Wear resistance of crowns made from different CAM/CAD materials

ObjectivesThe aim of this laboratory study was to evaluate the wear resistance of crowns made from current computer-aided design and computer-aided manufacture (CAD/CAM) materials. In addition, the abrasion of the steatite antagonist against these materials was compared.MethodsIdentically shaped crowns of lithium disilicate, zirconia-reinforced lithium disilicate and a polymer-infiltrated ceramic network (PICN) were fabricated with an occlusal thickness of 1.5 mm and a lateral wall thickness of 1.2 mm (n = 8). The crowns were cemented with a dual-polymerizing luting resin on composite resin dies. Using spherical steatite antagonists, all specimens were loaded with 49 N for 1,200,000 cycles in a mastication simulator with additional thermocycling. After 120,000, 240,000, 480,000, 960,000, and 1,200,000 cycles, precision impressions were made and investigated with a laser scanning microscope. The vertical and volume substance loss was measured. Additionally, the substance loss of the antagonists was evaluated after 1,200,000 loading cycles.ResultsNo significant difference (p > 0.05) was found in the median volume loss of the test materials after 1,200,000 cycles (lithium disilicate: 0.405 mm³, PICN: 0.362 mm³, zirconia-reinforced lithium disilicate: 0.340 mm³). The vertical substance loss of PICN (157 μm) was significantly lower (p ≤ 0.05) than that of lithium disilicate (201 μm) and zirconia reinforced lithium disilicate (191 μm). However, the substance loss of steatite against zirconia-reinforced lithium disilicate (0.191 mm³) was significantly lower (p ≤ 0.05) than against lithium disilicate (0.296 mm³) and PICN (0.531 mm³).SignificanceAll three CAD/CAM materials showed wear resistance that seems appropriate for clinical application. Also, the abrasion of the antagonist looks promising.     

Clinical performance of occlusal onlays made of lithium disilicate ceramic in patients with severe tooth wear up to 11 years

ObjectivesEvaluation of survival and complication rate of monolithic occlusal onlays made of lithium disilicate ceramic used in patients with severe tooth wear up to 11 years of clinical service.MethodsIn a prospective non-randomized clinical study 7 patients (4 male, 3 female; median age: 44.3 ± 6.56 years old) were restored full mouth with a total of 103 adhesively bonded occlusal onlays made of lithium disilicate ceramic (IPS e.max Press, Ivoclar Vivadent, Schaan, Liechtenstein). All restorations were examined during annual recall visits using periodontal parameters according to the modified United States Public Health Service (USPHS) criteria: (a) marginal discoloration, (b) secondary caries, (c) marginal integrity, (d) surface texture, (e) restoration fracture, and (f) occlusal wear, rating with Alpha, Bravo and Charlie over an observation period up to 11 years (68–139 months; median: 94.9 ± 26.1 months). Data was statistically analyzed using the Kaplan–Meier estimation.ResultsMonolithic lithium disilicate occlusal onlays presented a 100% survival rate. Four restorations within one patient (3.9%) presented marginal discoloration, one after 60 and three after 108 months (all rated Bravo). One restoration (1%) showed a marginal crack formation (technical complication) after 120 months, rated Bravo. No biological complication, debonding or secondary caries could be found and tested periodontal parameters showed excellent results.SignificanceBased on the analyzed data up to 11 years, monolithic occlusal onlays made of lithium disilicate ceramic can be considered as a reliable treatment option for full-mouth rehabilitations in patients with severe tooth wear.     

