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Effects of pretreatments and hydrothermal aging on biaxial flexural strength of lithium di-silicate and Mg-PSZ ceramics
Accepted Manuscript Title: Effects of pretreatments and hydrothermal aging on biaxial flexural strength of lithium di-silicate and Mg-PSZ ceramics Authors: Maria Andr´e DDS Wen Kou PhD, DDS G¨oran Sj¨ogren PhD, DDS Professor Anders Sundh PhD, DDS Manager PII: DOI: Reference:
S0300-5712(16)30178-6 http://dx.doi.org/doi:10.1016/j.jdent.2016.09.002 JJOD 2664
To appear in:
Journal of Dentistry
Received date: Revised date: Accepted date:
13-4-2016 20-8-2016 12-9-2016
Please cite this article as: Andr´e Maria, Kou Wen, Sj¨ogren G¨oran, Sundh Anders.Effects of pretreatments and hydrothermal aging on biaxial flexural strength of lithium di-silicate and Mg-PSZ ceramics.Journal of Dentistry http://dx.doi.org/10.1016/j.jdent.2016.09.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Effects of pretreatments and hydrothermal aging on biaxial flexural strength of lithium disilicate and Mg-PSZ ceramics Maria André1, Wen Kou2, Göran Sjögren3, Anders Sundh4
1
DDS, Dental Materials Science, Faculty of Medicine, Umeå University, SE-901 87 Umeå, Sweden.
2
PhD, DDS, Dental Materials Science, Faculty of Medicine, Umeå University, SE-901 87 Umeå, Sweden.
3
PhD, DDS, Professor, Dental Materials Science, Faculty of Medicine, Umeå University, SE-901 87 Umeå, Sweden.
4
PhD, DDS, Manager of Research and Development, Cad.esthetics AB, SE-931 21 Skellefteå, Sweden.
Corresponding author: Professor Göran Sjögren Dental Materials Science Faculty of Medicine Umeå University SE-901 87 Umeå
2 (35) Sweden E-mail: [email protected]
Key words: accelerated aging, ceramic, CAD/CAM, magnesia-stabilized zirconia. Running head; A study of the strength of two different types of ceramic
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Effects of pretreatments and hydrothermal aging on biaxial flexural strength of lithium di-silicate and Mg-PSZ ceramics
Key words: accelerated aging, ceramic, CAD/CAM, magnesia-stabilized zirconia.
4 (35) Running title; Biaxial flexural strength of two different types of ceramic
5 (35) Abstract Objectives: To evaluate the effect of specimen thickness, pretreatment and hydrothermal aging on the biaxial flexural strength (BFS) of lithium di-silicate glass (e.max Cad) and magnesia-stabilized zirconia (ZirMagnum) ceramic discs. Methods: The e.max Cad discs was studied: i) crystallized, ii) crystallized and glazed and iii) crystallized, glazed and unglazed side etched with hydrofluoric acid. The ZirMagnum discs were studied: i) as delivered, ii) after sandblasting and iii) after heat treatment similar to veneering. Hydrothermal aging was simulated by autoclave treatment. Results: The BFS of all the ZirMagnum specimens was superior (p<0.001) to all the e.max Cad specimens. Glazing the 0.4 mm e.max Cad discs reduced (p<0.05) their BFS compared with the unglazed 0.8 mm specimens, whereas glazing of 0.8 mm discs had no influence (p>0.05) on the strength. Etching and autoclaving of e.max Cad did not affect (p>0.05) the BFS. For ZirMagnum sandblasting with 0.2 MPa or 0.6 MPa did not influence the biaxial flexural strength (p>0.05), whereas heat treatment reduced (p<0.01) the BFS of 0.6 MPa sandblasted ZirMagnum. Autoclaving reduced the strength (p<0.05) compared with ZirMagnum as delivered, whereas autoclaving of the 0.6 MPa sandblasted and heat-treated specimens did not influence (p>0.05) the BFS. Glazing, etching and sandblasting increased (p<0.05) surface roughness. Conclusions: The effects of glazing, heat treatment, aging and mechanical treatment of the materials evaluated should be considered since their strength could be affected. Clinical significance: Mechanical properties of restorations made from prefabricated ceramic blocks could be affected of various treatments and could change over time.
