Stress distribution on teeth restored with veneers using various incisal preparation designs: A 3D finite element analysis study.

STATEMENT OF PROBLEM: The type of veneer preparation is often chosen according to the patient's tooth structure and occlusion. Taking biomechanics into account in this decision-making process provides the clinician with more technical information on how to improve the clinical longevity of restorations. However, biomechanical analyses of veneer preparation designs are sparse. PURPOSE: The purpose of this 3-dimensional (3D) finite element analysis with microcomputed tomography (µCT) data obtained from realistic models was to assess the influence of different preparations for ceramic and composite resin veneers on restoration and resin layer stress distribution. MATERIAL AND METHODS: Four replicas of a central incisor were printed and prepared for laminate veneers with 4 different incisal edge designs: shoulder (SH), palatal chamfer (PC), palatal chamfer and oblique fracture involving the distal angle (OF-PC), and palatal chamfer involving horizontal incisal fracture (IF-PC). After fabrication and cementation of the veneers, the restored replicas were assessed with µCT, and 3D finite element models were built. A 100-N load was applied on the palatal surface at 60 and 125 degrees relative to the longitudinal axis. Maximum principal stress and stress distribution on the veneers, cement layer, and tooth structure were calculated and analyzed. RESULTS: The SH preparation exhibited better stress distribution than the PC preparation, and the cement layer and the veneer were subjected to lower stress. The IF-PC preparation had better stress distribution than the OF-PC. The shoulder and IF-PC showed higher stress on laminate veneers, but lower stress on the cement layer. Ceramic veneers exhibited lower stress than composite resin veneers. CONCLUSIONS: The different incisal preparations for laminate veneers influenced stress distribution on restorations and on the resin cement layer. The shoulder type preparation showed better stress distribution and the composite resin veneers showed unfavorable results compared with the ceramic veneers.

High-viscosity glass-ionomer cement or composite resin for restorations in posterior permanent teeth? A systematic review and meta-analyses.

OBJECTIVES: Answer the PICO question: Do class I and II posterior restorations in permanent teeth placed with high-viscosity glass-ionomer cement (HV-GIC) fail more than composite resin (CR) restorations? DATA: The study was registered in the PROSPERO database (CRD42020138290). Randomized and controlled clinical trials, comparing the performance of HV-GIC and CR in load bearing cavities of posterior permanent teeth were included. Cochrane risk of bias tool and GRADE were used to assess the quality and certainty of the evidence. Meta-analyses were performed for clinical outcomes on USPHS and FDI criteria for 12-, 24- and 36-months follow-ups. SOURCES: PubMed, Scopus and Web of Science were last searched on April 2, 2022, without language or date restrictions. Reference lists of primary studies and their related article link in PubMed were manually searched. STUDY SELECTION: Ten studies were included, while data from 8 were used for the meta-analyses. A total of 849 HV-GIC and 800 CR restorations were followed. The primary outcome was the fracture/retention of the restoration, with a comparable performance for both materials on all follow-ups. The 36 months follow-up for class I restorations (longest) showed risk difference of -0,00 (95%CI -0,03 to 0,03; p = 0,98) and no heterogeneity (p = 0,98, I2=0%). The certainty of the evidence is moderate, as all included studies were at an uncertain risk of bias. CONCLUSIONS: HV-GIC and CR presented comparable clinical performance in posterior permanent teeth up to 36 months. HV-GIV wear in class I restorations followed by 24 months was the only poorer result compared to CR. CLINICAL SIGNIFICANCE: Conservative load bearing cavities in permanent posterior teeth can be restored with HV-GIC with comparable clinical performance to CR expected at least up to 3 years. HV-GIC is a valuable direct restorative option for posterior teeth in high caries risk patients, in which CR is frequently associated with failure.

Relationship between battery level and irradiance of light-curing units and their effects on the hardness of a bulk-fill composite resin.

