Impact of machining on the flexural fatigue strength of glass and polycrystalline CAD/CAM ceramics

Strength-limiting damage and defects of dental CAD/CAM full-contour zirconia ceramics

OBJECTIVE

This study aimed to compare the four-point flexural strength of CAM-milled and sintered (as-sintered, AS) specimens with those of high-polished (HP) specimens using chairside polishing systems to simulate clinical surface conditions.

METHODS

Seven full-contour zirconia CAM/CAM blanks with various yttria contents (3, 4, 5 mol%) including three high-translucent groups (5Y) were selected to prepare flexural specimens. The bend bar specimens (2.0 × 4.0 × 25.0 mm3) were fabricated by using STL file and dental CAM machine with the respective zirconia blanks (98 mm ϕ and 10-14 mm in height). Twelve bar specimens were machined from one zirconia puck and a total of 24 specimens were prepared from each group. The pre-sintered bar specimens were sintered by using a dental zirconia furnace at 1530-1550 °C for 2 h according to the instructions. All sintered specimens were divided into two groups: as-sintered (AS) group and high-polished (HP) groups (n = 12). HP groups were subjected to polishing one surface of specimens using a three-step polishing system and finally finished with diamond polishing. After cleaning and drying, the flexural strength of all specimens was determined by a fully articulating four-point flexure fixture consisting of a 1/4-point test configuration with an inner/outer span of 10/20 mm. Statistical differences between AS and HP groups were conducted with Weibull analysis. The fractured surfaces of zirconia specimens were observed using a field emission SEM and EDS to detect failure origins.

RESULTS

The mean AS flexural strength values were significantly lower than those of HP counterparts. However, Weibull moduli expressing the reliability of HP groups were generally decreased although not significantly in comparison to their AS. The fracture of the AS specimens mostly originated from extrinsic CAM-milling defects, while the HP specimens were fractured from intrinsic subsurface or volume defects including pores, large grain clusters, inclusions, and corner-located critical flaws. Two high-translucent (5Y) zirconia groups were not affected in their strength and reliability after polishing, whereas one 5Y zirconia significantly increased its strength but significantly lowered its reliability.

SIGNIFICANCE

The extrinsic and intrinsic strength-limiting defects should be considered in evaluating the flexural strength and reliability of dental CAD/CAM zirconia ceramics for full-contour restorations. For the materials tested in this study, more optimized processing of blanks and milling protocols of pre-sintered zirconia blanks should be developed including post-sintering surface finishing to reduce the flaw population regulating strength and reliability which will affect the survivability of dental zirconia prostheses.

Impact of try-in paste removal on the fatigue behavior of bonded lithium disilicate ceramics

This in vitro study assessed the effectiveness of three cleaning protocols (air-water spray, 37% phosphoric acid, or Ivoclean) on lithium disilicate restorations' fatigue behavior after try-in paste application, compared to a clean condition. Lithium disilicate discs (IPS e.max CAD, Ivoclar) with Ø-= 12 mm and 1 mm thickness were prepared from prefabricated CAD-CAM blocks, polished, subjected to CAD-CAM milling topography simulation and crystallization. After, etching with 5% hydrofluoric acid and the application of try-in paste (Variolink try-in paste shade white; load of 2.5 N for 5 min) was performed. Discs that received try-in paste were divided into three groups according to the removal protocol: SPRAY - air-water spray for 30 s; HPO - active application of 37% phosphoric acid for 60 s; IVOC - application of Ivoclean for 20 s. Control group (CTRL group) did not receive the try-in paste application. Half of the specimens (n= 15) were tested in the baseline condition (24 h up to 7 days), and the others underwent 25,000 thermal cycles (5 - 55 °C) + 210 days of distilled water storage (37 °C). Additional specimens (n= 3) underwent monotonic testing (1 mm/min). Fatigue testing involved a cyclic fatigue approach (20 Hz, initial load = 100 N - 5000 cycles, step size = 50 N - 10,000 cycles) until a visible crack appeared. Fractographic and topographic analyses were performed. Fatigue data were statistically analyzed with two-way ANOVA, Kaplan-Meier log-rank (Mantel-Cox), and independent t-test (α= 0.05). In the baseline condition, the IVOC group resulted in a superior fatigue behavior compared to the CTRL and SPRAY groups, but similar to the HPO group. The HPO and SPRAY presented a similar fatigue behavior to the CTRL group. It was noticed a decrease in fatigue behavior after aging, which resulted in all the cleaning protocols leading to similar fatigue behavior compared to the CTRL group. On the SPRAY group surface, try-in pastes remnants were noticed. In summary, despite a detrimental impact at baseline conditions, all tested cleaning protocols seem proper to remove the try-in paste from the ceramic's surface in the long-term evaluation.

Effect of abutment screw-access hole on the fatigue performance of implant-supported lithium-disilicate luted simplified restorations

The present study aimed to evaluate the impact of the existence of an abutment screw-access hole and the filling effects on the fatigue mechanical behavior of a luted lithium-disilicate glass-ceramic. Seventy-two discs (Ø = 10 mm, 1.0 mm in thickness) of lithium disilicate (IPS e.max CAD, Ivoclar AG) were obtained from prefabricated blocks. Thirty-six abutment specimens of an opaque zirconia (Yz - IPS e.max ZirCAD, Ivoclar AG) and titanium (Ti - Luminesse Ti-Cam discs, Talladium Inc.) were confectioned, and allocated according to 6 groups: Yz and Ti rigid (without screw access hole); Yz unfilled, Yz filled, Ti unfilled and Ti filled (with the screw access (Ø = 2.5 mm) in the center). For the unfilled groups, only a polytetrafluoroethylene tape was used. Resin composite (Tetric N-Ceram, Ivoclar AG) was applied to the screw access hole for the filled groups (Yz and Ti). A cyclic fatigue test was carried out (load of 200 N, 10,000 cycles each; 20 Hz of frequency, step size of 100 N until failure detection (radial/cone crack). The fatigue failure load (FFL) and number of cycles until failure (CFF) were recorded for statistical purposes. The stress distribution (MPa) was evaluated by finite element analysis. A statistically positive effect of the abutment material and the presence of the screw access hole was observed (p ≤ 0.05). The rigid groups (without screw access holes) depicted almost 100% of survival after the fatigue tests. Among the other groups, the Yz-filled group showed the best performance (p ≤ 0.05), followed by the Yz unfilled group. The Ti groups depicted lower values of FFL and CFF, with the Ti unfilled group showing the most unfavorable fatigue behavior (p ≤ 0.05). The lowest tensile stress concentration in the restorative material was observed with the use of rigid abutments, the filled groups depicted intermediate values, while unfilled groups showed the highest stress concentration (Yz rigid = 306.3 MPa; Ti rigid = 310.4 MPa < Yz filled = 490.7 MPa; Ti filled = 498.9 MPa < Yz unfilled = 707.6 MPa; Ti unfilled = 719.7 MPa). Therefore, the presence of a screw-access hole decreases the mechanical performance of a lithium disilicate ceramic regardless of the abutment material. In the presence of a screw-access hole, zirconia abutments depicted a higher fatigue failure load when compared with titanium. The filling of the abutment screw-access hole with resin composite increased the mechanical performance of the simulated restoration.

Effect of printing layer orientation and polishing on the fatigue strength of 3D-printed dental zirconia

OBJECTIVE

The aim of the study was to evaluate the influence of surface polishing and printing layer orientation on the fatigue behaviour of 3 mol% yttria-stabilized zirconia (3Y-TZP) by stereolithography (SLA) in comparison with subtractive manufacturing.

MATERIALS AND METHODS

60 experimental zirconia bar-shaped specimens were 3D-printed (P) via SLA, and 30 specimens were milled (M) from commercial zirconia block (Lava™ Frame, 3 M ESPE AG). All specimens had the same dimensions (1 mm × 1 mm x 12 mm) after sintering. The 3D-printed specimens were randomly divided according to printing orientations: parallel or perpendicular to the tensile surface in the fatigue test. The specimens were subsequently submitted to two surface finishing protocols (n = 15/gr): unpolished or polished. Their phase compositions were analysed by X-ray diffraction. The fatigue behaviour was evaluated by a stepwise approach.

RESULTS

The milled and both 3D-printed groups showed similar phase compositions for the as-sintered condition. Considerable amounts of rhombohedral phase were detected after polishing. Milled unpolished samples presented significantly higher fatigue strength than 3D-printed unpolished samples. Polishing did not improve the fatigue strength for milled zirconia but was advantageous for the 3D-printed specimens. 3D-printed specimens with parallel printing-layer orientation were significantly stronger than specimens with perpendicular layers regardless of surface finishing.

CONCLUSION

The manufacturing techniques had a significant influence on the fatigue strength of 3Y-TZP, but not on the phase compositions of the surface. The polishing protocol showed different effects on 3Y-TZP fatigue strength and induced phase transition of the 3Y-TZP from Tetragonal to Rhombohedral. The best fatigue strength was achieved through milling using an unpolished surface and SLA-printed layers that were parallel to the tensile surface, followed by polishing.

