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Mechanical behavior of endocrowns fabricated with different CAD-CAM ceramic systems
RESEARCH AND EDUCATION
Mechanical behavior of endocrowns fabricated with different CAD-CAM ceramic systems Nereu Roque Dartora, MS, DDS,a Izabela Cristina Maurício Moris, PhD, MS, DDS,b Stephanie Francoi Poole, DDS,c Ataís Bacchi, PhD, MS, DDS,d Manoel Damião Sousa-Neto, PhD, MS, DDS,e Yara Terezinha Silva-Sousa, PhD, MS, DDS,f and Erica Alves Gomes, PhD, MS, DDSg An emphasis on minimally ABSTRACT invasive principles combined Statement of problem. The mechanical behavior of ceramic endocrowns is unclear. with adhesive materials has Purpose. The purpose of this in vitro and 3-dimensional finite element analysis (3D-FEA) study was led to the development of new to evaluate the mechanical behavior of endodontically treated teeth restored with ceramic options for the restoration of endocrowns made by using different computer-aided design and computer-aided manufacturing 1 ,2 endodontically treated teeth, (CAD-CAM) systems. including the single-piece , Material and methods. Sixty mandibular human molars were endodontically treated, prepared for endocrown.3 4 These are endocrowns, and divided into 4 groups (n=15) according to the following various ceramic systems: fabricated with the crown and leucite-based glass ceramic (LC group), lithium disilicate-based glass ceramic (LD group), glass the core as a single unit ceramic based on zirconia-reinforced lithium silicate (LSZ group), and monolithic zirconia (ZR anchored to the internal group). After adhesive bonding, the specimens were subjected to thermomechanical loading and portion of the pulp chamber, then to fracture resistance testing in a universal testing machine. The failure mode of the thereby achieving macrospecimens was qualitatively evaluated. Three-dimensional FEA was performed to evaluate the stress distribution in each group. Data were analyzed by using a 1-way ANOVA and the Tukey mechanical retention provided HSD test (a=.05). by the walls of the pulp chamber and micromechanical Results. Statistically significant differences among the groups were observed (P<.05). The outcomes retention by means of adheof the LC, LD, and LSZ groups were similar (1178 N, 1935 N, and 1859 N) but different from those of the ZR group (6333 N). The LC and LD groups had a higher ratio of restorable failures, while LSZ and sive bonding.5,6 These restoZR had more nonrestorable failures. Fractographic analysis indicated a regular failure pattern in the rations are particularly ZR group and irregular failure patterns in the other groups. Three-dimensional FEA revealed similar indicated for endodontically values and stress pattern distributions among the groups. treated posterior teeth with Conclusions. The mechanical performance of monolithic zirconia was better than that of the other extensive loss of the crown, ceramic endocrowns considered in this research; however, monolithic zirconia presented a higher weakened axial walls, limited rate of catastrophic tooth structure failure. (J Prosthet Dent 2020;-:---) interocclusal space, and/or 2 ,7 ,8 short clinical crowns. as compared with fiber or metal posts.6,10 In addition, Ceramic molar endocrowns have greater retention 9 in compressive tests, molars restored with endocrowns and stability and are less prone to fracturing because have been reported to be more resistant to fracture they lead to a decrease in dentin tensile stress levels This research received support from São Paulo State Research Foundation (FAPESP) (research grant numbers 2016/25311-7). a Postgraduate student, School of Dentistry, University of Ribeirão Preto (UNAERP), Ribeirão Preto, Brazil. b Professor, School of Dentistry, University of Ribeirão Preto (UNAERP), Ribeirão Preto, Brazil. c Postgraduate student, School of Dentistry, University of Ribeirão Preto (UNAERP), Ribeirão Preto, Brazil. d Professsor, Meridional Faculty (IMED), School of Dentistry, Passo Fundo, Brazil. e Professor, Department of Restorative Dentistry, Dental School of Ribeirão Preto, University of São Paulo (FORP-USP), Ribeirão Preto, Brazil. f Professor, School of Dentistry, University of Ribeirão Preto (UNAERP), Ribeirão Preto, Brazil. g Professor, School of Dentistry, University of Ribeirão Preto (UNAERP), Ribeirão Preto, Brazil.
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Table 1. Details of ceramic materials used
Clinical Implications Lithium-reinforced and leucite-reinforced glass ceramics are suitable materials for endocrowns. Dentists should also consider monolithic zirconia as a promising material.
than conventional crowns with intraradicular retainers.3 Monolithic endocrowns have been fabricated with different systems,11-16 including computer-aided design and computer-aided manufacturing (CAD-CAM).17 Leucite-reinforced glass ceramic has satisfactory optical properties and higher flexural strength than feldspathic ceramics.18,19 Lithium disilicate is a good option for endocrown fabrication, allowing for adhesive or conventional cementation,20 with optical properties similar to those of natural teeth20 and appropriate flexural strength.21 Lithium silicate with zirconium dioxide has been reported to provide increased flexural strength when compared with lithium disilicate.19,22,23 Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) offers adequate biocompatibility, satisfactory esthetics, and high fracture toughness and flexural strength.24-28 However, information about the behavior of Y-TPZ when used in the fabrication of endocrowns is lacking. The purpose of this in vitro and 3-dimensional finite element analysis (3D-FEA) study was to evaluate the mechanical behavior of endodontically treated teeth restored with ceramic endocrowns made by using different CAD-CAM systems. The null hypothesis was that the different ceramic materials used in the fabrication of endocrowns would not affect the mechanical strength, fracture pattern, stress values, or distribution schemes in endodontically treated teeth. MATERIAL AND METHODS After approval by the appropriate research ethics committee (C.A.A.E.: 55753216.3.0000.5319), 60 mandibular human molars extracted because of periodontal disease or for orthodontic reasons and with similar root lengths and mesiodistal and buccolingual dimensions stored in 0.1% thymol solution at 4 C were selected for this study. The teeth were sectioned parallel to the occlusal surface by using a diamond disk (15LC; Buehler) at a low speed with a sectioning machine (Isomet 1000; Buehler) 2 mm above the cementoenamel junction (CEJ). The endodontic treatment was performed as previously described.9 After 7 days of storage at 37 C and 100% humidity, the roots were embedded 1 mm from the CEJ in a plastic cylinder (25 mm diameter, 20 mm height) in the long axis, with THE JOURNAL OF PROSTHETIC DENTISTRY
Group
Material
Composition
Manufacturer
LC
Leucite-reinforced vitreous ceramic (IPS Empress CAD)
Components: SiO2 Additional content: Al2O3, K2O, Na2O, and other oxides.
