Optical properties of dental zirconia, bovine dentin, and enamel-dentin structures.

OBJECTIVE: To evaluate the optical properties and the relative translucency parameter of Ceramill ZI White (3Y-TZP) and Ceramill Zolid FX White (5Y-PSZ) zirconia ceramic systems and compare them with those of the bovine dentin and enamel/dentin structures. MATERIALS AND METHODS: 3Y-TZP and 5Y-PSZ zirconia ceramic systems were evaluated. A 0.5-mm-thick 3Y-TZP (3Y-NC.5), 0.5-mm-thick (5Y-NC.5), and 1.4-mm-thick (5Y-C.14) were used. A 0.5-mm-thick dentin specimens and 1.4-mm-thick enamel/dentin specimens (n = 5) were obtained from anterior bovine maxillary teeth. Scattering, absorption, transmittance, and albedo coefficient were calculated using Kubelka-Munk's model. Data were statistically analyzed using Kruskal-Wallis and Mann-Whitney tests (p < 0.001), and goodness-of-fit coefficient (GFC). Relative translucency parameter differences were evaluated using translucency thresholds. RESULTS: Reflectance, scattering, absorption, and transmittance properties were wavelength dependent. Good matches (GFC ≥ 0.999) in spectral reflectance were observed between 0.5-mm-thick dentin and 1.4-mm-thick enamel/dentin, and 3Y-NC.5 and 5Y-NC.5. Scattering was the main optical extinction process during light interaction with zirconia and dental structures, as indicated by albedo coefficient. Translucency differences were acceptable only for 3Y-NC.5 and the dentin structure, and 5Y-C.14 and the enamel/dentin structure. CONCLUSIONS: Optical properties of 3Y-TZP and 5Y-PSZ dental zirconia differed from each other and from bovine dental structures. Nevertheless, 3Y-TZP showed similar relative translucency parameter to bovine dentin. CLINICAL SIGNIFICANCE: To achieve the best esthetic results in restorative dentistry, it is crucial for clinicians to know about the optical properties of 3Y-TZP and 5Y-PSZ and to be able to compare these properties with those of dental structures.

The influence of smoking on the effectiveness of at-home bleaching: A prospective clinical study.

OBJECTIVES: To evaluate the effect of smoking on tooth whitening and color change after at-home bleaching. MATERIALS AND METHODS: This prospective clinical study evaluated two groups of participants: NS-non-smokers (n = 24) and S-smokers (n = 16). All subjects were treated with at-home bleaching using 22% carbamide peroxide (CP) for 1 h/day for 14 days. Color coordinates (L*-luminosity, a*-red-green axis and b*-yellow-blue axis) were obtained before bleaching (D0-baseline), 1 day (D1), 15 days (D15), and 1 month (D30) after bleaching. The whiteness index (WID ) and CIEDE2000 color differences (ΔE00 ) were calculated for S and NS. Color changes between S and NS subjects for specific teeth (CI-Central Incisors, LI-Lateral Incisors, and C-Canines) were analyzed using the Mann-Whitney test (α = 0.05). RESULTS: There was no significant difference in WID between S and NS subjects at baseline. NS participants showed higher WID than S at D1, D15, and D30 (p < 0.05). NS showed higher ΔWID for CI and C between D0 and the other evaluation times (D1, D15, and D30) compared to S. Smokers showed higher darkening of CI than NS in ΔWID D30-D1 (p < 0.05). Color changes showed similar overall behavior for ΔWID . For all evaluation times, ΔWID and ΔE00 values were below the corresponding acceptability thresholds. CONCLUSION: At-home bleaching was effective on smokers and nonsmokers. Smoking negatively affected color stability after bleaching. ΔE00 and ΔWID were acceptable for both groups. CLINICAL SIGNIFICANCE: Although smoking negatively affected color stability after bleaching, the observed changes were clinically acceptable.

Assessment of the survival and success rates of lithium disilicate crowns after different surface finishing procedures: An in vitro study.

