Flexural Strength of Different Monolithic Computer-Assisted Design and Computer-Assisted Manufacturing Ceramic Materials upon Different Thermal Tempering Processes

Color Characteristics of High Yttrium Oxide-Doped Monochrome and Multilayer Partially Stabilized Zirconia upon Different Sintering Parameters

OBJECTIVES

 Sintering influences the optical properties of zirconia. This study examined the effect of altering sintering temperature and time of monochrome (Mo) and multilayer (Mu) 5 mol% yttria-partially stabilized zirconia (5Y-PSZ) on color characteristics.

MATERIALS AND METHODS

 Three hundred specimens (width × length × thickness = 10 × 20 × 2 mm) were prepared from Mo and Mu (with cervical [C], middle [M], and incisal [I] region) 5Y-PSZ and randomly sintered at decreasing (Td: 1,450°C), regular (Tr: 1,500°C), and increasing (TI: 1,550°C) sintering temperature, with extremely short (He: 10 minutes), ultrashort (Hu: 15 minutes), short (Hs: 30 minutes), and regular (Hr: 135 minutes) sintering time (n = 15/group). Color appearance (EW ), translucency parameter (TP), contrast ratio (CR), opalescence parameter (OP), and color appearance difference (∆E diff) were evaluated in the CIE L*a*b* system. Microstructures were evaluated by scanning electron microscope (SEM) and X-ray diffractometer (XRD).

STATISTICAL ANALYSIS

 Analysis of variance (ANOVA) and Bonferroni comparisons were determined for significant differences (p < 0.05).

RESULTS

 Significant differences in color parameters upon zirconia type, sintering temperature, and sintering time, and their interactions were indicated (p < 0.05). Increasing sintering temperature and extended sintered time resulted in larger grain, reduced tetragonal-to-monoclinic phase transformation, and significantly increased the TP and OP, but decreased the CR and ∆E diff (p < 0.05). Decreasing sintering temperature and time led to clinically unacceptable color appearance.

CONCLUSION

 Mo was found to be more translucent than Mu. To achieve the most favorable optical properties, increasing sintering temperature and extending sintering time are recommended. Decreasing sintering temperature is not suggested. However, shortened sintering time is feasible, but it needs sintering with increasing sintering temperature to achieve a promising color appearance.

Flexural strength of novel glass infiltrated monochrome and multilayer high yttrium oxide containing zirconia upon various sintered cooling rates

PURPOSE

The sintering technique and cooling strategy influence the strength of zirconia. This study examined the impact of altering the cooling rate of glass-infiltrated monolayer and multilayer 5 mol% yttria-partially stabilized zirconia (5Y-PSZ) on their strength.

MATERIALS AND METHODS

One-hundred eighty (180) specimens (width × length × thickness = 10 × 20 × 2 mm) were prepared using monolayer (Mo: Cercon-xt) and multilayer (Mu: Cercon-xt ML) 5Y-PSZ. Randomly distributed specimens (n = 15/group) were sintered with traditional (T) versus glass infiltrated (G) technique and cooled down with different cooling rates: slow (S: 5°C/min), normal (N: 35°C/min), and fast (F: 70°C/min). Four-point bending test was used to measure flexural strength (σ). Microstructures were investigated by scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). Three-way ANOVA and Tamhane comparisons were determined for a significant difference of σ (p < 0.05). Weibull analysis was determined for Weibull modulus (m).

RESULTS

The highest σ (MPa) was seen for GMuS (696.8 ± 69.8). Mo-PSZ and Mu-PSZ showed no significant difference in σ. G-sintering presented significantly higher σ (659.9 ± 79.3) than T-sintering (426.0 ± 63.7). S-cooling (560.9 ± 126.1) had the highest σ. The highest m-value was observed in GMuN (12.1 ± 3.8). A significant difference in σ was indicated due to cooling rates and sintering techniques (p < 0.05).

CONCLUSIONS

Glass infiltration significantly enhanced strength through elastic gradience. F-cooling reduced grain size, impaired grain boundary integration, and increased the tetragonal to monoclinic phase transition, significantly decreasing flexural strength in traditional sintering. Nevertheless, F-cooling was recommended for glass-infiltrated 5Y-PSZ to enhance strength while reducing processing time.

Flexural Strength of Different Monolithic Computer-Assisted Design and Computer-Assisted Manufacturing Ceramic Materials Upon Accelerated Aging

OBJECTIVES

 The durability of ceramic is crucial, which is probably influenced by aging. This study evaluated the effect of aging on flexural strength of different ceramics.