Objectives: This study evaluated the relationship between the battery charge level and irradiance of light-emitting diode (LED) light-curing units (LCUs) and how these variables influence the Vickers hardness number (VHN) of a bulk-fill resin. Materials and Methods: Four LCUs were evaluated: Radii Plus (SDI), Radii-cal (SDI), Elipar Deep Cure (Filtek Bulk Fill, 3M Oral Care), and Poly Wireless (Kavo Kerr). Irradiance was measured using a radiometer every ten 20-second activations until the battery was discharged. Disks (4 mm thick) of a bulk-fill resin (Filtek Bulk Fill, 3M Oral Care) were prepared, and the VHN was determined on the top and bottom surfaces when light-cured with the LCUs with battery levels at 100%, 50% and 10%. Data were analyzed by 2-way analysis of variance, the Tukey's test, and Pearson correlations (α = 5%). Results: Elipar Deep Cure and Poly Wireless showed significant differences between the irradiance when the battery was fully charged versus discharged (10% battery level). Significant differences in irradiance were detected among all LCUs, within each battery condition tested. Hardness ratios below 80% were obtained for Radii-cal (10% battery level) and for Poly Wireless (50% and 10% battery levels). The battery level showed moderate and strong, but non-significant, positive correlations with the VHN and irradiance. Conclusions: Although the irradiance was different among LCUs, it decreased in half of the devices along with a reduction in battery level. In addition, the composite resin effectiveness of curing, measured by the hardness ratio, was reduced when the LCUs' battery was discharged.

In-office dental bleaching with violet light emitting diode: bleaching efficacy and pulpal temperature rise.

Objectives: This study evaluated the bleaching efficacy of different in-office protocols associated with violet light emitting diode (V-LED), and measured the pulpal temperature rise caused by V-LED with or without gel application. Materials and Methods: Bovine incisors were distributed in 4 groups (n = 10): VL - V-LED; HP - 35% hydrogen peroxide (control); HYB - hybrid protocol, V-LED applied without gel for 10 irradiation cycles followed by V-LED applied with gel for another 10 irradiation cycles; and HPVL - gel and V-LED applied for 20 irradiation cycles. Three bleaching sessions were performed with 7-day intervals. Bleaching efficacy was evaluated with Δ E a b * , ΔE 00 and ΔWID . Data were recorded at baseline, 7, 14, 21 and 70 days. For pulpal temperature rise, thermocouples were placed inside the pulp chamber of human incisors. To determine intrapulpal temperature, the teeth were irradiated with V-LED with or without application of bleaching gel. Color difference data were analyzed by 2-way repeated measures ANOVA and Tukey's test. Pulpal temperature was analyzed by t-test (α = 5%). Results: VL exhibited lower color ( Δ E a b * and ΔE 00) and whiteness changes (ΔWID ) than the other groups. HPVL presented higher color change values than HYB. HYB and HPVL showed not different ΔWID values; and HP showed the highest whiteness changes at all times. There were significant differences comparing ΔT with gel (8.9°C) and without gel application (7.2°C). Conclusions: HPLV was more efficient than HYB. The 2 protocols with VL showed similar results to control. Gel application combined with VL promoted higher pulpal temperature than to the no gel group.

Is the clinical performance of composite resin restorations in posterior teeth similar if restored with incremental or bulk-filling techniques? A systematic review and meta-analysis.

OBJECTIVES: A systematic review was performed to analyze the clinical performance of class I and II restorations in posterior teeth placed with the incremental or the bulk-filling techniques. The primary outcome was retention/fracture rate, and the secondary outcomes evaluated were anatomical form, surface texture, color match, marginal adaption, marginal discoloration, caries, and postoperative sensitivity. METHODS: Electronic and manual searches were performed for randomized clinical trials comparing the clinical performance of composite resin restorations in posterior teeth placed with the incremental or the bulk-filling techniques. The Cochrane Collaboration risk of bias tool was used to assess the quality of the studies and the GRADE tool was used to access the quality of the evidence. RESULTS: Fourteen studies were included in this systematic review and most of them had unclear risk of bias. The risk difference (RD) for retention/fracture was 0.00 (95%CI = - 0.01, 0.01; p = 0.86) for 1-1.5 years of follow-up; 0.00 (95%CI = - 0.02, 0.02; p = 0.88) for 2-3 years of follow-up; 0.05 (95%CI = - 0.08, 0.18; p = 0.46) for 5 or more years of follow-up. The RD for postoperative sensitivity was 0.04 (95%CI = - 0.02, 0.10; p = 0.18) for up to 30 days; 0.00 (95%CI = - 0.01, 0.02; p = 0.63) for 1-1.5 years of follow-up; and 0.00 (95%CI = - 0.01, 0.02; p = 0.71) for 2-3 years of follow-up. For the other secondary outcomes, no significant differences were observed (p > 0.05) between the restorative techniques. The certainty of evidence was graded as moderate. CONCLUSIONS: The clinical performance of class I and II restorations in posterior teeth is similar when placed with the incremental and bulk-filling techniques. CLINICAL RELEVANCE: Based on the results of this study, posterior restorations placed with bulk-filling technique present satisfactory clinical performance, which is similar to direct restorations placed with the conventional incremental technique, considering various follow-up periods evaluated. TRIAL REGISTRATION:  CRD42018108450.