Manufacture,characterisation and properties of novel fluorcanasite glass–ceramics

ObjectiveThe aim of this study was to investigate the manufacture and characterisation of different compositions of fluorcanasite glass–ceramics with reduced fluorine content and to assess their mechanical and physical properties.MethodsThree compositional variations (S80, S81 and S82) of a fluorcanasite glass were investigated. Differential thermal analysis (DTA) and X-ray diffraction (XRD) identified crystallisation temperatures and phases. X-ray fluorescence (XRF) determined the element composition in the glass–ceramics. Different heat treatments [2 h nucleation and either 2 or 4 h crystallisation] were used for the glasses. Scanning electron microscopy (SEM) examined the microstructure of the cerammed glass. The chemical solubility, biaxial flexural strength, fracture toughness, hardness and brittleness index of S81 and S82 fluorcanasite were investigated with lithium disilicate (e.max CAD, Ivoclar Vivadent) as a commercial comparison. Statistical analysis was performed using one-way ANOVA with Tukey's multiple comparison tests (P < 0.05). Weibull analysis was employed to examine the reliability of the strength data.ResultsAll compositions successfully produced glasses. XRD analysis confirmed fluorcanasite formation with the S81 and S82 compositions, with the S82 (2 + 2 h) showing the most prominent crystal structure. The chemical solubility of the glass–ceramics was significantly different, varying from 2565 ± 507 μg/cm² for the S81 (2 + 2 h) to 722 ± 177 μg/cm² for the S82 (2 + 2 h) to 37.4 ± 25.2 μg/cm² for the lithium disilicate. BFS values were highest for the S82 (2 + 2 h) composition (250 ± 26 MPa) and lithium disilicate (266 ± 37 MPa) glass–ceramics. The fracture toughness was higher for the S82 compositions, with the S82 (2 + 2 h) attaining the highest value of 4.2 ± 0.3 MPa m^1/2(P = 0.01). The S82 (2 + 2 h) fluorcanasite glass–ceramic had the lowest brittleness index.ConclusionThe S82 (2 + 2 h) fluorcanasite glass–ceramic has acceptable chemical solubility, high biaxial flexural strength, fracture toughness and hardness.Clinical significanceA novel glass–ceramic has been developed with potential as a restorative material. The S82 (2 + 2 h) has mechanical and physical properties that would allow the glass–ceramic to be used as a machinable core material for veneered resin-bonded ceramic restorations.     

Effect of processing induced particle alignment on the fracture toughness and fracture behavior of multiphase dental ceramics

ObjectiveTo investigate the processing induced particle alignment on fracture behavior of four multiphase dental ceramics (one porcelain, two glass–ceramics and a glass-infiltrated–alumina composite).MethodsDisks (Ø12 mm × 1.1 mm-thick) and bars (3 mm × 4 mm × 20 mm) of each material were processed according to manufacturer instructions, machined and polished. Fracture toughness (K_Ic) was determined by the indentation strength method using 3-point bending and biaxial flexure fixtures for the fracture of bars and disks, respectively. Microstructural and fractographic analyses were performed with scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction.ResultsThe isotropic microstructure of the porcelain and the leucite-based glass–ceramic resulted in similar fracture toughness values regardless of the specimen geometry. On the other hand, materials containing second-phase particles with high aspect ratio (lithium disilicate glass–ceramic and glass-infiltrated–alumina composite) showed lower fracture toughness for disk specimens compared to bars. For the lithium disilicate glass–ceramic disks, it was demonstrated that the occurrence of particle alignment during the heat-pressing procedure resulted in an unfavorable pattern that created weak microstructural paths during the biaxial test. For the glass-infiltrated–alumina composite, the microstructural analysis showed that the large alumina platelets tended to align their large surfaces perpendicularly to the direction of particle deposition during slip casting of green preforms.SignificanceThe fracture toughness of dental ceramics with anisotropic microstructure should be determined by means of biaxial testing, since it results in lower values.     

