Extended glaze firing improves flexural strength of a glass ceramic

Effect of staining techniques and repeated firing cycles on translucency, color and biaxial flexural strength of advanced lithium disilicate containing Virgilite crystals

BACKGROUND

The repeated firings can enhance shade matching, translucency, and strength; however, they may also lead to color shifts. Previous research suggests that multiple firings enhance these properties to a certain extent; however, the impact of staining techniques remains underexplored. The aim of this study is to investigate the effect of staining techniques and multiple firings on the translucency, color and biaxial flexural strength of advanced lithium disilicate ALD containing Virgilite crystals.

METHODS

Sixty-three discs of ALD (CEREC Tessera®) were divided into 3 groups based on staining techniques (n = 21); group CO (glaze only), group SC (single-step characterization), and group DC (double-step characterization). The discs were then subjected to either 2, 4, or 6 firing cycles, resulting in 9 groups (n = 7): COII, COIV, COVI, SCII, SCVI, DCII, DCIV, and DCVI. Relative translucency parameter (RTP), color change (ΔE), and biaxial flexural strength were measured, then discs were analyzed using SEM. Data were statistically analyzed using ANOVA, Bonferroni correction, and Spearman's correlation (α = 0.05).

RESULTS

Repeated firing and staining techniques significantly affected translucency, color change, and biaxial flexural strength (p < 0.001). Translucency increased with firings, highest in CO and lowest in DC. ΔE increased with firings, highest for DC and lowest in CO. The biaxial flexural strength of the CO group remained stable across firing cycles, with no significant changes. The SC group, initially the weakest, showed a significant increase, reaching its peak after six cycles. The DC group had high strength in the fourth cycle, with a significant difference observed between the second and fourth cycles. By the sixth cycle, all groups showed comparable strength with no significant differences.

CONCLUSIONS

Within the limitation of this study, firing cycles and staining techniques impact the properties of ALD. More firing cycles enhance translucency but increase color change. Repeated firing, particularly with the double-step characterization technique, significantly improved biaxial flexural strength up to the fourth cycle, demonstrating its superior performance over the single-step characterization technique.

Effect of extrinsic pigmentation and multiple firing cycles on the properties of a zirconia-containing lithium silicate ceramic

PURPOSE

To assess the impact of staining and multiple firings on the mechanical, optical, and surface characteristics of zirconia-containing lithium silicate ceramics (ZLS).

MATERIALS AND METHODS

Ninety ZLS discs (Suprinity, VITA Zahnfabrick) were divided according to the "Number of firings" protocol: Ctr-control, no characterization; SC-single firing cycle (for characterization, crystallization and staining simultaneously); and DC-double firing cycle (crystallization firing cycle was performed separately from the staining firing). Extrinsic pigmentation was performed to replicate the characterization of a monolithic restoration. A layer of stain and glaze was applied with a fine brush to the ceramic surface. Specimens were fired two or four cycles, designed to simulate additional staining firings for potential characterization adjustments, but without applying new stain, resulting in six groups (n = 15): CtrII, CtrIV, SCII, SCIV, DCII, and DCIV. X-ray diffraction, color, translucency, and surface roughness analysis were also performed, and the samples were subjected to the biaxial flexural strength test. Data were subjected to one-way ANOVA, Tukey's test (α = 0.05), and Weibull analysis.

RESULTS

No changes in the diffractogram pattern were found, irrespective of the number of firings. The translucency (p = 0.02) and roughness (p < 0.01) of DCIV were significantly reduced compared to DCII. DCIV also showed the highest flexural strength (325.52AMPa), followed by SCII (271.64BMPa).

CONCLUSIONS

Repeated firings increased the flexural strength of ZLS, where the double-step characterization technique demonstrated better statistical results than the other groups. Moreover, despite the ceramic showing good color stability, repeated firings significantly compromised its translucency.

Mechanical and optical properties of a borosilicate glass used to improve the finishing of 3Y-TZP restorations

Borosilicate glass was developed to enhance the mechanical behavior and smoothness of dental zirconia as an alternative to conventional glaze. This study assessed the mechanical and optical properties of 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) coated with borosilicate glass or a commercial glaze fired for an extended period of time. Disc-shaped 3Y-TZP zirconia specimens (Zpex, Tosoh) were sintered at 1550°C for 2 hours. The specimens were divided into three groups: as-sintered (control, C); commercial glaze (G); and borosilicate glass (SL). The glaze and borosilicate glass were applied over the zirconia and fired for 20 minutes at 950°C and 1200°C, respectively. Biaxial flexural strength, fractography, X-ray diffraction (XRD), roughness (Ra and Rz), fracture toughness (Vickers indentation method), color difference (∆E00), and translucency (TP00) analyses were conducted. The t-test or the one-way ANOVA and Tukey's tests were used to analyze the data (α = 0.05). Flexural strength data were subjected to the Weibull analysis. The SL group exhibited the highest flexural strength (1025.8 MPa), whereas the C (859.41 MPa) and G (816.0 MPa) groups exhibited similar values. The SL group also had the highest characteristic strength. The fracture origin in all groups was on the zirconia surface. XRD analysis revealed that the specimens from the SL group contained tetragonal, cubic, and monoclinic phases. The SL group presented the lowest surface roughness. Fracture toughness in the SL group was lower than in the C group, but similar to that observed in the G group. The translucency and color differences observed in the G and SL groups were similar. Borosilicate glass enhanced the flexural strength of 3Y-TZP, promoted the smoothest surface, and exhibited optical properties similar to those of the glaze.