Extended glaze firings for porcelain-veneered zirconia: Effects on the mechanical and optical behavior.

OBJECTIVE: This study evaluated the effect of dwell time (conventional or extended) and cooling protocol (fast or slow) of self-glaze firings on the mechanical (flexural strength and crack propagation) and optical (color and translucency) properties of a porcelain-veneered zirconia system. METHODS: Bilayer disc-shaped samples were prepared (Vita VM9 + In-Ceram YZ) and divided according to the final thermal treatment: glaze firing followed by slow cooling (furnace opening at 200 °C) (G-S) or fast cooling (furnace opening at 600 °C) (G-F, manufacturer-recommended protocol), extended glaze firing (15 min of dwell time) followed by slow cooling (EG-S) or fast cooling (EG-F), or no thermal treatment (CTRL). Porcelain roughness (Ra and Rz) was measured before and after glaze firings. Color (ΔE00) and translucency (TP00) alteration were also evaluated. Flexural strength was measured with the piston-on-three-ball test and crack propagation analysis was performed after Vickers indentations. Complementary analyzes of crystalline phase and scanning electron microscopy were carried out. RESULTS: Significant effect of dwell time was observed, with extended glaze leading to higher flexural strength and shorter crack lengths. Cracks of EG groups were observed to end in clusters of crystals. Color and translucency changed below perceptibility thresholds. All treatments led to a smoother surface and EG groups reached the lowest Rz values. An extra SiO2 peak was revealed in control and EG groups. No effect of cooling protocol was found. SIGNIFICANCE: Extended glaze firing was able to improve the resistance to crack initiation and propagation of porcelain-veneered zirconia without clinically perceptible changes in optical properties.

Novel speed sintered zirconia by microwave technology.

OBJECTIVE: Continuous efforts have been made to hasten the zirconia densification process without compromising properties. This study evaluated the long-term structural durability of microwave speed-sintered zirconia (MWZ) relative to a conventionally sintered zirconia (CZ). METHODS: As-machined dental 3Y-TZP discs (Ø12 × 1.2 mm) were speed sintered at 1450 °C for 15 min using an industrial microwave oven, while conventional sintering was conducted in a standard dental furnace at 1530 °C for 2 h. Both were followed by natural cooling. The total sintering time was 105 min for MWZ and 600 min for CZ. Groups were compared regarding density, grain size, phase composition, and fracture resistance. Structural durability was investigated employing two fatigue protocols, step-stress and dynamic fatigue. RESULTS: Compared to CZ, MWZ exhibited a slightly lower density (MWZ = 5.98 g/cm3, CZ = 6.03 g/cm3), but significantly smaller grain sizes (MWZ = 0.53 ± 0.09 µm, CZ = 0.89 ± 0.10 µm), lower cubic-zirconia contents (MWZ = 15.3%, CZ = 22.7%), and poorer translucency properties (TP) (MWZ = 13 ± 1, CZ = 29 ± 0.8). However, the two materials showed similar flexural strength (MWZ = 978 ± 112 MPa, CZ = 1044 ± 161 MPa). Additionally, step-stress testing failed to capture the fatigue effect in 3Y-TZP, whereas dynamic fatigue revealed structural degradation due to moisture-assisted slow-crack-growth (SCG). Finally, MWZ possessed a slightly higher Weibull modulus (MWZ = 7.9, CZ = 6.7) but similar resistance to SCG (MWZ = 27.5, CZ = 24.1) relative to CZ. SIGNIFICANCE: Dental 3Y-TZP with similar structural durability can be fabricated six-times faster by microwave than conventional sintering.