Does speed-sintering affect the optical and mechanical properties of yttria-stabilized zirconia? A systematic review and meta-analysis of in-vitro studies

Zirconia restorations are increasingly popular in dental treatment. Yttria-stabilized zirconia (YSZ) needs to be sintered for clinical applications and novel speed-sintering protocols are being developed for chairside treatments. Whether the properties of speed-sintered YSZ meet clinical requirements, however, remains unclear. Therefore, we conducted a systematic review and meta-analysis on the influence of speed-sintering on the optical and mechanical properties of dental YSZ according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 guidelines. A literature search was conducted using PubMed, Embase, and Web of Science databases for relevant articles published between January 1, 2010 and February 28, 2022 in English, Chinese, or Japanese. After full-text evaluation and quality assessment, 26 articles were selected. Meta-analysis revealed that speed-sintering does not significantly affect the CIEDE2000-based translucency parameter, contrast ratio, three-point flexural strength, biaxial flexural strength, or fracture toughness of YSZ (p < 0.01) compared to conventional sintering. However, the CIELab-based translucency parameter of conventionally sintered YSZ is higher than that of speed-sintered YSZ. The descriptive analysis indicated that speed-sintering does not affect the hardness of YSZ compared to that of conventionally sintered YSZ. The results indicate that speed-sintering is suitable for preparing YSZ for dental restorations.

Load-bearing capacity under fatigue of bonded-yttria tetragonal zirconia polycrystals and -yttria-stabilized zirconia: Effects of the viscosity of a dual-cured resin cement

This study aimed to evaluate the effect of low and high viscosities of dual-cured resin cement on the mechanical fatigue behavior of yttria tetragonal zirconia polycrystals (3Y-TZP) and yttria-stabilized zirconia (4YSZ) adhesively luted to a dentin analogue (glass fiber-reinforced epoxy resin). Ceramic discs were randomly divided into four groups (n = 20) based on the following study factors: dual-cured resin cement viscosities (low and high) and zirconia microstructure (3Y-TZP and 4YSZ). The discs were treated by air abrasion with aluminum oxide particles (50 μm), followed by the application of primer, and then luted with high or low viscosity resin cement to the dentin analogue. Subsequently, the luted sets underwent a step-stress fatigue test, which involved an initial load of 200 N, step increments of 100 N, 10,000 cycles per step, and a frequency of 20 Hz. Data on fatigue failure load (FFL) and the number of cycles for failure (CFF) were collected and analyzed using survival tests, including Kaplan-Meier and Mantel-Cox analyses, as well as Weibull analysis. Additionally, topography analysis, fractographic features, bonding interface analysis, and Raman spectroscopy were performed. The results revealed that 3Y-TZP exhibited superior fatigue behavior compared to 4YSZ, regardless of the viscosity of the resin cement used for luting. Among all groups, 3Y-Low exhibited the best fatigue performance, while 4YSZ luted with low or high viscosity resin cements yielded the lowest fatigue behavior (FFL). There were no significant differences in Weibull modulus among the groups. After air abrasion, both ceramics showed similar topography. Raman analysis indicated that the surface of 3Y-TZP ceramics prior to sintering had a monoclinic phase, which transitioned predominantly to tetragonal phase peaks after sintering. A similar transition was observed in 4YSZ ceramics. In summary, 3Y-TZP exhibited superior mechanical fatigue behavior compared to 4YSZ. The influence of resin cement viscosity on fatigue behavior was more pronounced in 3Y-TZP, with low-viscosity resin cement enhancing its performance. However, the mechanical fatigue behavior of 4YSZ was less affected by the viscosity of the dual-cured resin cement, showing similar results with both low and high viscosities. In conclusion, 3Y-TZP demonstrated superior mechanical fatigue behavior compared to 4YSZ. The impact of resin cement viscosity on fatigue behavior was more pronounced in 3Y-TZP, with low-viscosity resin cement enhancing its performance. Conversely, the mechanical fatigue behavior of 4YSZ was less sensitive to the viscosity of the dual-cured resin cement, resulting in similar outcomes with both low and high viscosities.

Advanced lithium disilicate: A comparative evaluation of translucency and fatigue failure load to other ceramics for monolithic restorations

The aim of this in vitro study was to evaluate the surface roughness, translucency, fatigue failure load (FFL), and number of cycles for fatigue failure (CFF) of a recently released lithia-based material called advanced lithium disilicate and three other ceramics indicated for monolithic restorations. First, ALD (advanced lithium disilicate, CEREC Tessera, Dentsply Sirona), LD (lithium disilicate, IPS e. max CAD, Ivoclar), LS (lithium silicate-disilicate, Suprinity, Vita Zahnfabrik), and 4Y-PSZ (Yttria-stabilized zirconia, IPS e.max ZirCAD MT, Ivoclar) discs (n = 15, Ø = 10 mm and thickness = 1.0 mm) were fabricated from CAD/CAM blocks/discs, A2 shade. The discs were sintered/crystallized and subsequently analyzed by a rugosimeter (Mitutoyo SJ-410) to determine Ra and Rz surface roughness parameters. Next, they were evaluated to determine the translucency parameter (TP) using a bench-top spectrophotometer (SP60, EX-Rite). The discs were subsequently cemented to glass fiber epoxy resin discs, and the specimens were tested under cyclic loading (Step-test), immersed in distilled water at a frequency of 20 Hz, with an initial cyclic load of 200 N for 5,000 cycles and increments of 50 N every 10,000 cycles until failure. Fatigue failure load (FFL) and number of cycles for fatigue failure (CFF) were recorded for subsequent Kaplan Meier analysis, with post-hoc Mantel-Cox and Weibull analysis (α = 0.05). Complementary fractographic, topographic and energy dispersive spectroscopy analyses (EDS) were performed. 4Y-PSZ showed higher survival (p < 0.05), with higher FFL and CFF (1077 N; 180,333 cycles), followed by LD (980 N; 161,000 cycles), LS (937 N; 152,333 cycles) and ALD (910 N; 147,000 cycles). No differences were observed between the tested groups regarding Weibull modulus. ALD presented TP (28.14) equal to DL (28.27) and higher than LS (25.51). All lithia-based materials had higher translucency than 4Y-PSZ (TP = 8.62) (p < 0.05). ALD appears to have a similar elemental composition to LD and LSD for oxygen and silicon. ALD and LSD have a similar zirconium content. Fractures originated on the cemented surface of the ceramic discs. Lithia-based ceramics showed lower surface roughness, with ALD (Ra = 0.04 μm; Rz = 0.66 μm) showing the lowest values (p < 0.05). Despite showing lower FFL when compared to LD and 4Y-PSZ, ALD has compatible translucency and mechanical fatigue performance with its indication for fabricating monolithic, anterior and posterior adhesively cemented single-unit restorations. However, further studies are needed to substantiate its clinical performance.

Marginal Discrepancy of Five Contemporary Dental Ceramics for Anterior Restorations

OBJECTIVES

 This study aimed to compare marginal accuracy of five contemporary all-ceramic crowns indicated for anterior restorations.

MATERIALS AND METHODS

 A master die of maxillary central incisor was prepared for all-ceramic crown and duplicated to produce 50 replicas of epoxy resin material. Five ceramic materials were used to mill the crowns (n = 10). All crowns were manufactured following the same digital workflow; same master die, scanning unit and design software, and the recommended manufacturing protocol. Final seating of crown was secured by a small droplet of temporary cement on its incisal edge. Marginal accuracy was evaluated by scanning electronic microscope with a magnification of 300 × . Vertical marginal gap was measured for each crown at predefined four points.

STATISTICAL ANALYSIS

 One-way analysis of variance was used to test differences between groups and Tukey test was used for multiple comparisons between group combinations. A level of significance at 95% was set for all statistics.

RESULTS

 The highest mean marginal gap and mean maximum gap calculated were for the e.max CAD crowns (49.2 µm, 87.6 µm), while the lowest values were for the Cercon xt crowns (10.2 µm, 21.7 µm). The mean marginal gap and the mean maximum gap of the e.max CAD crowns were statistically significantly greater than those of all other groups (p < 0.05). However, the differences between all other combinations were insignificant (p > 0.05).

CONCLUSION

 Marginal accuracy of lithium disilicate crowns is clinically acceptable. Zirconia and zirconia-reinforced lithium silicate materials can produce a greater level of marginal accuracy compared to lithium disilicate.

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.

Edge chipping damage in lithium silicate glass-ceramics induced by conventional and ultrasonic vibration-assisted diamond machining

OBJECTIVES

Diamond machining of lithium silicate glass-ceramics (LS) induces extensive edge chipping damage, detrimentally affecting LS restoration functionality and long-term performance. This study approached novel ultrasonic vibration-assisted machining of pre-crystallized and crystallized LS materials to investigate induced edge chipping damage in comparison with conventional machining.

METHODS

The vibration-assisted diamond machining was conducted using a five-axis ultrasonic high-speed grinding/machining machine at different vibration amplitudes while conventional machining was performed using the same machine without vibration assistance. LS microstructural characterization and phase development were performed using scanning electron microscopy (SEM) and x-ray diffraction (XRD) techniques. Machining-induced edge chipping depths, areas and morphology were also characterized using the SEM and Java-based imaging software.