Effect of three different preparation designs on the marginal adaptation of indirect overlay restoration fabricated from lithium disilicate ceramic material: An in-vitro comparative study

ObjectivesMarginal adaptation is considered one of the key factors influencing the success of indirect restorations. This study aimed to estimate the marginal fit of lithium disilicate overlays with three distinct preparation designs before and after cementation.MethodsThirty maxillary first premolars were divided into the hollow chamfer design (HCD¹) group, butt-joint design (BJD²) group, and conventional occlusal box design (COD³) group (n = 10 each). The samples were scanned using an intra-oral scanner, and overlays were fabricated using computer-assisted design and milled on a computer-assisted machine. The finished restorations were luted using a self-adhesive resin RelyX Ultimate. The marginal gap was assessed using a digital microscope with 230X magnification power. Statistical analysis was conducted using analysis of variance and post hoc (Bonferroni correction) tests, assuming a significance level of 5%.ResultsThe HCD and BJD groups recorded significantly lower marginal gap, (11.39 ± 0.72, 16.29 ± 0.75) and (11.59 ± 0.75, 16.93 ± 0.65) respectively, than the COD group (24.57 ± 1.18, 34.45 ± 1.09) both pre- and post-cementation.ConclusionThis study demonstrated that modification of tooth preparation plays a significant role in the marginal adaptation of the lithium disilicate overlays. The gap was smaller with the HCD and BJD than with the COD, with a statistically significant difference.     

In vitro enamel erosion and abrasion-inhibiting effect of different fluoride varnishes

ObjectiveTo investigate the erosion and abrasion inhibiting effect of CPP-ACP/NaF and xylitol/NaF varnishes.MethodsBovine enamel samples (n = 40) were exposed to the following treatments (n = 10): NaF varnish (Duraphat^®, positive control); CPP-ACP/NaF varnish (MI varnishTM); xylitol/NaF (Profluorid^®) or distilled and deionized water (MilliQ^®, negative control). The samples were submitted for 3 days to 4 cycles/day of erosion (5 min in Sprite Zero) and 2 cycles of abrasion/day after the first and last erosive challenge, with a toothbrush machine and slurries of a placebo toothpaste for 15 s (50 strokes/s). Among the cycles and after the last daily cycle, the specimens remained in artificial saliva. The change in the enamel surface was evaluated by using 3D non-contact optical profilometry with surface roughness (Ra and Sa values) and tooth structure loss (TSL) measurements. Scanning electron microscopy (SEM) assessed the enamel topographic characteristics. Differences in the Ra, Sa and TSL among treatments were tested using one-way ANOVA followed by the Tukey test.ResultsAll varnishes promoted better results for Ra and Sa values than the negative control (p = 0.0001), without difference among them (p > 0.05). However, CPP-ACP/NaF varnish stimulated fewer TSL (7.09 ± 0.70 μm) compared to NaF varnish (10.33 ± 1.36 μm, p = 0.002), xylitol/NaF varnish (9.96 ± 0.41 μm, p = 0.007) and the negative control (18.38 ± 3.32 μm, p = 0.0001).ConclusionA single-application of fluoride topical varnishes was effective in reducing enamel wear. The CPP-ACP/NaF varnish had the best effect against enamel loss from an erosion-abrasion challenge.     

An in situ and ex situ study of the microstructural evolution of a novel lithium silicate glass-ceramic during crystallization firing

ObjectiveTo elucidate the compositional and microstructural developments of a novel lithium silicate glass-ceramic during its crystallization cycle.MethodsBlocks of a lithium silicate glass-ceramic (Obsidian®, Glidewell Laboratories) were cut into 1 mm thick plates and polished to 1 μm finish. Some of them were crystallized prior to polishing. Firstly, ex situ compositional and microstructural characterizations of both the pre- and post-crystallized samples were performed by wavelength dispersive X-ray fluorescence, field-emission scanning electron microscopy, and X-ray diffractometry. Secondly, the pre-crystallized samples were subjected to in situ compositional and microstructural characterizations under non-isothermal heating by simultaneous thermogravimetry/differential scanning calorimetry, X-ray thermo-diffractometry, and field-emission scanning electron thermo-microscopy.ResultsThe microstructure of pre-crystallized Obsidian® consists of an abundant population of perlitic-like/dendritic lithium silicate (Li₂SiO₃) nanocrystals in a glass matrix. Upon heating, the residual glassy matrix does not crystallize into any form of SiO₂; elemental oxides do not precipitate unless over-heated above 820 °C; and the Li₂SiO₃ nanocrystals do not react with the glassy matrix to form typical lithium disilicate (Li₂Si₂O₅) crystals. Nonetheless, the Li₂SiO₃ nanocrystals grow and spheroidize through the solution-reprecipitation process in the softened glass, and new lithium orthophosphate (Li₃PO₄) nanocrystals precipitate from the glass matrix.SignificanceThe identification of compositional and microstructural developments of Obsidian® indicates that, by controlling the firing conditions, it is possible to tailor its microstructure, which in turn could affect its mechanical and optical properties, and ultimately its clinical performance.     