MATERIALS AND METHODS

 One-hundred twenty ceramic discs (Ø 12 mm, 1.5 mm thickness) were prepared from zirconia-reinforced lithium silicate (ZLS, C), lithium disilicate (LS2, E), precolored yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP, Ip), and customized color Y-TZP (Ic). Samples were randomly divided into two groups for accelerated aging (A) between 5 and 55°C water baths, 30-second immersing time each, for 10,000 cycles, and nonaged group (N), serving as control. Biaxial flexural strength (σ) was evaluated utilizing the piston-on-three-balls at 0.5 mm/min speed. Analysis of variance and Tukey comparisons were determined for significant differences (α = 0.05). Weibull analysis was applied for survival probability, Weibull modulus (m), and characteristic strength (σo). Microstructures were evaluated with scanning electron microscopy and X-ray diffraction (XRD).

RESULTS

 The highest σ and σo were seen for IcN, followed by IcA, IpN, IpA, EN, CA, CN, and EA, respectively. CN showed the highest m, while EA showed the lowest m. Significant differences of σ for each ceramic were indicated (p < 0.05). Aging caused a significant difference in σ (p < 0.05). XRD showed t→m phase transformation of Ip and Ic after aging.

CONCLUSION

 Aging affected strength of ceramics. Comparable strength between LS2 and ZLS was evidenced, but both were less strength than Y-TZP either aging or non-aging. Comparable strength between precolored Y-TZP and customized color Y-TZP was indicated. Better resisting aging deterioration of Y-TZP than LS2 and ZLS is suggested for fabrication restorative reconstruction.

Effect of sintering programs and surface treatments on monolithic zirconia

PURPOSE

To investigate the effect of sintering programs and surface treatments on surface properties, phase transformation and flexural strength of monolithic zirconia.

MATERIALS AND METHODS

Zirconia specimens were sintered using three distinct sintering programs [classic (C), speed (S), and superspeed (SS)] (n = 56, each). One sample from each group underwent scanning electron microscopy (SEM) and grain size analysis following sintering. Remaining samples were divided into five subgroups (n = 11) based on the surface treatments: control (CL), polish (P), glaze (G), grind + polish (GP), and grind + glaze (GG). One sample from each subgroup underwent SEM analysis. Remaining samples were thermally aged. Monoclinic phase volume, surface roughness, and three-point flexural strength were measured. Monoclinic phase volume and surface roughness were analyzed by Kruskal-Wallis and Dunn tests. Flexural strength was analyzed by two-way ANOVA and Weibull analysis. The relationships among the groups were analyzed using Spearman's correlation analysis.

RESULTS

Sintering program, surface treatment, and sintering × surface treatment (P ≤ .010) affected the monoclinic phase volume, whereas the type of surface treatment and sintering × surface treatment affected the surface roughness (P < .001). Type of sintering program or surface treatment did not affect the flexural strength. Weibull analysis revealed no significant differences between the m and σo values. Monoclinic phase volume was positively correlated with surface roughness in the SGG and SSP groups.

CONCLUSION

After sintering monolithic zirconia in each of the three sintering programs, each of the surface treatments can be used. However, for surface quality and aging resistance, G or GG can be recommended as a surface finishing method.

Optical Characteristics of Monochrome and Multilayer Fully Stabilized Zirconia Upon Sintered Cooling Speed

OBJECTIVES

 Firing protocols influence optical properties of dental ceramics. Effects of varying cooling rates of monochrome and multilayer 5 mol% yttria-stabilized tetragonal polycrystalline (5YTZP) on optical properties are subjected for investigation.

MATERIALS AND METHODS

 Ninety specimens (width, length, thickness = 10 × 20 × 2 mm) were prepared from monochrome (Mo: Cercon xt) and multilayer (Mu: Cercon xt ML with cervical (C) and incisal (I) zoning) 5YTZP. Specimens were sintered and randomly treated with three cooling rates (n = 15/group): slow (S: 5°C/min), normal (N: 35°C/min), and fast (F: 70°C/min). Color appearance (∆EW), color appearance difference (∆E diff), translucency parameter (TP), contrast ratio (CR), and opalescence parameter (OP) were evaluated in CIEL*a*b* (Commission International de I'Eclairage) system. ∆E diff was achieved from the coordinate difference of specimen to VITA classic shade A2. Microstructures and compositions were evaluated by scanning electron microscope and energy dispersive spectroscopy. Monoclinic (m), tetragonal (t), and cubic (c) phases were investigated with X-ray diffraction.

STATISTICAL ANALYSIS

 An analysis of variance and Bonferroni multiple comparisons were determined for significant differences (p < 0.05).