LD
Lithium disilicatereinforced vitreous ceramic (IPS and max CAD)
Components: SiO2 Ivoclar Vivadent AG Additional content: Li2O, K2O, MgO, Al2O3, P2O5, and other oxides.
LSZ
Vitreous ceramic reinforced with lithium silicate and zirconium oxide (VITA Suprinity PC)
ZrO2: 8%-12% SiO2: 56%-64% Li2O: 15%-21% La2O3: 0.1% Pigments: <10% Other oxides: >10%
VITA Zahnfabrik H. Rauter GmbH & Co KG
ZR
Monolithic zirconia (ZirkOM SI)
ZrO2: 94.39% Y2O3: 5.30% Other oxides: 0.31%
Qinhuangdao Aidite HighTechnical Ceramics Co Ltd
Ivoclar Vivadent AG
autopolymerized acrylic resin (Jet; Artigos Odontológicos Clássico Ltda). Each specimen was mounted in a high-speed milling machine (Dentsply Sirona) and prepared by using a diamond rotary instrument (2136; KG Sorensen) under water cooling. A shoulder with a width between 2.2 mm and 2.7 mm was prepared. Internally, the preparation was approximately 5 mm in depth, 4 mm in the buccolingual direction, and 6 mm in the mesiodistal direction. The axial walls showed an internal taper of 8 to 10 degrees. A barrier of glass ionomer cement (Ketac Molar; 3M ESPE) was applied to the pulp chamber.9 The specimens were divided into 4 groups (n=15) according to the different CAD-CAM systems (Table 1). The endocrowns were fabricated by using a CADCAM system (Cerec 3; Dentsply Sirona) and were milled (CEREC InLab MC XL System; Dentsply Sirona). For bonding, the internal surface was etched with 5% hydrofluoric acid (IPS Ceramic Etching Gel; Ivoclar Vivadent AG) for 60 seconds for the LC group and 20 seconds for the LD and LSZ groups, while, for the ZR group, endocrowns were airborne-particle abraded (Rocatec system; 3M ESPE). A silane coupling agent (RelyX Ceramic Primer; 3M ESPE) was applied and dried. The surfaces of the dental preparations were treated with 37% phosphoric acid (FGM Produtos Odontológicos), 2 consecutive layers of adhesive system (Adper Single Bond Plus; 3M ESPE) were applied and light polymerized at 650 mW/cm2 power density (Radii Plus; SDI), and they were cemented with dual-polymerizing resin cement (RelyX Ultimate; 3M ESPE). The specimens were stored in relative humidity at 37 C for 7 days; subjected to thermomechanical loading testing in a pneumatic mastication simulator (Biopdi) at a frequency of 5 Hz, starting with a load of 80 N, followed by stages of 120, 160, 200, 240, 280, and 320 N, with a maximum of 20 000 cycles each9; and, simultaneously, thermocycled in a water bath at 5 C Dartora et al
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Table 2. Mechanical properties of investigated materials Material E (GPa) n
Table 3. Fracture resistance values (N) Groups
Mean
Standard Deviation
Leucite-reinforced vitreous ceramic
65.3
0.20
30
LC
1178 A
273
Lithium disilicate
102.5
0.21
30
LD
1935 A
530
Zirconium oxideereinforced lithium silicate
102.9
0.19
30
LSZ
1859 A
588
Monolithic zirconia
206.3
0.24
30
ZR
6333 B
2391
Bone marrow
1.37
0.30
31
Dentin
18.6
0.31
32
Periodontal ligament
0.05
0.45
33
0.45
34
1.4×10
and 55 C. A 6-mm diameter metal sphere was used as the antagonist. Then, the specimens were loaded to fracture in a universal testing machine (Biopdi) until permanent deformation or failure. The crosshead speed was 0.5 mm/min, and a compressive load was axially applied with a load cell of 4.9 kN. The maximum load was recorded in Newtons. The failure mode of the specimens was qualitatively evaluated by using a stereomicroscope (Leica DFC295 connected to a Leica S8 APO; Leica Microsystems) at ×40 magnification and classified as Type I, endocrown fracture; Type II, restorable remaining tooth structure, fracture above the bone crest level (BCL); or Type III, nonrestorable remaining tooth structure, fracture below the BCL. Subsequently, the fractured specimens were ultrasonically cleaned in isopropyl alcohol for 10 minutes and then in distilled water for 10 minutes, dried, and examined under a scanning electron microscope (SEM) (EVO 50H Electron Microscope; Carl Zeiss AG, Zeiss) to determine the mode of failure based on the origin of the fracture and the principles of fractography.29 The Shapiro-Wilk statistical test for normality and the Levene test for homogeneity revealed normal distributions for the data. The fracture resistance data were subjected to 1-way ANOVA and the Tukey HSD test (a=.05). Analyses were performed by using a statistical software program (IBM SPSS Statistics, v20.0; IBM Corp). The 3D geometry of the endodontically treated molars was obtained by computed microtomography (SkyScan 1174v2; Bruker-microCT) and imported into a CAD software program (Rhinoceros 5.0 Educational, NURBS Modeling for Windows; McNeel North America) for modeling the endocrown, radicular dentin, gutta percha, periodontal ligament (0.2-mm thick), and cylindrical block (25 mm in diameter and 20 mm in height). The FE mesh was obtained (SimLab 2017.2.1; Altair/HyperWorks) with linear elements Type Tet10, with a total of 504 659 nodes and 304 632 elements. The mechanical properties of the materials (Young modulus E and Poisson ratio v) were obtained from published data (Table 2). Four models reproducing the different CAD-CAM ceramics used to fabricate the endocrowns in the in vitro analysis were obtained. All materials were considered linearly elastic, isotropic, and homogeneous. A static axial Dartora et al
Different letters indicate statistical differences (P<.05).