RESULTS

All machining-induced edge chipping damages resulted from brittle fractures. The damage scales, however, depended on the material microstructures; mechanical properties associated with the fracture toughness, critical strain energy release rates, brittleness indices, and machinability indices; and ultrasonic vibration amplitudes. Pre-crystallized LS with more glass matrix and lithium metasilicate crystals yielded respective 1.8 and 1.6 times greater damage depths and specific damage areas than crystallized LS with less glass matrix and tri-crystal phases in conventional machining. Ultrasonic machining at optimized amplitudes diminished such damages by over 50 % in pre-crystallized LS and up to 13 % in crystallized LS.

SIGNIFICANCE

This research highlights that ultrasonic vibration assistance at optimized conditions may advance current dental CAD/CAM machining techniques by significant suppression of edge chipping damage in pre-crystallized LS.

High-versus low-viscosity resin cements: Its effect on the load-bearing capacity under fatigue of a translucent zirconia

The purpose of the present study was to characterize the elastic modulus and Poisson's ratio of a resin cement with distinct viscosities, and to evaluate their impact on the static and fatigue strength of a translucent zirconia (4Y-PSZ) after air-abrasion surface treatment. Bar-shaped specimens of two different viscosities of resin cement (high and low) were obtained (25 × 10 × 3 mm). Sonelastic and Maxwell principles tests were performed to determine the elastic modulus and Poisson's ratio of each resin cement. Disc-shaped specimens of 4Y-PSZ were made (Ø = 15 mm, 1.2 mm in thickness) for the mechanical tests and allocated into groups according to two factors: surface treatment (presence or absence of air-abrasion with alumina particles; 45 μm grain-size); cement (absence, low or high viscosity). The static (n = 10) and cyclical (n = 15) biaxial flexural strength tests were performed by piston-on-three-balls geometry. A fatigue strength test was executed (20 Hz, initial stress of 60 MPa [12% of the mean static biaxial flexural strength], followed by increments of 25 MPa [5% of the mean static biaxial flexural strength] at each step of 10,000 cycles until the failure). The obtained data were analyzed by Weibull analysis. Survival rates were tabulated by the Kaplan-Meier test. Complementary analyses of surface roughness, topography, cross-sectional interfacial zone, fractography, and zirconia crystalline content (X-ray diffraction) were also performed. The evaluated resin cements with high and low viscosity presented similar elastic modulus (13.63 GPa; 12.74 GPa) and Poisson's ratio (0.32; 0.30), respectively. The air-abraded groups depicted higher mechanical strength of the zirconia ceramics than non-abraded groups (p˂ 0.05), regardless of the resin cement. 4Y-PSZ adhesively bonded to a high or low viscosity resin cement have statistically similar behavior (p˃ 0.05). The mechanical structural reliability of the 4Y-PSZ was not affected by the factors. Therefore, resin cement with high and low viscosity presented similar properties and potential to fill the zirconia surface, and did not affect the mechanical behavior of 4Y-PSZ. However, the air-abrasion surface treatment increased the static and fatigue flexural strength of the translucent zirconia.

Resin cement coating reverts the machining damage on the flexural fatigue strength of lithium disilicate glass-ceramic

This study evaluated the effect of resin cement coating with high and low viscosities on the flexural fatigue strength of machined lithium disilicate glass-ceramic. Discs (IPS e.max CAD; Ivoclar Vivadent) were prepared and divided according to the surface condition (machining [M]-CEREC inLab; and polishing [P]-laboratory procedures), resin cement coating (with or without), and cement viscosity (high [H] and low [L]). The ceramic bonding surface was etched/primed by a one-step primer application followed by resin cement application (Variolink N base + high or low viscosity catalyst; Ivoclar Vivadent). Biaxial flexural fatigue strength was evaluated on a piston-on-three-ball set by the step-test method (n = 15) (initial stress: 60 MPa; incremental steps: 20 MPa; 10,000 cycles/step, at 20 Hz). Weibull statistics were used for fatigue data. Contact angle, topographic, and fractographic analysis were also performed. Machining produced statistically lower contact angle than polishing and a significant detrimental effect on the fatigue behavior (σ₀ M = 247.2 [246.9-268.3]; σ₀ P = 337.4 [297.8-382.4]). Machined groups followed by resin cement coating (σ₀ MH = 297.9 [276.0-321.5]; σ₀ Ml = 301.2 [277.1-327.4]) behaved similarly to the polished and coated groups (σ₀ PH = 342.0 [308.9-378.5]; σ₀ PL = 357.3 [324.7-393.1]), irrespective of the cement viscosity. Therefore, cement coating has able to revert the detrimental effects of the machining on the fatigue strength of lithium disilicate glass-ceramic. High and low viscosity cements behaved similarly in the improvement of CAD-CAM lithium disilicate fatigue strength.

Flexural Strength Properties of Five Different Monolithic Computer-Aided Design/Computer-Aided Manufacturing Ceramic Materials: An In Vitro Study

Objective

The purpose of this in vitro study is to compare the flexural strength and Weibull modulus of 5 different monolithic computer-aided design/computer-aided manufacturing (CAD/CAM) ceramics.

Methods

A total of 50 specimens were fabricated, 10 from each of the following materials: lithium disilicate-based ceramic (IPS e.max CAD), zirconia -reinforced lithium-silicate ceramic (Vita Suprinity), leucite-based glass ceramic (IPS Empress CAD), and two zirconia-based ceramics (Zenostar and CopraSmile). The specimens were 4 mm wide, 2 mm thick, and 16 mm long. Flexural strength test was executed using a universal testing machine (Model 5980, Instron Industrial Products, Norwood, MA, USA). The two-parameter Weibull distribution function was used to analyze the variability of flexural strength values. Statistical analysis was performed on SPSS Version 23 (IBM Corp., Armonk, NY, USA) using one-way analysis of variance (ANOVA) and post-hoc Tukey’s test.

Results

Suprinity had the highest Weibull modulus value, while Empress CAD displayed the lowest value. One-way ANOVA showed significant difference in the flexural strength between the different materials tested (p<0.05). Post-hoc analysis revealed significant differences among all the test groups in terms of flexural strength. Zenostar presented the highest mean flexural strength value (1033.90 MPa), while Empress CAD had the lowest value.

Conclusion

High-translucency zirconia had superior flexural properties than translucent zirconia, lithium disilicate ceramics, zirconia-reinforced lithium silicate ceramics, and leucite-based glass ceramics.

Ceramic surface conditioning, resin cement viscosity, and aging relationships affect the load-bearing capacity under fatigue of bonded glass-ceramics

This study aimed to evaluate the influence of ceramic surface treatments, resin cement viscosities, and storage regimens on the fatigue performance of bonded glass-ceramics (lithium disilicate, LD; feldspathic, FEL). Ceramic discs (Ø = 10 mm; thickness = 1.5 mm) were allocated into eight groups per ceramic (n = 15), considering three factors: "ceramic surface treatment" in two levels - 5% hydrofluoric acid etching and silane-based coupling agent application (HF), or self-etching ceramic primer (E&P); "resin cement viscosity" in two levels - in high or low viscosity; and "storage regimen" in two levels - baseline, 24 h to 5 days; or aging, 180 days + 25,000 thermal cycles. Adhesive luting was performed onto glass fiber-reinforced epoxy resin discs (Ø = 10 mm; thickness = 2 mm) and the bonded assemblies were subjected to cyclic fatigue tests: initial load = 200 N; step-size = 25 N (FEL) and 50 N (LD); 10,000 cycles/step; 20 Hz. Scanning electron microscopy (SEM) inspections were performed. Regarding the LD ceramic, the fatigue behavior was reduced after aging for HF_HIGH and E&P_LOW conditions, while stable performance was observed for HF_LOW and E&P_HIGH. Regarding the FEL results, aging negatively affected HF_HIGH, E&P_HIGH, and E&P_LOW, being that only the HF_LOW condition presented a stable behavior. The failure initiated from defects on the etched surface of the ceramics, where the cross-sectional analysis commonly revealed unfilled areas. Long-term aging might induce a decrease in mechanical behavior. The 'ceramic microstructure/surface conditioning/resin cement viscosity relationships' modulate the fatigue performance of lithium disilicate and feldspathic glass-ceramics.

Polishing the bonding surface, before or after crystallization, does not alter the fatigue behavior of bonded CAD-CAM lithium disilicate

The aim of this study was to assess if the finishing/polishing of the bonding surface of lithium disilicate ceramic, prior to or after crystallization, would affect the fatigue behavior of a bonded restorations. For this, lithium disilicate ceramic (IPS e.max CAD) discs (n = 15) were milled and randomly divided into 3 groups: CAD-CAM group which remained untouched; PRE group which received a finishing/polishing protocol (OptraFine system) prior to its crystallization; and POST group, which received the treatment after its crystallization. After surface treatments, ceramic and glass-fiber reinforced epoxy resin discs were paired and bonded using a resin cement (Multilink N). A cyclical fatigue test was conducted (frequency 20 Hz, initial load 200 N for 5000 cycles, step-size of 100 N for 10,000 cycles/step) until failure occurrence. Surface roughness and topography were analyzed. An initial descriptive analysis of surface roughness, FFL and CFF was performed to obtain the mean, standard deviation and confidence interval values (SPSS v. 21, SPSS Inc.) for statistical analysis. Roughness data was using one-way ANOVA with Tukey's post hoc test (α = 0.05), while the fatigue data was submitted to survival analysis with Kaplan-Meier test (α = 0.05) and Weibull modulus (Weibull++, Reliasoft). Neither the finishing/polishing procedure of the bonding surface, nor the moment (prior to or after crystallization), affected the fatigue behavior of bonded milled lithium disilicate. There were also no differences for mechanical reliability among conditions. Despite this, finishing/polishing reduced surface roughness and led to smoother topography. Finishing/polishing the bonding surface of milled lithium disilicate, before or after crystallization, does not alter the fatigue behavior of the bonded restorative set, although there is some influence on roughness and topography.

