Cyclic contact fatigue resistance of ceramics for monolithic and multilayer dental restorations

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Cyclic contact fatigue resistance of ceramics for monolithic and multilayer dental restorations Rodrigo Alessandretti, Marcia Borba, Alvaro Della Bona ∗ Postgraduate Program in Dentistry, Dental School, University of Passo Fundo, Brazil

a r t i c l e

i n f o

a b s t r a c t

Article history:

Objective. To evaluate the cyclic contact fatigue resistance and failure mode of ceramics for

Received 19 September 2019

monolithic and multilayer restorations.

Received in revised form

Methods. Ceramic structures (10 mm × 1.8 mm) were fabricated as follows (n = 28): (1) CAD-

3 February 2020

on- trilayer structure composed of Y-TZP (yttria stabilized tetragonal zirconia polycrystal-

Accepted 4 February 2020

IPS e.max ZirCAD) infrastructure, fusion glass–ceramic (IPS e.max CAD Crystall/Connect)

Available online xxx

and lithium disilicate-based glass–ceramic (IPS e.max CAD); (2) ZFC- bilayer structure composed of Y-TZP infrastructure veneered by a fluorapatite glass–ceramic (IPS e.max Ceram); (3)

Keywords:

LDC- monolithic lithium-disilicate glass–ceramic (IPS e.max CAD); and (4) YZW- monolithic

Ceramic

Y-TZP (Zenostar Zr Translucent). All ceramics structures were bonded to a dentin analog

Failure

substrate (G10). Specimens were submitted to cyclic contact fatigue test in a pneumatic

Fatigue

cycling machine with 80 N load and 2 Hz frequency in distilled water at 37 ◦ C. Test was

Multilayer structures

interrupted after 104 , 105 , 5 × 105 and 106 cycles and the presence or absence of failure was

Structural reliability

recorded. Fatigue data were analyzed using Kaplan–Meier (log rank) and Holm–Sidak tests (˛ = 0.05). The relationship between the type of crack leading to failure and the experimental group was analyzed using chi-square test (˛ = 0.05). Results. There was no statistical difference between CAD-on and YZW groups (p = 0.516), which presented the highest survival rates after cyclic loading, followed by ZFC and LDC groups (p < 0.01). There was a significant relationship between type of crack and experimental group (p < 0.001). LDC specimens showed the greatest frequency of radial cracks, while cone cracks were more prevalent for ZFC and CAD-on specimens. Significance. Monolithic Y-TZP (YZW) showed similar fatigue resistance to CAD-on multilayer specimens, but different failure mode. Monolithic lithium disilicate glass–ceramic (LDC) and Y-TZP conventionally veneered by glass–ceramic (ZFC) showed lower survival time under fatigue. © 2020 The Academy of Dental Materials. Published by Elsevier Inc. All rights reserved.

1.

Introduction

Studies have reported that all-ceramic restorations, especially veneered zirconia-frameworks, exhibit chipping, cracking, or delamination of the porcelain [1–3]. These clinical failures

occur due to some factors, such as: low fracture toughness and fracture strength of the veneering porcelain [4–6], inadequate porcelain sintering and cooling rate [7–11], low bond strength between infrastructure and veneering porcelain and thermal incompatibility between materials [3,8,12–14].

∗

Corresponding author at: School of Dentistry, University of Passo Fundo, Campus I, BR285, km 171, Passo Fundo, RS 99052-900, Brazil. E-mail address: [email protected] (A. Della Bona). https://doi.org/10.1016/j.dental.2020.02.006 0109-5641/© 2020 The Academy of Dental Materials. Published by Elsevier Inc. All rights reserved.

Please cite this article in press as: Alessandretti R, et al. Cyclic contact fatigue resistance of ceramics for monolithic and multilayer dental restorations. Dent Mater (2020), https://doi.org/10.1016/j.dental.2020.02.006