100
Percentage (%) of Failure
Gutta percha
−1
Reference
75
50
25
0
LC
LD
LSZ
ZR
Groups Type III
Type II
Type I
Figure 1. Percentage (%) of failure type in groups studied. LC, leucitebased glass ceramic; LD, lithium disilicate-based glass ceramic; LSZ, glass ceramic based on zirconia-reinforced lithium silicate; ZR, monolithic zirconia.
load of 200 N was applied at 3 points of centric occlusion on the occlusal surface of the endocrown. As a boundary condition, the nodes of the base and side face of the cylindrical block were fixed, assuming x=y=z=0. All structures of the models were considered perfectly joined, without adhesion failures or interpositioning. The von Mises stress criterion was used to evaluate the stress values and distribution patterns. RESULTS One-way ANOVA revealed a statistically significant difference among the groups (F[3,56]=54 528; P<.05). The Tukey HSD test revealed that the ZR group had the highest fracture resistance values, statistically different from the other groups (P<.05) (Table 3). The analysis of failure revealed a higher percentage of restorable fractures in the LC and LD groups (Fig. 1) and unrestorable fractures in the LSZ and ZR groups (Fig. 2). The SEM revealed that the fracture surface originated in the occlusal surface, particularly at the point of loading, in all groups. In the occlusal surface, the presence of porosities in the LC and LD groups and cracks in the LC group were noted, but no porosities in the LSZ and ZR groups. For the fracture features, the LC group had irregular fractures typical of vitreous ceramics in several THE JOURNAL OF PROSTHETIC DENTISTRY
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Figure 2. Occlusal view of failure mode for each studied group. A, LCdfailure Type I. B, LDdfailure Type I. C, LSZdfailure Type III. D, ZRdfailure Type III. LC, Leucite-based glass ceramic; LD, Lithium disilicate-based glass ceramic; LSZ, Glass ceramic based on zirconia-reinforced lithium silicate; ZR, Monolithic zirconia.
ceramic fragments (Fig. 3); the LD group had irregular fractures with fewer steps than the LC and LSZ groups, presenting as a vitreous ceramic (Fig. 4); the LSZ group had cracks, fractures with more steps but in only 1 plane, with several ceramic fragments, as in the LC group (Fig. 5); and the ZR group had fewer steps in the surface of the ceramic material, while the fracture of the tooth was characterized as a regular fracture but with a rougher surface, which is a characteristic of crystalline ceramics (Fig. 6). A higher level of stress was observed in the loading application point and the region of the angle between the pulp and axial walls of the endocrown and root dentin among the groups (Fig. 7). Quantitatively, the von Mises stress values were similar among the groups (LC: 636 MPa; LD: 631 MPa; LSZ: 631 MPa; and ZR: 626 MPa). DISCUSSION The first null hypothesis was rejected because the different ceramic materials affected both the fracture mechanical strength and the fracture pattern of the endodontically treated teeth. The second null hypothesis THE JOURNAL OF PROSTHETIC DENTISTRY
was accepted because the von Mises stress values and distribution patterns were similar among the groups. The vitreous ceramics used in this study (LC, LD, and LSZ) were similar in terms of mechanical strength. Considering the mean fracture resistance of the groups studied and correlating it with occlusal force in healthy molars (which varies from 441 N to 981 N in individuals with normal occlusion and parafunctional habits),35 these materials may be indicated for the fabrication of endocrowns. However, the LC group had a mean resistance strength value close to that of the parafunctional occlusal force, suggesting that preference be given to the other materials for restoration in patients with bruxism.36 LD had a mean value similar to that found in previous studies.9 Meanwhile, LSZ had maximum resistance values similar to those values obtained for molar crowns produced with CAD-CAM and a thickness of 1.5 mm.17 According to the manufacturer, 8% to 10% by weight of zirconium oxide should be added to provide greater mechanical resistance to the material19,22,23; however, this was not observed in the present study. The thermomechanical loading performed before the fracture Dartora et al
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Figure 3. Scanning electron microscope images of fracture surface testing for leucite-based glass ceramic. A, B, Fractography showed that fracture originated in occlusal surface at point of loading. C, D, Features of fractographic analysis. (A, C, D, original magnification ×40; B, original magnification ×100).