Qualitative SEM analysis of fracture surfaces for dental ceramics and polymers broken by flexural strength testing and crown compression

PURPOSE

To perform qualitative analysis using scanning electron microscopy (SEM) of fracture surfaces for ceramic and polymeric dental materials broken via standardized flexural and crunch-the-crown (CTC) tests.

MATERIALS AND METHODS

Zirconia, glass-ceramic, and polymeric (Trilor; TRI, Juvora; JUV, Pekkton; PEK) materials were loaded using crowns for CTC tests, discs (zirconia and glass-ceramics) for piston-on-3 ball tests, bars (polymer) for 3-point bend tests, and bars (zirconia, glass-ceramics) for 4-point bend tests. SEM was used to characterize the fracture surfaces and identify fracture surface features (e.g., origin, mist, hackle, and the direction of crack propagation [DCP]). Electron dispersive spectroscopy was used to identify the local chemistry.

RESULTS

Fracture surface features were found to be less visually apparent for glass-ceramics than zirconia. For zirconia bars, fractures originated roughly midway between the corner and center for processing defects related to sintering. Fractures originated at the bottom corners of glass-ceramic bars (void or surface flaw) and PEK bars (surface flaw). TRI bar failures exposed glassy fibers. Fracture features were generally less discernable for discs compared to bars for zirconia and glass-ceramics. Ceramic crowns fractured into 2 to 3 pieces, with fractures originating at the occlusal surface and clear evidence for the DCP. Failures of TRI and JUV specimens (bars and crowns) were less catastrophic than for the ceramics, with exposed fibers (TRI) and surface cracks (JUV). PEK crown and bar fractures presented dimple (ductile) features formed due to microvoid coalescence followed by brittle crack propagation.

CONCLUSIONS

The critical flaws responsible for failure initiation were a function of material composition and test configuration. Fractographic analysis can reveal problems associated with the manufacturing of materials, their handling, grinding and finishing/polishing procedures, the structural design and choice of material, and the quality of the final laboratory-delivered restoration.

A brief review on fatigue test of ceramic and some related matters in Dentistry

The characteristics of dental ceramics have been extensively studied over the years to provide highly qualified materials for use in prosthetic restorations. The ability to adhere to dental substrates, outstanding aesthetics (translucency, color, and substrate masking ability) and improved mechanical properties provide these materials with optical features and high strength to withstand masticatory stimuli. Different classifications are adopted, and it is generally considered that glass-ceramics have better optical characteristics due to the high glass content, and polycrystalline ceramics have superior strength favored by their densified and organized crystals, hampering crack growth. This knowledge was largely built-up during years of scientific research through different testing methodologies, but mainly employing static loads. It is important to not only take into account the intensity of loads that these materials will be exposed to, but also the effect of the intermittence of cyclic load application leading to mechanical fatigue and the influence of factors related to the crack origin and their propagation under this condition. Furthermore, the bonding surface of ceramic restorations requires surface treatments that improve the bond strength to luting agents; however, these treatments require caution because of their potential to produce defects and affect the structural behavior. Moreover, ceramic restorations often require internal adjustments for proper seating or external adjustments for fitting the occlusal contact with the antagonist. In this sense, finishing/polishing protocols may alter the defect population, as luting agents may also interact by filling in the superficial defects on the restoration intaglio surface. Thus, the balance among all these factors will define the performance of a restorative setup, as well as the posterior exposure to the humid environment and the masticatory stimuli (cyclical loading), which may favor developing slow and subcritical growth of cracks in ceramic materials and the degradation of the bond interface. Therefore, it is essential that the concepts which explain the fatigue mechanism are understood, as well as the crack propagation and failure patterns of restorative ceramic materials.

Flexural strengths, failure load, and hardness of glass-ceramics for dental applications

STATEMENT OF PROBLEM

Glass-ceramics are often selected for use in dental restorations based upon advertised flexural strengths obtained from standardized tests on prefabricated specimens (bars and disks); these may not accurately reflect their performance in dental applications.

PURPOSE

The purpose of this in vitro study was to determine and compare 4-point flexural strength, biaxial flexural strength, hardness, and crown failure loads for 3 commercially available glass-ceramics.

MATERIAL AND METHODS

Specimens were pressed and prepared from 3 brands of glass-ceramics: Celtra (CEL), IPS e.max (EMA), and Lisi (LIS). Rectangular bars, circular disks, and fully contoured crowns were created (n=15 specimens per glass-ceramic, 45 specimens per geometry, totaling 135 specimens). Disks were tested for biaxial flexural strength by using a piston-on-3-ball (POB) test, while bars were tested for 4-point flexural strength by using 4-point bending (4PB) and Vickers hardness (VH) tests. Crown failure loads were assessed in "crunch-the-crown" (CTC) tests. The results were analyzed by using general linear modeling, the Pearson correlation coefficient, and Weibull analysis.

RESULTS

The general linear modeling revealed significant differences (P<.05) in the failure load for crown specimens (EMA>LIS>CEL), the 4-point flexural strength (EMA>LIS>CEL), and the biaxial flexural strength (EMA>LIS=CEL). The disk specimens had higher flexural strengths than the bar specimens for CEL and EMA materials. LIS had a higher Weibull modulus than EMA and CEL for bar and crown specimens. CEL had a higher Weibull modulus than LIS and EMA for disk specimens. There was no correlation among the VH (R²=0.86 and P=.24), biaxial flexural strength (R²=0.84 and P=.26), and crown failure load. However, there was a high correlation between the failure load (crown specimens) and 4-point flexural strength (bar specimens) (R²=0.99 and P=.03).

CONCLUSIONS

The 4-point flexural strength correlated significantly with crown failure load.

In-lab simulation of CAD/CAM grinding and intaglio surface treatments of 4YSZ monolithic restorations: Effect on its load-bearing capacity under fatigue

OBJECTIVES

To evaluate the effect of in-lab simulation of CAD/CAM grinding and intaglio surface treatments on the surface characteristics (topography and roughness) and fatigue behavior of adhesively luted 4YSZ simplified restorations.

METHODS

Ceramic discs (Ø = 10 mm, thickness = 1 mm) were randomly allocated into 6 groups considering: "In-lab simulation of CAD/CAM grinding" (ground or polished) and "intaglio surface treatments": Ctrl (without surface treatment), AlOx (aluminum oxide air abrasion) or GLZ (glaze spray application). The surface roughness of all samples was measured, the treated discs received a ceramic primer, were luted with resin cement onto a dentin analogue material (woven glass-reinforced epoxy resin) and tested under a cyclic fatigue test (step-stress approach, n = 15; 1.4 Hz, 10,000 cycles/step, step-size of 100N starting at 200N until failure). A complementary analysis was performed to corroborate the findings in the fatigue test that the glaze fill defects increase the mechanical properties of the ceramic. To do so, bars (n= 10; 1.0 × 1.0 × 12 mm; considering the groups: N-ID: non-indented; ID: indented; ID-GLZ: indented plus glaze spray application) were indented in a vickers hardness tester to produce a crack pattern, treated with glaze or not, and then submitted to flexural strength tests (FS). Fractographic and topographic analysis were performed.

RESULTS

In-lab simulation of CAD/CAM grinding decreased the fatigue failure load of the 4YSZ ceramic when comparing polished and ground groups, regardless of surface treatment. GLZ induced better fatigue performance compared to the air abrasion, regardless of the grinding condition (ground or polished surface). The results of the flexural strength test corroborated the findings in the fatigue test, as the ID-GLZ group presented superior FS than the ID group, however both had inferior FS than N-ID. There is an inverse association between roughness and fatigue failure load, as the higher the surface roughness, the lower the fatigue failure load. Failures in the fatigue and flexural strength tests started from the face subjected to tensile stresses.

CONCLUSION

In-lab simulation of CAD/CAM grinding had a detrimental effect on the fatigue behavior of 4YSZ and glaze spray induced better 4YSZ performance compared to the air abrasion. The intaglio surface treatments differently influenced the 4YSZ fatigue performance, however, only glaze spray can reverse the damage caused by the grinding.