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Few all-ceramic systems were introduced attempting to minimize such problems. An option is to produce monolithic restorations using crystal-reinforced glass–ceramics, which have good mechanical and optical behaviors, but have limitations on clinical indications (e.g. long spam bridges). More recently, the composition and microstructure of zirconiabased ceramics were modified resulting in more translucent materials, offering an additional alternative to produce monolithic restorations [15–17]. Another option is to replace the porcelain conventionally used to veneer the zirconia framework by a ceramic with better mechanical properties and good bonding to zirconia. This is the case of CAD-on technique (Ivoclar® ), where the yttria stabilized tetragonal zirconia polycrystal (Y-TZP) framework and the lithium disilicate-based glass–ceramic veneer are fabricated using CAD/CAM technology and fused together with a low-fusion glass–ceramic, resulting in a trilayer ceramic restoration [18–21]. Clinically, dental restorations need to withstand the oral environment with variable conditions of pH, temperature, and humidity, and stress induced by mastication and parafunctional habits [22–24]. Moreover, ceramics are susceptible to slow crack growth (SCG) in humid environment and to fatigue mechanisms that can degrade its fracture resistance and reduce the restoration lifetime [22,25]. SCG involves the stable growth of cracks at stress intensity factor (KI ) levels lower than those necessary for the crack to become unstable (KIc ). Stresscorrosion results from a water-assisted rupture and cleavage of siloxane bonds at the crack tip under mechanical loads [22–24]. Ceramics with high glass content, such as felspathic porcelain and fluorapatite glass–ceramic, are more susceptible to SCG. Ceramics with higher crystalline content, such as lithium disilicate-based glass–ceramic and Y-TZP, are more resistant to stress-corrosion, but they can experience additional cyclic fatigue damage accumulation ahead of the crack tip in the form of localized microplasticity, or microcracking, often refer to as intrinsic fatigue mechanisms. Extrinsic mechanisms of fatigue could also be present and act in the wake behind the crack tip, reducing the effect of a crack-tip shielding process [26,27]. Therefore, fatigue failure happens at lower loads than predicted by fast fracture [1,22]. All-ceramic crowns show various fracture modes. For monolithic ceramic crowns, catastrophic failures are usually associated to radial cracks originated in the cementation surface, especially in occlusal and marginal areas. Otherwise, for multi-layered crowns, chipping and delamination are mostly associated to the veneer material and initiated from cracks introduced during axial and off-axial occlusal contacts (i.e. cone, partial cone and median cracks) [1–3]. In addition, fatigue degradation has been associated with progressive surface wear due to abrasion and attrition [27]. Thus, it is important to evaluate different monolithic and multi-layered ceramic systems under fatigue challenging as an attempt to reproduce failure mechanisms reported for allceramic restorations [25–29]. Therefore, the objective of this study was to evaluate the cyclic contact fatigue resistance and failure mode of monolithic and multilayer ceramic structures, testing the hypothesis that monolithic zirconia and CAD-on multilayer structures show higher survival rates after cyclic loading than monolithic lithium disilicate glass–ceramic and Y-TZP conventionally veneered by glass–ceramic.

2.