RESULTS

 ΔEW of MoF was highest (66.04 ± 1.86), while MuN-I was lowest (62.60 ± 0.86). TP and OP of MoS were highest at 2.85 ± 0.11, and 2.25 ± 0.10, while MuF-I was lowest at 2.16 ± 0.10 and 1.60 ± 0.12. CR of MuF-I was highest (0.948 ± 0.005), while MoS was lowest (0.936 ± 0.005). ΔEdiff of MoF was highest (3.83), while MuN-I was lowest (0.93). Limited grain growth and m-phase composition were indicated upon fast cooling. There were significant differences for all color parameters due to varied materials, cooling rates, and their interactions (p < 0.05) except for interaction in ∆EW and OP.

CONCLUSIONS

 Translucency of monochrome and multilayer 5YTZP were different, possibly due to colorant additives. Incisal layer of multilayer 5YTZP was perfectly matched with VITA shade. Increasing cooling speed resulted in smaller grain size, t-m transformation, and finally lower translucency and opalescence. Therefore, to achieve most favorable optical properties, slow cooling rate is recommended.

Flexural Strength of CAD/CAM Lithium-Based Silicate Glass-Ceramics: A Narrative Review

Amongst chairside CAD/CAM materials, the use of lithium-based silicate glass-ceramics (LSGC) for indirect restorations has recently been increasing. Flexural strength is one of the most important parameters to consider in the clinical selection of materials. The aim of this paper is to review the flexural strength of LSGC and the methods used to measure it.

METHODS

The electronic search was completed within PubMed database from 2 June 2011 to 2 June 2022. English-language papers investigating the flexural strength of IPS e.max CAD, Celtra Duo, Suprinity PC, and n!ce CAD/CAM blocks were included in the search strategy.

RESULTS

From 211 potential articles, a total of 26 were identified for a comprehensive analysis. Categorization per material was carried out as follows: IPS e.max CAD (n = 27), Suprinity PC (n = 8), Celtra Duo (n = 6), and n!ce (n = 1). The three-point bending test (3-PBT) was used in 18 articles, followed by biaxial flexural test (BFT) in 10 articles, with one of these using the four-point bending test (4-PBT) as well. The most common specimen dimension was 14 × 4 × 1.2 mm (plates) for the 3-PBT and 12 × 1.2 mm (discs) for BFT. The flexural strength values for LSGC materials varied widely between the studies.

SIGNIFICANCE

As new LSGC materials are launched on the market, clinicians need to be aware of their flexural strength differences, which could influence the clinical performance of restorations.

The effects of heating rate and sintering time on the biaxial flexural strength of monolithic zirconia ceramics

The strength of zirconia ceramic materials used in restorations is dependent upon sintering. Varying sintering protocols may affect the biaxial flexural strength of zirconia materials. This in vitro study was conducted to investigate the effects of sintering parameters on the biaxial flexural strength of monolithic zirconia. Two different monoblock zirconia ceramics were used. Following coloration, samples of both types of ceramics were divided into groups according to whether or not biaxial flexural strength testing was performed directly after sintering or following thermocycling. Biaxial flexural strength data was analysed with a Shapiro Wilk normality test, followed by 1-way ANOVA, Tukey post hoc tests for inter-group comparisons, and paired samples t-tests for intra-group comparisons. A significant difference was found between the biaxial flexural strengths of Zircon X and Upcera ceramics before thermocycling (p<0.05). In both Zircon X and Upcera ceramic groups, the thermocycling process created a significant difference in the biaxial flexural strength values of the ceramic samples in Group 6 (p<0.05) which had the slowest heating rate and longest holding time. The zirconia ceramics have higher BFS at higher heating rates either before or after thermocycling. The holding time has significant effects on thermocycling and flexural strength. The zirconia achieved its optimum strength when it sintered at longer time regardless of heating rates.

The ability of mouthguards to protect veneered teeth: A 3D finite element analysis

BACKGROUND/AIMS

Professional and amateur athletes might have veneer restorations. The aim of this study was to investigate the protective effect of mouthguards on veneered anterior restorations.