Effect of glazing and thermocycling on the fracture toughness and hardness of a New fully crystallized aluminosilicate CAD/CAM ceramic material

BACKGROUND

The mechanical properties of fully crystallized lithium aluminosilicate ceramics may be influenced by intraoral temperature variations and postmilling surface treatment. The purpose of this study is to explore the interplay among glazing, thermocycling, and the mechanical characteristics (namely, fracture toughness and hardness) of fully crystallized lithium aluminosilicate ceramics.

METHODS

Bending bars (n = 40) cut from LisiCAD blocks (GC, Japan) were randomly assigned to glazed or unglazed groups (n = 20) and subjected to the single edge v-notch beam method to create notches. A glazing firing cycle was applied to the glazed group, while the unglazed group was not subjected to glazing. Half of the specimens (n = 10) from both groups underwent thermocycling before fracture toughness testing. The fracture toughness (KIC) was evaluated at 23 ± 1 °C using a universal testing machine configured for three-point bending, and the crack length was measured via light microscopy. Seven specimens per group were selected for the hardness test. Hardness was assessed using a Vickers microhardness tester with a 1 kg load for 20 s, and each specimen underwent five indentations following ISO 14705:2016. The Shapiro-Wilk and Kolmogorov-Smirnov tests were used to evaluate the normality of the data and a two-way ANOVA was utilized for statistical analysis. The significance level was set at (α = 0.05).

RESULTS

Regardless of the thermocycling conditions, the glazed specimens exhibited significantly greater fracture toughness than did their unglazed counterparts (P < 0.001). Thermocycling had no significant impact on the fracture toughness of either the glazed or unglazed specimens. Furthermore, statistical analysis revealed no significant effects on hardness with thermocycling in either group, and glazing alone did not substantially affect hardness.

CONCLUSIONS

The impact of glazing on the fracture toughness of LiSiCAD restorations is noteworthy, but it has no significant influence on their hardness. Furthermore, within the parameters of this study, thermocycling was found to exert negligible effects on both fracture toughness and hardness.

Effect of repeated firing on the topographical, optical, and mechanical properties of fully crystallized lithium silicate-based ceramics

STATEMENT OF PROBLEM

The influence of different firing protocols on the topographical, optical, and mechanical properties of fully crystallized computer-aided design and computer-aided manufacturing (CAD-CAM) lithium silicate-based glass-ceramics (LSCs) for dental restorations remains unclear.

PURPOSE

The purpose of this in vitro study was to investigate the effect of different firing regimens on the surface roughness, gloss, Martens hardness, indentation modulus, biaxial flexural strength, and crystalline structure of fully crystallized CAD-CAM LSCs and the effect of their interposition on the irradiance of a light-polymerization unit.

MATERIAL AND METHODS

Three fully crystallized CAD-CAM LSC blocks were evaluated (N=150): lithium disilicate (Initial LiSi Blocks; LS), zirconia-reinforced silicate (Celtra Duo; CD), and lithium aluminum disilicate (CEREC Tessera; CT). Specimens were allocated to 5 subgroups according to their firing protocol. LSC roughness (Sa) was measured with an optical profilometer, and gloss (GU) was detected with a gloss meter. Martens hardness (HM) and indentation modulus (EIT) data were obtained from a hardness testing machine. The irradiance of a light-polymerization unit and transmittance of LSCs were measured with an instrument (Managing Accurate Resin Curing-Light Collector; BlueLight analytics, Inc) subsequent to ceramic interposition. Crystalline phases were analyzed by X-ray diffraction, and biaxial flexural strength (σ) was determined by the ball-on-3-ball method in a universal testing machine followed by Weibull analysis to calculate characteristic strength (σ0) and Weibull modulus (m). Two-way ANOVA and Tukey HSD post hoc tests (α=.05) were used to analyze the data.

RESULTS

Statistically significant differences were found among different treatment groups based on Sa, GU, HM, and EIT values (P<.001). Delivered irradiance was significantly reduced following CT (P<.01) and glazed LSC (P<.005) interposition. CD displayed highest biaxial flexural strength and reliability after 1 firing cycle (σ=568.2 MPa, m=16.8) CONCLUSIONS: The type of material and firing regimens had a significant effect on the topographical, optical, and mechanical properties of fully crystallized CAD-CAM LSCs. Glazing significantly reduced delivered irradiance, Martens hardness, and biaxial flexural strength.