Materials and methods

Different ceramics and manufacturing techniques were used to obtain four types of ceramic structures (10 mm diameter × 1.8 mm thickness), simulating the configuration of ceramic restorations, as described below (n = 28): - CAD-on: Multilayer structure made of a zirconia (Y-TZP; IPS e.max ZirCAD, Ivoclar Vivadent, Schaan, Liechtenstein) framework (1.0 mm) and a lithium disilicate-based glass–ceramic (LDC; IPS e.max CAD, Ivoclar Vivadent, Schaan, Liechtenstein) veneer (0.7 mm), fused together with a glass–ceramic (G; IPS e.max CAD Crystall./Connect, Ivoclar Vivadent, Schaan, Liechtenstein). Y-TZP and LDC blocks were turned into cylindrical structures by machining (Ferdimat CA51H, Sao Jose dos Campos, Brazil) under water cooling. The cylindrical structures were cut into disc-shaped specimens using a diamond disc under water cooling in a metallographic cutting machine (Struers Minitron, Copenhagen, Denmark). Specimens were polished (Struers Abramin, Copenhagen, Denmark) up to 800- and 1200-graded silicon carbide papers under water cooling. Y-TZP discs were sintered (Zirkonofen 600/V2, ZirkonZahn, Gais, South Tyrol, Italy) according to manufacturer’s instructions (Table 1). The disc-shaped Y-TZP and LDC were fused according to manufacturer’s instructions (Table 1), as described in a previous study [18]. The thickness of G layer was estimated in 0.1 mm, since the final thickness of the CAD-on structure was 1.8 mm. - ZFC: Bilayer structure made of Y-TZP (IPS e.max ZirCAD) framework (1.0 mm) veneered with a fluorapatite glass–ceramic (0.7 mm) (FC - IPS e.max Ceram), obtained by the traditional layering technique (powder/liquid). Y-TZP structure was fabricated as described for CAD-on. Before applying FC, a thin layer (about 0.1 mm) of IPS e.max ZirLiner (Ivoclar Vivadent, Schaan, Liechtenstein) was applied on the Y-TZP surface and fired (Programat EP5000) according to manufacturer’s instructions (Table 1). The Y-TZP discs were placed into a silicone matrix (Zetaplus, Zhermack SpA, Badia Polesine, Italy) and a FC layer was applied using the conventional layering technique. The ceramic veneer was sintered according to manufacturer’s instructions (Table 1). -YZW: Monolithic structure (1.8 mm thick) of zirconia (Zenostar Zr Translucent, Wieland Dental, Rosbach vor der Höhe, Germany). A cylindrical pattern was scanned (Cerec InLab, Sirona Dental Company, Bensheim, Germany) and data was transferred to the milling machine (InLab MC X5, Sirona Dental Company, Bensheim, Germany) for multi-cylinder YZW machining. YZW cylinders were cut into discs with a metallographic cutting machine (Struers Minitron, Copenhagen, Denmark) using diamond disc under water cooling. Disc-shaped specimens were sintered (Zirkonofen 600/V2) according to manufacturer’s instructions (Table 1). -LDC: Monolithic structure (1.8 mm thick) of lithium disilicate-based glass–ceramic (IPS e.max CAD) fabricated as described for CAD-on. LDC structures were crystallized (Programat EP5000) according to manufacturer’s instructions (Table 1). Ceramic structures were bonded to a fiber-reinforced epoxy resin-based (G10- NEMA G10, International Paper, Hampton, SC, USA), which served as a dentin analog substrate [18,30].

Please cite this article in press as: Alessandretti R, et al. Cyclic contact fatigue resistance of ceramics for monolithic and multilayer dental restorations. Dent Mater (2020), https://doi.org/10.1016/j.dental.2020.02.006

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Table 1 – Description of materials used in the study, their composition and firing cycle. Material

Composition

Firing cycle

IPS e.max Zircad MO* IPS e.max CAD Crystall./Connect*

ZrO2 , Y2 O3 , HfO2 , Al2 O3 , SiO3, and other oxides Oxides, water, butandiol and chloride

IPS e.max CAD*

SiO2 , Li2 O, K2 O, P2 O5 , ZrO2 , ZnO, other and coloring oxides

IPS e.max Zirliner*

SiO2 , Al2 O3 , Na2 O, K2 O, CaO, P2 O5 , F, other oxides and pigments

IPS e.max Ceram*

SiO2 , Al2 O3 , Na2 O, K2 O, KnO, CaO, P2 O5 , F, other oxides and pigments

Zenostar Zr Translucent¥

ZrO2 , HfO2 , Y2 O3 and other oxides

Heating rate 8 ◦ C/min; firing temperature 1500 ◦ C for 120 min; cooling rate 8 ◦ C/min. CAD-on technique: Pre-drying temperature 403 ◦ C for 2 min; heating rate 30 ◦ C/min; firing temperature (1) 820 ◦ C for 2 min and firing temperature (2) 840 ◦ C for 7 min; vacuum start temperature 550 ◦ C; vacuum finish temperature 820 ◦ C. Monolithic: Pre-drying temperature 403 ◦ C for 6 min; heating rate 60 ◦ C/min; firing temperature 840 ◦ C; vacuum start temperature 550 ◦ C; vacuum finish temperature 820 ◦ C. Pre-drying temperature 403 ◦ C for 4 min; heating rate 40 ◦ C/min; firing temperature 960 ◦ C; vacuum start temperature 450 ◦ C; vacuum finish temperature 959 ◦ C. Pre-drying temperature 403 ◦ C for 4 min; heating rate 40 ◦ C/min; firing temperature 750 ◦ C; vacuum start temperature 450 ◦ C; vacuum finish temperature 749 ◦ C. Heating rate 5 ◦ C/min; firing temperature 1450 ◦ C for 120 min; cooling rate 5 ◦ C/min.