Simulation of CAD/CAM milling on lithium disilicate: Mechanical and topographic analyses of surface grinding different protocols

The aim of this study was to evaluate the topography and the fatigue performance of lithium disilicate glass-ceramic after surface grinding through different laboratory protocols used to simulate the Computer-aided design/Computer-aided manufacturing (CAD/CAM) milling. Ceramic discs (IPS e.max CAD, Ø = 13.5 mm × 1.2 mm of thickness) were produced through different methodologies: milling in CAD/CAM system (CAD/CAM group); produced in-lab with a polished surface (POL group); or produced through in-lab methods and randomly distributed into five groups according to different grinding protocols to simulate the CAD/CAM milling [grinding with a CAD/CAM bur coupled to a mandrel (CAD/CAM Bur group); fine diamond bur using oscillatory movements (DBO group); fine diamond bur in x and y axes of the disc (DBXY group); #60-grit silicon carbide sandpaper (SiC group); and #60-grit wood sandpaper (WS group)]. The specimens were fatigue tested (n = 15) according to the step-stress method (initial load: 60 N; step-size: 20 N; 10,000 cycles/step; 20 Hz frequency). A roughness analysis was performed on all specimens, while fractal dimension (FD) and fractography were performed on representative samples. The Kaplan-Meier analysis showed that the POL (293.3 N) group presented better fatigue performance (higher load and number of cycles for failure) (p < 0.05) than the other groups (CAD/CAM = 222.7 N; CAD/CAM Bur = 181.3 N; DBO = 184.0 N; DBXY = 192.0 N; SiC = 182.6 N; WS = 182.6 N). For roughness, only the SiC (Ra = 1.616; Rz = 10.465) and WS (Ra = 1.673; Rz = 10.655) groups produced statistically similar Ra (μm) and Rz (μm) values to the CAD/CAM (Ra = 1.628; Rz = 9.571) group (p > 0.05). The surface created by CAD/CAM milling and POL group exhibited more complexity (FD) higher values than the experimental groups. For the ceramic surface topography images, the CAD/CAM milling visibly produced a uniform surface compared to the other groups; however, the POL group was the smoothest. The DBO, DBXY, SiC, and WS groups resulted in similar characteristics of surface topography. Therefore, although the SiC and WS groups showed similar roughness to the control group (CAD/CAM), no in-lab simulation method was fully capable to mimic the mechanical performance of the CAD/CAM-milled lithium disilicate glass-ceramic.

Effect of pigmentation techniques on the fatigue mechanical behavior of a translucent zirconia for monolithic restorations

This study aims to characterize the effect of pre- and/or post-sintering pigmentation techniques on the fatigue behavior of a 4YSZ ceramic (4 mol% yttrium stabilized zirconia). First, 4YSZ ceramic discs (IPS e.max ZirCAD, 15 mm diameter, and 1.2 mm thickness) were obtained and allocated according to the 'pigmentation technique' factor into 5 groups: Ctrl - unshaded ceramic disc (IPS e.max ZirCAD BL); Manuf - ceramic discs shaded by the manufacturer at block formation (IPS e.max ZirCAD MT A2); Brush - unshaded ceramic disc pigmented in the pre-sintered stage using a brush; Stain - unshaded ceramic disc pigmented in the post-sintering stage using a stain glaze (shade A2); Brush + Stain - combination of Brush and Stain techniques. The specimens were sintered and analyzed by a spectrophotometer (VITA Easyshade) for color differences (ΔE₀₀ calculated according to CIEDE, 2000) to ensure that they present the same perceived color (ΔE₀₀ ≤ 1.77, acceptability threshold). The specimens (n = 15) were submitted to biaxial flexural fatigue testing using a cyclic fatigue method (frequency of 20 Hz; 10,000 cycles per step; initial stress of 200 MPa; and step-size of 25 MPa) until specimen fracture. Fatigue strength (FS) and number of cycles until failure (CFF) were recorded with statistical purposes. Fractographic, complementary topography, elemental and roughness analyses were performed. The Ctrl shown the highest FS, CFF, and survival rates compared to the other conditions (p < 0.05), being only statistically similar to Brush. Stain showed the lowest FS, CFF and survival rates (p < 0.05). The Manuf and Brush + Stain shown intermediary performance. In regards of Weibull moduli, Brush shown the lowest values (lowest structural reliability), for both FS and CFF, being only statistically similar to Manuf. Topography analysis shown a uniform size of zirconia crystals in Ctrl, Manuf and Brush. In addition, a smooth glassy surface with some spots of impregnated pigment was observed in the groups where glaze was applied (Stain and Brush + Stain). This finding was corroborated by elemental analysis. Despite that, Stain and Brush + Stain presented the roughest surface (p < 0.05). In summary, the pigmentation techniques used to provide a Vita classical A2 color shade have a detrimental effect on the mechanical fatigue properties of a 4YSZ ceramic, except for the technique performed with the brush in the block pre-sintering stage. Despite this, the brush technique presents great variability in mechanical performance, resulting in lower structural reliability.

The influence of roughness on the resistance to impact of different CAD/CAM dental ceramics

This study aimed to investigate the effect of surface roughness (polished vs. CAD/CAM milling simulation) on impact strength of five dental ceramics for manufacturing CAD/CAM monolithic restorations. Specimens of five ceramics (FC- feldspathic glass-ceramic; PICN- polymer-infiltrated ceramic-network; ZLS- zirconia-reinforced lithium silicate glass-ceramic; LD- lithium disilicate glass-ceramic; YZ- yttria-stabilized tetragonal zirconia polycrystal ceramic) to be tested under impact (15×10×2mm3; n= 15) were divided into two groups, according to surface treatment: polishing (pol) and grinding (gri) as CAD/CAM milling simulation. Impact strength was tested using the Dynstat method. Roughness, topographic, fractographic and finite element analyses were performed. The impact strength data were analyzed by Weibull, and Pearson correlation was used to correlate roughness and impact strength data. The CAD/CAM milling simulation led to significantly (p<0.05) greater roughness (Ra and Rz) and statistically reduced the impact strength for PICN (polPICN= 4.59 to griPICN= 1.09; ±76% decrease), for LD (polLD= 17.69 to griLD= 10.09; ±43% decrease) and for YZ (polYZ= 74.99 to griYZ= 20.67; ±72% decrease) ceramics; and also promoted a more irregular topography with scratches and grooves. Fractographic and FEA analyses depicted the origin of failure at the higher stress concentration side during the impact test, where the pendulum impacted. The CAD/CAM milling simulation significantly decreased the impact strength of the evaluated ceramic materials.

Cyclic fatigue tests on non-anatomic specimens of dental ceramic materials: A scoping review

The aim of the present scoping review was to identify and discuss the methods, testing parameters, and characteristics used to induce cyclic fatigue on non-anatomic dental ceramic specimens. In vitro studies written in English which evaluated commercially-available non-anatomic dental ceramic specimens subjected to mechanical cyclic fatigue were selected. The search was performed in the PubMed, Scopus and Web of Science databases. The initial search yielded 1,636 articles, of which 81 were included. Based on the collected data, most of the included studies evaluated dental ceramic specimens cemented to supporting substrate (n= 42; 51.9%); used step-stress (n= 35; 42.2%) accelerated fatigue test, loading frequencies above 10 Hz (n= 31, 35.6%), stainless steel (n = 28, 32.6%) load applicator with spherical shaped tip 40 mm diameter (n= 25, 30.9%); applied only axial loads (n= 77, 95.1%); and considered a wet testing environment (n= 65, 78.3%). The definition of test geometry, method, and testing parameters must be cautiously considered according to the study objective and the scenario that is simulated. Accelerated fatigue tests, load frequencies up to 20 Hz, a 40 mm stainless steel spherical load applicator and a wet testing environment are the major common defined parameters presented in the existing literature. More studies exploring the influence of such factors on fatigue mechanism are necessary.

Flexural properties of three lithium disilicate materials: An in vitro evaluation

Objective

The goal of this study was to investigate the flexural strength, Young’s modulus and Weibull modulus of two heat-pressed and one CAD/CAM processed lithium disilicate (LD) ceramics.

Material and methods

A total of 45 specimens with dimensions 16 × 4 × 1.2 ± 0.2 mm were fabricated out of three LD ceramics. For heat-pressed LD specimens, acrylate polymer blocks were cut and divided into two groups (n = 15 per group); a GC LiSi Press LD group (LP) and an IPS e.max Press group (EP). Specimens for each group were pressed corresponding to the manufacturer’s recommendations. For the CAD-CAM Group (EC), IPS e.max CAD blocks were cut to obtain specimens (n = 15) to the desired dimensions. Flexural strength and Young’s modulus tests were executed using a universal testing machine. A one-way ANOVA and post-hoc Tuckey’s tests were applied to analyze the results (p ≤ 0.05).

Results

Regarding flexural strength, the EC group showed higher statistically substantial difference than the EP and LP groups (p = 0.001), while there was no pronounced difference between the EP and LP groups (p = 0.065). For Young’s modulus test, all the three tested groups had no statistically substantial difference (p = 0.798).

Conclusion

The IPS e.max CAD group had higher mechanical performance than the IPS e.max Press and GC LiSi Press groups.

