Microtensile Bond Strength of Lithium Disilicate Ceramics to Resin Adhesives Moustafa N. Abousheliba / Donia Sleemb
Purpose: To evaluate the influence of the internal structure of lithium disilicate glass ceramics (LDC) on the microtensile bond strength to a resin adhesive using two surface treatments. Materials and Methods: Milling blocks of three types of LDC were sectioned (4 mm thick) using a precision cutting machine: IPS Empress 2 (conventional LDC), IPSe.max CAD (a refined crystal high strength LDC), and Celtra (zirconia reinforced LDC). Cut specimens received crystallization heat treatment as suggested by the manufacturers. Two surface treatments were performed on each group: hydrofluoric acid etching (HF) and airborne particle abrasion using 50-μm glass beads, while the as-cut surface served as control. Treated surfaces were examined using scanning electron microscopy (SEM). The disks were coated with a silane primer and bonded to pre-aged resin composite disks (Tetric EvoCeram) using a resin adhesive (Variolink II) and then stored in water for 3 months. Bonded specimens were sectioned into micro-bars (1 x 1 x 6 mm) and microtensile bond strength test (MTBS) was performed. Data were analyzed using two-way ANOVA and Tukey’s post-hoc test (α = 0.05). Results: Statistical analysis revealed significant differences in microtensile bond strength values between different LDCs (F = 67, p < 0.001), different surface treatments (F=232, p < 0.001), and interaction between LDC and surface treatments (F = 10.6, p < 0.001). Microtensile bond strength of Celtra ceramic (30.4 ± 4.6 MPa) was significantly higher than both IPS Empress 2 (21.5 ± 5.9 MPa) and IPSe.max ceramics (25.8 ± 4.8 MPa), which had almost comparable MTBS values. SEM images demonstrated homogenous glassy matrix and reinforcing zirconia fillers characteristic of Celtra ceramic. Heat treatment resulted in growth and maturation of lithium disilicate crystals. Particle abrasion resulted in abrasion of the glass matrix and exposure of lithium disilicate crystals, while HF etching produced a microrough surface, which resulted in higher MTBS values and reduction in the percentage of adhesive failure for all groups. Conclusions: Within the limitations of this study, bond strength to lithium disilicate ceramics depends on proper surface treatment and on the chemical composition of the glass ceramic. Keywords: MTBS, bond, lithium disilicate, SEM. J Adhes Dent 2014; 16: 547–552. doi: 10.3290/j.jad.a33249
T
he addition of lithium dioxide crystals to glass ceramics resulted in great improvement in its mechanical properties and provided a material that has multiple clinical applications.14 Lithium disilicate ceramics (LDCs) have two components: silica, which serves as the glassy matrix, and lithium oxide (Li2O) crystals, which serve as a flux used to lower the processing temperature of the glassy matrix from approximately 2000°C to 1100°C.
a
Professor, Materials Science Department, Faculty of Dentistry, Alexandria University, Egypt. Designed methodology, analyzed data, wrote manuscript, contributed substantially to discussion.
b
Student Researcher, Fine Measurements Laboratory, Materials Science Department, Faculty of Dentistry, Alexandria University, Egypt. Prepared specimens, performed instrumentations, reviewed literature.
Correspondence: Dr. Moustafa N. Aboushelib, Materials Science Department, Faculty of Dentistry, Champollion St, Azarita, Alexandria, Egypt. Tel: +20-109002-0505. e-mail: [email protected]
Vol 16, No 6, 2014
Submitted for publication: 18.03.14; accepted for publication: 05.11.14
LDCs have an unusual microstructure, consisting of small interlocking, plate- or needle-like crystals that are randomly oriented, act as crack stoppers, and provide a substantial increase in flexural strength compared to conventional glass ceramics. Fabrication of LDC begins with the formation of the glass matrix. The raw materials (Li2CO3 + SiO2) are combined and heated to a temperature above the liquidus line (1100°C to 1400°C) releasing CO2, a process known as homogenization. The molten glass is poured into a mold and annealed at 400°C to relax internal stresses and prevent cracking. To gain the required strength, the glass is heated in a process known as nucleation, where spontaneous nucleation occurs, the desired nuclei population is achieved (70% of total volume), and lithium disilicate crystals reach maturity size, a state known as partial crystallinity. Reheating of LDC resulted in an increase in its flexural strength due to denser packing of the crystals.8 547
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The first generation of lithium disilicate glass ceramic (IPS Empress 2, Ivoclar Vivadent; Schaan, Liechtenstein) was fabricated using the pressing technique, where melted glass pellets were injected under pressure to fill the mold cavity.8 Today, computer-assisted design and milling technology (CAD/CAM) is used to mill prefabricated blocks of the required shade and translucency. The packing volume – percent of crystals in relation to matrix volume – of the first generation was 60% lithium disilicate crystals (5 to 6 μm in length and 1 μm in diameter) in addition to the much smaller secondary lithium orthophosphate crystals (0.1 to 0.3 μm). The second generation of LDC (IPSe.max, Ivoclar Vivadent) was based on controlling the crystallization process, resulting in more homogeneous and finer crystals, which increased its flexural strength from 330 MPa to 440 MPa through a process known as double nucleation, in which fine lithium meta-silicate crystals (40% Li2O3, average size 0.5 μm) are precipitated in a first step. The resulting glass ceramic demonstrated excellent processing properties that are ideal for CAD/CAM. A second heat treatment (840°C to 850ºC) is performed where the meta-silicate phase is completely dissolved in the glass and mature lithium disilicates (Li2Si2O5) start to crystallize, yielding a 70% crystal volume (1.5 μm average size). Today, the material is available as both pressing pellets and CAD/CAM blocks of various colors and translucencies.15 The newest generation of LDC (Celtra, Degudent; Hanau, Germany) incorporates zirconia particles as reinforcing fillers with the intention of improving fracture resistance through crack interruption. The success of LDCs depends on establishing a strong bond between the ceramic material and the tooth structure, especially for non-retentive restorations such as ceramic veneers. Establishing a strong, durable bond to LDCs depends on understanding the internal structure of the material in order to properly select a suitable surface treatment and resin adhesive of choice.10 At the moment, little is known about the internal structure and proper surface treatment of zirconia-reinforced lithium disilicate ceramic. The aim of this study was to evaluate the microtensile bond strength (MTBS) of three lithium disilicate glass ceramics to a resin adhesive using different surface treatments. The proposed hypothesis was that the internal structure of LDCs would influence bond strength to resin adhesives.