STATEMENT OF PROBLEM: Evidence is limited for the impact of clinical adjustments and polishing on the longevity of glazed lithium disilicate restorations. PURPOSE: The purpose of this in vitro study was to evaluate the influence of surface finishing on the survival and success rates of lithium disilicate restorations based on fatigue resistance and failure mode. MATERIAL AND METHODS: Lithium disilicate (IPS e.max CAD) maxillary premolar crowns (N=54) were cemented on a dentin analog. The restorations were divided into 3 groups: overglaze (OG), abrasion (GA), and abrasion and polishing (AP). The crowns were submitted to cyclic fatigue in 37 oC water at 100 N and 2 Hz in 2 lifetimes. The load was applied to the occlusal surface by using anatomic pistons to simulate a clinical tripod occlusal contact. After cycling, the crowns were examined for failure (cracking, chipping, or catastrophic fractures) under optical and scanning electron microscopy. Cracking was considered either a structural failure (success analysis) or a survival (clinical criteria - survival analysis). Data were analyzed by using the log rank Kaplan-Meier and Holm-Sidak tests (α=.05). RESULTS: Surface finishing had no influence on the structural integrity of lithium disilicate, with similar success rates (P=.720). The calculated survival rate was higher for AP than that for other groups (P=.028). Cracking was found for GA and AP crowns, mostly initiating from the external surface. Chipping occurred in all experimental groups, and AP crowns did not show catastrophic failures. CONCLUSIONS: Although surface treatments had no influence on the success of lithium disilicate, polishing showed a positive effect on the survival rate of the crowns based on the clinical implications of cracking (no need for replacement).

Influence of piston material on the fatigue behavior of a glass-ceramic.

STATEMENT OF PROBLEM: The lack of standardization regarding the loading piston material used in fatigue tests could limit the interpretation of study findings. PURPOSE: The purpose of this in vitro study was to evaluate the effect of the piston material on the fatigue behavior of a lithium disilicate glass-ceramic. MATERIAL AND METHODS: Plate-shaped, 1.2-mm-thick, lithium disilicate glass-ceramic specimens were cemented onto a dentin analog substrate with resin cement. The specimens were divided into 4 groups according to the piston material used in the fatigue test (n=30): metal, glass fiber-reinforced epoxy resin, ceramic, and human tooth. The fatigue test was performed in a mechanical cycling machine by using the boundary technique at 2 Hz in distilled water at 37 °C. The fatigue data were analyzed by using the Weibull distribution and a lifetime-inverse power law relationship. Failures were evaluated with fractography and transillumination. RESULTS: The Weibull modulus (ß) was similar among groups. The exponent of crack growth (n) was significantly greater for glass fiber-reinforced epoxy resin and tooth groups than for metal and ceramic; therefore, the probability of failure (Pf) of glass-ceramic specimens loaded by resin and tooth pistons depended more on load amplitude. Specimens tested with tooth showed the highest value of K (characteristic lifetime), which is an indication of greater survival. Radial crack was the only failure mode observed for all experimental groups. CONCLUSIONS: The piston material influenced the fatigue survival of the lithium disilicate glass-ceramic. The glass fiber-reinforced epoxy resin piston closely simulated the fatigue behavior induced by the human tooth on the evaluated glass-ceramic.

Fatigue resistance of polymeric restorative materials: effect of supporting substrate.

The objective of this study was to investigate the effect of mismatch between the elastic properties of substrate and restorative material on the fatigue resistance and stress distribution of multilayer structures. The tested hypotheses were that (1) both an indirect composite resin (IR) and a polymer-infiltrated ceramic network (PICN) would show a higher survival rate after cyclic loading when cemented to a substrate with a high elastic modulus (E); and (2) PICN structures would have higher survival rates than IR structures regardless of the supporting substrate. Blocks of PICN and IR were cut to obtain 1.0-mm-thick sections, which were cemented to substrates with different E values: c, core resin cement (low E); r, composite resin (intermediate E); and m, metal (nickel-chromium alloy; high E). The resulting 6 groups of specimens (n = 20 per group) were subjected to a cyclic fatigue test (106 cycles). Stress distribution was verified using finite element analysis, and the risk of failure was estimated. Fatigue data were analyzed using Kaplan-Meier and Holm-Sidák tests. The χ2 test was used to evaluate the type of crack. The groups IRc, IRr, and PICNm had the highest survival rates after cyclic loading and were statistically similar to each other. Their survival rates were significantly greater than those of the IRm, PICNr, and PICNc groups (P < 0.001), which were all significantly different from each other (P < 0.001). There was a significant relationship between the experimental group and type of crack (P < 0.001). Specimens cemented to core resin cement and composite resin substrates showed predominantly radial cracks, while those cemented to nickel chromium alloy had predominantly cone cracks. The risk of failure values revealed that PICN was more sensitive to the type of substrate than IR. PICN has greater fatigue-resistant behavior when cemented to a substrate with a high E value, while IR has superior performance when substrates with lower and intermediate E values are used.