Influence of testing environment on static fatigue behavior of a glass and a polycrystalline ceramic

It aims on evaluate the effect of the test environment on static fatigue behavior of lithium disilicate-based (LD), and yttrium oxide-stabilized zirconia (YSZ) ceramics. Specimens of LD (IPS e.max CAD, Ivoclar Vivadent) and YSZ (IPS e.max ZirCAD MO, 3 mol% Y2O3, Ivoclar Vivadent) were randomly allocated into three groups: tested in air, inert (paraffin oil, Sigma Aldrich) or distilled water. The static fatigue test (n=15) was performed using a piston-on-three ball assembly, adapted from ISO 6872, as follows: starting load 100 N for LD and 300 N for YSZ; loading application time set to 1 hour for each loading step; step size of 50 N for LD and 100 N for YSZ, applied successively until fracture. Data from static fatigue strength (MPa) and time to fracture (hours) were recorded. Fractographic analysis was executed. Survival analysis corroborates absence of influence of environment on static fatigue outcomes (fatigue strength, time to fracture and survival rates) for YSZ. For LD, specimens tested in air presented statistically superior survival rate and static fatigue strength (p= 0.025). In regards of time to fracture, LD tested in air were superior than when tested in distilled water (p=0.019) or inert (p=0.017) environments. No statistical differences for Weibull modulus were observed. Failures started on the tensile stress surface. Thus, the test environment did not affect slow crack growth (SCG) mechanisms during static fatigue test of YSZ ceramics, but it plays a significant role for the static fatigue behavior of lithium disilicate-based glass ceramics, indicating a high susceptibility to SCG.

Load-bearing capacity under fatigue and FEA analysis of simplified ceramic restorations supported by Peek or zirconia polycrystals as foundation substrate for implant purposes

The fatigue behavior and FEA analysis of different ceramic materials cemented over distinct substrates for implant-supported crowns were evaluated in this study. Discs of 10 mm in diameter of both restorative and substrate materials were made and randomly allocated into pairs (n = 15) considering the two study factors: 'restorative ceramic material' (1 mm thickness) - polymer-infiltrated ceramic network (PICN), lithium disilicate (LD), zirconia-reinforced lithium silicate (ZLS), or translucent zirconia (TZ); and 'foundation substrate' (2 mm thickness) - polyetheretherketone (Peek) or yttrium-stabilized zirconia (YZ). Adhesive cementation was made with a dual cure resin cement. Fatigue testing was run using the step-stress methodology: initial load of 200 N for 5000 cycles, followed by steps of 10,000 cycles starting at 400 N up to 2800 N or until failure, step size of 200 N, frequency of 20 Hz. Data were analyzed by the Kaplan Meier and log-rank post-hoc tests. Fractography analysis (stereomicroscope and SEM) and FEA were also performed. Both factors under study and their interaction statistically influenced the fatigue failure load (FFL), cycles for failure (CFF) and survival rates (p < 0.001). The restorative materials bonded to YZ had higher FFL and CFF than when adhering to Peek, while restorative materials with more crystalline content (TZ and ZLS) showed higher FFL and CFF than LD and PICN. The fractography analysis showed that all materials bonded to YZ resulted in failures starting at the occlusal surface (Hertzian cone cracks), while materials bonded to Peek had radial cracks from the ceramic-cement intaglio surface. FEA analysis showed that tensile stress concentration decreased in the intaglio surface when testing the restorative material over a stiffer (YZ) foundation substrate. In addition, the higher the restorative material's crystalline content, the more the stress is concentrated within the material (TZ > ZLS ≥ LD > PICN) when bonded to the same foundation substrate. Thus, it concluded that a stiffer foundation substrate (YZ) enhances the load-bearing capacity under fatigue of the restorative set; that restorative materials with higher crystalline content results in higher fatigue performance of the set, regardless of the foundation used; and that the foundation material influences the failure pattern of the restorative set.

Monolith Dental Bridge by Soft Machining of Dried Ceramic Dough

All-ceramic fixed restorations are gaining popularity because of increased esthetic consciousness in the younger population. Monolith ceramic crowns and bridges are preferred to withstand masticatory loads where the availability of occlusal height is limited. Machining green blanks of ceramic with organic binders have been studied to reduce loss of final strength, cost, and wastage associated with milling partial sintered and sintered blanks. Notches on the surface/edge associated with green milling will disappear after sintering in contrast to sintered state machining where diamond polishing is essential. In this study, we explored a novel ceramic dough process to form a green body of alumina or 3 Mol% yttria-stabilized zirconium oxide (YSZ), and computer numerical control (CNC) machining was performed on the dried dough. Micro Computer Tomography analysis of the bridges after sintering revealed a negligible void volume, 0.06–0.08% of the total volume, with randomly dispersed voids. Precision analysis of the sintered bridges with respect to the reference file resulted in a deviation range of +0.56 to −0.79 mm, with negligible deviation on the cementation surface. The green machined surface had a roughness profile of 1.2–1.7 μm after machining and 2.2–2.4 μm after sintering, as revealed by 3D profilometry.

Effect of different surface treatments on optical, colorimetric, and surface characteristics of a lithium disilicate glass-ceramic

OBJECTIVE

To evaluate the effect of surface treatments on optical, colorimetric, and surface characteristics of lithium disilicate glass-ceramic.

MATERIALS AND METHODS

Specimens (n = 5, IPS e.max CAD) were randomly allocated to the following treatments: mirror-polished: SiC papers; as-cut: mimicking CAD-CAM milling; ground: 90-120 μm-grit diamond bur; ground polished: ground, finished (46-30 μm-grit diamond bur), polished (diamond cups, brush and diamond paste); ground glazed: ground, glazed; ground polished glazed: association of methods. CIELAB color coordinates were obtained by a spectrophotometer. CIEDE2000 color differences (ΔE₀₀ ) and the translucency parameter (TP₀₀ ) were calculated. Light transmittance was assessed with a colorimeter. Surface characteristics (topography and roughness) were analyzed. Statistical differences for each condition and outcome were detected using one-way ANOVA with Tukey's post-hoc test (α = 0.05).

RESULTS

TP₀₀ data show statistical reduction after grinding (p < 0.05), which was only restored with polishing (solely or with glazing). ΔE₀₀ shows that grinding results in perceptible variations in color (above 0.81), which were restored after all post-processing protocols (exception to only glaze application in contact with a black background). Light transmittance data corroborated such performance. Polishing and glazing reduced roughness and improved surface topography.

CONCLUSION

Grinding statistically increased roughness, reduced translucency, light transmittance through the ceramic, and resulted on color differences. On contrary, polishing (followed or not by glazing) reduced roughness and enhanced ceramic translucency and light transmittance. Glaze also reduced roughness, but it still presented reduced translucency. The positioning (facing up or down) of the ceramic treated surface influenced the considered outcomes.

CLINICAL SIGNIFICANCE

Grinding with diamond burs results in a deleterious impact to the optical, colorimetric and surface characteristics of lithium disilicate ceramic. Thus, polishing (followed or not by glazing) is recommended for optical and topographical enhancements when lithium disilicate monolithic restorations require occlusal adjustments.

In-lab simulation of CAD/CAM milling of lithium disilicate glass-ceramic specimens: Effect on the fatigue behavior of the bonded ceramic

The aim of this study was to evaluate the effect of in-lab simulation procedures performed on a lithium disilicate ceramic luted to a dentin-analogue material regarding the fatigue performance and topographic changes. Lithium disilicate ceramic (IPS e.max CAD) discs (Ø = 13.5 mm and 1.5 mm of thickness) were produced in different ways: milled in a CAD/CAM system (CAD/CAM - control group); mirror-polished (POL group); produced in-lab and ground with #60 silicon carbide paper (SiC group); with #60 wood sandpaper (WS group); with a fine diamond bur (DB group); or with a CAD/CAM bur adapted in a handpiece with a custom mandrel (MANDREL group). The ceramic discs were adhesively luted (Multilink N) onto dentin analogue discs (Ø = 12 mm and 2 mm of thickness) and fatigue testing (n = 19 discs) was performed by step-stress methodology (initial load of 200 N; step-size of 50 N; 10,000 cycles per step; 20 Hz). Surface roughness and contact angle analysis were also performed. According to Kaplan-Meier and post-hoc Mantel-Cox (log-rank), distinct fabrication methods affected the fatigue performance of bonded glass-ceramic discs (p< 0.001). The CAD/CAM group presented the lowest fatigue failure loads (1250 N) and number of cycles for failure (185,000), while the POL groups obtained the highest results (1752 N; 284,444 cycles). The in-lab groups had intermediate values (1355 - 1526 N; 206,052 - 238,684 cycles). Polished specimens presented the lowest roughness values (Ra = 0.041 μm), while the SiC (1.604 μm), WS (1.701 μm), and MANDREL (1.867 μm) groups showed statistically similar roughness values to the CAD/CAM group (1.738 μm). Despite differences before etching, the contact angle was similar among the milled and simulated groups after etching, except for the polished group. Even with some topographic similarities, the tested in-lab simulation methods were not able to mimic the milled specimens in terms of fatigue findings, leading to distinct magnitude of overestimations of the results.

Effect of milling, fitting adjustments, and hydrofluoric acid etching on the strength and roughness of CAD-CAM glass-ceramics: A systematic review and meta-analysis

STATEMENT OF PROBLEM

Whether procedures performed before the cementation of computer-aided design and computer-aided manufacturing (CAD-CAM) glass-ceramic restorations, including milling, fitting adjustment, and hydrofluoric acid etching introduce defects on the ceramic surface that affect the mechanical and surface properties is unclear.

PURPOSE

A systematic review and meta-analysis were conducted to assess the effect of milling, fitting adjustments, and hydrofluoric acid etching (HF) on the flexural strength and roughness (Ra) of CAD-CAM glass-ceramics.