MATERIALS AND METHODS Preparation of the Specimens CAD/CAM milling blocks of three types of LDC – lithium disilicate glass ceramic core material (IPSEmpress 2, Ivoclar Vivadent; Schaan, Liechtenstein); 2. refined crystal lithium disilicate glass ceramic (IPSe.max CAD, Ivoclar Vivadent); 3. dispersed zirconia filled lithium disilicate glass ceramic (Celtra, Degudent) – were mounted in a precision cutting device (Isomet 1000, Buehler; Lake Bluff, IL, USA) and sectioned using a diamond548
coated disk under water cooling. Cut specimens (4 mm thick) received maturation heat treatment recommended by the manufacturers (Austromat 3001, Dekema DentalKeramiköfen; Freilassing, Germany). Surface Treatments Cut disks received one of the following surface treatments: airborne particle abrasion (P-G 400, Hornisch + Rieth; Winterback, Germany) using 50-μm glass particles (1.5 bar, 15-s blasting time at 10 cm distance) or hydrofluoric acid etching applied for 60 s, followed by washing under distilled water and air-jet drying for 30 s. As-cut surfaces served as control. Specimens were ultrasonically cleaned in 70% ethyl alcohol for 12 min and dried at 80°C to remove moisture. Bonding Procedure A silane coupling agent (Monobond Plus, Ivoclar Vivadent) was applied using a microbrush and allowed to dry for 1 min. A second coat was applied as previously mentioned. Resin composite disks (3 mm thick) were prepared by incremental packing of composite resin material (Tetric EvoCeram, shade A3, Ivoclar Vivadent) in a plastic mold. The disks were bonded to the prepared LDC disks using a resin adhesive (Variolink II, A2, Ivoclar Vivadent) which was generously applied on the disks, followed by application of fixed load (500 g) and light polymerization using a high-intensity (1200 mw/ cm2) LED unit (Bluephase, soft start mode, Ivoclar Vivadent). The unit was recharged and calibrated using a blue-light intensity meter. Microtensile Bond Strength Test (MTBS) The bilayered specimens were sectioned into micro-bars (1 mm2 in cross section) using a diamond-coated disk under water cooling (Isomet 1000; Buehler). Twenty sound micro-bars were obtained from the cut specimens and stored in distilled water for 3 months at 37°C (n = 20). Micro-bars were glued to a custom-made attachment unit using a resin adhesive (Syntac, Ivoclar Vivadent) and subjected to tensile force using a universal testing machine at a crosshead speed of 0.5 mm/min (Instron 6022; Instron; High Wycombe, UK). The load cell (250 N) was calibrated using standardized weights, and the crosshead speed was monitored using a digital caliper (Millitron; Feinprüf Perthen; Göttingen, Germany). Material properties are summarized in Table 1. The fractured surface of each micro-bar was examined under a stereomicroscope (SZ 11, Olympus; Osaka, Japan) at 120X magnification to evaluate the failure mode, which was categorized as adhesive (fracture line through the resin adhesive) or cohesive (fracture line through resin composite or ceramic material). Scanning Electron Microscopy (SEM) The internal structure of different LDCs was examined using high magnification SEM imaging. Twelve intact sections were cut from CAD/CAM blocks (2 mm thick). Cutting marks were removed by polishing the sections using ascending grits of silicon carbide paper (600-, The Journal of Adhesive Dentistry
Aboushelib and Sleem
Table 1
Properties of used materials and application technique
Material
Manufacturer
Composition
Application
Ultradent porcelain etch
Ultradent; South Jordan, UT, USA
9.5% buffered HF, carrier gel
Apply continuous coat on dry surface for 60 s. Frosty white surface appears after drying.
Variolink II
Ivoclar Vivadent; Schaan, Liechtenstein
Dimethacrylate resin, silica, 1-μm barium glass and YbF3 filler
Properly mix two pastes and immediately apply on silanated ceramic substrate. Mixed material has 60 min working time before initial setting.
Monobond Plus
Ivoclar Vivadent
4% trimethoxy silane, 2.5% disulfide methacrylate, 2.5% phosphonic acid dimethacrylate, 96% ethanol
Apply for 60 s, then blot dry to evaporate solvent after partial hydrolysis of monomer.
Syntac adhesive
Ivoclar Vivadent
Polyethylene glycol dimethacrylate, DEG-DMA, maleic acid, glutaraldehyde, water, dimethylketone
Apply thin, even coat with microbrush, then light polymerize. Shiny surface appears after polymerization.
800-, 1000-, and 1200-grit) on a custom made rotating metallographic polishing device. The sections were ultrasonically cleaned in a bath of 80% ethyl alcohol for 15 min and dried at 80°C for 60 min to remove surface debris. Sections were thermally etched (heating at 600°C for 20 min) to improve detection of internal structure, gold sputter coated, and prepared for SEM examination (XL 30, Phillips; Eindhoven, The Netherlands) at the following intervals: before maturation heat treatment, at the middle of maturation heat treatment program, and after completion of heat treatment. Levene’s test of equality of error variances was performed to test the null hypothesis that error variance in bond strength value is equivalent among all tested groups. Two-way ANOVA was selected to analyze the data, with one within-group factor (LDC type, 3 levels) and one between-group factor (3 different surfaces). Tukey’s post-hoc test was selected for pair-wise comparisons (α = 0.05). The sample size (n = 20) was based on a power analysis study designed to detect medium effect size differences (F = 0.25). Data were digitally examined and analyzed using computer software (SPSS 14.0; Chicago, IL, USA).