resistance testing may have caused instability in the phases of the lithium silicate reinforced with zirconium oxide This resulted in an increase in local residual stresses that were relieved during cooling by means of microcracks, leading to statistically similar mechanical strength resistance values between LC and LD.19 The ZR group had the highest fracture strength resistance value (6333 N), which was better than the fracture resistance of a healthy molar (3901 N).36 This finding is related to its high fracture toughness and flexural strength, obtained from a composition mainly containing crystalline particles and suggesting its appropriateness in extensive restorations.19,24,25 Analyzing the fracture pattern and determining the nature of the stress distribution are as important as considering the fracture load, as, in clinical practice, after the failure occurs, the tooth is assessed to determine whether the remaining structure is repairable or not.15 With regard to the patterns of endocrown failures, the LC and LD groups had higher percentages of Type I and Type II failures, which are considered restorable fractures. However, the LSZ and ZR groups had a high percentage Dartora et al
of unrestorable fractures.15 The addition of zirconium oxide particles provided to the LSZ a fracture pattern similar to that of ZR. However, ZR had higher maximum fracture resistance than LSZ. The high fracture resistance of a material is not the single determining factor in the observation of high rates of catastrophic failures. In this study, the LSZ group had an 85% catastrophic failure rate and a fracture resistance level of approximately 30% of that of the ZR group. Even though the ZR group had a high percentage of catastrophic failures, they generally occurred under a load that not even patients with bruxism could achieve. The better mechanical results suggest that the fabrication of ZR endocrowns can improve success rates. In the analysis of von Mises stress, the FE models had similar stress distribution patterns among the groups,14 with higher stress observed between the pulp base of the endocrown and the radicular dentin.9 FEA is a complementary numerical tool because it allows the user to identify the regions with the highest stress concentration, as well as those most prone to failure. However, THE JOURNAL OF PROSTHETIC DENTISTRY
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Figure 4. SEM images of fracture surface testing for lithium disilicate-based glass ceramic. A, B, Fractography showed that fracture originated in occlusal surface at point of loading. C, D, Features of fractographic analysis. (A, C, D, original magnification ×40; B, original magnification ×100).
Figure 5. SEM images of fracture surface testing for glass ceramic based on zirconia-reinforced lithium silicate. A, B, Fractography showed that fracture originated in occlusal surface at point of loading. C, D, Features of fractographic analysis. (A, B, C, D, original magnification ×40). THE JOURNAL OF PROSTHETIC DENTISTRY
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Figure 6. SEM images of fracture surface testing for monolithic zirconia. A, B, Fractography showed that fracture originated in occlusal surface at point of loading. C, D, Features of fractographic analysis. (A, C, D, original magnification ×40; B, original magnification ×100).
it cannot predict fracture patterns nor their progression among the materials. Simulations are used to provide estimates of the survival rate and to predict catastrophic failures in monolithic ceramic restoration.26 In this study, the cyclic isometric loading protocol associated with thermal cycling promoted a gradual increase of the load on the restored tooth to accommodate the restoration material to the dental substrate until it reached a more critical occlusal load level that could lead to failure of the assembly. The SEM images showed high rates of porosity and cracks in the surface of the endocrowns in the LC, LD, and LSZ groups, which, probably, was influenced by the thermomechanical loading process. Laboratory studies like the present investigation have inherent limitations, and the results should be interpreted with caution. However, a correlation was established between the results obtained in the mechanical tests and finite element method; endocrowns made of lithium-reinforced and leucite-reinforced glass ceramics may be considered the most suitable materials for the rehabilitation of endodontically treated teeth, while Dartora et al
materials containing zirconium oxide warrant further examination with regard to the reliability of their use. Additionally, the results of the present study could be used as a basis for future research and might be relevant eventually in clinical practice in the reconstruction of dental crowns with endocrowns. CONCLUSIONS Based on the findings of this in vitro and FEA study, the following conclusions were drawn: 1. Vitreous ceramics reinforced with leucite and lithium disilicate had similar levels of mechanical resistance and a higher percentage of restorable failures. 2. Although vitreous ceramics reinforced with lithium silicate and zirconium oxide had a level of mechanical resistance statistically similar to that of ceramics containing leucite and lithium disilicate, a higher rate of catastrophic dental failures was noted. 3. In relation to the fracture resistance, the mechanical performance of monolithic zirconia was better than that of the other ceramic materials evaluated. THE JOURNAL OF PROSTHETIC DENTISTRY
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Figure 7. von Mises stress distribution of each model (sagittal view). A, Leucite-based glass ceramic (LC); B, Lithium disilicate-based glass ceramic (LD); C, Glass ceramic based on zirconia-reinforced lithium silicate (LSZ); D, Monolithic zirconia (ZR). Highest stress values in gray; lowest stress values in blue.