Effects of CAD/CAM ceramics and thicknesses on translucency and color masking of substrates

ObjectiveTo compare the translucency and masking of zirconia-reinforced lithium silicate with lithium disilicate glass-ceramic and hybrid composite.Materials and methodsOne hundred and twenty disc-shaped specimens were fabricated using Enamic, Celtra Duo, IPS E.max CAD, and Suprinity. Twelve groups were prepared according to material type and thickness (0.5, 1.0, and 1.5 mm). The specimens were tested over five substrates: composite resin-A3.5 shade (A3.5 control group), zirconia-A1 shade (ZR), nickel–chromium alloy (NC), black (B), and white (W). A spectrophotometer operating at wavelengths from 360 to 750 nm and a view-area size of 9.53 mm was used for color measurement.ResultsMean color difference values (ΔΕ) were found to be lowest in Suprinity and highest in IPS E.max. CAD. Color difference values were significantly related to substrate shade. The A3.5 substrate revealed the significantly lowest ΔΕ values of all the substrates (P = 0.05). Regarding the translucency parameters for ceramics of different thicknesses, a significant difference was observed among all groups except in Suprinity.ConclusionsDifferent ceramic types with different thicknesses and substrate strongly affect translucency and masking.Clinical significanceFabricating durable aesthetic restorations that also meets patients’ expectations is among the most significant challenges in dentistry. With high demand for more natural restorations, ceramics based on computer-aided design (CAD) and computer-aided manufacture (CAM) became popular and exhibited excellent results. However, various factors such as abutment shade, luting cement color, and ceramic type, thickness, and translucency may affect the prosthesis shade. The objective of this study was to evaluate the effects of these factors on the final shades of CAD/CAM-based ceramic restorations.     

In-vitro wear of natural tooth surface opposed with zirconia reinforced lithium silicate glass ceramic after accelerated ageing

Objective

To evaluate the effect of different pH media on zirconia-reinforced lithium silicate glass ceramic and how they interact with opposing dentition after being aged in different pH cycling and high temperature conditions.

Mechanical properties,aging stability and translucency of speed-sintered zirconia for chairside restorations