Table 2 Microtensile bond strength (MPa) and failure mode of different test groups Ceramic type
Surface treatment
MTBS (MPa)
Failure mode
Empress 2A
As milled
18.8 ± 1.6
80% adhesive 20% cohesive in resin composite
Particle abrasion
24.3 ± 1.7
50% adhesive 50% cohesive in resin composite
Hydrofluoric etching
32.2 ± 2.9
10% adhesive 90% cohesive in resin composite
As milled1
21.7 ± 2.4
85% adhesive 15% cohesive in resin composite
Particle abrasion1
24.5 ± 2.3
75% adhesive 25% cohesive in resin composite
Hydrofluoric etching
31.4 ±2.9
40% adhesive 60% cohesive in resin composite
As milled
24.7 ±1.4
100% adhesive
Particle abrasion2
32.6 ± 1.3
75% adhesive 25% cohesive in resin composite
Hydrofluoric etching2
34 ± 2.8
30% adhesive 70% cohesive
IPSe.maxA
RESULTS Celtra
Levene’s test of equality (F = 1.6, p < 0.112) indicated homogenous distribution of bond strength values among all tested groups. Partial eta squared value and observed power were 0.96 and 1, respectively. Statistical analysis revealed that the internal structure of LDCs significantly influenced microtensile bond strength values (F = 67, p < 0.001). Microtensile bond strength of Celtra ceramic was significantly higher (30.4 ± 4.6 MPa) than that of IPS Empress 2 (21.5 ± 5.9 MPa) and IPSe.max ceramic (25.8 ± 4.8 MPa), which had comparable MTBS values. Regarding the influence of surface treatment, hydrofluoric acid etching (32.5 ± 2.7 MPa) produced significantly higher (F = 232, p < 0.001) MTBS values compared to particle abrasion (27.1 ± 4.2 MPa) and as-cut surfaces (21.7 ± 3 MPa) for all test groups. There was also a significant interaction (F = 10.6, p < 0.001) between ceramic type and surface treatment. Vol 16, No 6, 2014
Same superscript letter indicates no statistically significant difference in MTBS between ceramic materials. Same superscript numeral indicates no statistically significant difference in MTBS between surface treatments.
Failure mode was also influenced by the type of surface treatment. For all tested ceramics, the highest percentage of adhesive failure was associated with the as-milled surface. The percentage of adhesive failures was reduced when airborne particle abrasion was performed. The lowest percentage of adhesive failures was observed with hydrofluoric acid etching. These data are summarized in Table 2. 549
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1 μm
0,5 μm
Fig 1a SEM image of IPS.empress 2 at beginning of crystallization heat treatment (35,000X magnification).
Fig 1b SEM image of IPS.empress 2 after crystallization heat treatment (15,000X magnifcation). Observe larger crystal size compared to IPS.e.max ceramic.
0,5 μm
Fig 2 SEM image (35,000X magnification) demonstrating mature lithium disilicate crystals of IPS.e.max ceramic (compare smaller crystal size with IPS. empress 2). Fig 3a SEM image demonstrating internal structure of Celtra ceramic at early stage of crystallization heat treatment. Growth of lithium disilicate crystals starts to fill the glass matrix (arrows), while zirconia fillers fill the glass matrix (35,000X magnification).
0,5 μm
1 μm
Fig 3b SEM image of Celtra ceramic after crystallization heat treatment. Glass matrix is filled with mature crystals of lithium disilicate (20,000X magnification). Fig 4a SEM image of Celtra ceramic after HF etching demonstrating surface microroughness (2000X).
10 μm
Scanning electron microscopic imaging of IPS Empress 2 before crystallization heat treatment demonstrated a glassy matrix filled with immature lithium dioxide crystals (Fig 1a). After crystallization heat treatment, the reinforcing crystals reached mature size (Fig 1b). Images of IPSe.max ceramic revealed homogenous, smaller, and denser lithium disilicate crystals (Fig 2). Celtra ceramic demonstrated a glassy phase rich in lithium disilicate crystals, similar to those of IPSe.max. and characteristic dispersed zirconia fillers (Figs 3a and 3b). HF etching produced a honeycomb-like microrough, porous surface (Fig 4a), while particle abrasion with glass beads caused abrasion of the glassy matrix (Fig 4b). 550
10 μm
Fig 4b SEM image of particle-abraded Celtra demonstrating abraded glassy matrix and exposure of needle-like crystals (2000X).