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biomechanical behavior of endodontically treated premolars. J. Endod 2008;34:1015-9. 34. Friedman CM, Sandrik JL, Heuer MA, Rapp GW. Composition and mechanical properties of gutta-percha endodontic points. J Dent Res 1975;54: 921-5. 35. Vallittu PK, Kononen M. Biomechanical aspects and material properties. 1st ed. Stockholm: Gothia Fortbildning; 2013. p. 116-30. 36. De Abreu RA, Pereira MD, Furtado F, Prado GP, Mestriner WJR, Ferreira LM. Masticatory efficiency and bite force in individuals with normal occlusion. Arch Oral Biol 2014;59:1065-74. Corresponding author: Dr Erica Alves Gomes School of Dentistry, University of Ribeiraeo Preto Av Costábile Romano, 2.201 Ribeiraeo Preto, SP CEP 14096-900 BRAZIL Email: [email protected] Acknowledgments The authors wish to thank Dr Ricardo Faria Ribeiro of the Laboratory of Biomechanical Studies in Prosthodontics and Implants at the Department of Dental Materials and Prosthodontics, School of Dentistry of Ribeirão Preto, University of São Paulo (FORP-USP), Brazil, for technical support and to Gustavo Dartora of the School of Dentistry, Meridional Faculty (IMED), Passo Fundo, RS, Brazil, for the help with the design and manufacture of restorations. Copyright © 2019 by the Editorial Council for The Journal of Prosthetic Dentistry. https://doi.org/10.1016/j.prosdent.2019.11.008
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Mechanical behavior of endocrowns fabricated with different CAD-CAM ceramic systems Nereu Roque Dartora, MS, DDS,a Izabela Cristina Maurício Moris, PhD, MS, DDS,b Stephanie Francoi Poole, DDS,c Ataís Bacchi, PhD, MS, DDS,d Manoel Damião Sousa-Neto, PhD, MS, DDS,e Yara Terezinha Silva-Sousa, PhD, MS, DDS,f and Erica Alves Gomes, PhD, MS, DDSg An emphasis on minimally ABSTRACT invasive principles combined Statement of problem. The mechanical behavior of ceramic endocrowns is unclear. with adhesive materials has Purpose. The purpose of this in vitro and 3-dimensional finite element analysis (3D-FEA) study was led to the development of new to evaluate the mechanical behavior of endodontically treated teeth restored with ceramic options for the restoration of endocrowns made by using different computer-aided design and computer-aided manufacturing 1 ,2 endodontically treated teeth, (CAD-CAM) systems. including the single-piece , Material and methods. Sixty mandibular human molars were endodontically treated, prepared for endocrown.3 4 These are endocrowns, and divided into 4 groups (n=15) according to the following various ceramic systems: fabricated with the crown and leucite-based glass ceramic (LC group), lithium disilicate-based glass ceramic (LD group), glass the core as a single unit ceramic based on zirconia-reinforced lithium silicate (LSZ group), and monolithic zirconia (ZR anchored to the internal group). After adhesive bonding, the specimens were subjected to thermomechanical loading and portion of the pulp chamber, then to fracture resistance testing in a universal testing machine. The failure mode of the thereby achieving macrospecimens was qualitatively evaluated. Three-dimensional FEA was performed to evaluate the stress distribution in each group. Data were analyzed by using a 1-way ANOVA and the Tukey mechanical retention provided HSD test (a=.05). by the walls of the pulp chamber and micromechanical Results. Statistically significant differences among the groups were observed (P<.05). The outcomes retention by means of adheof the LC, LD, and LSZ groups were similar (1178 N, 1935 N, and 1859 N) but different from those of the ZR group (6333 N). The LC and LD groups had a higher ratio of restorable failures, while LSZ and sive bonding.5,6 These restoZR had more nonrestorable failures. Fractographic analysis indicated a regular failure pattern in the rations are particularly ZR group and irregular failure patterns in the other groups. Three-dimensional FEA revealed similar indicated for endodontically values and stress pattern distributions among the groups. treated posterior teeth with Conclusions. The mechanical performance of monolithic zirconia was better than that of the other extensive loss of the crown, ceramic endocrowns considered in this research; however, monolithic zirconia presented a higher weakened axial walls, limited rate of catastrophic tooth structure failure. (J Prosthet Dent 2020;-:---) interocclusal space, and/or 2 ,7 ,8 short clinical crowns. as compared with fiber or metal posts.6,10 In addition, Ceramic molar endocrowns have greater retention 9 in compressive tests, molars restored with endocrowns and stability and are less prone to fracturing because have been reported to be more resistant to fracture they lead to a decrease in dentin tensile stress levels This research received support from São Paulo State Research Foundation (FAPESP) (research grant numbers 2016/25311-7). a Postgraduate student, School of Dentistry, University of Ribeirão Preto (UNAERP), Ribeirão Preto, Brazil. b Professor, School of Dentistry, University of Ribeirão Preto (UNAERP), Ribeirão Preto, Brazil. c Postgraduate student, School of Dentistry, University of Ribeirão Preto (UNAERP), Ribeirão Preto, Brazil. d Professsor, Meridional Faculty (IMED), School of Dentistry, Passo Fundo, Brazil. e Professor, Department of Restorative Dentistry, Dental School of Ribeirão Preto, University of São Paulo (FORP-USP), Ribeirão Preto, Brazil. f Professor, School of Dentistry, University of Ribeirão Preto (UNAERP), Ribeirão Preto, Brazil. g Professor, School of Dentistry, University of Ribeirão Preto (UNAERP), Ribeirão Preto, Brazil.
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Table 1. Details of ceramic materials used
Clinical Implications Lithium-reinforced and leucite-reinforced glass ceramics are suitable materials for endocrowns. Dentists should also consider monolithic zirconia as a promising material.
than conventional crowns with intraradicular retainers.3 Monolithic endocrowns have been fabricated with different systems,11-16 including computer-aided design and computer-aided manufacturing (CAD-CAM).17 Leucite-reinforced glass ceramic has satisfactory optical properties and higher flexural strength than feldspathic ceramics.18,19 Lithium disilicate is a good option for endocrown fabrication, allowing for adhesive or conventional cementation,20 with optical properties similar to those of natural teeth20 and appropriate flexural strength.21 Lithium silicate with zirconium dioxide has been reported to provide increased flexural strength when compared with lithium disilicate.19,22,23 Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) offers adequate biocompatibility, satisfactory esthetics, and high fracture toughness and flexural strength.24-28 However, information about the behavior of Y-TPZ when used in the fabrication of endocrowns is lacking. The purpose of this in vitro and 3-dimensional finite element analysis (3D-FEA) study was to evaluate the mechanical behavior of endodontically treated teeth restored with ceramic endocrowns made by using different CAD-CAM systems. The null hypothesis was that the different ceramic materials used in the fabrication of endocrowns would not affect the mechanical strength, fracture pattern, stress values, or distribution schemes in endodontically treated teeth. MATERIAL AND METHODS After approval by the appropriate research ethics committee (C.A.A.E.: 55753216.3.0000.5319), 60 mandibular human molars extracted because of periodontal disease or for orthodontic reasons and with similar root lengths and mesiodistal and buccolingual dimensions stored in 0.1% thymol solution at 4 C were selected for this study. The teeth were sectioned parallel to the occlusal surface by using a diamond disk (15LC; Buehler) at a low speed with a sectioning machine (Isomet 1000; Buehler) 2 mm above the cementoenamel junction (CEJ). The endodontic treatment was performed as previously described.9 After 7 days of storage at 37 C and 100% humidity, the roots were embedded 1 mm from the CEJ in a plastic cylinder (25 mm diameter, 20 mm height) in the long axis, with THE JOURNAL OF PROSTHETIC DENTISTRY
Group
Material
Composition
Manufacturer
LC
Leucite-reinforced vitreous ceramic (IPS Empress CAD)
Components: SiO2 Additional content: Al2O3, K2O, Na2O, and other oxides.