Damage sensitivity of dental zirconias to simulated occlusal contact.

OBJECTIVE: Mechanical damages can occur from dental restoration processing and fitting, or while it is in-service. This study evaluates the damage sensitivity of translucent zirconia (5Y-PSZ) relative to conventional 3Y-PSZ following mouth-motion simulations at various loads. METHODS: 5Y-PSZ and 3Y-PSZ discs were adhesively bonded to a dentin-like substrate and divided into groups according to the load (50 N or 200 N) and number of cycles (up to 106) used in the chewing simulation. Specimens were mounted with 30° inclination in an electrodynamic mouth-motion simulator, and subjected to contact-slide-liftoff cyclic loading in water. Surface and sub-surface damages were analyzed using a sectioning technique. After the simulation, specimens were removed from the substrate and loaded with the damaged surface in tension for biaxial strength testing to assess their damage tolerance. RESULTS: The strength of both ceramics underwent significant degradation after mouth-motion simulations. For 5Y-PSZ, the strength degradation was greater (∼60%) and occurred at a lower number of cycles than 3Y-PSZ. Herringbone cracks emerged on 3Y-PSZ and 5Y-PSZ surfaces under a 200-N load after 50 and 10 cycles, respectively. Meanwhile at a 50-N load, cracks formed at ∼1000 cycles in both ceramics. Further increasing the number of cycles only had moderate effects on the strength of both ceramics, despite an increase in surface and sub-surface damage. More significantly, a 50-N occlusal load can debase the zirconia strengths as much as a 200-N load. SIGNIFICANCE: Surface flaws produced during the chewing simulation are capable of significant strength degradation in zirconia, even after a small number of low-load cycles.

Wear behavior and microstructural characterization of translucent multilayer zirconia.

OBJECTIVE: To characterize the composition, microstructure and wear properties of a multilayer translucent zirconia relative to the conventional 3Y-TZP. METHODS: Two types of ceramics were evaluated: a multilayer zirconia (MULTI, IPS e.max ZirCAD Multi, Ivoclar Vivadent) and a control 3Y-TZP (IPS e.max ZirCAD LT, Ivoclar Vivadent). Pre-sintered CAD-CAM blocks were cut, ground, sintered and polished to 1 µm finish. The phase fraction and grain size were measured using XRD and FE-SEM. For wear testing (n = 12), square-shaped specimens (16 × 16 × 1 mm) were adhesively bonded to a dentin analog. Sliding wear tests were performed using a spherical zirconia antagonist (r = 3.15 mm), with 30 N load at 1.5 Hz for 500,000 cycles in water. Optical and scanning electron microscopes and 3D laser scanner were used for quantitative wear analyses. Data were analyzed using Student's t-test (α = 0.05). RESULTS: For MULTI, the enamel layer had the highest cubic content and the largest grain size, followed by the two transition layers, and the dentin layer. 3Y-TZP showed the smallest grain size and cubic content. A significant amount of wear was observed in both materials up to 50,000 cycles until it reached a plateau. MULTI showed higher volume loss and greater wear depth than 3Y-TZP (p < 0.01). The higher volume loss was associated with extensive lateral fracture, leading to material spalling from the surface of cubic-containing zirconias. SIGNIFICANCE: The wear pattern in multi-layered zirconia was more severe than 3Y-TZP. Additionally, the different layers of the multi-layered zirconia had similar wear behavior.

The bleaching efficacy of carbamide peroxide gels containing potassium nitrate desensitizer.