METHODS

A nonlinear dynamic analysis was performed to simulate conditions during an impact with or without a custom-made mouthguard. Using a computer-aided design (CAD) software, a slice of a human maxilla was designed containing an upper right central incisor. The model was composed of mucosa, cortical bone, trabecular bone, periodontal ligament, dentin, enamel, and pulp tissue. The enamel was prepared (feather design), restored with an indirect veneer (1.0 mm thickness), and duplicated to simulate both conditions with or without a mouthguard (4 mm thickness). Both models were subdivided into finite elements using the computer-aided engineering (CAE) software. Frictionless contacts were used, and an impact was simulated in which a rigid sphere hit the model at 1 m s^-1 . Fixation was defined at the base of the bone. The elastic modulus of the veneer was assessed by using five different restorative materials (resin composite, hybrid ceramic, zirconia-reinforced lithium silicate, lithium disilicate, and zirconia). Von Mises stress, minimal principal stress, and maximum principal stress (in MPa) were obtained and plotted for visual comparison.

RESULTS

Von-Mises results showed higher stress concentrations in the veneer's cervical labial region for models without a mouthguard. Observing the quantitative results for each model, the highest compressive (709 MPa) and tensile (58 MPa) stresses occurred in the situation without a mouthguard with a zirconia veneer, while the lowest occurred in resin composite veneer with a mouthguard (8 and 5 MPa). The mouthguard was able to reduce the stresses in the tooth structure and it also reduced the risk of fracture in all conditions.

CONCLUSIONS

Mouthguards were beneficial in reducing the effects of dental trauma regardless of the restorative material used to manufacture the indirect veneer, since they act by dampening the generated stresses during the trauma event. Equal impact stresses on a mouthguard will lead to higher stresses in veneered teeth with more rigid restorative materials leading to a less protective effect.

Factory Crystallized Silicates for Monolithic Metal-Free Restorations: A Flexural Strength and Translucency Comparison Test

Flexural strength (FS) and translucency (Contrast Ratio-CR) of three different factory crystallized silica-based glass ceramics, Celtra Duo (CD), N!ce (NI) and Li-Si Block, a lithium disilicate, IPS e.max CAD (LD), and a leucite-reinforced feldspathic ceramic, Empress CAD (EM), in two different translucencies (HT and LT) for use in chairside dental restorations have been compared. CAD blocks of the materials were cut into beams and tiles and processed following manufacturers’ instructions. The beams were tested (3-PBT) to determine flexural strength, Weibull characteristic strength, and Weibull modulus; and tiles were tested to determine CR. All data were statistically analyzed. In addition, SEM analysis of the materials was performed. Differences in flexural strength (FS) and translucency (CR) between the materials were found to be statistically significant. FS decreased as follows (MPa): LDHT 350.88 ± 19.77 (a) = LDLT 343.57 ± 18.48 (a) > LSLT 202.15 ± 17.41 (b) = LSHT 196.93 ± 8.87 > NIHT 186.69 ± 13.06 (c) = CDLT 184.73 ± 13.63 (c) = CDHT 174.15 ± 21.76 (c) = NILT 172.12 ± 11.98 (c) > EMHT 131.16 ± 13.33 (e) = EMLT 127.65 ± 11.09. CR decreased as follows (mean ± sd): CDLT 74.1 ± 1.1 (a); LSLT 74.0 ± 1.1 (ab); NILT 73.3 ± 0.8 (ab); EMLT 73.0 ± 1.5 (ab); NIHT 72.4 ± 1.0 (bc); LDLT 71.3 ± 1.1 (bc); LTHT 65.2 ± 0.9 (de); LSHT 63.8 ± 1.1 (def); EMHT 636 ± 1.2 (ef); CDHT 62.2 ± 0.8 (f). Our findings show that factory-crystallized lithium silicate glass ceramics fulfill ISO standards for Classes 1 and 2. Therefore, they can be considered viable alternatives to produce single-unit restorations with a chairside procedure not requiring thermal treatment.

Biaxial Flexural Strength of Different Monolithic Zirconia upon Post-Sintering Processes

OBJECTIVE

 Different post-sintering processes are expected to be a reason for alteration in the strength of zirconia. This study evaluated the effect of post-sintering processes on the flexural strength of different types of monolithic zirconia.

MATERIALS AND METHODS

 A total of 120 classical- (Cz) and high-translucent (Hz) monolithic zirconia discs (1.2 mm thickness and 14 mm in Ø) were prepared, sintered, and randomly divided into four groups to be surface-treated with (1) as-glazed (AG); (2) finished and polished (FP); (3) finished, polished, and overglazed (FPOG); and (4) finished, polished, and heat-treated (FPHT) technique (n = 15). Biaxial flexural strength (σ) was determined on a piston-on-three ball in a universal testing machine at a speed of 0.5 mm/min.

STATISTICAL ANALYSIS

 Analysis of variance, and post hoc Bonferroni multiple comparisons were determined for significant differences (α = 0.05). Weibull analysis was applied for survival probability, Weibull modulus (m), and characteristic strength (σ₀). The microstructures were examined with a scanning electron microscope and X-ray diffraction.