Does glaze firing affect the strength of advanced lithium disilicate after simulated defects?

OBJECTIVE

To study the influence of glazing on strength repair of lithium disilicate glass-ceramics after defect incorporation in different production processing phases.

MATERIALS AND METHODS

Bar-shaped specimens (1 × 1 × 12 mm, n = 280; 20/group) made from different lithium disilicate ceramics (IPS e.max CAD, Ivoclar, "LD" or advanced lithium disilicate CEREC Tessera, Dentsply Sirona, "ALD") were exposed to 7 different protocols: crystallized without (c) and with glaze layer (cg), with a defect incorporated before crystallization without (ic) and with glaze layer (icg), with a defect after crystallization without (ci) or with glaze layer (cig), and defect incorporated after the glaze layer (cgi). The flexural strength was determined using the three-point bending test. Analysis of indented areas and fractured specimens was performed by scanning electron microscopy. Flexural strength data were evaluated by two-way ANOVA followed by Tukey tests (α = 5%).

RESULTS

Two-way ANOVA revealed a significant influence of ceramic (p < 0.001; F = 55.45), protocol (p < 0.001; F = 56.94), and the interaction protocol*ceramic (p < 0.001; F = 13.86). Regardless of ceramics, defect incorporation as final step resulted in the worst strength, while defects introduced before crystallization did not reduce strength. Glaze firing after defect incorporation led to strength repair for ALD, whereas such an effect was not evident for LD.

CONCLUSIONS

The advanced lithium disilicate must receive a glaze layer to achieve its highest strength. Defects incorporated in the pre-crystallized stage can be healed during crystallization. Defects should not be incorporated after glazing.

CLINICAL RELEVANCE

Clinical adjustments should be performed on pre-crystallized or crystalized restorations that receive a glazer layer afterwards.

Impact of glazing on wear, fracture load, and optical properties of a new fully crystallized lithium disilicate ceramic material

OBJECTIVES

Lithium disilicate (LDS) based glass ceramics are indispensable materials in the field of prosthetic dentistry due to their strength and excellent esthetics. Recently, novel fully crystallized LDS for the milling process have been introduced to the market, requiring only chairside polishing before delivery. Since limited data is available about subjecting this material to glazing, this study aimed to evaluate the wear properties, fracture resistance, and optical properties of the newly introduced fully crystallized LDS after glazing.

METHODS

Lithium disilicate glass-ceramics (fully crystallized (LiSiCAD) and partially crystallized types (EmaxCAD), of shade A3, were used in the present study. The fully crystallized specimens were subjected to a glazing firing cycle in a furnace (LiSiCAD-G) and compared to a polished fully crystallized counterpart (LiSiCAD-P, negative control) and a glazed partially crystallized LDS (EmaxCAD, positive control). Rectangular-shaped ceramic specimens (n = 10, 12 × 6.5 × 1.5 mm) and enamel antagonists were used to test the wear resistance in a specially designed wear machine built at the Department of Oral Technology, University of Bonn, Germany. The volumetric enamel loss of antagonists was measured by means of overlapping virtual 3D models derived from micro-CT scans for the teeth before and after the wear test. The weight loss of ceramic specimens in milligrams was measured after 100,000 and 200,000 wear cycles. A spectrophotometer was used to calculate the CIELAB color parameters of the ceramic specimens against black and white backgrounds to measure the translucency. Fracture resistance was also assessed after thermodynamic fatigue using a universal testing machine until fracture at a 1 mm/min crosshead speed. Statistical analysis was performed using one-way ANOVA and the significance level was set at α = 0.05.

RESULTS

EmaxCAD and LisiCAD-G groups showed significantly higher mean ceramic weight loss after 100,000 cycles (2 ± 0.3 mg and 1.93 ± 0.2 mg, respectively) than LisiCAD-P group (0.78 ± 0.19 mg). Glazed LiSiCAD specimens demonstrated significantly higher ΔE values from shade A3 compared to polished LiSiCAD. There was no significant difference in ΔE between LiSiCAD-G and EmaxCAD. For fracture resistance, no significant difference was observed between LiSiCAD-G and LiSiCAD-P before or after aging.

CONCLUSIONS

The glazed fully crystallized LDS was superior to the partially crystallized one with regard to wear resistance but showed more color changes than the polished fully crystallized one. Furthermore, polishing of fully crystallized LDS could still be considered a better option than glazing when aesthetics is a primary prerequisite.

Effect of staining layer on roughness after progressive wear of monolithic ceramics

PURPOSE

The aim of this study was to evaluate staining layer behavior applied to high-translucency zirconia (YZHT), feldspathic ceramics (FD), and zirconia-reinforced lithium silicate (ZLS) surfaces against different antagonists.