BACKGROUND: To evaluate the bleaching efficacy of at-home carbamide peroxide (CP) gels in two concentrations, containing or not a desensitizing agent. MATERIAL AND METHODS: Forty incisors were divided into four groups (n=10), according to gel concentrations (10% or 22%), and presence or not of 3% potassium nitrate in the gel. A thin layer of gel was applied to the buccal surface of each tooth for 2h/day for 4 weeks. Bleaching efficacy was measured using a spectrophotometer, and ∆E*ab, ∆E00 and ∆WID were calculated. Measurements were performed at baseline, 7, 14, 21, 28, and 35 days following the first gel application. Data were analyzed by two-way RM-ANOVA and Tukey's test (α=0.05). RESULTS: Regarding gel concentration or potassium nitrate inclusion, both gels resulted in color change above the perceptibility thresholds, which were similar between gels. Regarding time, significant differences were observed between color change values at 7 days and other time periods. ∆WID ranged from 3.8 to 9.6. Significant moderate to strong positive correlation was observed among the parameters. CONCLUSIONS: Nor the CP concentration, neither the inclusion of potassium nitrate in the gel, had influence on bleaching efficacy. All gels were effective and showed good results from the first weeks' application. Key words:Tooth bleaching, carbamide peroxide, desensitizer, potassium nitrate, color.

Cytotoxicity of contemporary resin-based dental materials in contact with dentin.

In this study, the cytotoxicity of different combinations of contemporary resin-based restoratives (adhesives, composites, luting agents) against human keratinocytes (HaCaT) was evaluated under two conditions, whether materials were applied to dentin or not. Adhesives (3-step etch-and-rinse/3ER: OptiBond FL; 2-step self-etch/2SE Clearfil SE Bond; Single Bond Universal/UNI), composites (conventional composite resin/CCR: Filtek Z350XT; flowable/FCR: Filtek Z350XT Flow; self-adhesive composite resin/SACR: Dyad Flow), and luting agents (conventional luting agent/CLA: Variolink-II; self-adhesive luting agent/SLA: RelyXU200) were combined according to their clinical use. Eluates from polymerized specimens applied to dentin were placed in contact with cells grown for 1 and 7 d. The controls were defined by cells without material contact. Cell viability was determined using MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide)] assay. C=C conversion was investigated using Fourier-transform infrared spectroscopy. After 1 d of incubation, when dentin was not present, 2SE yielded the highest cell viability, whereas 3ER, UNI, and SACR showed higher cell viability in the presence of dentin. After 7 d, when dentin was absent, 2SE and CLA achieved significantly higher cell viability. The presence of dentin resulted in a drastically higher cell viability for all materials, except 2SE and CLA. UNI had the lowest C=C conversion. The presence of dentin was a significant factor, which resulted in higher cell viability than what was seen for the material specimens per se. All materials resulted in a lower viability of HaCaT than what was seen under the no-material control conditions, with effects mainly limited to the first 24 h.

Non-silicate nanoparticles for improved nanohybrid resin composites.

OBJECTIVE: Zirconia and alumina nanoparticles were coated with a silica-rich layer (ALSI and ZRSI) and used to prepare experimental nanohybrid resin composites, which were characterized and compared to a control commercial resin composite (Filtek Z350 XT). METHODS: Silica nanoparticles with sizes compatible to ALSI (Aerosil 150) and ZRSI (Aerosil OX 50) were tested as references. The volume of nanoparticles was equivalent across the composites, which also had consistent content of glass microparticles. CC conversion, viscosity, depth of cure, surface topography, hardness, opacity, radio-opacity, and edge chipping resistance (ReA) were tested after 24 h. Flexural strength (σf) and fracture toughness (KIC) were also tested after 15 K thermal cycles. Data were analyzed using one-way or two-way ANOVA and Tukey's test (α = 0.05). RESULTS: ALSI and ZRSI yielded resin composites with lower viscosity and more irregular nanoagglomerates compared to nanosilica-based composites. CC conversion and depth of cure were lower for ZRSI composite, which had higher opacity, radio-opacity, and hardness. ReA was higher for ALSI composite. Composites with ALSI and ZRSI showed stable σf after aging, whereas the control and Aerosil 150 resin composites showed significant degradation. The commercial and nanosilica-based composites showed up to 42% reduction in KIC after aging, whereas resin composites with ZRSI and ALSI showed a more stable KIC. SIGNIFICANCE: ALSI and ZRSI generated nanohybrid resin composites with improved and/or more stable physical properties compared with nanosilica-based and commercial composites. This study suggests that changing the composition of nanofillers is a simple method to enhance the performance of nanohybrid composites.