MATERIAL AND METHODS

Literature searches were performed up to June 2020 in the PubMed/MEDLINE, Web of Science, and Scopus databases, with no publication year or language limits. The focused question was "Do milling, fitting adjustments, and hydrofluoric acid etching affect the flexural strength and roughness of CAD-CAM glass-ceramics?" For the meta-analysis, flexural strength and Ra data on milling, fitting adjustment, and HF etching versus control (polishing) were analyzed globally. A subgroup analysis assessed the effect of etching parameters (HF concentration and time) on the flexural strength and roughness of CAD-CAM glass-ceramics with different microstructures. Comparisons were performed with random-effect models at 5% significance.

RESULTS

Fourteen studies from 2764 potentially relevant records were included in the qualitative syntheses, and 12 in the meta-analysis. Milling and fitting adjustments increased roughness and reduced the flexural strength of CAD-CAM glass-ceramics. The effect of HF etching was dependent on the glass-ceramic microstructure, HF concentration, and etching time. For feldspathic- and leucite-reinforced ceramics, HF 5% applied for between 30 and 120 seconds increased roughness without affecting flexural strength. For lithium disilicate glass-ceramics, HF concentrations greater than 4.9% used for 20 seconds or more reduced the strength without affecting the surface roughness.

CONCLUSIONS

The flexural strength of CAD-CAM glass-ceramic is reduced by grinding procedures such as milling and fitting adjustment. Ceramic microstructure, HF concentration, and etching time determined the effect of hydrofluoric acid etching on the flexural strength and surface roughness of glass-ceramic materials.

Effects of thermal and mechanical cycling on the mechanical strength and surface properties of dental CAD-CAM restorative materials

STATEMENT OF PROBLEM

The properties of dental computer-aided design and computer-aided manufacturing (CAD-CAM) materials vary. Studies regarding the effects of aging on the properties of these materials are lacking.

PURPOSE

The purpose of this in vitro study was to evaluate the changes in the mechanical and surface properties of different CAD-CAM materials after thermocycling and mechanical loading.

MATERIAL AND METHODS

In total, 150 bar-shaped specimens (17.0×4.0×2.0 mm) were prepared from feldspathic glass-ceramic (VM; Vitablocs Mark II), lithium disilicate glass-ceramic (EX; IPS e.max CAD), zirconia-reinforced lithium silicate glass-ceramic (CD; Celtra Duo), polymer-infiltrated ceramic network (VE; Vita Enamic), and resin-nanoceramic (CS; Cerasmart). Each type was divided into 2 groups (n=15; each). One group was subjected to thermocycling in distilled water at 5 °C to 55 °C for 6000 cycles and 50 N mechanical loading for 1.2×10⁶ cycles. The other group was stored in 37 °C water for 24 hours. Nanoindentation hardness, Young modulus, and 3-point flexural strength were measured for the analyses of the mechanical properties. Surface roughness, surface microstructure, and elemental composition were measured to analyze the surface characteristics. Statistical analyses were performed with 1-way ANOVA with the Tukey HSD post hoc test, independent samples t test, Kruskal-Wallis test with Bonferroni post hoc test, Mann-Whitney U test, and 2-way ANOVA (α=.05).

RESULTS

Before and after aging, CS exhibited the lowest hardness (1.20 to 1.04 GPa) and Young modulus (13.76 to 13.48 GPa) values (P<.05). EX exhibited the highest flexural strengths (393.43 to 391.86 MPa), and VM exhibited the lowest (109.98 to 112.73 MPa) values (P<.05). CS exhibited the highest surface roughness (S_a and S_q; 10.60 to 28.82, 14.21 to 38.27 nm) values (P<.05). After aging, the hardness and Young modulus of VM, EX, and VE decreased significantly (P<.001). No significant difference was observed in the flexural strengths of the CAD-CAM materials (P>.05). Significant increases were observed in the surface roughness of all the materials (P<.05), with altered microstructures. Except for the flexural strength, the mechanical properties and surface characteristics of the CAD-CAM materials were significantly affected by the material type after aging.

CONCLUSIONS

Before and after aging, resin-nanoceramic exhibited the lowest hardness and Young modulus, and the highest surface roughness. Lithium disilicate glass-ceramic exhibited the highest flexural strength and feldspathic glass-ceramic exhibited the lowest value. After aging, increased surface roughness and microstructure alterations were observed. Significant interactions between aging process and material type were found for the mechanical properties and surface characteristics except for the flexural strength.

Step-stress vs. staircase fatigue tests to evaluate the effect of intaglio adjustment on the fatigue behavior of simplified lithium disilicate glass-ceramic restorations

The aim of the study was to compare the outcomes for the fatigue mechanical behavior of bonded simplified lithium disilicate restorations, with and without an internal adjustment by grinding with diamond bur in running two fatigue tests: Staircase and Step-stress testing approaches. Ceramic discs (IPS e.max CAD) were prepared (Ø = 10 mm; thickness = 1.0 mm), submitted to an in-lab simulation of CAD/CAM milling (#60 SiC paper) and allocated into 2 groups according to the internal adjustment by grinding of the cementation surface: no adjustment (CTRL); or grinding with a coarse diamond bur (GR). Adhesive cementation (Multilink N) was performed onto epoxy resin discs (Ø = 10 mm; thickness = 2 mm) after ceramic/epoxy surface treatments. The cemented assemblies of each group were randomly assigned into 2 subgroups considering two fatigue tests (n = 15): Staircase - SC (250,000 cycles; 20 Hz), or Step-stress - SS (10,000 cycles per step; 20 Hz). Roughness, topographic and fractographic analyses were additionally performed. Statistical analyses were carried out using the Dixon and Mood method for Staircase data, and Kaplan-Meier and Mantel-Cox (log-rank) tests for Step-stress data. Ceramic restorations having its intaglio surface ground (GR group: SC test = 306.67 N; SS test = 646.67 N) presented lower fatigue failure load (FFL) values than the CTRL group (SC test = 879.28 N; SS test = 1090.00 N), regardless of the fatigue testing approach. The percentage of mean FFL decrease comparing the CTRL to GR group was higher for SC (65.1%) than the SS (40.7%) approach. However, a different total number of cycles was applied for each method. Both fatigue tests were able to detect the negative effect of internal adjustments of lithium disilicate glass-ceramic simplified restorations on their mechanical behavior. Therefore, both methods can be applied for similar evaluations (fatigue testing for ceramic restorations).

Surface Roughness of Milled Lithium Disilicate With and Without Reinforcement After Finishing and Polishing: An In Vitro Study

PURPOSE

To determine the efficacy of various finishing and polishing techniques on the surface roughness of two computer-aided design/computer-aided manufacturing (CAD/CAM) materials, lithium disilicate (IPS e.max), lithium disilicate reinforced with lithium aluminosilicate (Straumann^® n!ce™), and a stackable low-fusing nanofluorapatite glass ceramic (Ceram).

MATERIALS AND METHODS

Flat specimens (n = 12) per treatment group were fabricated 2 mm thick, 15 mm in length, and 12mm in width. Samples were either glazed or polished. Glazing was accomplished with either Ivoclar IPS e.max CAD crystall glaze spray or IPS e.max Ceram glaze paste, according to manufacturer instructions. Three different polishing systems were tested: Brasseler Dialite HP, Ivoclar OptraFine, and Komet LD/ZR. Polishing was performed using a Kavo adjustable slow speed electric contra-angle handpiece mounted to an oscillating Toothbrush Dentifrice Assessment Instrument. Surface roughness data was collected using a benchtop stylus profilometer and analyzed for statistical significance using two-way ANOVA (α = 0.05). Representative scanning electron micrograph images were generated for all samples.

RESULTS

Overall there was no significant difference in Ra when comparing types of ceramic (p = 0.9315, F = 0.071). However, there was a statistically significant difference when comparing groups of finishing treatments (p < 0.001, F = 113.5) and also when comparing finishing treatment with ceramic type (p < 0.001, F = 11.13). No significant difference was found with IPS e.max CAD crystall glaze spray on Straumann^® n!ce™ versus IPS e.max Ceram glaze paste on IPS e.max Ceram (p = 0.8745) or IPS e.max CAD crystall glaze spray on IPS e.max versus IPS e.max Ceram glaze paste on IPS e.max Ceram (p = 0.3373). Significant differences in Ra of Straumann^® n!ce™ were found when comparing Brasseler with Ivoclar (p = 0.0014) and Ivoclar with Komet (p = 0.047). No significant difference was observed between Brasseler and Komet (p = 0.8099).

CONCLUSIONS

It appears that the degree of surface roughness depends upon the specific finishing system and ceramic combination used. Straumann^® n!ce™ is more efficiently polished using Brasseler Dialite HP or Komet LD/ZR polishing systems. Ivoclar crystal glaze spray was found to be equally as effective on Straumann^® n!ce™ and IPS e.max as IPS e.max Ceram glaze paste on IPS e.max Ceram.