METHODS AND MATERIALS

Monolithic ceramic discs (n = 120) (ø 12 mm; thickness, 1.2 mm; ISO 6872) were obtained, 30 from YZHT and FD, and 60 from ZLS CAD/CAM blocks (staining layer applied before or after the crystallization procedure). The specimens were divided into 12 subgroups (n = 10) according to the antagonists: steatite, polymer-infiltrated ceramic, or zirconia. Mechanical cycling (1.5 × 104  cycles; 15 N; horizontal displacement, 6 mm; 1.7 Hz) and flexural strength tests (1 mm/min-1000 kg cell) were performed. The differences between final and initial roughnesses (Ra, Rz, and Rsm), the mass loss, and the flexural strength data were individually analyzed by two-way ANOVA and Tukey's test (α = 0.05).

RESULTS

The roughnesses of all ceramics did not present a statistically significant difference before wear simulation: Ra (p = 0.3348), Rz (p = 0.5590), and Rsm (p = 0.5330). After the wear simulation, the Ra parameter was not affected by an interaction between ceramic and antagonist (p = 0.595). The Rz and Rsm parameters were affected only by the antagonist pistons (both, p = 0.000). The ceramics used in this study showed statistically significant differences in mass loss after the wear test (p < 0.0001). The additional firing (2 steps) of the ZLS2 led to a higher lost mass quantity.

CONCLUSION

All ceramics presented similar initial roughnesses and similar roughnesses after the wear simulation. The zirconia antagonist showed better performance against ceramics with high crystalline content.

CLINICAL SIGNIFICANCE

It is clear that restorative materials must be carefully selected by dental practitioners according to indications, properties, and antagonists. The steatite antagonist, that is, an enamel analog, showed better performance against vitreous ceramics, while the zirconia antagonist showed better performance against ceramics with high crystalline content. Wear affects the surface roughnesses of the ceramics. Additional firing for the staining of the zirconia-reinforced lithium silicate ceramic led to a greater loss of mass.

Effect of glazing technique and firing on surface roughness and flexural strength of an advanced lithium disilicate

OBJECTIVE

The objective of this study was to investigate the effects of glazing technique and firing on the surface roughness and flexural strength of an advanced lithium disilicate (ALD) and lithium disilicate (LD).

METHODS

Eight groups of bar-shaped specimens (1 mm × 1 mm × 12 mm, N=160, 20/group) were manufactured from ALD (CEREC Tessera, Dentsply Sirona) and LD (IPS e.max CAD, Ivoclar). The specimens were then submitted to various posttreatments: crystallization (c), crystallization followed by a second firing (c-r), crystallization with glaze in one step (cg), and crystallization followed by a glaze layer firing (c-g). Surface roughness was measured by means of a profilometer, and flexural strength was determined using a three-point bending test. Surface morphology, fractography, and crack healing analysis were conducted using scanning electron microscopy.

RESULTS

Refiring (c-r) did not affect the surface roughness (Ra) while applying glaze at both cg and c-g procedures increased the roughness. ALDc-g (442.3 ± 92.5 MPa) promoted higher strength than ALDcg (282.1 ± 64.4 MPa), whereas LDcg (402.9 ± 78.4 MPa) was stronger than LDc-g (255.5 ± 68.7 MPa). Refiring completely closed the crack in ALD, but it had a limited effect on LD.

CONCLUSIONS

Two-step crystallization and glazing improved ALD strength compared to the one-step protocol. Refiring and one-step glazing do not increase LD's strength, while two-step glazing has a negative effect.

CLINICAL RELEVANCE

Besides both materials being lithium-disilicate glass ceramics, the glazing technique and firing protocol affected their roughness and flexural strength differently. A two-step crystallization and glazing should be the first choice for ALD, while for LD, glazing is optional and when necessary, should be applied in one-step.

Highly crystallized glass-ceramics from high content gold tailings via a one-step direct cooling method

Highly crystalline glass-ceramics were successfully manufactured via a one-step direct cooling method using Shuangqishan (Fujian, China) gold tailings as raw materials. A series of glass-ceramics were prepared by controlling the gold tailings addition and post-treatment. X-ray diffraction results show that the crystalline phase of glass-ceramics samples with high tailing addition content (65–80 wt%) is akermanite phase (Ca₂MgSi₂O₇). By contrast, the main phase of 60 wt% and 55 wt% tailings addition samples is diopside (CaMgSi₂O₆) crystalline phase. In addition, although glass-ceramics have typical fracture characteristics of brittle materials, the crack propagation in the fracture process is disturbed by grains, resulting in the deviation of the fracture path in terms of macroscopic and microcosmic observation. Based on the investigation of samples with different tailings additions, glass-ceramics with 60 wt% tailings contents show excellent mechanical properties with a density of 2.89 g cm⁻³, a Vickers hardness value of 8.17 GPa, and a flexural strength of 116 MPa after 950 °C heat treatment. This study further confirms the possibility of using Shuangqi Mountain gold tailings as the raw materials for highly crystalline glass-ceramics, which shows great potential for application in mass production.

Highly crystalline glass-ceramics were successfully manufactured via a one-step direct cooling method using Shuangqishan (Fujian, China) gold tailings as raw materials.

