cement bond strengths to zirconia are affected by specific bonding strategies.