Influence of smoking on oral cells genotoxicity after at-home bleaching using 22% carbamide peroxide: a cohort study.

OBJECTIVE: This study evaluated the influence of smoking on the oral cells genotoxicity before and after at-home bleaching using 22% carbamide peroxide (CP). MATERIALS AND METHODS: This is a prospective observational analytics cohort study which evaluated nonsmokers (NS; n = 24) and smokers (S; n = 16) patients. At-home bleaching was performed using 22% CP gel in individual trays for 1 h per day for 14 days in both groups. Scrapped cells from marginal gums were collected before the bleaching treatment (D0-baseline) and 1 day (D1), 15 days (D15), and 1 month (D30) after its finishing. Cells were stained with Giemsa 10%, and the micronucleus (MN) and metanuclear alterations (MA) were counted by a trained operator in 1000 cells per patient. The collections and data analysis occurred blindly. Data was analyzed by Kruskal-Wallis, Dunn, and Mann-Whitney test (α = 0.05). RESULTS: MN frequency was not influenced by smoking or bleaching. An increase of MA was observed between D0 and D30 for both groups (p < 0.001); however, no statistical difference was found between NS and S (p > 0.05) in the evaluation times. CONCLUSION: Smoking associated with 22% carbamide peroxide gel for at-home bleaching does not show genotoxic potential analyzed by the MN counts. However, a significant increase of MA was found for smokers and nonsmokers. CLINICAL RELEVANCE: Despite of the increase in MA, smoking associated with 22% CP peroxide at-home bleaching showed no important genotoxic potential (MN) for oral cells. Therefore, at-home bleaching treatment is safe for nonsmokers and smokers even with a high carbamide peroxide concentration of 22%.

Predictors of orthodontic treatment conclusion: A historical cohort study.

INTRODUCTION: The study investigated which patient and orthodontic treatment factors act as predictors for the conclusion of the ongoing treatment in a dental clinic of a specialization program in Orthodontics. METHODS: Data were collected from the records of patients treated from 1997 to 2015. Potential predictors for treatment conclusion were investigated: patient-related factors (PRFs) and treatment-related factors (TRFs). PRFs were sex, age, face balance, Angle malocclusion classification, open bite, denture, facial pattern, facial profile, buccal corridor, crossbite, maxillary deficiency, and sagittal mandibular behavior; and TRFs were therapeutic approaches, treatment modality, extractions, and Bolton discrepancy. The initial and final treatment dates were collected. Descriptive data analysis, univariate, and multivariate logistic regression were performed (5% significance). RESULTS: Of the 903 records, 561 patients were included in the study. It was demonstrated that starting the treatment at a young age (PRF) and the presence of crossbite (TRF) are predictive factors for the treatment conclusion. A vertical facial pattern (dolichofacial or brachyfacial) and a greater number of extractions for orthodontic reasons may contribute positively to the conclusion of the treatment. The frequency of treatment inconclusion was higher during the first 2 years of treatment (more than 50% of the patients that initiated the treatment). CONCLUSIONS: Young age at the beginning of treatment and the presence of crossbite malocclusion can increase the chance of treatment conclusion.

Intraoral repair of a chipped porcelain-zirconia restoration.