DISCUSSION Resin bonding of all-ceramic restorations is known to reduce risk of fracture during function. A strong bond between a ceramic restoration and the tooth structure provides good support for the restoration and actively transmits functional loads through the bonded interface.3 Interfacial fracture toughness, also known as resistance of the bonded interface to rupture, depends on the surface treatment of the ceramic substrate16 and the chemistry of the selected adhesive.17,23 Several factors interact in the process of establishing a strong bond between two different materials. The first The Journal of Adhesive Dentistry
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factor is the mechanical properties of the substrate material. Celtra and IPSe.max ceramics have smaller, denser lithium disilicate crystals, which increased their stiffness and flexural strength. Previous studies established a direct correlation between the elastic modulus of the bonding substrate and the established bond strength, which explains the higher bond strength observed compared to the weaker IPS Empress 2.1,12 Several studies reported an average shear bond strength value between 13 and 15 MPa between IPS Empress 2 and resin adhesives;2,6,7,21,20 however, other studies reported higher values using the microtensile bond strength test,9,13,15 which is in agreement with the present data. The MTBS test was proven to be more sensitive in detecting small differences in bond strengths and in material ranking compared to different types of shear bond strength testing, which suffered from the problem of stress concentration at the loading point.24,28 Due to the presence of significant differences in MTBS values between different LDCs, the proposed hypothesis was accepted. The second factor is the surface roughness of the bonding substrate. SEM images of airborne-particle– abraded specimens demonstrated abrasion of both the glassy matrix and the reinforcing crystals, without creation of three-dimensional retentive features (Fig 4b). Airborne particle abrasion is a routine step used to remove the reaction layer around pressed LDC, but it can be selectively used on the fitting surface of CAD/CAM restorations. On the other hand, HF etching produced characteristic honeycomb irregularities and created a microporous surface (Fig 4a) by partially dissolving the glass phase, leaving behind an active surface rich in silica.11,21,23,18 For the three tested LDCs, HF etching produced the highest MTBS values and was associated with the lowest percent of adhesive failure after three months of water storage.6,26 The third factor is establishing a strong chemical bond between the substrate and the resin adhesive of choice, which is the function of the silane primer. The three LDCs used here have a silica rich layer (30 wt% to 40 wt%) available for direct bonding with the silane primer. The bond protects the surface from the hydrolytic effect of water and increases bond strength to resin adhesive.4,29 Different methods have been used to accelerate drying of the applied silane primer, but they did not result in increasing bond strength compared to letting dry at room temperature.27 New surface treatments using Er:YAG laser and irradiation with pulsed ErCr:YSGG laser improved bond strength to lithium disilicate ceramic when used at 1.5 and 2.5 W, while higher energy levels failed to increase bond strength due to surface destruction of the material.22 Special attention should be paid during selection of surface treatment if lithium disilicate ceramics are to be bonded to core buildup materials in order to achieve successful clinical performance.5,17 Different etching agents were also previously tested, but HF remains the ideal etchant for that purpose.19 During try-in of LDCs, reconditioning of contaminated ceramic Vol 16, No 6, 2014
surfaces with 5% HF was proven to be the most effective method to clean contaminated surfaces,11,20 but extended application of HF could lead to deterioration of the mechanical properties of LDCs.16 The interaction between the composition of LDCs and surface treatment had a significant influence on both MTBS and mode of failure. To improve its mechanical properties, Celtra ceramic was reinforced with zirconia fillers acting as crack stoppers. The presence of zirconia requires the use of a phosphate monomer in order to establish a chemical bond with resin adhesive, which may require the application of a di-functional primer designed to bond to silica and zirconia phases. This ceramic was associated with the highest bond strength values compared to IPSe.max and IPS Empress 2 materials.18 The highest recorded bond strength was observed with Celtra ceramic in combination with HF etching, indicating the effectiveness of this method in increasing microroughness. Similar findings were observed for IPSe.max CAD ceramic using the same resin adhesive, while a fluorosilicate ceramic demonstrated the highest bond strength maintaining its machined surface.30 The type of resin adhesive is known to influence the bond strength to lithium disilicate ceramics as well.17 While a high initial bond strength could be a tempting parameter for selection of a resin adhesive, the durability of the established bond is the true indicator of the expected performance of resin adhesives.23 Fatigue and thermocycling are known to increase percentage of adhesive failure, which could lead to debonding failure of nonretentive restorations. After three months of water storage, the bond strength of HF-etched LDC in combination with silane primer produced a strong, durable bond with the resin adhesive employed.25
CONCLUSIONS Within the limitations of this study, chemical and structural differences were found between the studied lithium disilicate ceramics which influenced microtensile bond strength to a resin adhesive. Hydrofluoric acid etching in combination with silane primer remains the gold standard for bonding to lithium disilicate glass ceramics.
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Borges GA, Sophr AM, de Goes MF, Sobrinho LC, Chan DC. Effect of etching and airborne particle abrasion on the microstructure of different dental ceramics. J Prosthet Dent 2003;89:479-488. Cekic-Nagas I, Canay S, Sahin E. Bonding of resin core materials to lithium disilicate ceramics: the effect of resin cement film thickness. Int J Prosthodont 2010;23:469-471. Chung KH, Liao JH, Duh JG, Chan DC. The effects of repeated heatpressing on properties of pressable glass-ceramics. J Oral Rehabil 2009;36:132-141. Colares RC, Neri JR, Souza AM, Pontes KM, Mendonca JS, Santiago SL. Effect of surface pretreatments on the microtensile bond strength of lithium-disilicate ceramic repaired with composite resin. Braz Dent J 2013;24:349-352. Della Bona A, Anusavice KJ, Mecholsky JJ, Jr. Failure analysis of resin composite bonded to ceramic. Dent Mater 2003;19:693-699. Dilber E, Yavuz T, Kara HB, Ozturk AN. Comparison of the effects of surface treatments on roughness of two ceramic systems. Photomed Laser Surg;30:308-314. El Zohairy AA, de Gee AJ, de Jager N, van Ruijven LJ, Feilzer AJ. The influence of specimen attachment and dimension on microtensile strength. J Dent Res 2004;83:420-424. Ergun G, Cekic I, Lassila LV, Vallittu PK. Bonding of lithium-disilicate ceramic to enamel and dentin using orthotropic fiber-reinforced composite at the interface. Acta Odontol Scand 2006;64:293-299. Gehrt M, Wolfart S, Rafai N, Reich S, Edelhoff D. Clinical results of lithium-disilicate crowns after up to 9 years of service. Clin Oral Investig;17:275-284. Guarda GB, Correr AB, Goncalves LS, Costa AR, Borges GA, Sinhoreti MA, Correr-Sobrinho L. Effects of surface treatments, thermocycling, and cyclic loading on the bond strength of a resin cement bonded to a lithium disilicate glass ceramic. Oper Dent 2012;38:208-217. Hooshmand T, Parvizi S, Keshvad A. Effect of surface acid etching on the biaxial flexural strength of two hot-pressed glass ceramics. J Prosthodont 2008;17:415-419. Hooshmand T, Rostami G, Behroozibakhsh M, Fatemi M, Keshvad A, van Noort R. Interfacial fracture toughness of different resin cements bonded to a lithium disilicate glass ceramic. J Dent 2011;40:139-145. Kiyan VH, Saraceni CH, da Silveira BL, Aranha AC, Eduardo Cda P. The influence of internal surface treatments on tensile bond strength for two ceramic systems. Oper Dent 2007;32:457-465. Klosa K, Boesch I, Kern M. Long-term bond of glass ceramic and resin cement: evaluation of titanium tetrafluoride as an alternative etching agent for lithium disilicate ceramics. J Adhes Dent 2013;15:377-383. Klosa K, Wolfart S, Lehmann F, Wenz HJ, Kern M. The effect of storage conditions, contamination modes and cleaning procedures on the resin bond strength to lithium disilicate ceramic. J Adhes Dent 2009;11: 127-135.