ObjectiveTo evaluate the performance of zirconia ceramics sintered in a speed sintering induction furnace by comprehensive understanding of their optical and mechanical properties, microstructure, phase composition and aging stability, in comparison to ceramics sintered in a conventional furnace.MethodsSpeed sintered (SS) Katana STML_SS (Kuraray Noritake) (total thermal cycle/sintering time/dwell temperature: 30 min/16 min/1560 °C) and CEREC Zirconia (CEREC Zr_SS) (Dentsply Sirona) (15 min/2 min/1578 °C) were compared to conventionally sintered (CS) Katana STML_CS (6.8 h/2 h/1550 °C) and inCoris TZI_CS (4 h/2 h/1510 °C). The translucency parameter (TP) and contrast ratio (CR) were measured with a spectrophotometer. The chemical composition of the materials was determined by XRF and phase composition was characterized using XRD. Hydrothermal aging behavior was evaluated by measuring the tetragonal-to-monoclinic ZrO₂ phase transformation after accelerated hydrothermal aging in steam at 134 °C. The indentation fracture toughness, Vickers hardness and biaxial strength of the sintered ceramics were assessed.ResultsSpeed and conventionally sintered zirconia revealed similar density, microstructure, average strength and hydrothermal aging stability. Both Katana STML_SS/CS 5Y-PSZ ceramics were characterized with a higher content of cubic phase (≈53 wt%), which resulted in a higher amount of Y₂O₃ in the remaining tetragonal ZrO₂ phases compared to the 3Y-TZP CEREC Zr_SS and inCoris TZI_CS (8 and 20 wt%, respectively). The sintering program did not affect the hydrothermal aging behavior of Katana STML_SS and CEREC Zr_SS. TP of Katana STML_SS (TP ≈ 32) was not affected by speed sintering, while the translucency of CEREC Zr_SS (TP = 14) was significantly reduced. Hardness, fracture toughness and Weibull characteristic strength of Katana STML_SS and CEREC Zr_SS also reached the optimal level, but speed sintering substantially lowered their mechanical reliability.SignificanceSpeed sintering of 3Y-TZP and 5Y-PSZ in a speed sintering induction oven appeared suitable for clinical applications. However, further studies should focus on improving of translucency and mechanical reliability of the speed-sintered zirconia ceramics.     

The effect of chewing simulation on surface roughness of resin composite when opposed by zirconia ceramic and lithium disilicate ceramic

Objective

To assess the change in surface roughness of nanohybrid resin composite (Tetric EvoCeram) after antagonist wear against monolithic zirconia and lithium disilicate ceramics through a simulated chewing test using a three-dimensional (3D) profilometer.

Surface characterization of dental ceramics and initial streptococcal adhesion in vitro

ObjectivesThe aim of this in vitro study was to investigate the surface properties of dental ceramic materials belonging to different ceramic classes, and to correlate the findings to the initial adherence of three oral streptococcal strains.MethodsRectangular specimens were prepared from different ceramic materials (glass/lithium disilicate glass/glass-infiltrated zirconia/partially sintered zirconia/hipped zirconia ceramic) and polished; surface roughness (Ra) was determined. Glass plates were used as a control. Specimens were incubated with phosphate-buffered saline or an artificial saliva (protein mixture; 2 h, 37 °C). Surface free energy (γ^t) and its polar (γ^p) and disperse (γ^d) contribution were determined prior to and after artificial saliva exposure. Uncoated and protein-coated specimens were incubated with Streptococcus gordonii DSMZ 6777, Streptococcus oralis DSMZ 20068 or Streptococcus sanguinis DSMZ 20068 suspension for 2.5 h at 37 °C (n = 15 for each treatment and strain). Adherent streptococci were quantified fluorometrically.ResultsThe lithium disilicate glass ceramic showed the highest values for Ra; the lowest values were found for the glass ceramic, the partially sintered zirconia and the hipped zirconia ceramic. Protein coating caused a significant increase in γ^t and γ^p, but not in the control material. The control material showed higher values for streptococcal adhesion than all ceramic materials. After protein coating, only slight and random differences in streptococcal adhesion were found between the various ceramic materials.SignificanceDental ceramic materials show differences in terms of Ra, γ^t and initial streptococcal adhesion; however, correlations between surface properties and streptococcal adhesion were poor.     