6 (35) 1. Introduction Ceramics have been an alternative for dental restorations for many years but their brittle nature [1,2] has previously limited their application to the anterior region and to use as veneering material in ceramic-fused-to-metal restorations. The clinical properties of ceramics desirable in dentistry, e.g. esthetics, biocompatibility, chemical resistance and low plaque retention, have encouraged the development of ceramic materials with improved fracture resistance [3]. Today modern manufacturing processes, such as strictly controlled prefabrication of homogenous ceramic blocks and computer-aided design/computer-aided manufacturing (CAD/CAM) techniques, make it possible to handle ceramic materials in a way that was impossible earlier [4-7]. Stronger and tougher dental ceramic materials are nowadays available to be used as all-ceramic single-to-multiple units in anterior and posterior regions [8-11]. Two types of ceramics currently used in dentistry are glass-ceramics and zirconia oxide ceramics [10,12]. Glass-ceramics consist mainly of silica-based glass matrix reinforced with crystals which prevents crack propagation and offers high esthetic quality regarding color and translucency. These have been used in dentistry since the 1950s [1]. Among the advantages of glass-ceramics is that the silica content allows etching and adhesive cementation [13]. During the 1990s another type of glass-ceramic became available for dentistry, the lithium di-silicate glass-ceramic [14,15]. This is a monolithic ceramic, which can be processed using the CAD/CAM technique [14] in selectable colors and finished with painting and glazing. The indications are said to be inlays, onlays, single crowns and three-unit fixed partial dentures (FPDs) [15]. Zirconia oxides offer excellent mechanical properties and are considered among the most fracture resistant dental ceramic materials. Zirconia is mainly used as core material veneered with feldspathic or glass ceramics to improve the esthetics and reduce
7 (35) grinding on antagonists. The fracture resistance of zirconia depends mainly on the crystallographic form and transformation toughening properties. Pure zirconia can exist in three different phases; cubic (c), tetragonal (t) and monoclin (m) [16,17]. The cubic phase occurs at a temperature of >2370OC, the tetragonal phase between 1170 OC – 2370OC and the monoclin phase between 0OC – 1170OC [16,17]. During cooling an increased amount of the t m phase occurs, which unless controlled can be catastrophic for the mechanical properties of the material [17]. In order to control the phase transformation stabilizing oxides such as yttria, magnesia, hafnia or ceria can be added to zirconia. This is known as partially stabilized zirconia (PSZ) [16,17]. The tetragonal phase will then remain at room temperature but transformation to monoclinic phase can still occur, so-called low-temperature degradation (LTD) or aging, among other situations in occasion of stress, could stop crack propagation and thereby improve the fracture resistance of the material [18]. One type of zirconia used as biomaterial is magnesia-stabilized zirconia (Mg-PSZ), used for example in odontology and orthopedics [5,9,10,19]. Mg-PSZ is one of the toughest zirconia-based ceramics [20] and is usually sintered at ~1800OC. It has a grain size ~30-60 µm [16,21,22]. One interesting property of Mg-PSZ as a biomaterial is that it is said to be more resistant to LTD than yttria-partially stabilized zirconia (Y-TZP) [19,23,24]. Since it has been demonstrated that various pretreatments, such as veneering, heat treatment and sandblasting could affect the properties of zirconia ceramics [5,25,26] and it has been stated that Mg-PSZ exhibits good resistance to LTD and aging [23] it was of interest to study the effect on the strength of Mg-PSZ of various pretreatments used in the production of dental restorations. In addition, it has recently been stated that in studies on the effect of various pretreatments and aging on lithium di-silicate glassceramics various testing parameters have been used making direct comparisons between the studies difficult [27]. The aim of the
8 (35) present work was, therefore, to study the effect of various pretreatments and hydrothermal aging on the fracture strength of lithium di-silicate and Mg-PSZ ceramics.