LD
Lithium disilicatereinforced vitreous ceramic (IPS and max CAD)
Components: SiO2 Ivoclar Vivadent AG Additional content: Li2O, K2O, MgO, Al2O3, P2O5, and other oxides.
LSZ
Vitreous ceramic reinforced with lithium silicate and zirconium oxide (VITA Suprinity PC)
ZrO2: 8%-12% SiO2: 56%-64% Li2O: 15%-21% La2O3: 0.1% Pigments: <10% Other oxides: >10%
VITA Zahnfabrik H. Rauter GmbH & Co KG
ZR
Monolithic zirconia (ZirkOM SI)
ZrO2: 94.39% Y2O3: 5.30% Other oxides: 0.31%
Qinhuangdao Aidite HighTechnical Ceramics Co Ltd
Ivoclar Vivadent AG
autopolymerized acrylic resin (Jet; Artigos Odontológicos Clássico Ltda). Each specimen was mounted in a high-speed milling machine (Dentsply Sirona) and prepared by using a diamond rotary instrument (2136; KG Sorensen) under water cooling. A shoulder with a width between 2.2 mm and 2.7 mm was prepared. Internally, the preparation was approximately 5 mm in depth, 4 mm in the buccolingual direction, and 6 mm in the mesiodistal direction. The axial walls showed an internal taper of 8 to 10 degrees. A barrier of glass ionomer cement (Ketac Molar; 3M ESPE) was applied to the pulp chamber.9 The specimens were divided into 4 groups (n=15) according to the different CAD-CAM systems (Table 1). The endocrowns were fabricated by using a CADCAM system (Cerec 3; Dentsply Sirona) and were milled (CEREC InLab MC XL System; Dentsply Sirona). For bonding, the internal surface was etched with 5% hydrofluoric acid (IPS Ceramic Etching Gel; Ivoclar Vivadent AG) for 60 seconds for the LC group and 20 seconds for the LD and LSZ groups, while, for the ZR group, endocrowns were airborne-particle abraded (Rocatec system; 3M ESPE). A silane coupling agent (RelyX Ceramic Primer; 3M ESPE) was applied and dried. The surfaces of the dental preparations were treated with 37% phosphoric acid (FGM Produtos Odontológicos), 2 consecutive layers of adhesive system (Adper Single Bond Plus; 3M ESPE) were applied and light polymerized at 650 mW/cm2 power density (Radii Plus; SDI), and they were cemented with dual-polymerizing resin cement (RelyX Ultimate; 3M ESPE). The specimens were stored in relative humidity at 37 C for 7 days; subjected to thermomechanical loading testing in a pneumatic mastication simulator (Biopdi) at a frequency of 5 Hz, starting with a load of 80 N, followed by stages of 120, 160, 200, 240, 280, and 320 N, with a maximum of 20 000 cycles each9; and, simultaneously, thermocycled in a water bath at 5 C Dartora et al
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Table 2. Mechanical properties of investigated materials Material E (GPa) n
Table 3. Fracture resistance values (N) Groups
Mean
Standard Deviation
Leucite-reinforced vitreous ceramic
65.3
0.20
30
LC
1178 A
273
Lithium disilicate
102.5
0.21
30
LD
1935 A
530
Zirconium oxideereinforced lithium silicate
102.9
0.19
30
LSZ
1859 A
588
Monolithic zirconia
206.3
0.24
30
ZR
6333 B
2391
Bone marrow
1.37
0.30
31
Dentin
18.6
0.31
32
Periodontal ligament
0.05
0.45
33
0.45
34
1.4×10
and 55 C. A 6-mm diameter metal sphere was used as the antagonist. Then, the specimens were loaded to fracture in a universal testing machine (Biopdi) until permanent deformation or failure. The crosshead speed was 0.5 mm/min, and a compressive load was axially applied with a load cell of 4.9 kN. The maximum load was recorded in Newtons. The failure mode of the specimens was qualitatively evaluated by using a stereomicroscope (Leica DFC295 connected to a Leica S8 APO; Leica Microsystems) at ×40 magnification and classified as Type I, endocrown fracture; Type II, restorable remaining tooth structure, fracture above the bone crest level (BCL); or Type III, nonrestorable remaining tooth structure, fracture below the BCL. Subsequently, the fractured specimens were ultrasonically cleaned in isopropyl alcohol for 10 minutes and then in distilled water for 10 minutes, dried, and examined under a scanning electron microscope (SEM) (EVO 50H Electron Microscope; Carl Zeiss AG, Zeiss) to determine the mode of failure based on the origin of the fracture and the principles of fractography.29 The Shapiro-Wilk statistical test for normality and the Levene test for homogeneity revealed normal distributions for the data. The fracture resistance data were subjected to 1-way ANOVA and the Tukey HSD test (a=.05). Analyses were performed by using a statistical software program (IBM SPSS Statistics, v20.0; IBM Corp). The 3D geometry of the endodontically treated molars was obtained by computed microtomography (SkyScan 1174v2; Bruker-microCT) and imported into a CAD software program (Rhinoceros 5.0 Educational, NURBS Modeling for Windows; McNeel North America) for modeling the endocrown, radicular dentin, gutta percha, periodontal ligament (0.2-mm thick), and cylindrical block (25 mm in diameter and 20 mm in height). The FE mesh was obtained (SimLab 2017.2.1; Altair/HyperWorks) with linear elements Type Tet10, with a total of 504 659 nodes and 304 632 elements. The mechanical properties of the materials (Young modulus E and Poisson ratio v) were obtained from published data (Table 2). Four models reproducing the different CAD-CAM ceramics used to fabricate the endocrowns in the in vitro analysis were obtained. All materials were considered linearly elastic, isotropic, and homogeneous. A static axial Dartora et al
Different letters indicate statistical differences (P<.05).