Effect of staining on the mechanical, surface and biological properties of lithium disilicate

•

Staining and glazing procedures affects the microbial adherence and surface roughness.

•

Stained and glazed surfaces result in higher wear than polished ceramic surfaces.

•

The stain and glaze layer on the ceramic surface promotes lower strength value.

Purpose

To simulate biodegradation and wear of stained and glazed CAD lithium disilicate ceramic, and evaluate their effects on the microbial adherence and mechanical and surface properties of lithium disilicate ceramic

Materials and methods

160 lithium disilicate ceramic discs were fabricated and divided in eight groups according to manual stain and glaze application with a fine paint brush (without stain and glaze; with stain and glaze) and aging procedures (no aging; wear at 30 N load, 1.7 Hz, 3 × 10⁵ cycles; biodegradation by exposure to microcosm biofilm; biodegradation + wear; biodegradation + wear). Profilometry was performed to determine the surface roughness and the wear consequences. Biaxial flexural strength test was performed, and a Streptococcus mutans adherence test was conducted to evaluate the number of colony forming units.

Results

Unaged samples with and without stain and glaze presented the lowest values of surface roughness (p < 0.001), but after aging (wear, biodegradation, or both), the samples in the stain and glaze groups were rougher than those in the no stain and glaze groups (p < 0.001). The stain and glaze groups showed the highest volume of wear after aging (p = 0.04), and had the lowest flexural strength values (p < 0.01), irrespective of the aging method. The aging method did not affect the flexural strength (p = 0.06). The number of colonies forming units was higher for biodegradation + no stain and glaze, biodegradation + wear + no stain and glaze, no aging + stain and glaze, biodegradation + stain and glaze, and biodegradation + wear + stain and glaze. The lowest values were observed for no aging + no stain and glaze.

Conclusion

The staining and glazing of lithium disilicate increased the surface wear and bacterial adherence, and decreased biaxial flexural strength of the material. When exposed to S. mutans, surface roughness increased, and biodegradation favored bacterial adherence.

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).

Evaluation of the Bond Strength of Densely Sintered Ceramics Subjected to Extended Firing

Introduction

Critical failures in ceramic materials can be caused by the processing mode, which includes all steps taken in the manufacture of a ceramic part, from molding to firing.

Purpose

To evaluate the effect of extended firing on bond strength in densely sintered ceramics of the zirconium reinforced lithium silicate, lithium disilicate, and feldspathic ceramic.

Materials and Methods

Three types of ceramics were evaluated: zirconium reinforced lithium silicate, lithium disilicate, feldspathic ceramic. A total of 6 ceramic blocks, two for each material were used in the study. Each block was cut into four square sections. A total of 24 ceramic surfaces were randomly distributed into 6 groups (n = 4 surfaces per group) divided according to the variables: heat treatment: conventional firing or extended firing; test time: immediate (24 hours after cementation) or longevity (after 1000 cycles of thermocycling). The bond strength tests were performed in a semi-universal test machine for microshear bond strength. For data analysis, the Shapiro-Wilk test was performed to evaluate the normality between the groups measured, and regarding homoscedasticity (homogeneity of variances) by the Bartlett test. The comparisons between the groups were made using the nonparametric Kruskal-Wallis test.

Results

There was no statistically significant difference of the resistance values in relation to the thermal treatment methods (conventional firing and extended firing) in any ceramic group (p <0.05). Among the times (immediate and long), there was a statistically significant difference (p <0.05), with higher resistance values for immediate time.

Conclusion

Extended firing did not influence the micro-shear bond strength of zirconium reinforced lithium silicate, lithium disilicate, feldspathic ceramic.

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

OBJECTIVE

This study evaluated the effect of dwell time (conventional or extended) and cooling protocol (fast or slow) of self-glaze firings on the mechanical (flexural strength and crack propagation) and optical (color and translucency) properties of a porcelain-veneered zirconia system.

METHODS

Bilayer disc-shaped samples were prepared (Vita VM9 + In-Ceram YZ) and divided according to the final thermal treatment: glaze firing followed by slow cooling (furnace opening at 200 °C) (G-S) or fast cooling (furnace opening at 600 °C) (G-F, manufacturer-recommended protocol), extended glaze firing (15 min of dwell time) followed by slow cooling (EG-S) or fast cooling (EG-F), or no thermal treatment (CTRL). Porcelain roughness (Ra and Rz) was measured before and after glaze firings. Color (ΔE₀₀) and translucency (TP₀₀) alteration were also evaluated. Flexural strength was measured with the piston-on-three-ball test and crack propagation analysis was performed after Vickers indentations. Complementary analyzes of crystalline phase and scanning electron microscopy were carried out.

RESULTS

Significant effect of dwell time was observed, with extended glaze leading to higher flexural strength and shorter crack lengths. Cracks of EG groups were observed to end in clusters of crystals. Color and translucency changed below perceptibility thresholds. All treatments led to a smoother surface and EG groups reached the lowest Rz values. An extra SiO₂ peak was revealed in control and EG groups. No effect of cooling protocol was found.