∗ ¥

Ivoclar Vivadent, Schann, Liechtenstein. Information from manufacturers. Wieland Dental, Ivoclar Vivadent; Schaan, Liechtenstein. Information from manufacturers.

That is, 25-mm in diameter G10 cylinders were cut in slices of 4 mm thick using a diamond disc on a metallographic cutting machine (Struers Minitron, Copenhagen, Denmark) under water cooling. Aluminum oxide particles (D50 = 110 ␮m; Renfert do Brasil, Ribeirão Preto, Brazil) were used to sandblast (20 s) the bonding surface of G10 with a pressure of 2 bars from a 10-mm distance. The bonding surface of all ceramic structures was also sandblasted as described for G10. Both G10 and ceramic structures were sonically cleaned in a distilled water bath (5 min), water-sprayed and air-dried before the adhesive process. The adhesive system (Primers A and B, Ivoclar Vivadent, Schaan, Liechtenstein) was applied on the sandblasted surface of G10. Bonding surface of the ceramics was treated with silane (Monobond S, Ivoclar Vivadent, Schann, Liechtenstein) for 3 min and air-dried for 10 s. The dual resin cement containing phosphate monomer (MDP) (Multilink N, Ivoclar Vivadent, Schaan, Liechtenstein) was mixed and applied on the treated surfaces of G10 and ceramic, which were bonded together and placed under a 750 g load for 1 min. Excess cement was removed with a brush (cavibrush, FGM, Joinvile, SC, Brazil) and specimens were light-activated for 40 s from each side using a light curing unit (Bluephase N, Ivoclar Vivadent, Schann, Liechtenstein; 1.200 mW/cm2 ). Specimens were stored in 37 ◦ C distilled water for 24 h prior to fatigue tests in a pneumatic cycling machine (BioPDI, Biocycle, São Carlos, Brazil) with 2 Hz frequency in distilled water at 37 ◦ C. A load of 80 N was applied to the ceramic surface with a spherical stainless steel metal piston (diameter: 6 mm) [31]. Test was interrupted after 104 , 105 , 5 × 105 e 106 cycles and the presence or absence of failure was recorded. The fatigue protocol (load and number of cycles) was defined based on data from a pilot study. Failure was reported when cracking, chipping or catastrophic fracture was detected. Survived specimens showed no evidence of crack or fracture. Transillumination and optical microscopy were used to examine the specimens and failure report followed fractographic principles. Cracks leading to failure were classified as follows: radial

Fig. 1 – Kaplan–Meier survival curves for the experimental groups.

crack, cone crack, and combined (when both radial and cone cracks were presented) [18]. Fatigue data were analyzed using Kaplan–Meier and Holm–Sidak survival test (˛ = 0.05). The relationship between experimental group and the type of crack was analyzed with Chi-square test (˛ = 0.05).

3.

Results

Fig. 1 shows the Kaplan–Meier survival curves for the experimental groups. There was no statistical difference (p = 0.516)

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Table 2 – Mean survival time (number of cycles) with respective 95% confidence intervals (95% CI) for each experimental group. Groups

Mean survival time*

95% CI

CAD-on ZFC LDC YZW

591,071.00A 16,428.00C 237,178.00B 626,785.00A

425,136.00–757,006.00 7,685.00–25,171.00 111,675.00–362,680.00 455,139.00–798,431.00

∗

Values followed by same letter in the column are not statistically different (p > 0.05).

between the survival curves of YZW and CAD-on, which had the highest fatigue survival rates, followed by LDC and ZFC that were significantly different from each other (p < 0.01). The mean survival time and 95% confidence intervals (95% CI) of the experimental groups are shown in Table 2. 57% of YZW specimens and 42% of CAD-on specimens survived the fatigue test (106 cycles). Only 4 LDC specimens (14%) and none of the ZFC specimens survived the fatigue test. There was a significant relationship between experimental group and type of crack leading to failure (p < 0.001) (Table 3). LDC showed the highest frequency of radial cracks (Fig. 2A), while ZFC and CAD-on groups showed the highest frequency of cone cracks (Fig. 2C and D). Combined cracks, in which both radial and cone cracks were presented, were more frequent for YZW specimens (Fig. 2B). Most specimens showed cracks without chipping and catastrophic failure of the structure, especially for CAD-on and ZFC groups. Representative images of the different type of cracks are shown in Fig. 2A–D.

4.