Influence of the application of a pre-sintered surface augmentation on zirconia and lithium disilicate bonding using an adhesive composite resin cement

ObjectiveThe purpose of this laboratory study is to evaluate the application of a pre-sintered surface augmentation to zirconia (Zir) and lithium disilicate (LDS) ceramics on the delamination strength of adhesive resin cement. The applied surface augmentation was the ruling of lines to the pre-sintered surface of the ceramics.MethodsNinety milled Zir and sixty pressed LDS specimens (3 mm × 0.5 mm × 25 mm) were created and divided into five groups (n = 30). Group 1: Zir no surface treatment (control Zir-NT); Group 2: Zir airborne particle abraded (Zir-APA) with 30 μm CoJet; Group 3: Zir pre-sintered surface augmentation (Zir-SA); Group 4: LDS etched (control LDS-etched) and; Group 5: LDS with pre-sintered surface augmentation and etching (LDS-SA). A resin adhesive cement (3 mm × 1 mm × 8 mm) was then applied and cured to the ceramic specimens. The delamination strength values of the resin cement from the ceramic were recorded. The delamination strength data were analysed statistically using one-way ANOVA and Turkey post hoc analysis.ResultsThe mean delamination strength and standard deviation, when comparing only the Zir-SA to the resin cement were statistically different (p < 0.001); Zir-SA 63.42 ± 11.85, Zir-NT 26.82 ± 12.07, and Zir-APA 48.11 ± 17.85 MPa. Comparison between LDS groups were not significantly different (p = 0.193); LDS-etched 33.49 ± 16.07 and LDS-SA 28.83 ± 10.15 MPa. The delaminated Weibull modulus was highest for surface augmentation Zir specimens (m = 13.56) but decreasing to less than half for Zir-APA (m = 6.27) and Zir-NT (m = 5.68). The Weibull values for the LDS-SA and LDS-etched specimens was 5.63 and 3.38 respectively.SignificanceIncorporating the pre-sintered surface augmentation to zirconia improved the delamination strength and reliability of Zir to the resin cement but not for LDS.     

Press-on force during polishing of resin composite restorations

ObjectiveTo measure the press-on force during the polishing of composite restorations carried out by 10 dentists in a clinically simulated procedure.MethodsComposite restorations (Tetric EvoCeram Bulk Fill) were placed in standardized Class II two-surface cavities in first upper acrylic molars. The surfaces were roughened by sandblasting (50 μm, 1 bar). The tooth was mounted on a tailor-made device with a 3D force sensor (Kistler, Z21134-300, 10H z). Ten dentists (7 male, 3 female) polished one Class II restoration each using the one-step polishing system OptraPol together with a dental handpiece and water spray. The dentists were allowed to use all shapes of the polishing system (small flame, large flame, cup, lens). During polishing, the press-on forces measured for up to 2 minutes. Simultaneously, the polishing procedure was recorded with a digital camera to correlate the forces with the polishing shapes and movements.ResultsIn total, 17,999 force measurements were available for analysis. The mean forces of all operators varied between 0.77(±0.63) N and 2.23(±1.48) N; the difference was statistically significant (ANOVA, post-hoc Tukey B, p < 0.05). All dentists exerted maximum forces higher than 3 N (between 3.3 N and 18.3 N). Force values exceeded 2 N during 25% of the polishing time. Female dentists polished with a statistically significant lower force than male dentists (ANOVA, p < 0.001). Polishing with the large flame and the cup generated significantly higher forces than polishing with the small flame and the lens (ANOVA, post hoc Tukey B, p < 0.01).ConclusionsThe press-on forces applied during polishing varied significantly between dentists and within the same dentist. In about 25% of the polishing time, forces were above the 2 N limit, which is recommended by some manufacturers as the maximum polishing force. Test institutes and manufacturers should evaluate the polishing performance of polishing instruments with various press-on forces.     