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21. Kumbuloglu O, Lassila LV, User A, Toksavul S, Vallittu PK. Shear bond strength of composite resin cements to lithium disilicate ceramics. J Oral Rehabil 2005;32:128-133. 22. Kursoglu P, Motro PF, Yurdaguven H. Shear bond strength of resin cement to an acid etched and a laser irradiated ceramic surface. J Adv Prosthodont 2013;5:98-103. 23. Marocho SM, Özcan M, Amaral R, Bottino MA, Valandro LF. Effect of resin cement type on the microtensile bond strength to lithium disilicate ceramic and dentin using different test assemblies. J Adhes Dent 2013;15:361-368. 24. Mirmohammadi H, Aboushelib MN, Salameh Z, Feilzer AJ, Kleverlaan CJ. Innovations in bonding to zirconia based ceramics: Part III. Phosphate monomer resin cements. Dent Mater 2010;26:786-792. 25. Nagai T, Kawamoto Y, Kakehashi Y, Matsumura H. Adhesive bonding of a lithium disilicate ceramic material with resin-based luting agents. J Oral Rehabil 2005;32:598-605. 26. Panah FG, Rezai SM, Ahmadian L. The influence of ceramic surface treatments on the micro-shear bond strength of composite resin to IPS Empress 2. J Prosthodont 2008;17:409-414. 27. Pereira CN, Buono VT, Mota JM. The influence of silane evaporation procedures on microtensile bond strength between a dental ceramic and a resin cement. Indian J Dent Res 2010;21:238-243. 28. Phrukkanon S, Burrow MF, Tyas MJ. Effect of cross-sectional surface area on bond strengths between resin and dentin. Dent Mater 1998;14:120-128. 29. Pisani-Proenca J, Erhardt MC, Valandro LF, Gutierrez-Aceves G, BolanosCarmona MV, Del Castillo-Salmeron R, Bottino MA. Influence of ceramic surface conditioning and resin cements on microtensile bond strength to a glass ceramic. J Prosthet Dent 2006;96:412-417. 30. Pollington S, Fabianelli A, van Noort R. Microtensile bond strength of a resin cement to a novel fluorcanasite glass-ceramic following different surface treatments. Dent Mater 2010;26:864-872.
Clinical relevance: The composition of lithium disilicate glass ceramic affects the bond strength to resin adhesives, which must be taken into consideration in clinical situations.
The Journal of Adhesive Dentistry
Purpose: To evaluate the influence of the internal structure of lithium disilicate glass ceramics (LDC) on the microtensile bond strength to a resin adhesive using two surface treatments. Materials and Methods: Milling blocks of three types of LDC were sectioned (4 mm thick) using a precision cutting machine: IPS Empress 2 (conventional LDC), IPSe.max CAD (a refined crystal high strength LDC), and Celtra (zirconia reinforced LDC). Cut specimens received crystallization heat treatment as suggested by the manufacturers. Two surface treatments were performed on each group: hydrofluoric acid etching (HF) and airborne particle abrasion using 50-μm glass beads, while the as-cut surface served as control. Treated surfaces were examined using scanning electron microscopy (SEM). The disks were coated with a silane primer and bonded to pre-aged resin composite disks (Tetric EvoCeram) using a resin adhesive (Variolink II) and then stored in water for 3 months. Bonded specimens were sectioned into micro-bars (1 x 1 x 6 mm) and microtensile bond strength test (MTBS) was performed. Data were analyzed using two-way ANOVA and Tukey’s post-hoc test (α = 0.05). Results: Statistical analysis revealed significant differences in microtensile bond strength values between different LDCs (F = 67, p < 0.001), different surface treatments (F=232, p < 0.001), and interaction between LDC and surface treatments (F = 10.6, p < 0.001). Microtensile bond strength of Celtra ceramic (30.4 ± 4.6 MPa) was significantly higher than both IPS Empress 2 (21.5 ± 5.9 MPa) and IPSe.max ceramics (25.8 ± 4.8 MPa), which had almost comparable MTBS values. SEM images demonstrated homogenous glassy matrix and reinforcing zirconia fillers characteristic of Celtra ceramic. Heat treatment resulted in growth and maturation of lithium disilicate crystals. Particle abrasion resulted in abrasion of the glass matrix and exposure of lithium disilicate crystals, while HF etching produced a microrough surface, which resulted in higher MTBS values and reduction in the percentage of adhesive failure for all groups. Conclusions: Within the limitations of this study, bond strength to lithium disilicate ceramics depends on proper surface treatment and on the chemical composition of the glass ceramic. Keywords: MTBS, bond, lithium disilicate, SEM. J Adhes Dent 2014; 16: 547–552. doi: 10.3290/j.jad.a33249
T
he addition of lithium dioxide crystals to glass ceramics resulted in great improvement in its mechanical properties and provided a material that has multiple clinical applications.14 Lithium disilicate ceramics (LDCs) have two components: silica, which serves as the glassy matrix, and lithium oxide (Li2O) crystals, which serve as a flux used to lower the processing temperature of the glassy matrix from approximately 2000°C to 1100°C.
a
Professor, Materials Science Department, Faculty of Dentistry, Alexandria University, Egypt. Designed methodology, analyzed data, wrote manuscript, contributed substantially to discussion.
b
Student Researcher, Fine Measurements Laboratory, Materials Science Department, Faculty of Dentistry, Alexandria University, Egypt. Prepared specimens, performed instrumentations, reviewed literature.