Flexural strength and crystalline stability of a monolithic translucent zirconia subjected to grinding, polishing and thermal challenges.

The objective of this study is to investigate the magnitude of structural degradation of a monolithic translucent zirconia caused by clinically relevant grinding and polishing procedures, when associated or not with low temperature degradation (LTD), induced by accelerated hydrothermal aging using autoclave or thermocycling Ninety disks (Ø12 × 1 mm) were prepared from dental zirconia for monolithic restorations (Vipi Block Zirconn Translucent, Vipi). The specimens were divided into 3 groups (n = 30) according to surface treatment: As Sintered (untreated), Grind (diamond bur), Grind + Polish (diamond bur + polish); and then subdivided according to aging method (n = 10): Baseline (no aging), Autoclave (134°C, 2.2 kgf/cm2 pressure for 5 h), and Thermocycling (200,000 cycles, 5°C and 55°C, for 15 s each). Roughness, biaxial flexural strength and percentage of monoclinic phase were evaluated. Regarding surface treatment, the Grind group presented higher roughness and greater flexural strength compared to As Sintered group, while Grind + Polish showed intermediate roughness and flexural strength similar to Grind group. Aging had little effect on roughness, but yielded a significant reduction in flexural strength. Tetragonal to monoclinic phase transformation was observed in all groups, caused by both mechanical stresses (grinding and polishing) and LTD, which was similarly induced by the traditional autoclave method, as well as the thermocycling method The use of diamond burs to grind zirconia surface may result in deleterious effects on the surface quality of monolithic zirconia restorations, yet has a potential toughening effect by phase transformation. However, when zirconia is exposed to LTD, regardless of the surface treatment, degradation of the surface quality and strength are observed.

Probing the interfacial strength of novel multi-layer zirconias.

OBJECTIVES: The rapidly increasing use of zirconia-based CAD/CAM multi-layer structures in dentistry calls for a thorough evaluation of their mechanical integrity. This work examines the effect of the multi-layering architecture as well as variations in composition and inclusion of pigments among the layers on the flexural strength of multi-layer zirconias. METHODS: A modified 4-point bending test, aided by a Finite Element Analysis (FEA), was used to probe the interfacial strength of 3 classes of yttria-partially-stabilized zirconia: Ultra Translucent Multi-Layer (UTML-5Y-PSZ), Super Translucent Multi-Layer (STML-4Y-PSZ), Multi-Layer (ML-3Y-PSZ). In accord with the size limitation (22-mm height) of CAD/CAM pucks, test samples were prepared in the form of "long" (25×2×3mm) and "short" (17.8×1.5×2mm) beams. Homogeneous beams (both long and short) were produced from either the Enamel (the lightest shade) or Dentin (the darkest shade) layer, whereas multi-layer beams (short beam only) were obtained by cutting the pucks along their thickness direction, where the material components of various shades were stacked. RESULTS: The Enamel and Dentin layers exhibited similar flexural strength for a given material class, with ML amassing the highest strength (800-900MPa) followed by STML (560-650MPa) and UTML (470-500MPa). The 3 classes of multi-layer zirconia showed a trade-off between strength and translucency, reflecting different yttria contents in these materials. The failure stress of the cross-sectional multi-layer beams was, however, ∼30% lower than that of their Enamel or Dentin layer counterparts, regardless of material tested. SIGNIFICANCE: The weakness of interfaces is a drawback in these materials. Additionally, when measuring strength using short beam flexure, friction between the specimen and supporting pins and accuracy in determining loading span distances may lead to major errors.

Radiofrequency therapy in esthetic dermatology: A review of clinical evidences.