SIGNIFICANCE

Extended glaze firing was able to improve the resistance to crack initiation and propagation of porcelain-veneered zirconia without clinically perceptible changes in optical properties.

Effects of multiple firing processes on the mechanical properties of lithium disilicate glass-ceramics produced by two different production techniques

STATEMENT OF PROBLEM

Repeated firings cause materials to be exposed to additional heat treatments. The effect of these additional heat treatments on the mechanical properties of lithium disilicate glass-ceramics is not fully known.

PURPOSE

The purpose of this in vitro study was to determine the effects of repeated firing on the mechanical properties of lithium disilicate glass-ceramics produced by 2 different techniques, press and computer-aided design and computer-aided manufacturing (CAD-CAM).

MATERIAL AND METHODS

Eighty rectangular (25×4×2 mm) lithium disilicate glass-ceramic specimens were used in this study, 40 produced by heat pressing and 40 by milling, and divided into 4 groups (n=10) with a different number of veneer porcelain firings (1 to 4). After firing, the Vickers hardness, flexural strength (3-point bend test), and fracture toughness were determined, and the specimens were analyzed with an environmental scanning electron micrograph. Data were analyzed with analysis of variance (ANOVA) (α=.05).

RESULTS

The repeat firing processes did not affect the flexural strength of the specimens in either group (P>.05), while the surface hardness and fracture toughness were significantly changed (P<.05).

CONCLUSIONS

Increasing the number of firings adversely affected the mechanical properties of lithium disilicate glass-ceramics.

In vitro performance and fracture resistance of pressed or CAD/CAM milled ceramic implant-supported screw-retained or cemented anterior FDPs

PURPOSE

This study investigated the in-vitro performance of anterior implant-supported fixed dental prostheses (FDP). The effect of ceramics, fabrication, finalization and the presence of a screw-channel wa s investigated.

METHODS

Identical anterior ceramic FDPs (tooth 11-13; n=80) were milled (Lithiumdisilicate (LiSiCAD, emaxCAD, Ivoclar-Vivadent), Lithiumaluminiumsilicate (LiAlSi, experimental material) or pressed (Lithiumdisilicate (LiSiPress, emaxPress, Ivoclar-Vivadent), Lithiumsilicate (ZLS, CeltraPress, Dentsply Sirona). FDP-groups (n=8 per material and group) simulated a cemented or screw-retained approach. After cementation or screwing on titanium abutments, thermal cycling and mechanical loading (TCML) was performed on all restorations to mimic 5-year clinical performance. Performance and fracture force were determined and failures were analyzed.

STATISTICS

(Kolmogorov-Smirnov-test, one-way-ANOVA; post-hoc-Bonferroni, multivariate-regression, α=0.05).

RESULTS

All FDPs survived TCML without aging, cracks, fractures or chipping. For FDPs without screw channel fracture values varied between with 839.8±112.3N (LiAlSi glazed) and 1485.9±232.6N (LiSiCAD). With screw channel, fracture results varied between 701.4±220.1N (LiALSi glazed) and 1516.3±253.7N (LiSipress). The type of material had a significant influence on the fracture results (LiSi>ZLS>LiAlSi; p≤0.012). Fabrication and finalization had no influence on the results. A screw channel did not significantly (p≥0.135) reduce the fracture force of the FDPs. Type of failure was mostly characterized by a fracture of the connector (LiSi, LiAlSi) or the abutment (ZLS, LiAlSi).

CONCLUSIONS

FDPs survived TCML without failures indicating that the in vitro performance was not influenced by the tested parameters. Composition of ceramic material has significant influence on the fracture resistance of implant supported LiSi based FDPs. Screw channel, fabrication or finalization did not weaken the FDPs.

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

OBJECTIVE

To evaluate the composition, flexural strength and fatigue behaviour of lithium disilicate ceramic (LD) after repeated firings and different staining techniques.

METHODS

LD discs were fabricated and divided according to number of firing cycles and staining technique: CO - control, discs were crystallized (850°C/10min); SC - single-step characterization - crystallization and staining (applied with a thin brush) were performed in a single step with one firing cycle (850°C/10min); and DC - double-step characterization - crystallization firing cycle was performed first (850°C/10min), followed by staining firing cycle (770°C/90s). 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, 5Hz) were performed. Data were subjected to one-way ANOVA and Tukey's test (α=0.05).

RESULTS

EDS 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.

SIGNIFICANCE

Repeated 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.

Effect of staining and repeated firing on the surface and optical properties of lithium disilicate

OBJECTIVE

Evaluate the effect of staining and repeated firings on color, translucency and surface proprieties of CAD/CAM lithium disilicate (LD).