Correspondence: Dr. Moustafa N. Aboushelib, Materials Science Department, Faculty of Dentistry, Champollion St, Azarita, Alexandria, Egypt. Tel: +20-109002-0505. e-mail: [email protected]
Vol 16, No 6, 2014
Submitted for publication: 18.03.14; accepted for publication: 05.11.14
LDCs have an unusual microstructure, consisting of small interlocking, plate- or needle-like crystals that are randomly oriented, act as crack stoppers, and provide a substantial increase in flexural strength compared to conventional glass ceramics. Fabrication of LDC begins with the formation of the glass matrix. The raw materials (Li2CO3 + SiO2) are combined and heated to a temperature above the liquidus line (1100°C to 1400°C) releasing CO2, a process known as homogenization. The molten glass is poured into a mold and annealed at 400°C to relax internal stresses and prevent cracking. To gain the required strength, the glass is heated in a process known as nucleation, where spontaneous nucleation occurs, the desired nuclei population is achieved (70% of total volume), and lithium disilicate crystals reach maturity size, a state known as partial crystallinity. Reheating of LDC resulted in an increase in its flexural strength due to denser packing of the crystals.8 547
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The first generation of lithium disilicate glass ceramic (IPS Empress 2, Ivoclar Vivadent; Schaan, Liechtenstein) was fabricated using the pressing technique, where melted glass pellets were injected under pressure to fill the mold cavity.8 Today, computer-assisted design and milling technology (CAD/CAM) is used to mill prefabricated blocks of the required shade and translucency. The packing volume – percent of crystals in relation to matrix volume – of the first generation was 60% lithium disilicate crystals (5 to 6 μm in length and 1 μm in diameter) in addition to the much smaller secondary lithium orthophosphate crystals (0.1 to 0.3 μm). The second generation of LDC (IPSe.max, Ivoclar Vivadent) was based on controlling the crystallization process, resulting in more homogeneous and finer crystals, which increased its flexural strength from 330 MPa to 440 MPa through a process known as double nucleation, in which fine lithium meta-silicate crystals (40% Li2O3, average size 0.5 μm) are precipitated in a first step. The resulting glass ceramic demonstrated excellent processing properties that are ideal for CAD/CAM. A second heat treatment (840°C to 850ºC) is performed where the meta-silicate phase is completely dissolved in the glass and mature lithium disilicates (Li2Si2O5) start to crystallize, yielding a 70% crystal volume (1.5 μm average size). Today, the material is available as both pressing pellets and CAD/CAM blocks of various colors and translucencies.15 The newest generation of LDC (Celtra, Degudent; Hanau, Germany) incorporates zirconia particles as reinforcing fillers with the intention of improving fracture resistance through crack interruption. The success of LDCs depends on establishing a strong bond between the ceramic material and the tooth structure, especially for non-retentive restorations such as ceramic veneers. Establishing a strong, durable bond to LDCs depends on understanding the internal structure of the material in order to properly select a suitable surface treatment and resin adhesive of choice.10 At the moment, little is known about the internal structure and proper surface treatment of zirconia-reinforced lithium disilicate ceramic. The aim of this study was to evaluate the microtensile bond strength (MTBS) of three lithium disilicate glass ceramics to a resin adhesive using different surface treatments. The proposed hypothesis was that the internal structure of LDCs would influence bond strength to resin adhesives.
MATERIALS AND METHODS Preparation of the Specimens CAD/CAM milling blocks of three types of LDC – lithium disilicate glass ceramic core material (IPSEmpress 2, Ivoclar Vivadent; Schaan, Liechtenstein); 2. refined crystal lithium disilicate glass ceramic (IPSe.max CAD, Ivoclar Vivadent); 3. dispersed zirconia filled lithium disilicate glass ceramic (Celtra, Degudent) – were mounted in a precision cutting device (Isomet 1000, Buehler; Lake Bluff, IL, USA) and sectioned using a diamond548
coated disk under water cooling. Cut specimens (4 mm thick) received maturation heat treatment recommended by the manufacturers (Austromat 3001, Dekema DentalKeramiköfen; Freilassing, Germany). Surface Treatments Cut disks received one of the following surface treatments: airborne particle abrasion (P-G 400, Hornisch + Rieth; Winterback, Germany) using 50-μm glass particles (1.5 bar, 15-s blasting time at 10 cm distance) or hydrofluoric acid etching applied for 60 s, followed by washing under distilled water and air-jet drying for 30 s. As-cut surfaces served as control. Specimens were ultrasonically cleaned in 70% ethyl alcohol for 12 min and dried at 80°C to remove moisture. Bonding Procedure A silane coupling agent (Monobond Plus, Ivoclar Vivadent) was applied using a microbrush and allowed to dry for 1 min. A second coat was applied as previously mentioned. Resin composite disks (3 mm thick) were prepared by incremental packing of composite resin material (Tetric EvoCeram, shade A3, Ivoclar Vivadent) in a plastic mold. The disks were bonded to the prepared LDC disks using a resin adhesive (Variolink II, A2, Ivoclar Vivadent) which was generously applied on the disks, followed by application of fixed load (500 g) and light polymerization using a high-intensity (1200 mw/ cm2) LED unit (Bluephase, soft start mode, Ivoclar Vivadent). The unit was recharged and calibrated using a blue-light intensity meter. Microtensile Bond Strength Test (MTBS) The bilayered specimens were sectioned into micro-bars (1 mm2 in cross section) using a diamond-coated disk under water cooling (Isomet 1000; Buehler). Twenty sound micro-bars were obtained from the cut specimens and stored in distilled water for 3 months at 37°C (n = 20). Micro-bars were glued to a custom-made attachment unit using a resin adhesive (Syntac, Ivoclar Vivadent) and subjected to tensile force using a universal testing machine at a crosshead speed of 0.5 mm/min (Instron 6022; Instron; High Wycombe, UK). The load cell (250 N) was calibrated using standardized weights, and the crosshead speed was monitored using a digital caliper (Millitron; Feinprüf Perthen; Göttingen, Germany). Material properties are summarized in Table 1. The fractured surface of each micro-bar was examined under a stereomicroscope (SZ 11, Olympus; Osaka, Japan) at 120X magnification to evaluate the failure mode, which was categorized as adhesive (fracture line through the resin adhesive) or cohesive (fracture line through resin composite or ceramic material). Scanning Electron Microscopy (SEM) The internal structure of different LDCs was examined using high magnification SEM imaging. Twelve intact sections were cut from CAD/CAM blocks (2 mm thick). Cutting marks were removed by polishing the sections using ascending grits of silicon carbide paper (600-, The Journal of Adhesive Dentistry
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Table 1
Properties of used materials and application technique
Material
Manufacturer
Composition
Application
Ultradent porcelain etch
Ultradent; South Jordan, UT, USA
9.5% buffered HF, carrier gel
Apply continuous coat on dry surface for 60 s. Frosty white surface appears after drying.