BACKGROUND: Chronological skin aging causes the modification of genetic material through enzymes and proteins changes. The process reduces cellular proliferation, along with loss of tissue elasticity, reduced ability to regulate aqueous exchanges, and inefficient tissue replication. Appearance is negatively affected by cumulative changes in coloration, texture, and elasticity over time. The increase in the population's average life expectancy boosts the search for cosmetic therapies that can delay aging, mostly for the noninvasive modalities. Among the various options, radiofrequency therapy is a technique that can help reduce the effects of skin aging. AIM: Therefore, this study aims to review clinical evidence provided by scientific literature on the benefits of using radiofrequency therapy in reducing skin aging effects. METHODS: A review of the literature concerning skin aging, characteristics of radiofrequency therapy, and radiofrequency therapy in the treatment of skin laxity and mechanism of action was conducted using PubMed. RESULTS: The included studies have suggested that the mechanism of radiofrequency action is heating the dermis while preserving the epidermis. This heating causes immediate collagen denaturation, which is followed by the formation of new collagen, naturally providing skin tightening and greater elasticity. CONCLUSION: Even when used as single therapeutic modality, radiofrequency seems to meet the expectations in reducing the effects of skin aging.

Zirconia-reinforced lithium silicate crowns: Effect of thickness on survival and failure mode.

OBJECTIVE: To evaluate the reliability and failure mode of zirconia-reinforced lithium silicate (ZLS) molar crowns of different thicknesses. METHODS: Monolithic ZLS molar crowns (0.5mm, 1.0mm, and 1.5 mm thickness) were modeled and milled using a CAD/CAM system (n = 21/group). Crowns were cemented on dentin-like epoxy resin replicas with a resin cement. The specimens were subjected to single load-to-failure test for step-stress profiles designing. Mouth-motion step-stress accelerated-life test was performed under water by sliding an indenter 0.7 mm lingually down on the distobuccal cusp until specimen fracture or suspension. Use level probability Weibull curves and reliability were calculated and plotted. Polarized-light optical microscope and scanning electron microscope (SEM) were used to characterize fracture patterns. RESULTS: Irrespective of crown thickness, beta (ß) values were higher than 1 and fatigue accelerated failures. While 0.5 mm ZLS crowns exhibited a significant reduction in the probability of survival at 200N, 300N and 400 N mission loads (69%, 41% and 19%, respectively), no significant difference was observed between 1.0 mm and 1.5 mm crowns. Both thicknesses have maintained the survivability at approximately 90%. Failure primarily comprised bulk fracture where radial cracks originated from the cementation surface beneath the indenter loading trail and propagated towards the cervical margin. SIGNIFICANCE: 1.5 mm- and 1.0 mm-thickness monolithic ZLS crowns presented higher probability of survival compared to 0.5 mm crowns. Bulk fracture was the chief failure mode, regardless of thickness.

New multi-layered zirconias: Composition, microstructure and translucency.

OBJECTIVES: To fully realize the range of indication and clinical advantages of the new multi-layered zirconias, a comprehensive understanding of their chemical composition, microstructure, low temperature degradation (LTD) resistance, and translucency properties is paramount. METHODS: A zirconia system (Katana, Kuraray Noritake), including 3 distinct grades of multi-layered zirconias, was selected for study: Ultra Translucent Multi-layered zirconia (UTML), Super Translucent Multi-layered zirconia (STML), and Multi-layered zirconia (ML). For different materials and their individual layers, the chemical composition, zirconia phase fractions, and microstructure were determined by X-ray fluorescence (XRF), X-ray diffraction (XRD), and field emission scanning electron microscopy (FE-SEM). Also, their resistance to LTD and translucency properties were characterized. RESULTS: Our findings revealed no major differences amongst layers, but the 3 materials were very distinct-UTML: 5Y-PSZ (5 mol% yttria-partially-stabilized zirconia) with ˜75 wt% cubic content and a 4.05 (±0.85) µm average grain size, STML: 4Y-PSZ with ˜65 wt% cubic content and a 2.81 (±0.17) µm average grain size, and ML: 3Y-PSZ with <50 wt% cubic content and a 0.63 (±0.03) µm average grain size. After water aging at 120 °C for 12 h, greater monoclinic content was found in ML. UTML and STML did not show detectable monoclinic phase. The translucency was similar among layers, and also between UTML and STML, which were superior to ML. SIGNIFICANCE: For each multi-layered zirconia grades, the layers are only differed in pigment types and contents, which yield remarkably natural shade gradients. Also, despite the significant compositional difference between STML and UTML, both materials showed similar translucencies.

The progressive wear and abrasiveness of novel graded glass/zirconia materials relative to their dental ceramic counterparts.