OBJECTIVE: Ceramic fracture is an undesirable outcome of the rehabilitation with fixed partial dentures (FPD), mainly because it may involve additional cost and clinical time for intraoral repair or replacement of the restoration. This clinical report describes a 5 years survival intraoral repair of a chipped porcelain veneered zirconia framework restoration using a resin-based composite. CLINICAL CONSIDERATIONS: A FPD of porcelain veneered zirconia was made. After 18 months, the FPD presented a porcelain chip (porcelain fracture without exposure to the zirconia structure) on the buccal side of the pontic. An epoxy resin replica of the fractured surface was obtained and was examined under scanning electron microscopy. Fracture origin was found at the cervical area of the pontic. Intraoral repair by bonding the chipped fragment back in place was performed. After 15 days, the porcelain fragment debonded without patient knowledge and the fragment was lost. Then, intraoral repair using composite resin to restore the fractured area was performed and is still in function to date. CONCLUSIONS: Based on the 5-years survival of the performed intraoral repair, the composite resin reconstruction technique has shown to be an adequate alternative treatment for fractured FPD. CLINICAL SIGNIFICANCE: A resin composite repair of the fracture site can be performed in one clinical session, using much less time and cost than for the replacement of FPD. This clinical case survived 5 years to date.

The critical bond strength of orthodontic brackets bonded to dental glass-ceramics.

OBJECTIVES: To evaluate the critical bond strength (σ) of ceramic and metal brackets to a lithium disilicate-based glass-ceramic. MATERIALS AND METHODS: Two hundred and forty ceramic specimens (IPS e-max CAD) were randomly distributed in 12 experimental groups (n = 20). Two ceramic brackets (monocrystalline, BCm; and polycrystalline, BCp) and a metal bracket (BM) were bonded to glass-ceramic specimens after one of the following surface treatments: HF-hydrofluoric acid applied for 60 s; S-silane applied for 3 min; HFS-HF followed by S; and MDP-application of an adhesive containing a phosphate monomer (MDP). All brackets were bonded to the treated glass-ceramic using a resin cement, stored in 37 °C water for 48 h before shear bond strength testing. Optical (OM) and scanning electron (SEM) microscopies were used for fractographic analysis. Data was statistically analyzed using Kruskal-Wallis and Student-Newman-Keuls (α = 0.05). RESULTS: BCm bonded to glass-ceramic treated with either HFS or HF showed the highest median σ values, respectively, 10.5 MPa and 8.5 MPa. In contrast, the BCp bonded to glass-ceramic treated with MDP showed the lowest median σ value (0.8 MPa), which was not statistically different from other MDP-treated groups. CONCLUSIONS: The failure mode was governed by the glass-ceramic surface treatment, not by the bracket type. Quantitative (σ values) and qualitative (fracture mode) data suggested a minimum of 5 MPa for brackets bonded to glass-ceramic, which is the lower critical limit bond strength for a comprehensive orthodontic treatment. CLINICAL RELEVANCE: Bonding brackets to glass-ceramic requires micromechanical retention.

Effect of supporting substrate on the failure behavior of a polymer-infiltrated ceramic network material.

STATEMENT OF PROBLEM: Restorative materials are cemented on different types of substrates, such as dentin, metal, and glass-fiber posts with composite resin cores. PURPOSE: The purpose of this in vitro study was to evaluate the failure behavior after cycling fatigue of a polymer-infiltrated ceramic network material (PICN; VITA ENAMIC) cemented on different supporting substrates. MATERIAL AND METHODS: PICN plates (N=80) were obtained from computer-assisted design and computer-assisted manufacturing (CAD-CAM) blocks and cemented with a resin cement to 4 different supporting substrates (n=20): (1) human dentin (PICNDen); (2) dentin analog (PICNDenAn); (3) nickel-chromium alloy (PICNNiCr); and (4) composite resin plus fiberglass post (PICNRc). For comparison, the fracture behavior of a feldspathic ceramic (FelDenAn; VITABLOCS Mark II) and an indirect composite resin (ResDenAn; Opallis LAB Resin) cemented to the DenAn substrate was investigated (n=20). Thus, specimens were composed of the restorative material layer (1-mm thick) resin cemented (0.1-mm-thick layer) to a 2-mm-thick supporting substrate. All specimens were subjected to mechanical cycling (MC) using a pneumatic cycling machine (500 000 cycles, 2 Hz, 50 N). Specimens that did not fracture during cycling were tested under compression using a universal testing machine at a cross-head speed of 0.5 mm/min until the sound of the first crack was detected using an acoustic system. Failure data were statistically evaluated using Weibull distribution. Failures were classified as radial crack, cone crack, combined, and catastrophic fracture. RESULTS: All FelDenAn specimens were fractured during MC. Only 4 PICNRc specimens survived MC, so their fracture load data were not statistically analyzed. PICNNiCr showed the greatest characteristic load (L0) value, followed by ResDenAn. Groups PICNDenAn and PICNDen showed lower and similar L0 but statistically different Weibull modulus (m). There was a significant relationship between experimental group and failure mode (P<.001). FelDenAn and PICNRc had a higher frequency of radial cracks, whereas PICNNiCr failed from cone cracking. CONCLUSIONS: The supporting substrate influenced the failure behavior of PICN. When the substrate had a higher elastic modulus than the restorative material, better mechanical behavior was observed.