MATERIAL AND METHODS

One hundred eighty LD discs were made (ISO 6872) and distributed in different groups (n = 20): control (CO)-no treatment; single-step characterization (SC)-crystallization and staining fired together; and double step characterization (DC)-crystallization and then staining. The samples were submitted to two, four, or six firings, resulting in nine groups: COII, COIV, COVI, SCII, SCIV, SCVI, DCII, DCIV, and DCVI. The color and translucency were measured by a reflectance spectrophotometer. Surface roughness (Ra) and Vickers nano-hardness were also measured. ANOVA and Tukey statistical tests were used (α = .05).

RESULTS

Only CO and DC demonstrated significant color alterations (ΔE₀₀ > 1.8). SC and DC did not show changes in translucency by the number of firings (P > .05); however, for CO (P = .02) these values increased. Nano-hardness was similar in all groups (P > .05). Ra values indicated differences due to the type of characterization (SC presented the highest values) and number of firings (CO and DC groups) (P < .01).

CONCLUSION

SC promoted color, translucency, and roughness stability after repeated firings.

CLINICAL SIGNIFICANCE

The single characterization technic in CAD/CAM lithium disilicate presents good color, translucency, and hardness stability, which promote predictable results to monolithic restoration.

Influence of different surface finishing techniques on machinable feldspathic and leucite-reinforced ceramics

The effect of surface finishing techniques on machinable ceramics to obtain optimum smoothness is unclear. The aim of the study was to evaluate the effects of surface finishing techniques on machinable feldspathic and leucite-reinforced ceramics. Forty specimens were divided into four subgroups according to surface finishing techniques and ceramic structure. A profilometer was used to evaluate surface roughness. A spectrophotometer was used to obtain the CIE L*, a* and b* coordinates. All specimens were subjected to a three-point bending test to determine flexural strength. All data were analyzed with two-way ANOVA and Tukey's HSD tests (p<0.05). Polishing techniques were found to be more effective than the glazing method to obtain smooth surfaces. Glazing technique increased the flexural strength of leucite-reinforced ceramics. Finishing procedures have different effects on the success of the restorations and must be considered by the clinicians.

Evaluation of Wear Resistance of Dental Chairside CAD/CAM Glass Ceramics Reinforced by Different Crystalline Phases

The mechanical properties of crystalline phase of glass ceramics are critical. This study aimed to evaluate wear resistance of different crystalline-reinforced dental chairside computer-aided design/computer-aided manufacturing (CAD/CAM) glass ceramics. Materials of feldspar (Vita Mark II, VM), leucite (IPS Empress CAD, EC), lithium disilicate (IPS e.max CAD, EX), lithium disilicate enriched with zirconia (Vita Suprinity, VS), and enamel were embedded, grounded, and polished, respectively. Samples were indented with a Vickers hardness tester to test the fracture resistance (KIC). Two-body wear tests were performed in a reciprocal ball-on-flat configuration under artificial saliva. The parameters of load force (50 N), reciprocating amplitude (500 μm), frequency (2 Hz), and the test cycle (10,000 cycles) were selected. Specimen microstructure, indentation morphology, and wear scars were observed by scanning electron microscope (SEM), optical microscopy, and three-dimensional profile microscopy. EX, VS, and EC demonstrated significantly higher KIC values than the enamel, while ceramic materials showed smaller wear depth results. Cracks, massive delamination, and shallow plow were seen on the enamel worn scar. Long deep plow, delamination, and brittle cracks are more common for VM and EC, and short shallow plow and smooth subsurface are the characteristics of EX and VS. Greater fracture toughness values indicated higher wear resistances of the materials for the test glass ceramics. The CAD/CAM glass ceramics performed greater wear resistance than enamel. Feldspar- and leucite-reinforced glass ceramics illustrated better wear resistance similar to enamel than lithium disilicate glass ceramics, providing amicable matching with the opposite teeth.

Effect of sandblasting, etching and resin bonding on the flexural strength/bonding of novel glass-ceramics

OBJECTIVES

To process novel leucite glass-ceramics and test the effects of surface treatment and resin bonding on the biaxial flexural strength (BFS) and shear bond strength (SBS).

METHODS

Alumino-silicate glasses were ball-milled, and heat treated to form leucite glass-ceramics (LG-C, OLG-C), then sintered into ingots. Ingots were heat extruded into a refractory mould to form disc specimens (1.3×14mm diameter). IPS e.max^® was used as a commercial comparison. Glass-ceramic test groups were sandblasted (Groups. 1, 4, 6), sandblasted, etched and adhesively bonded (Groups. 2, 5, 7) or lapped, etched and adhesively bonded (Groups. 3, 8). Specimens were adhesively bonded with Monobond S, followed by the application of Variolink II^® cement and light curing. BFS testing was at 1mm/min and SBS testing at 0.5mm/min. Samples were characterised using XRD, SEM and profilometry.