OBJECTIVE: To investigate the wear behavior of novel graded glass/zirconia materials and their abrasiveness to the antagonist relative to homogeneous zirconias (polished or glazed) and a glass-ceramic. METHODS: Graded glass/zirconia specimens were prepared by sintering with concurrent glass-infiltration of pre-sintered zirconia (3Y-TZP) with a polished or as-machined surface. Monolithic zirconia samples were sintered and their surfaces were polished or glazed (as-machined). Glass-ceramic samples were obtained and the surface polished. All specimens were subjected to chewing simulations with a steatite antagonist (r = 3 mm) and a cyclic load of 50 N. Quantitative measurements of wear and roughness were performed on ceramics and antagonists for prescribed number of cycles. Damage sustained in ceramics and antagonists was analyzed by SEM. RESULTS: The polished zirconia presented little to no variation in wear depth (2 µm) and roughness (0.06 µm). Graded and glazed zirconia experienced a rapid increase in wear depth while the superficial glass layer was present (until 1000 cycles), but showed little variations afterwards - at 450k cycles ∼15 µm for graded and 78 µm for glazed zirconia. The glass-ceramic presented the greatest wear depth (463 µm) and roughness (1.48 µm). Polished zirconia, polished graded zirconia and glazed zirconia yielded significantly lower volumetric wear (∼3 mm3) of the antagonist than as-machined graded zirconia and glass-ceramic (∼5 mm3). SIGNIFICANCE: Polished graded zirconia and polished zirconia presented little wear and roughness, as well as yielded reduced antagonist wear. Glassy materials are both more susceptible to wear and more abrasive to the antagonist relative to zirconia.

Do thermal treatments affect the mechanical behavior of porcelain-veneered zirconia? A systematic review and meta-analysis.

OBJECTIVES: A systematic review of in vitro studies was conducted to assess the effect of thermal treatments on flexural strength or critical load to failure of porcelain-veneered zirconia (PVZ). SOURCES: Literature searches were performed up to June 2018 in PubMed/MEDLINE, Scopus and Web of Science databases, with no publication year or language limits. DATA: From 393 relevant studies, 21 were selected for full-text analysis, from which 7 failed to meet the inclusion criteria. The 14 remaining papers were included in the systematic review: 8 for meta-analysis and 6 restricted to descriptive analyses. Hand searching of reference lists resulted in no additional papers. STUDY SELECTION: In vitro studies using PVZ specimens testing the influence of thermal treatments on the fracture resistance to monotonic or cyclic loading. Papers evaluating cooling rate were divided into those applying fast cooling from above the porcelain glass transition temperature (Tg), or from below it. Meta-analyses were performed separately for flexural strength and critical load to failure, using random effects at a 5% significance level. CONCLUSIONS: Delaying furnace opening at a temperature below the porcelain Tg is advised for PVZ restorations, in order to improve their fracture resistance. Additional information is required to confirm the apparent beneficial effect of self-glaze and repeated veneer firings on the mechanical properties of these restorations. Finally, in order to obtain conclusive and relevant evidence regarding thermal treatments and the fracture resistance of PVZs, future studies should concentrate on anatomically-correct crown specimens.

Accurate and Efficient Thermal Stress Analyses of Functionally Graded Solids Using Incompatible Graded Finite Elements.

Functionally graded materials have found a wide usage in high temperature applications. The smooth transition from one material to another, in graded materials, may reduce thermal stresses, residual stresses and stress concentration factors as well as utilize properties of both materials. To perform accurate and efficient finite element analysis for heat transfer and transient thermal stress analyses in two-dimensional functionally graded materials, incompatible graded finite elements are developed and verified. User-defined subroutines in ABAQUS are developed to address the gradation of material properties within an element. An emphasis is made on an incompatible graded finite element (QM6) which is accurate and efficient compared to linear four-node (Q4) and quadratic eight-node (Q8) elements. With the help of posteriori error estimation, a critical comparison is made among three types of solid elements. Modified 6-node (QM6) incompatible graded elements provide better accuracy than Q4 elements and take less computational time than Q8 elements, thereby showing QM6 as an optimal element for engineering analysis.