RESULTS

 The mean ± standard deviation value of σ (MPa), m, and σ₀ were 1,626.43 ± 184.38, 9.51, and 1,709.79 for CzAG; 1,734.98 ± 136.15, 12.83, and 1,799.17 for CzFP; 1,636.92 ± 130.11, 14.66, and 1,697.63 for CzFPOG; and 1,590.78 ± 161.74, 10.13, and 1,663.82 for CzFPHT; 643.30 ± 118.59, 5.59, and 695.55 for HzAG; 671.52 ± 96.77, 3.28, and 782.61 for HzFP; 556.33 ± 122.85, 4.76, and 607.01 for HzFPOG; and 598.36 ± 57.96, 11.22, and 624.89 for HzFPHT. The σ was significantly affected by the post-sintering process and type of zirconia (p < 0.05), but not by their interactions (p > 0.05). The Cz indicated a significantly higher σ than Hz. The FP process significantly enhanced σ more than other treatment procedures.

CONCLUSION

 Post-sintering processes enabled an alteration in σ of zirconia. FP enhanced σ, while FPOG and FPHT resulted in a reduction of σ. Glazing tends to induce defects at the glazing interface, while heat treatment induces a phase change to tetragonal, both resulted in reducing σ. Finishing and polishing for both Cz and Hz monolithic zirconia is recommended, while overglazed or heat-treated is not suggested.

Surface Roughness and Translucency of Various Translucent Zirconia Ceramics after Hydrothermal Aging

OBJECTIVE

 This study investigated the effect of hydrothermal aging on surface roughness and translucency of various translucent zirconia materials.

MATERIALS AND METHODS

 Four types of zirconia were tested. Group 1 was translucent zirconia with no cubic structure. Group 2, 3, and 4 included cubic-containing zirconia with different amounts of cubic structures (less than 30%, 30-50%, and more than 50%, respectively). Each group contained 15 disk-shape specimens with dimensions of 15 mm in diameter and 1 mm in thickness. As-sintered surface roughness, translucency parameter, and contrast ratio were evaluated in the two different sessions, before and after aging.

STATISTICAL ANALYSIS

 Two-way repeated measures ANOVA with Bonferroni test was used to analyze statistically significant difference in those tested parameters. Phase structure before and after aging was analyzed by X-ray diffraction analysis (XRD).

RESULTS

 Groups 1 and 2 showed significant increased surface roughness after aging while groups 3 and 4 showed no alteration of surface. There was no effect of aging on translucency in all groups. After aging, group 1 and 2 presented monoclinic structure (16.63 and 5.01%, respectively).

CONCLUSION

 Hydrothermal aging caused phase transformation and increasing surface roughness in group 1 and 2 but did not affect translucency in all groups.

Influence of the Multiple Layers Application and the Heating of Silane on the Bond Strength between Lithium Disilicate Ceramics and Resinous Cement

Objective This study aimed to evaluate the bond strength between lithium disilicate ceramic and resinous cement when silane (Prosil, FGM) was applied in different amounts of layers under heating or not.

Materials and Methods Sixty IPS E-max CAD ceramic (Ivoclar) was used. They were conditioned with 10% hydrofluoric acid for 20 seconds. The samples were distributed in six groups ( n = 10): 1Sil, 1 layer of silane without heating; 1SilAq, 1 layer of silane with heating; 2Sil, 2 layers without heating; 2SilAq, 2 layers with heating; 3Sil, 3 layers without heating; and 3SilAq, 3 layers with heating. After each layer, a jet of cold air was applied for 20 seconds in groups 1Sil, 2Sil, 3Sil, and jet of hot air (50°C) in groups 1SilAq, 2SilAq, and 3SilAq. Subsequently, an adhesive layer was applied, and fourcylinders were made on the ceramic with a resin cement AllCemVeneer and photoactivated for 20 seconds. The samples were stored at 37°C for 24 hours and analyzed to the microshear test at EMIC.

Statistical Analysis Data were submitted to ANOVA and Tukey’s test (α = 0.05).

Results The results showed that there was no statistical interaction between the factors studied. The “heating” factor was not statistically significant; however, the “silane layers” factor showed differences between groups. The analysis of the results showed that the use of one (66%) or two layers (67%) of silane regardless of heating, produced higher values of bond strength, when compared with the group of three layers (62%).

Conclusion The use of silane with one or two layers provided a greater bond strength between lithium disilicate ceramic and resinous cement and that the heating did not influence the results.