Discussion

There is a wide discussion in the literature regarding on what type of ceramic material and restoration configuration could provide the best long-term clinical behavior [1,22,32]. Different strategies have been suggested aiming to reduce the failure rates of all-ceramic restorations, which involves either simplifying the restoration configuration (monolithic restorations) using ceramics with higher fracture toughness and good optical properties (translucent monolithic zirconia) or modifying the veneering material and technique to obtain a more favorable mechanical behavior for the multilayer restoration (e.g., CAD-on technique) [18–21]. In the present study both strategies were investigated, YZW and CAD-on, which showed the greatest survival rates in fatigue, confirming such literature trend and accepting the study hypothesis. The evaluated monolithic zirconia (Zenostar T) is composed of 3 mol% of yttria and less than 15% of cubic phase, characterized as second-generation zirconia, to which the strategy to increase translucency included increasing density through sintering at higher temperatures and reducing the concentration of alumina additives [16,17]. Thus, this ceramic has mechanical properties, such as flexural strength and fracture toughness, similar to the zirconia-based ceramics traditionally used as framework materials (firstgeneration zirconia). Yet, third-generation zirconia-based ceramics, which were not investigated in the present study, have an increased content of yttria stabilizer and a greater

fraction of cubic phase (> 25%), which improves translucency but worsens the mechanical behavior [16,17]. Therefore, the eminent development of translucent zirconia for monolithic restorations could expand its indication offering an additional option for the anterior teeth, where lithium disilicate-based glass–ceramic is widely used because of its good mechanical and optical properties. Nevertheless, lithium disilicate-based glass–ceramic has lower flexural strength, chipping resistance and load to fracture than first and second-generation zirconia-based ceramics. In addition, the present study showed that lithium disilicate monolithic (LDC) specimens were more prone to fail under fatigue than monolithic zirconia (YZW). Specimens from LDC group showed a high frequency of radial cracks. When applying a compressive load to the surface of a stiff monolithic ceramic structure (e.g., LDC) cemented onto an elastic substrate (e.g., G10, which serves as dentin analog), the substrate deformation (flexure) generates tensile stresses at the bonding interface and on the ceramic intaglio surface, where radial cracks are originated [18,22,33]. This rationale could also be applied to YZW specimens. However, due to the superior mechanical properties of zirconia, at a constant load, it requires a greater number of cycles to fail, compared to LDC. Thus, fatigue cumulative damage may result in a radial crack in the intaglio surface accompanied by a cone crack in the surface of the zirconia in contact with the piston (combined crack), where there is also a high concentration of stresses. Fracture is often a complex competition between various failure modes. Especially in fatigue, it is difficult to predict or conclude which crack is the primary source of the catastrophic fracture. Outer cone cracks and radial cracks are susceptible to SCG; while inner cone cracks are also prone to the additional effect of hydraulic pumping from pressureinduced intrusion of water during cyclic loading [27,33]. Yet, clinically, most catastrophic failures of all-ceramic monolithic crowns did not initiate from damage at the occlusal surface, but rather from radial cracks located in the ceramic intaglio surface [30,34]. It has been reported that multilayer structures have different failure mode compared to monolithic structures because the stress distribution is influenced by the flexural strength, elastic modulus and thickness of the materials [11,18,21,22]. Veneer ceramics are prone to cone cracks at the loading contact and to radial cracks from the interface with the framework ceramic, which is also susceptible to radial cracks from the intaglio surface [33]. In the present study, which uses a clinically relevant load level (80 N), cracking was the predominant failure mode for multilayer structures (CAD-on and ZFC), with cone cracks into the veneer ceramic the most frequent type of crack. Reports on multi-layered structures having Y-TZP as infrastructure, submitted to low loads and high number of cycles, have showed that inner cone cracks are dominant [33]. Such observations are in agreement with some reports on multilayer all-ceramic restoration failures that showed veneer chipping or delamination associated with contact damage [30,33,34]. Even though the failure modes were similar for the multilayer structures (CAD-on and ZFC), the fatigue behavior was different. CAD-on specimens showed greater fatigue resis-

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Table 3 – Survival rate, frequency of each type of crack and presence of fragments after fatigue test of specimens from each experimental group. Groups

CAD-on ZFC LDC YZW ∗

Specimens survival (%)

12 (42%) 0 (0%) 4 (14%) 16 (57%)

Type of crack*

Failure mode

Radial

Cone

Combined

Cracking

3 (11%) 3 (11%) 13 (46%) 3 (11%)

11 (40%) 22 (78%) 4 (15%) 0 (0%)

2 (7%) 3 (11%) 7 (25%) 9 (32%)

12 (75%) 25 (89%) 11 (46%) 0 (0%)

Fracture Chipping

Catastrophic

0 (0%) 2 (7%) 0 (0%) 0 (0%)

4 (25%) 1 (4%) 13 (54%) 12 (100%)

p < 0.001 (Chi-square test).