How oral environment simulation affects ceramic failure behavior.

STATEMENT OF PROBLEM: Investigating the mechanical behavior of ceramics in a clinically simulated scenario contributes to the development of new and tougher materials, improving the clinical performance of restorations. The optimal in vitro environment for testing is unclear. PURPOSE: The purpose of this in vitro study was to investigate the failure behavior of a leucite-reinforced glass-ceramic under compression loading and fatigue in different simulated oral environment conditions. MATERIAL AND METHODS: Fifty-three plate-shaped ceramic specimens were produced from computer-aided design and computer-aided manufactured (CAD-CAM) blocks and adhesively cemented onto a dentin analog substrate. For the monotonic test (n=23), a gradual compressive load (0.5 mm/min) was applied to the center of the specimens, immersed in 37ºC water, using a universal testing machine. The initial crack was detected with an acoustic system. The fatigue test was performed in a mechanical cycling machine (37ºC water, 2 Hz) using the boundary technique (n=30). Two lifetimes were evaluated (1×106 and 2×106 cycles). Failure analysis was performed using transillumination. Weibull distribution was used to evaluate compressive load data. A cumulative damage model with an inverse power law (IPL) lifetime-stress relationship was used to fit the fatigue data. RESULTS: A characteristic failure load of 1615 N and a Weibull modulus of 5 were obtained with the monotonic test. The estimated probability of failure (Pf) for 1×106 cycles at 100 N was 31%, at 150 N it was 55%, and at 200 N it was 75%. For 2×106 cycles, the Pf increased approximately 20% in comparison with the values predicted for 1×106 cycles, which was not significant. The most frequent failure mode was a radial crack from the intaglio surface. For fatigue, combined failure modes were also found (radial crack combined with cone crack or chipping). CONCLUSIONS: Fatigue affects the fracture load and failure mode of leucite-reinforced glass-ceramic.

How does the piston material affect the in vitro mechanical behavior of dental ceramics?

STATEMENT OF PROBLEM: Variables involving the indication, manufacturing, and clinical use of ceramic restorations make the standardization of in vitro studies a challenge and raise questions as to the clinical validity of the resulting data. PURPOSE: The purpose of this in vitro study was to assess the effect of piston material on the fracture behavior of ceramics tested under compressive load. MATERIAL AND METHODS: Two ceramics were evaluated: a lithium disilicate-based glass-ceramic (D) and a feldspathic porcelain (P). Plate-shaped ceramic specimens (1.5-mm thick) were adhesively cemented onto a dentin analog substrate. The specimens from each ceramic were divided into 4 groups according to the piston material (n=20): metal (M) (stainless steel), composite resin (R) (NEMA-G10, fiber-reinforced epoxy resin), ceramic (C) (lithium disilicate-based glass-ceramic), and human tooth (T) (canine). A gradual compressive load (0.5 mm/min) was applied to the center of the specimen with a universal testing machine. The test was performed in 37°C distilled water, and the initial crack was detected by using an acoustic system. The fracture load values (N) were statistically analyzed with the Kruskal-Wallis and Dunn tests (α=.05). A finite element analysis (FEA) was also performed. RESULTS: Piston material had no influence on fracture load and failure mode of ceramic D. Ceramic P showed higher fracture load values when loaded with the composite resin piston. Ceramic P showed more combined failures (cone crack and radial crack) than D. The FEA showed a distinct stress distribution for R piston on P. Pistons C and T resulted in similar stress distribution, fracture load, and failure mode for both ceramics. CONCLUSIONS: The effect of piston material on the ceramic fracture behavior depends on the ceramic being evaluated.