Variolink II
Ivoclar Vivadent; Schaan, Liechtenstein
Dimethacrylate resin, silica, 1-μm barium glass and YbF3 filler
Properly mix two pastes and immediately apply on silanated ceramic substrate. Mixed material has 60 min working time before initial setting.
Monobond Plus
Ivoclar Vivadent
4% trimethoxy silane, 2.5% disulfide methacrylate, 2.5% phosphonic acid dimethacrylate, 96% ethanol
Apply for 60 s, then blot dry to evaporate solvent after partial hydrolysis of monomer.
Syntac adhesive
Ivoclar Vivadent
Polyethylene glycol dimethacrylate, DEG-DMA, maleic acid, glutaraldehyde, water, dimethylketone
Apply thin, even coat with microbrush, then light polymerize. Shiny surface appears after polymerization.
800-, 1000-, and 1200-grit) on a custom made rotating metallographic polishing device. The sections were ultrasonically cleaned in a bath of 80% ethyl alcohol for 15 min and dried at 80°C for 60 min to remove surface debris. Sections were thermally etched (heating at 600°C for 20 min) to improve detection of internal structure, gold sputter coated, and prepared for SEM examination (XL 30, Phillips; Eindhoven, The Netherlands) at the following intervals: before maturation heat treatment, at the middle of maturation heat treatment program, and after completion of heat treatment. Levene’s test of equality of error variances was performed to test the null hypothesis that error variance in bond strength value is equivalent among all tested groups. Two-way ANOVA was selected to analyze the data, with one within-group factor (LDC type, 3 levels) and one between-group factor (3 different surfaces). Tukey’s post-hoc test was selected for pair-wise comparisons (α = 0.05). The sample size (n = 20) was based on a power analysis study designed to detect medium effect size differences (F = 0.25). Data were digitally examined and analyzed using computer software (SPSS 14.0; Chicago, IL, USA).
Table 2 Microtensile bond strength (MPa) and failure mode of different test groups Ceramic type
Surface treatment
MTBS (MPa)
Failure mode
Empress 2A
As milled
18.8 ± 1.6
80% adhesive 20% cohesive in resin composite
Particle abrasion
24.3 ± 1.7
50% adhesive 50% cohesive in resin composite
Hydrofluoric etching
32.2 ± 2.9
10% adhesive 90% cohesive in resin composite
As milled1
21.7 ± 2.4
85% adhesive 15% cohesive in resin composite
Particle abrasion1
24.5 ± 2.3
75% adhesive 25% cohesive in resin composite
Hydrofluoric etching
31.4 ±2.9
40% adhesive 60% cohesive in resin composite
As milled
24.7 ±1.4
100% adhesive
Particle abrasion2
32.6 ± 1.3
75% adhesive 25% cohesive in resin composite
Hydrofluoric etching2
34 ± 2.8
30% adhesive 70% cohesive
IPSe.maxA
RESULTS Celtra
Levene’s test of equality (F = 1.6, p < 0.112) indicated homogenous distribution of bond strength values among all tested groups. Partial eta squared value and observed power were 0.96 and 1, respectively. Statistical analysis revealed that the internal structure of LDCs significantly influenced microtensile bond strength values (F = 67, p < 0.001). Microtensile bond strength of Celtra ceramic was significantly higher (30.4 ± 4.6 MPa) than that of IPS Empress 2 (21.5 ± 5.9 MPa) and IPSe.max ceramic (25.8 ± 4.8 MPa), which had comparable MTBS values. Regarding the influence of surface treatment, hydrofluoric acid etching (32.5 ± 2.7 MPa) produced significantly higher (F = 232, p < 0.001) MTBS values compared to particle abrasion (27.1 ± 4.2 MPa) and as-cut surfaces (21.7 ± 3 MPa) for all test groups. There was also a significant interaction (F = 10.6, p < 0.001) between ceramic type and surface treatment. Vol 16, No 6, 2014
Same superscript letter indicates no statistically significant difference in MTBS between ceramic materials. Same superscript numeral indicates no statistically significant difference in MTBS between surface treatments.
Failure mode was also influenced by the type of surface treatment. For all tested ceramics, the highest percentage of adhesive failure was associated with the as-milled surface. The percentage of adhesive failures was reduced when airborne particle abrasion was performed. The lowest percentage of adhesive failures was observed with hydrofluoric acid etching. These data are summarized in Table 2. 549
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1 μm
0,5 μm
Fig 1a SEM image of IPS.empress 2 at beginning of crystallization heat treatment (35,000X magnification).
Fig 1b SEM image of IPS.empress 2 after crystallization heat treatment (15,000X magnifcation). Observe larger crystal size compared to IPS.e.max ceramic.
0,5 μm
Fig 2 SEM image (35,000X magnification) demonstrating mature lithium disilicate crystals of IPS.e.max ceramic (compare smaller crystal size with IPS. empress 2). Fig 3a SEM image demonstrating internal structure of Celtra ceramic at early stage of crystallization heat treatment. Growth of lithium disilicate crystals starts to fill the glass matrix (arrows), while zirconia fillers fill the glass matrix (35,000X magnification).