Comparative effects of tungsten,diamond burs and laser for residual adhesive removal after orthodontic debonding on flexural strength,surface roughness and phase transformation of high-translucent zirconia: An in vitro study

ObjectivesInformation regarding the effects of orthodontic bracket debonding on zirconia restorations, and the preferred method for residual adhesive removal from the zirconia restoration surface is lacking. Thus, this study aimed to assess the effects of different methods of residual adhesive removal after orthodontic bracket debonding on flexural strength, surface roughness, and phase transformation of high-translucent (HT) zirconia.Materials and methodsThis in vitro study evaluated 72 bar-shaped HT zirconia specimens; 18 specimens were assigned to the control group. Metal brackets were bonded to the remaining specimens by resin cement. After bracket debonding, the residual adhesive on the surface of specimens was removed by three methods (n = 18): a 30-flute tungsten-carbide (TC) bur at low speed, an ultrafine diamond bur at high speed, and Er:YAG laser irradiation. The surface roughness (Ra and Rz) was measured. X-ray diffraction (XRD) analysis was carried out, and the flexural strength was measured as well. Data were statistically analyzed (α = 0.05).ResultsBefore polishing, all methods increased the Ra and Rz values (P < 0.05) except for the diamond bur yielding a Rz value comparable to that of the control group. The Ra values of the test groups were comparable after polishing, and still higher than that of the control group (P < 0.05). The flexural strength of all three test groups was comparable (P > 0.05), and significantly lower than that of the control group (P < 0.001). The monoclinic phase was not observed in any group.ConclusionsOrthodontic bracket debonding adversely affects the surface roughness and flexural strength of zirconia despite polishing.     

Effect of repeated firings and staining on the mechanical behavior and composition of lithium disilicate

ObjectiveTo evaluate the composition, flexural strength and fatigue behaviour of lithium disilicate ceramic (LD) after repeated firings and different staining techniques.MethodsLD discs were fabricated and divided according to number of firing cycles and staining technique: CO - control, discs were crystallized (850 °C/10 min); SC – single-step characterization – crystallization and staining (applied with a thin brush) were performed in a single step with one firing cycle (850 °C/10 min); and DC – double-step characterization – crystallization firing cycle was performed first (850 °C/10 min), followed by staining firing cycle (770 °C/90 s). Specimens were fired two, four or six times (one crystallization firing cycle and one, three or five staining firing cycles), resulting into 9 groups (n = 30): COII, COIV, COVI, SCII, SCIV, SCVI, DCII, DCIV and DCVI. The composition of the specimens was investigated (EDS, XRD, Raman spectroscopy), and the biaxial flexural strength (n = 10) and staircase tests (n = 20, 5 × 10⁴ cycles, 5 Hz) were performed. Data were subjected to one-way ANOVA and Tukey’s test (α = 0.05).ResultsEDS and XRD revealed amorphous content for stained groups. Biaxial flexural strength was not affected by repeated firings in any group, but stained groups presented lower flexural strength than control groups (p = 0.001). The fatigue limit results decreased in all groups compared to flexural strength. SC groups showed similar (SCII and SCIV) or even higher fatigue limits (SCVI) than the control groups, and DC showed the lowest fatigue limit values. SEM and Raman suggested that the interfaces between staining and the LD showed only an overlap for the DC groups, whereas for the SC it was suggested an interaction between the stain and the LD.SignificanceRepeated firings did not result in decreased lithium disilicate flexural strength.Staining affected flexural strength and also resulted in increased amorphous content in the characterized specimens. Single-step staining resulted in the highest fatigue limit.     

Influence of bonding surface and bonding methods on the fracture resistance and survival rate of full-coverage occlusal veneers made from lithium disilicate ceramic after cyclic loading