A Comparison of Torque Stress on Abutment Screw of External Hexagon and Morse Taper Implant.

AIM: This three-dimensional (3D) finite element analysis observed the stress distribution on the prosthetic screws on external hexagon implant and morse taper implant with different tightening loads. MATERIALS AND METHODS: For this, two different 3D models assembly were obtained from the manufacturer and transferred to finite element analysis software: External hex implant model (EHM), and Morse taper implant model (MTM), both compounds by 3.75 × 7 mm implant, abutment and abutment screw. Bolt pretension force was applied on the shaft next to the threads of the prosthetic abutment. Preload was calculated using the torque on the prosthetic screw as recommended by the manufacturer (EHM30 and MTM20) and 10 Ncm torque above the manufacturer recommendations (EHM40 and MTM30). Maximum von mises equivalent stresses were obtained on the screws. RESULTS: Preload values results were 243.18N (EHM30), 229.71N (MTM20), 324.24N (EHM40) and 344.57N (MTM30). In EHM30, EHM40 and MTM20 models the maximum stresses were below the yield strength of the abutment screw material. However, the maximum stress in MTM30 model was higher than the reference value. CONCLUSION: The torque loads above the manufacturer recommendations can cause plastic deformation in the MT abutment screw threads. The screws of morse taper implant can be more sensitive to higher loads than external hexagon implant. CLINICAL SIGNIFICANCE: The adequate torque can result in screw loosening, while fracture may occur if torque is excessive. Abutment screw suffers many screwing cycles in its lifetime in a way that some tightening forces are above manufacturer recommendations.

Influence of surface finishing on fracture load and failure mode of glass ceramic crowns.

STATEMENT OF PROBLEM: Ceramic restorations often require adjustments using diamond rotary instruments, which damage the glazed surface. The effect of these adjustments on the fracture behavior of these restorations is unclear. PURPOSE: The purpose of this in vitro study was to evaluate the influence of induced surface defects on the fracture load and mode of failure of lithium disilicate-based (LDS) glass ceramic restorations. MATERIAL AND METHODS: Premolar crowns were obtained from LDS computer-aided design and computer-aided manufacturing blocks (n=60) and glazed. The crowns were bonded to dentin analog dies and divided into 5 groups (n=12), as follows: glaze; abrasion (diamond rotary instrument 2135); abrasion and reglaze; abrasion and polishing (diamond rotary instrument 2135F, 2135 FF, and polishing devices); and polishing. The topography of the crowns was examined by scanning electron microscopy, and roughness was measured. A compressive load (0.5 mm/min) was applied by a piston to the center of the lingual cusp until fracture. The fracture load was recorded and data were statistically analyzed by ANOVA and the Tukey HSD test (α=.05). Fractured crowns were examined to determine the fracture origin. RESULTS: Polishing and/or reglazing resulted in lower roughness than for the abraded group (P<.05), which did not affect the fracture loads (P=.696). Catastrophic fracture with origin at the intaglio surface was the mode of failure for all the crowns. CONCLUSIONS: The experiment design successfully submitted the crowns to a clinical stress state, resulting in a clinically relevant failure. Reglazing or polishing were effective in reducing surface defects. Surface treatments had no effect on the immediate catastrophic failure of LDS crowns.

Effect of antioxidants on adhesive bond strength to bleached enamel.