0,5 μm
1 μm
Fig 3b SEM image of Celtra ceramic after crystallization heat treatment. Glass matrix is filled with mature crystals of lithium disilicate (20,000X magnification). Fig 4a SEM image of Celtra ceramic after HF etching demonstrating surface microroughness (2000X).
10 μm
Scanning electron microscopic imaging of IPS Empress 2 before crystallization heat treatment demonstrated a glassy matrix filled with immature lithium dioxide crystals (Fig 1a). After crystallization heat treatment, the reinforcing crystals reached mature size (Fig 1b). Images of IPSe.max ceramic revealed homogenous, smaller, and denser lithium disilicate crystals (Fig 2). Celtra ceramic demonstrated a glassy phase rich in lithium disilicate crystals, similar to those of IPSe.max. and characteristic dispersed zirconia fillers (Figs 3a and 3b). HF etching produced a honeycomb-like microrough, porous surface (Fig 4a), while particle abrasion with glass beads caused abrasion of the glassy matrix (Fig 4b). 550
10 μm
Fig 4b SEM image of particle-abraded Celtra demonstrating abraded glassy matrix and exposure of needle-like crystals (2000X).
DISCUSSION Resin bonding of all-ceramic restorations is known to reduce risk of fracture during function. A strong bond between a ceramic restoration and the tooth structure provides good support for the restoration and actively transmits functional loads through the bonded interface.3 Interfacial fracture toughness, also known as resistance of the bonded interface to rupture, depends on the surface treatment of the ceramic substrate16 and the chemistry of the selected adhesive.17,23 Several factors interact in the process of establishing a strong bond between two different materials. The first The Journal of Adhesive Dentistry
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factor is the mechanical properties of the substrate material. Celtra and IPSe.max ceramics have smaller, denser lithium disilicate crystals, which increased their stiffness and flexural strength. Previous studies established a direct correlation between the elastic modulus of the bonding substrate and the established bond strength, which explains the higher bond strength observed compared to the weaker IPS Empress 2.1,12 Several studies reported an average shear bond strength value between 13 and 15 MPa between IPS Empress 2 and resin adhesives;2,6,7,21,20 however, other studies reported higher values using the microtensile bond strength test,9,13,15 which is in agreement with the present data. The MTBS test was proven to be more sensitive in detecting small differences in bond strengths and in material ranking compared to different types of shear bond strength testing, which suffered from the problem of stress concentration at the loading point.24,28 Due to the presence of significant differences in MTBS values between different LDCs, the proposed hypothesis was accepted. The second factor is the surface roughness of the bonding substrate. SEM images of airborne-particle– abraded specimens demonstrated abrasion of both the glassy matrix and the reinforcing crystals, without creation of three-dimensional retentive features (Fig 4b). Airborne particle abrasion is a routine step used to remove the reaction layer around pressed LDC, but it can be selectively used on the fitting surface of CAD/CAM restorations. On the other hand, HF etching produced characteristic honeycomb irregularities and created a microporous surface (Fig 4a) by partially dissolving the glass phase, leaving behind an active surface rich in silica.11,21,23,18 For the three tested LDCs, HF etching produced the highest MTBS values and was associated with the lowest percent of adhesive failure after three months of water storage.6,26 The third factor is establishing a strong chemical bond between the substrate and the resin adhesive of choice, which is the function of the silane primer. The three LDCs used here have a silica rich layer (30 wt% to 40 wt%) available for direct bonding with the silane primer. The bond protects the surface from the hydrolytic effect of water and increases bond strength to resin adhesive.4,29 Different methods have been used to accelerate drying of the applied silane primer, but they did not result in increasing bond strength compared to letting dry at room temperature.27 New surface treatments using Er:YAG laser and irradiation with pulsed ErCr:YSGG laser improved bond strength to lithium disilicate ceramic when used at 1.5 and 2.5 W, while higher energy levels failed to increase bond strength due to surface destruction of the material.22 Special attention should be paid during selection of surface treatment if lithium disilicate ceramics are to be bonded to core buildup materials in order to achieve successful clinical performance.5,17 Different etching agents were also previously tested, but HF remains the ideal etchant for that purpose.19 During try-in of LDCs, reconditioning of contaminated ceramic Vol 16, No 6, 2014
surfaces with 5% HF was proven to be the most effective method to clean contaminated surfaces,11,20 but extended application of HF could lead to deterioration of the mechanical properties of LDCs.16 The interaction between the composition of LDCs and surface treatment had a significant influence on both MTBS and mode of failure. To improve its mechanical properties, Celtra ceramic was reinforced with zirconia fillers acting as crack stoppers. The presence of zirconia requires the use of a phosphate monomer in order to establish a chemical bond with resin adhesive, which may require the application of a di-functional primer designed to bond to silica and zirconia phases. This ceramic was associated with the highest bond strength values compared to IPSe.max and IPS Empress 2 materials.18 The highest recorded bond strength was observed with Celtra ceramic in combination with HF etching, indicating the effectiveness of this method in increasing microroughness. Similar findings were observed for IPSe.max CAD ceramic using the same resin adhesive, while a fluorosilicate ceramic demonstrated the highest bond strength maintaining its machined surface.30 The type of resin adhesive is known to influence the bond strength to lithium disilicate ceramics as well.17 While a high initial bond strength could be a tempting parameter for selection of a resin adhesive, the durability of the established bond is the true indicator of the expected performance of resin adhesives.23 Fatigue and thermocycling are known to increase percentage of adhesive failure, which could lead to debonding failure of nonretentive restorations. After three months of water storage, the bond strength of HF-etched LDC in combination with silane primer produced a strong, durable bond with the resin adhesive employed.25
CONCLUSIONS Within the limitations of this study, chemical and structural differences were found between the studied lithium disilicate ceramics which influenced microtensile bond strength to a resin adhesive. Hydrofluoric acid etching in combination with silane primer remains the gold standard for bonding to lithium disilicate glass ceramics.
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Clinical relevance: The composition of lithium disilicate glass ceramic affects the bond strength to resin adhesives, which must be taken into consideration in clinical situations.
The Journal of Adhesive Dentistry