100
Percentage (%) of Failure
Gutta percha
−1
Reference
75
50
25
0
LC
LD
LSZ
ZR
Groups Type III
Type II
Type I
Figure 1. Percentage (%) of failure type in groups studied. LC, leucitebased glass ceramic; LD, lithium disilicate-based glass ceramic; LSZ, glass ceramic based on zirconia-reinforced lithium silicate; ZR, monolithic zirconia.
load of 200 N was applied at 3 points of centric occlusion on the occlusal surface of the endocrown. As a boundary condition, the nodes of the base and side face of the cylindrical block were fixed, assuming x=y=z=0. All structures of the models were considered perfectly joined, without adhesion failures or interpositioning. The von Mises stress criterion was used to evaluate the stress values and distribution patterns. RESULTS One-way ANOVA revealed a statistically significant difference among the groups (F[3,56]=54 528; P<.05). The Tukey HSD test revealed that the ZR group had the highest fracture resistance values, statistically different from the other groups (P<.05) (Table 3). The analysis of failure revealed a higher percentage of restorable fractures in the LC and LD groups (Fig. 1) and unrestorable fractures in the LSZ and ZR groups (Fig. 2). The SEM revealed that the fracture surface originated in the occlusal surface, particularly at the point of loading, in all groups. In the occlusal surface, the presence of porosities in the LC and LD groups and cracks in the LC group were noted, but no porosities in the LSZ and ZR groups. For the fracture features, the LC group had irregular fractures typical of vitreous ceramics in several THE JOURNAL OF PROSTHETIC DENTISTRY
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Figure 2. Occlusal view of failure mode for each studied group. A, LCdfailure Type I. B, LDdfailure Type I. C, LSZdfailure Type III. D, ZRdfailure Type III. LC, Leucite-based glass ceramic; LD, Lithium disilicate-based glass ceramic; LSZ, Glass ceramic based on zirconia-reinforced lithium silicate; ZR, Monolithic zirconia.
ceramic fragments (Fig. 3); the LD group had irregular fractures with fewer steps than the LC and LSZ groups, presenting as a vitreous ceramic (Fig. 4); the LSZ group had cracks, fractures with more steps but in only 1 plane, with several ceramic fragments, as in the LC group (Fig. 5); and the ZR group had fewer steps in the surface of the ceramic material, while the fracture of the tooth was characterized as a regular fracture but with a rougher surface, which is a characteristic of crystalline ceramics (Fig. 6). A higher level of stress was observed in the loading application point and the region of the angle between the pulp and axial walls of the endocrown and root dentin among the groups (Fig. 7). Quantitatively, the von Mises stress values were similar among the groups (LC: 636 MPa; LD: 631 MPa; LSZ: 631 MPa; and ZR: 626 MPa). DISCUSSION The first null hypothesis was rejected because the different ceramic materials affected both the fracture mechanical strength and the fracture pattern of the endodontically treated teeth. The second null hypothesis THE JOURNAL OF PROSTHETIC DENTISTRY
was accepted because the von Mises stress values and distribution patterns were similar among the groups. The vitreous ceramics used in this study (LC, LD, and LSZ) were similar in terms of mechanical strength. Considering the mean fracture resistance of the groups studied and correlating it with occlusal force in healthy molars (which varies from 441 N to 981 N in individuals with normal occlusion and parafunctional habits),35 these materials may be indicated for the fabrication of endocrowns. However, the LC group had a mean resistance strength value close to that of the parafunctional occlusal force, suggesting that preference be given to the other materials for restoration in patients with bruxism.36 LD had a mean value similar to that found in previous studies.9 Meanwhile, LSZ had maximum resistance values similar to those values obtained for molar crowns produced with CAD-CAM and a thickness of 1.5 mm.17 According to the manufacturer, 8% to 10% by weight of zirconium oxide should be added to provide greater mechanical resistance to the material19,22,23; however, this was not observed in the present study. The thermomechanical loading performed before the fracture Dartora et al
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Figure 3. Scanning electron microscope images of fracture surface testing for leucite-based glass ceramic. A, B, Fractography showed that fracture originated in occlusal surface at point of loading. C, D, Features of fractographic analysis. (A, C, D, original magnification ×40; B, original magnification ×100).