OBJECTIVE: To evaluate the influence of antioxidants (ATX) resveratrol, winter's bark, green tea and yerba mate on the bond strength between bleached enamel and the nanohybrid composite resin. METHODOLOGY: Bovine incisor crowns (n = 132) were randomly divided into 22 groups (n = 6) according to the application times (5, 10, 15, 30, and 60 min) of each antioxidant. Teeth restored without previous bleaching or ATX constituted the non-bleached control group (NB Ctrl) (n = 6), and teeth restored after bleaching and without ATX represented the bleached control group (B Ctrl) (n = 6). The 35 % hydrogen peroxide was applied for 45 min (3 application of 15 min) to the buccal enamel surface. ATX was used after bleaching for the specified time of each group and removed with air-water spray. The enamel was etched with 37 % phosphoric acid (30 s) and rinsed with air-water spray. The adhesive resin was applied to the enamel dry surface. Teeth were restored using 1 mm composite resin increments (10 × 10 × 3 mm) and sectioned in test specimens of 6 mm in length and 1 mm2 in cross-sectional area submitted to microtensile bond strength test (0.5 mm/min). The load (N) at failure was recorded, and the bond strength (σt) was calculated (MPa). The fracture area was analyzed under optical microscopy, and failures were classified as cohesive, mixed, or adhesive. Data was evaluated by Kruskal-Wallis and Dunn tests (p ≤ 0.05). RESULTS: B Ctrl group presented lower σt than NB Ctrl (p < 0.001). Applying resveratrol for 5 or 10 min, winter's bark for 10 or 15 min, green tea for less than 15 min, and yerba mate for 15 min provided similar σt between bleached enamel and nanohybrid composite to the control group. CONCLUSION: Restorative procedures performed immediately after tooth bleaching compromises adhesion. Experimental antioxidants applied to bleached enamel can increase the immediate bond strength of restorations performed directly after bleaching, with similar values to those observed in unbleached enamel. CLINICAL SIGNIFICANCE: This study presents promising results to support the use of antioxidants on the recently bleached enamel to allow adhesive tooth restorations. The immediate bonding obtained using antioxidants was similar to the one achieved in non-bleached enamel in brief application times. Green tea extract and resveratrol were able to restore the bond strength to bleached enamel in a short application time of 5 min. The reduction in the required application time holds the potential to decrease the overall duration of the clinical section, offering clinical advantages and improving the feasibility of using antioxidants on the bleached enamel prior to adhesive procedures.

Failure behavior of zirconia crowns subjected to air abrasion with different particle sizes.

This study aimed to investigate the failure behavior of 3 mol.% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) prosthetic crowns air-abraded with aluminum oxide (AO) particles of different sizes. Ninety ceramic premolar crowns were produced with 3Y-TZP frameworks veneered with porcelain. Crowns were randomly divided into three groups, according to the size of the air abrasion AO particles (n = 30): (GC) untreated (control); (G53) 53 µm; (G125) 125 µm. Air abrasion was performed with 0.25 mpa pressure, 10-mm distance, for 10 s. Crowns were adhesively cemented to dentin analog abutments. Specimens were loaded in compression to failure, in 37oC distilled water, using a universal testing machine (n = 30). Fractographic analysis was performed using a stereomicroscope and SEM. The roughness of the crown's inner surface was evaluated using an optical profilometer (n = 10). Fracture load data were statistically analyzed with Weibull analysis and roughness data with Kruskal-Wallis (α = 0.05). GC had the lowest characteristic fracture load (L0), while G53 and G125 had higher and statistically similar L0 values. The Weibull modulus (m) was similar among groups. The failure modes observed were catastrophic failure and porcelain chipping. There were no differences between the roughness parameters for the experimental groups (p > 0.05). The size of the AO particles did not affect the fracture load and failure mode of 3Y-TZP crowns. Air abrasion with 53 µm and 125 µm particles resulted in a higher fracture load of ceramic crowns than the untreated group while maintaining their reliability and surface characteristics.

Analysis of topography, flexural strength, and microstructure of a lithium-disilicate glass-ceramic after surface finishing.