Quantify the Protein-Protein Interaction Effects on Adsorption Related Lubricating Behaviors of α-Amylase on a Glass Surface

Dental ceramic material is one of the widely preferred restorative materials to mimic the natural tooth enamel surface. However, it has continuously been degraded because of low wear resistance during mastication in the oral cavity. The friction involved was reduced by introducing the lubricant saliva protein layers to improve the wear resistance of the dental materials. However, little is understood regarding how the protein-protein interactions (PPI) influence the adsorbed-state structures and lubricating behaviors of saliva proteins on the ceramic material surface. The objective of this study is to quantify the influences of PPI effects on the structural changes and corresponding oral lubrications of adsorbed α-amylase, one of the abundant proteins in the saliva, on the dental ceramic material with glass as a model surface. α-Amylase was first adsorbed to glass surface under varying protein solution concentrations to saturate the surface to vary the PPI effects over a wide range. The areal density of the adsorbed protein was measured as an indicator of the level of PPI effects within the layer, and these values were then correlated with the measurements of the adsorbed protein's secondary structure and corresponding friction coefficient. The decreased friction coefficient value was an indicator of the lubricated surfaces with higher wear resistance. Our results indicate that PPI effects help stabilize the structure of α-amylase adsorbed on glass, and the correlation observed between the friction coefficient and the conformational state of adsorbed α-amylase was apparent. This study thus provides new molecular-level insights into how PPI influences the structure and lubricating behaviors of adsorbed protein, which is critical for the innovations of dental ceramic material designs with improved wear resistance.

Effect of different grinding protocols on surface characteristics and fatigue behavior of yttria-stabilized zirconia polycrystalline: An in vitro study

STATEMENT OF PROBLEM

Zirconia frameworks milled by computer-aided design and computer-aided manufacturing (CAD-CAM) often require clinical adjustments. In addition, zirconia prefabricated abutments can also require customization to achieve an adequate emergence profile. However, the influence of grinding adjustment on the surface characteristics and mechanical behavior of yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) and the best grinding protocol is unclear.

PURPOSE

The purpose of this in vitro study was to evaluate the effect of different grinding protocols on the surface characteristics, phase transformation, and mechanical behavior of Y-TZP for frameworks and implant abutments.

MATERIAL AND METHODS

Bar-shaped specimens were fabricated according to ISO 6872-2016 and divided into 3 groups: GC (control, untreated), GA (grinding and finishing with medium and fine diamond rotary instruments using high-speed handpiece under constant water cooling), and GB (grinding and finishing with coarse and medium diamond rotary instruments, respectively, using slow-speed handpiece without water cooling). After specimen grinding, the topography and surface roughness were evaluated by using a laser confocal microscope, the Young modulus was measured by the impulse excitation technique, and crystallographic phase transformation was analyzed by X-ray diffraction. Specimens were then submitted to step-stress accelerated life testing (n=18). The surface roughness and Young modulus results were analyzed by 1-way ANOVA and the Tukey honestly significant difference test (α=.05). The data of step-stress accelerated life testing were analyzed by the survival probability considering the number of cycles and force until fracture.

RESULTS

Statistically significant differences were found among groups considering surface roughness (GA>GB>GC) (P<.05) and Young modulus (GB>GA=GC) (P=.003). X-ray diffraction showed that grinding leads to phase transformation, GC showed only tetragonal phase, while GA and GB showed tetragonal and monoclinic phases. No statistically significant difference (P<.05) was found among groups submitted to the same loading profile when the survival probability was compared, but significant difference was found between the light and moderate loading (P=.002) and light and severe loading (P=.014) of GB when different loading profiles in each group were compared.

CONCLUSIONS

Although grinding protocols affected surface characteristics and promoted phase transformation, the mechanical behavior of Y-TZP was not impaired. Therefore, both the grinding protocols tested can be safely used based on the evaluated properties.

Influence of CAD/CAM Fabrication and Sintering Procedures on the Fracture Load of Full-Contour Monolithic Zirconia Crowns as a Function of Material Thickness

OBJECTIVE

The purpose of this in vitro study was to analyze the effect of computer-aided design/computer-aided manufacturing (CAD/CAM) fabrication and sintering procedures on the fracture load of monolithic zirconia crowns with different material thicknesses adhesively seated to methacrylate dies fabricated with stereolithography technology.

METHOD

Monolithic zirconia crowns were fabricated from inCoris TZI C material with a chairside CAD/CAM system (CEREC MCXL) comprising three material thicknesses (0.5/1.0/1.5 mm, n=8 each). Two CAD/CAM fabrication procedures (milling, MI; grinding, GR), two chairside sintering procedures (superspeed, SS; speedfire sintering, SF), and one labside sintering procedure (classic, CL) were evaluated. In total, 144 crowns were fabricated. Restorations were adhesively seated to methacrylate dies fabricated with SLA technology. Thermomechanical cycling (TCML) was performed before fracture testing. Loading forces until fracture were registered and statistically analyzed with one-way analysis of variance (ANOVA), post hoc Scheffé test, and three-way ANOVA (α=0.05).

RESULTS

Test groups showed statistically significant differences (p<0.05). The highest mean value was found for 1.5-mm crowns of group GR_SF with 3678.6 ± 363.9 N. The lowest mean value was found for group 0.5-mm crowns of group MI_SF with 382.4 ± 30.7 N. There was a significant three-way interaction effect between thickness, sintering, and processing [F(4,126)=9.542; p<0.001; three-way ANOVA, significance level α=0.05].

CONCLUSIONS

CAD/CAM fabrication and sintering procedures influence the maximum loading force of monolithic zirconia crowns with different material thicknesses. A material thickness of 0.5 mm should be considered as a critical thickness for monolithic zirconia crown restorations.

Newer vs. older CAD/CAM burs: Influence of bur experience on the fatigue behavior of adhesively cemented simplified lithium-disilicate glass-ceramic restorations

This study evaluated the effect of the CAD/CAM burs experience (newer vs older as consequence of the milling sequence) on fatigue failure load (FFL), number of cycles for failure (CFF), and survival rates of lithium disilicate glass-ceramic simplified restorations adhesively cemented to a dentin analogue substrate. Three sets of CAD/CAM burs were used to mill disc-shaped ceramic specimens (1 bur set - 18 milled discs with 10 mm diameter and 1.5 mm thickness), considering the bur experience as a result of the milling sequence to compose the study groups: G1-6 - discs obtained from the 1st to 6th milling of each bur set; G7-12 - specimens from the 7th to 12th milling; G13-18 - discs from the 13th to 18th. Discs of dentin analogue (G10, 10 mm diameter and 2.0 mm thickness) were made to serve as substrate (base material) and randomly assigned into pairs with the respective ceramic discs. Then, the ceramic discs were adhesively cemented onto the dentin analogue substrate, composing a three-layer specimen that mimics a monolithic restoration of a posterior tooth. Specimens were tested under stepwise fatigue approach: frequency = 20 Hz, 5000 cycles at maximum load of 400 N to accommodate the testing assembly, followed by incremental steps of 200 N with initial load ranging from 10 to 1000 N, to a maximum of 20,000 cycles/each step, until the occurrence of failure (radial crack). FFL and CFF were recorded at the end of the testing and subjected to statistical analysis. Supplementary roughness analysis of the milled surface was performed (n = 18) using a contact profilometer. Residual stress after milling and acid etching were accessed via X-ray Diffractometry analysis. FFL and CFF were not affected by increase on bur experience (no statistical differences among groups), despite that, it affected both Ra and Rz parameters (G1-6 had the smoothest surface). The residual stress concentration was negligible (milling did not induce residual stress concentration). It is concluded that the fatigue behavior of adhesively cemented lithium-disilicate glass-ceramic restorations was not influenced by CAD/CAM bur experience (newer vs older as consequence of the milling sequence), and so the residual stress concentration induced by milling was negligible.

The effect of hydrofluoric acid concentration on the fatigue failure load of adhesively cemented feldspathic ceramic discs

OBJECTIVE

This study investigated the influence of hydrofluoric acid (HF) etching at different concentrations on the fatigue failure load of adhesively cemented feldspathic ceramic discs (Vita Mark II). Besides, their effect on the micromorphology of ceramic surface was investigated.

METHODS

Eighty ceramic discs (ϕ=10 mm; thickness=1.5 mm) were cemented to epoxy supporting discs (ϕ=10 mm; thickness=2.0mm) using different surface conditioning methods (n=20): nonetched control (CTRL), or etched for 60s with different HF concentrations: 1% (HF1), 5% (HF5), or 10% (HF10). All the ceramic discs received a silane application (Monobond Plus). The epoxy discs were etched with 10% HF for 60s and received a primer coating (Multilink Primer A+B). Adhesively cementation was performed (Multilink Automix), and the assemblies (ceramic discs/epoxy discs) were subjected to cyclic loads in water by a staircase approach (500,000 cycles; 20Hz; initial load=290N; step size=30N). Fatigue failure load data were analyzed using 1-way ANOVA and post-hoc Tukey's tests (α=.05).

RESULTS

Mean failure load of the HF5 group (255.0±23.0N) was significantly lower; HF1 group (301.7±71.0N) presented intermediate values, and the highest values were achieved in CTRL (351.7±13.4N) and HF10 (341.7±20.6N) groups. All the failures were radial cracks starting from the bonding surface.

SIGNIFICANCE

In terms of fatigue failure load, etching with 1% and 5% HF had a deleterious effect on the fatigue behavior of an adhesively cemented feldspathic ceramic, while 10% HF had no negative influence.