resistance testing may have caused instability in the phases of the lithium silicate reinforced with zirconium oxide This resulted in an increase in local residual stresses that were relieved during cooling by means of microcracks, leading to statistically similar mechanical strength resistance values between LC and LD.19 The ZR group had the highest fracture strength resistance value (6333 N), which was better than the fracture resistance of a healthy molar (3901 N).36 This finding is related to its high fracture toughness and flexural strength, obtained from a composition mainly containing crystalline particles and suggesting its appropriateness in extensive restorations.19,24,25 Analyzing the fracture pattern and determining the nature of the stress distribution are as important as considering the fracture load, as, in clinical practice, after the failure occurs, the tooth is assessed to determine whether the remaining structure is repairable or not.15 With regard to the patterns of endocrown failures, the LC and LD groups had higher percentages of Type I and Type II failures, which are considered restorable fractures. However, the LSZ and ZR groups had a high percentage Dartora et al
of unrestorable fractures.15 The addition of zirconium oxide particles provided to the LSZ a fracture pattern similar to that of ZR. However, ZR had higher maximum fracture resistance than LSZ. The high fracture resistance of a material is not the single determining factor in the observation of high rates of catastrophic failures. In this study, the LSZ group had an 85% catastrophic failure rate and a fracture resistance level of approximately 30% of that of the ZR group. Even though the ZR group had a high percentage of catastrophic failures, they generally occurred under a load that not even patients with bruxism could achieve. The better mechanical results suggest that the fabrication of ZR endocrowns can improve success rates. In the analysis of von Mises stress, the FE models had similar stress distribution patterns among the groups,14 with higher stress observed between the pulp base of the endocrown and the radicular dentin.9 FEA is a complementary numerical tool because it allows the user to identify the regions with the highest stress concentration, as well as those most prone to failure. However, THE JOURNAL OF PROSTHETIC DENTISTRY
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Figure 4. SEM images of fracture surface testing for lithium disilicate-based glass ceramic. A, B, Fractography showed that fracture originated in occlusal surface at point of loading. C, D, Features of fractographic analysis. (A, C, D, original magnification ×40; B, original magnification ×100).
Figure 5. SEM images of fracture surface testing for glass ceramic based on zirconia-reinforced lithium silicate. A, B, Fractography showed that fracture originated in occlusal surface at point of loading. C, D, Features of fractographic analysis. (A, B, C, D, original magnification ×40). THE JOURNAL OF PROSTHETIC DENTISTRY
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Figure 6. SEM images of fracture surface testing for monolithic zirconia. A, B, Fractography showed that fracture originated in occlusal surface at point of loading. C, D, Features of fractographic analysis. (A, C, D, original magnification ×40; B, original magnification ×100).
it cannot predict fracture patterns nor their progression among the materials. Simulations are used to provide estimates of the survival rate and to predict catastrophic failures in monolithic ceramic restoration.26 In this study, the cyclic isometric loading protocol associated with thermal cycling promoted a gradual increase of the load on the restored tooth to accommodate the restoration material to the dental substrate until it reached a more critical occlusal load level that could lead to failure of the assembly. The SEM images showed high rates of porosity and cracks in the surface of the endocrowns in the LC, LD, and LSZ groups, which, probably, was influenced by the thermomechanical loading process. Laboratory studies like the present investigation have inherent limitations, and the results should be interpreted with caution. However, a correlation was established between the results obtained in the mechanical tests and finite element method; endocrowns made of lithium-reinforced and leucite-reinforced glass ceramics may be considered the most suitable materials for the rehabilitation of endodontically treated teeth, while Dartora et al
materials containing zirconium oxide warrant further examination with regard to the reliability of their use. Additionally, the results of the present study could be used as a basis for future research and might be relevant eventually in clinical practice in the reconstruction of dental crowns with endocrowns. CONCLUSIONS Based on the findings of this in vitro and FEA study, the following conclusions were drawn: 1. Vitreous ceramics reinforced with leucite and lithium disilicate had similar levels of mechanical resistance and a higher percentage of restorable failures. 2. Although vitreous ceramics reinforced with lithium silicate and zirconium oxide had a level of mechanical resistance statistically similar to that of ceramics containing leucite and lithium disilicate, a higher rate of catastrophic dental failures was noted. 3. In relation to the fracture resistance, the mechanical performance of monolithic zirconia was better than that of the other ceramic materials evaluated. THE JOURNAL OF PROSTHETIC DENTISTRY
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Figure 7. von Mises stress distribution of each model (sagittal view). A, Leucite-based glass ceramic (LC); B, Lithium disilicate-based glass ceramic (LD); C, Glass ceramic based on zirconia-reinforced lithium silicate (LSZ); D, Monolithic zirconia (ZR). Highest stress values in gray; lowest stress values in blue.
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biomechanical behavior of endodontically treated premolars. J. Endod 2008;34:1015-9. 34. Friedman CM, Sandrik JL, Heuer MA, Rapp GW. Composition and mechanical properties of gutta-percha endodontic points. J Dent Res 1975;54: 921-5. 35. Vallittu PK, Kononen M. Biomechanical aspects and material properties. 1st ed. Stockholm: Gothia Fortbildning; 2013. p. 116-30. 36. De Abreu RA, Pereira MD, Furtado F, Prado GP, Mestriner WJR, Ferreira LM. Masticatory efficiency and bite force in individuals with normal occlusion. Arch Oral Biol 2014;59:1065-74. Corresponding author: Dr Erica Alves Gomes School of Dentistry, University of Ribeiraeo Preto Av Costábile Romano, 2.201 Ribeiraeo Preto, SP CEP 14096-900 BRAZIL Email: [email protected] Acknowledgments The authors wish to thank Dr Ricardo Faria Ribeiro of the Laboratory of Biomechanical Studies in Prosthodontics and Implants at the Department of Dental Materials and Prosthodontics, School of Dentistry of Ribeirão Preto, University of São Paulo (FORP-USP), Brazil, for technical support and to Gustavo Dartora of the School of Dentistry, Meridional Faculty (IMED), Passo Fundo, RS, Brazil, for the help with the design and manufacture of restorations. Copyright © 2019 by the Editorial Council for The Journal of Prosthetic Dentistry. https://doi.org/10.1016/j.prosdent.2019.11.008
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