ObjectivesThe purpose of this laboratory study was to evaluate the influence of bonding method and type of dental bonding surface on fracture resistance and survival rate of resin bonded occlusal veneers made from lithium disilicate ceramic after cyclic loading.MethodsFourty-eight extracted molars were divided into three groups (N = 16) depending on the preparation: within enamel, within dentin/enamel or within enamel/composite resin filling. Lithium disilicate occlussal veneers were fabricated with a fissure-cusp thickness of 0.3–0.6 mm. Restorations were etched (5% HF), silanated and adhesively luted using a dual-curing luting composite resin. Test groups were divided into two subgroups, one using a only a self-etching primer, the other additionally etching the enamel with phosphoric acid. After water storage (37 °C; 21 d) and thermocycling (7500 cycles; 5–55 °C), specimens were subjected to dynamic loading in a chewing simulator (600,000 cycles; 10 kg/2 Hz). Surviving specimens were loaded until fracture using a universal testing machine.ResultsAll specimens survived artificial aging, several specimens showed some damage. ANOVA revealed that enamel etching provided statistically significantly (p ≤ 0.05) higher fracture resistance than self-etching when bonding to enamel and dentin. Self-etching provided statistically significant (p ≤ 0.05) higher fracture resistance for the enamel-composite group than for the enamel group. Enamel etching provided statistically significant (p ≤ 0.05) higher fracture resistance for the enamel and dentin group than for groups enamel and enamel-composite.SignificanceEtching enamel improved the fracture resistance of occlusal veneers when bonding to dentin and enamel and increased the survival rate when bonding to enamel.     

Effect of finishing/polishing techniques and low temperature degradation on the surface topography,phase transformation and flexural strength of ultra-translucent ZrO2 ceramic

ObjectiveTo investigate the effect of different surface finishing and polishing regimes and low temperature degradation on flexural strength, phase transformation and surface topography of ultra-translucent ZrO₂ ceramic.Methods300 (n = 15/group) of conventional zirconia (Z: Ice Zirkon Transluzent) and ultra-translucent zirconia (UT: Prettau Anterior) bar-specimens were made and divided according to the "Finishing/Polishing" - (C — Control, B — diamond rubber polishers, P — adjusting with burs, PB — adjusting with burs + diamond polishers, PG — adjusting with burs + glaze), "Low temperature Degradation (LTD)" (with or without a treatment at 127 °C, 1.7 bar/24 h). Then, a 3-point mini flexural test was performed in a universal testing machine (1 mm/min, 500 kgf load cell). SEM, EDS, XDR, AFM, optical profilometry and Weibull analysis were performed. Data were analyzed by 3-way ANOVA and Tukey’s post-test (5%).ResultsGroups ZPB_D (1670 ± 253 MPa), ZB_D (1664 ± 217 MPa), and ZB (1655 ± 3678 MPa) showed significantly higher flexural strength than the UTPG group (372 ± 56 MPa). The Weibull modulus was significantly higher for the ZP_D group compared to the UB, UC_D, UP_D and UPB_D, while UTB, UTC_D and UTP_D had the lowest value. Monoclinic phases were observed only in the conventional zirconia groups and were more evident after LTD. Diamond rubber polishers presented less roughness for both zirconias.SignificanceThe use of diamond rubber polishers is the most suitable finishing/polishing method for zirconia ceramic restorations and that final glazing reduces the fracture resistance of these materials.     

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

ObjectivesTo evaluate ultrashort-pulse laser (UPL) as a surface treatment for improved bond strength to Yttria-tetragonal zirconia polycrystalline (Y-TZP).MethodsFully-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 2.5 μm spacing (8 mJ, 10 kHz)(LSD); or single pulse laser surface lines with 2.5 μm spacing (4 mJ, 6.7 kHz)(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 48 h water incubation (37 °C) with or without subsequent thermocycling (5–55 °C, 5000 cycles).ResultsAll 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 ± 4.53 MPa > LSL: 31.84 ± 8.46 MPa > SIL: 19.95 ± 3.99 MPa = ALB: 19.51 ± 2.55 MPa > CTL: 14.51 ± 2.23 MPa). Thermocycling significantly reduced bond strength for all treatments in a surface treatment-dependent manner.SignificanceThe 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.     

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

ObjectiveTo evaluate the reliability and failure mode of zirconia-reinforced lithium silicate (ZLS) molar crowns of different thicknesses.MethodsMonolithic ZLS molar crowns (0.5 mm, 1.0 mm, 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.ResultsIrrespective of crown thickness, beta (β) 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 200 N, 300 N 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.Significance1.5 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.