Fig. 2 – Top view of failed specimens. (A) LDC specimen tested up to 100,000 cycles showing a radial crack. (B) YZW specimen showing combined failure mode at 500,000 cycles; a radial crack runs across the specimen cross-section while a partial cone crack can be observed on top surface. (C) Cone crack shown on top surface (LDC layer) of a CAD-on specimen tested up to 500,000 cycles. (D) Cone crack on top surface (FC layer) of a ZFC specimen at 10,000 cycles. Black arrows point to radial cracks (RC) initiated in the ceramic intaglio surface. White arrows point to the cone cracks (CC) in the top ceramic surface, originated from the contact with the loading piston. Images were obtained using transillumination with blue light (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

tance (42% survived the fatigue test) than ZFC specimens, which also showed a higher probability of failure in fatigue and a lower survival time compared to other groups. The low mechanical properties of the fluorapatite glass–ceramic (IPS

e.max Ceram) combined with inherent limitations from the fabrication technique (traditional layering) impaired its structural resistance to fatigue [22]. Notwithstanding, the layering technique still is widely used in dentistry, but it is sensitive to

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several factors, including the experience of the dental laboratory technician, the homogeneity of the ceramic paste (mixing powder and liquid) and the sintering conditions [8,22,23,26,35]. Moreover, CAD-on survival rates were also superior to monolithic LDC structures, mainly due to the reinforcing effect of the high strength high toughness zirconia infrastructure [33]. A previous study found that monolithic zirconia and CADon structures have similar load to failure and reliability [18], which is in agreement with the contact cyclic fatigue resistance data from the present study. Yet, conventionally veneered Y-TZP showed greater fracture load than monolithic LDC [18]; while in the present study LDC showed superior survival rate in fatigue compared to ZFC. As previously mentioned, fatigue failure behavior of ceramic structures is complex and often affected by several strength degradation mechanisms. The fluorapatite glass–ceramic used to veneer the Y-TZP infrastructure is more susceptible to stresscorrosion than lithium dissilicate-based glass–ceramic. In addition, SCG is not present during fast fracture experiments [23,24,26,27]. In the present study, the fatigue protocol and specimen configuration were designed to simulate the oral conditions and to reproduce the fabrication process used for all-ceramic restorations. Yet, aiming to standardize the surface conditions and the flaw population of the ceramic structures, the same cementation surface treatment was used for different materials. Therefore, LDC was not acid-etched as suggested by the manufacturer. In addition, the absence of thermal cycling to accelerate cement degradation, the absence of sliding movements during cyclic loading and the use of a simplified model to mimic ceramic restorations are study limitations. Moreover, the long-term success of ceramic restorations is not only influenced by the characteristics of the restoration itself (i.e. composition, processing, thickness), but also by the inherent features of the remaining dental structure and cementation protocol (adhesion quality) [36]. Thus, further studies should be conducted aiming to investigate additional factors involved in the success of all-ceramic restorations.

5.

Conclusion

Monolithic zirconia (YZW) showed similar fatigue resistance to CAD-on multilayer specimens, but different failure modes. Both strategies showed favorable fatigue behavior and are good options for prosthetic rehabilitation. Monolithic lithium disilicate-based glass–ceramic (LDC) showed intermediate fatigue resistance. The bilayer structure (ZFC) showed the lowest survival time under fatigue.

Acknowledgements This study was partially supported by Capes do Brasil, and CNPq do Brasil Grant #302587/2017-9. The authors thank Ivoclar Vivadent, Inc. for partially supported the materials used in the study.

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Please cite this article in press as: Alessandretti R, et al. Cyclic contact fatigue resistance of ceramics for monolithic and multilayer dental restorations. Dent Mater (2020), https://doi.org/10.1016/j.dental.2020.02.006