Glass-ceramic restorations often require adjustments using coarse diamond burs (D) which have the potential to introduce critical defects at the surface from which a crack can initiate and propagate until fracture. Surface finishing using fine and ultra-fine burs (F), polishing (P), or glazing (G) have the potential to reduce or eliminate these defects. PURPOSE: This study investigated the influence of finishing, polishing, and glazing on the roughness, flexural strength, and microstructure of a lithium disilicate glass-ceramic (LD). MATERIALS AND METHODS: LD CAD-CAM blocks were cut into 96 discs (1.2 mm x 12 mm). After crystallization, a layer of glaze was applied to the discs surface. The specimens were randomly divided into eight groups (n=12) for different surface treatments: G- glaze (control), GD, GDG, GDP, GF GFG, GFP, and GDFP. Mean roughness (Ra), mean square height (Rq) and maximum roughness (Rz) were measured. The biaxial flexural strength test was performed in a universal testing machine at 0.5 mm/min. Microstructural analysis was performed using X-ray diffraction patterns (XRD). Differences on the roughness obtained in distinct groups was analyzed by Kruskal-Wallis and Bonferroni tests (α = 0.05). The flexural strength tests were compared by one-way ANOVA. RESULTS: Glazed groups (G, GDG and GFG) presented the lowest surface roughness, lower crystallinity, and higher flexural strength than the other groups. Although polishing and glaze presented similar surface roughness, polishing protocol decreased the flexural strength compared to the control group G. CONCLUSION: clinically adjusted LD glass-ceramic restorations should be reglazed whenever possible to improve strength.

Effect of Aging on Fracture Load and Reliability of Cemented Glass-Ceramics.

PURPOSE: Lithium disilicate (LD) and lithium silicate (LS) glass-ceramics may show a different acid etching behavior and mechanical integrity after aging. This study evaluated the influence of aging on the fracture load and reliability of glass-ceramics after different etching protocols. MATERIALS & METHODS: Specimens were fabricated and divided according to the etching time (10% hydrofluoric acid (HF) for 20s, 40s, and 60s). Etched surfaces were examined under microscopy. The ceramics were resin cemented to a dentin analog material. Samples were tested after 24 h (I) (n=20) or stored in 37°C water for 1 year (A)(n=20). A compressive load (0.1 mm/min) was applied to the samples until failure was detected by acoustic emission. The influence of aging on the fracture load (Lf) was tested by 2-way ANOVA and Tukey tests (α=0.05). The characteristic fracture load (L0) and Weibull modulus (m) values were calculated. RESULTS: LS etching for 20 s resulted in the highest immediate Lf, which was significantly decreased after aging (P≤0.05). Water-storage had no effect in this glass-ceramic Lf etched for 40 and 60s (P0.05). For LD, the etching time had no significant effect on the immediate and aged Lf (P0.05). The Lf was significantly reduced after aging (A) for LD (P≤0.05). Radial cracks were the predominant failure mode. Surface topography was more regular after lower etching times for LD, at after higher times for LS. CONCLUSIONS: After 1-year water-assisted aging, the evaluated etching times had no influence on the load-bearing capacity and structural reliability of resin-bonded glass-ceramics.

Fatigue-life and stress distribution of a glass-ceramic under different loading conditions.

This study aimed to evaluate the effect of different loading conditions on the mechanical behavior and stress distribution of a leucite-reinforced glass-ceramic. Plate-shaped ceramic specimens were obtained from leucite-reinforced glass-ceramic (1.5 × 8.4 × 8.3 mm) and adhesively cemented to a dentin analog substrate. Monotonic and cyclic contact fatigue tests were performed to simulate sphere-to-flat contact, using a 6 mm diameter spherical piston; and flat-to-flat contact, using a 3 mm diameter flat piston. For the monotonic test (n=20), a gradual compressive load (0.5 mm/min) was applied to the specimen using a universal testing machine. Failure load data were analyzed with Weibull statistics. The cyclic contact fatigue test was performed using protocols (load and a number of cycles) defined by the boundary technique (n=30). Fatigue data were analyzed using an inverse power law relationship and Weibull-lifetime distribution. The stress distribution was investigated using Finite Element Analysis (FEA). The monotonic and the fatigue Weibull modulus were similar among the two contact conditions. In fatigue, the slow crack growth exponent was greater for sphere-to-flat contact, which indicates that the load level had a greater effect on the specimen's probability of failure. In conclusion, FEA showed different stress distribution for the tested loading conditions. The stress distribution and probability of fatigue failure of specimens tested in sphere-to-flat contact showed greater dependency to load level.