RESULTS

XRD confirmed tetragonal leucite in LG-C/OLG-C and lithium disilicate/lithium orthophosphate in IPS e.max^®. Mean BFS (MPa (SD)) were: Gp1 LG-C; 193.1 (13.9), Gp2 LG-C; 217.7 (23.0), Gp3 LG-C; 273.6 (26.7), Gp4 OLG-C; 255.9 (31); Gp5 OLG-C; 288.6 (37.4), Gp6 IPS e.max^®; 258.6 (20.7), Gp7 IPS e.max^®; 322.3 (23.4) and Gp8 IPS e.max^®; 416.4 (52.6). The Median SBS (MPa) were Gp1 LG-C; 14.2, Gp2 LG-C (10s etch); 10.6 and Gp3 IPS e.max^®; 10.8. Mean surface roughness was 5-5.1μm (IPS e.max^®) and 2.6μm (LG-C).

SIGNIFICANCE

Novel leucite glass-ceramics with reduced flaw size and fine microstructures produced enhanced BFS and SBS by resin bonding. These properties may be useful for the fabrication of minimally invasive aesthetic and fracture resistant restorations.

The effect of extended glaze firing on the flexural fatigue strength of hard-machined ceramics

STATEMENT OF PROBLEM

It is unclear whether an extended glaze firing could improve the long-term mechanical performance of densely sintered CAD-CAM ceramics.

PURPOSE

The purpose of this in vitro study was to analyze the effect of an extended glaze firing on the flexural fatigue strength (FFS) of densely sintered milled (hard-machined) leucite-based (LEU) and lithium disilicate-based (DIS) ceramics.

MATERIAL AND METHODS

Disks were machined from ceramic blocks and divided into 6 groups (n=20) according to the material, LEU or DIS, and to the applied glaze firing: manufacturer-recommended glaze (G group), extended glaze (EG group), and control/no firing (C group). The surface roughness of the disks was measured before and after firing by using a contact profilometer, and data were compared by paired sample tests. Specimens were submitted to fatigue by using the staircase test design in water (piston-on-3 balls; 500 000 cycles, 20 Hz, and sinusoidal loading). Mean (±SD) FFS values were then calculated and analyzed by using 1-way analysis of variance and post hoc Tukey test (α=.05).

RESULTS

Surface roughness did not change after the firing (P>.05). The highest FFS value in both ceramics was obtained after EG firing (LEU-EG=80.52 ±6.3 MPa; DIS-EG=147.25 ±10.5 MPa), which was statistically superior to G firing (LEU-G=73 ±6.8 MPa, P=.003; DIS-G=134.34 ±15.6 MPa; P=.023) and C group (LEU-C=61.94 ±6.3 MPa; P<.001; DIS-C=134.13 ±17.3 MPa; P=.023).

CONCLUSIONS

EG firing optimized the biaxial flexural fatigue strength of hard-machined leucite and lithium disilicate ceramics compared with conventional glaze firing.

Use of Feldspathic Porcelain Veneers to Improve Smile Harmony: A 3-Year Follow-up Report

Abstract This case report describes an esthetic treatment to improve the shape and alignment of the anterior teeth, reestablishing smile harmony, using feldspathic porcelain veneers. Results of clinical follow up after 36 months are also presented. The advantages, disadvantages and limitations of the technique are detailed with reference to the relevant literature. This suggests that the success of treatment depends on adequate conditions of bonding between the veneers and the tooth complex, which involves parameters such as the strength and durability of the bond interface. Therefore, the clinical success of feldspathic porcelain veneers depends on the accurate selection of cases and correct execution of clinical and laboratory procedures. The rehabilitation involved from first right premolar to the left with feldspathic porcelain veneers made on refractory dies. After the 3-year follow up, excellent clinical results and patient satisfaction were achieved.

Bioactive and inert dental glass-ceramics

The global market for dental materials is predicted to exceed 10 billion dollars by 2020. The main drivers for this growth are easing the workflow of dentists and increasing the comfort of patients. Therefore, remarkable research projects have been conducted and are currently underway to develop improved or new dental materials with enhanced properties or that can be processed using advanced technologies, such as CAD/CAM or 3D printing. Among these materials, zirconia, glass or polymer-infiltrated ceramics, and glass-ceramics (GCs) are of great importance. Dental glass-ceramics are highly attractive because they are easy to process and have outstanding esthetics, translucency, low thermal conductivity, high strength, chemical durability, biocompatibility, wear resistance, and hardness similar to that of natural teeth, and, in certain cases, these materials are bioactive. In this review article, we divide dental GCs into the following two groups: restorative and bioactive. Most restorative dental glass-ceramics (RDGCs) are inert and biocompatible and are used in the restoration and reconstruction of teeth. Bioactive dental glass-ceramics (BDGCs) display bone-bonding ability and stimulate positive biological reactions at the material/tissue interface. BDGCs are suggested for dentin hypersensitivity treatment, implant coating, bone regeneration and periodontal therapy. Throughout this paper, we elaborate on the history, processing, properties and applications of RDGCs and BDGCs. We also report on selected papers that address promising types of dental glass-ceramics. Finally, we include trends and guidance on relevant open issues and research possibilities. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 619-639, 2017.