9 (35) 2. Materials and methods 2.1 Preparation of specimens Using computer-aided machining (Cad.esthetics Softwear system, Cad.esthetichs AB, Skellefteå, Sweden) disc-shaped specimens were sliced from prefabricated blocks of a lithium di-silicate glass-ceramic (IPS e.max Cad, Ivoclar-Vivadent, Schaan, Lichtenstein. Batch no N79116) and a magnesia-stabilized zirconia ceramic, Mg-PSZ (ZirMagnum, Cad.esthetichs AB. Batch no SM-0936). The diameter of the discs was 13 mm. The thickness of the e.max Cad discs was 0.4 mm and 0.8 mm and of the ZirMagnum discs 0.25 mm, 0.4 mm and 1.3 mm. The specimens were ultrasonically (Bransonic B221; Bransonic Ultrasonic Co, Danbury, Conn., USA) cleaned for 10 minutes in tap water, rinsed in distilled water and air-dried. To study the effect of glazing and hydrofluoric acid (HFacid) on the strength of the e.max Cad the biaxial flexural fracture strength was determined after the e.max Cad discs were: i) crystallized, ii) crystallized and glazed, iii) crystallized, glazed and the unglazed side etched with hydrofluoric acid gel for 20 seconds (9.5% HF-acid, Ultradent Porcelain Etch, Ultradent Products Inc., South Jordan, Utah, USA. Batch no B3K28). Crystallization and glazing were carried out in accordance with the manufacturers’ instructions (Ivoclar-Vivadent). For glazing IPS e.max Cad Crystal/Glaze (Ivoclar-Vivadent. Lot no N74262) was used. According to the manufacturer’s information the composition (weight%) of the glazing paste was SiO2 60-65 %, K2O 15-19 %, Al2O3 6-10.5 % and other oxides, pigments 5.5-30 %. Crystallization and glaze firing were performed in one step. The firing temperature was 840 OC. To study the effect of glazing on the biaxial flexural fracture strength the e.max Cad specimens were loaded in different directions; one group were placed in the sample holder (The European Standard EN ISO 6872, 2008) [28] with the glazed surface of the specimens facing down (in tension) bearing on the three stainless-steel supporting balls, whereas the other group was placed in
10 (35) the same position but with the unglazed surface facing down, i.e., the glazed surface in compression and the unglazed surface in tension. The specimens etched with hydrofluoric acid were placed in the sample holder with the unglazed and etched surface in tension. To ensure that the thickness of the glazed layer of the e.max Cad specimens did not exceed 30 µm, and considering it to be a mono-layered construction [29-31], the thickness of the specimens was measured before and after glazing using a digital caliper (Solar Absolute Digmatic, Mitutoyo, Japan). The biaxial flexural fracture strength of the ZirMagnum specimens was studied i) as delivered, ii) after sandblasting one of the flat surfaces of the discs and iii) after a heat treatment similar to veneering. The sandblasting of one of the flat surfaces of the discs was done at a pressure of 0.2 MPa or 0.6 MPa using 110 µm Al2O3 applied for 90 sec at a distance of approximately 2 mm between the nozzle and the surface of the disc (Basic Quattro, Renfert GmbH, Hitzingen, Gemany). After sandblasting the ZirMagnum discs were ultrasonically (Bransonic B22) cleaned for 10 minutes in tap water, rinsed in distilled water and air-dried. During the biaxial flexural test the sandblasted specimens were placed in the sample holder with the sandblasted surface face down bearing on the three stainless-steel supporting balls, i.e., the sandblasted surface in tension. For both types of the ceramics, i.e. e.max Cad and ZirMagnum, all the groups examined in the present study comprised 10 specimens. 2.2 Hydrothermal aging Ten of the crystallized glazed and HF-etched e.max Cad discs and 10 of the 0.6 MPa sandblasted and heat-treated ZirMagnum discs were hydrothermally aged at 134OC and 0.2 MPa for 2 x 5 hours (Autoklav Typ GE 446, Getinge AB, Halmstad, Sweden). Between the two autoclave treatments the specimens were stored at room temperature for 16 hours.
11 (35) 2.3 Surface roughness After the specimens were ultrasonically cleaned (Bransonic B221) for 5 minutes in tap water, rinsed in distilled water and airdried the average surface roughness (Ra; µm) of the surface intended to be in tensile stress during the biaxial flexural testing was determined using a measuring profilometer (Taylor/Hobson Precision Form Taylor Surf 50, Taylor Hobson, IL, USA), stylus tip radius 2 μm, cut-off length 0.8 mm, evaluation length 4 mm and stylus speed 0.5 mm/sec. These settings were in accordance with the EN 623-4:2004 standard [32]. Ten parallel scans were made at 0.5 mm intervals resulting in 10 scans for each specimen, giving a total of 100 scans for each test group. 2.4 Biaxial flexural strength test A universal testing machine (Tinius Olsen H10K-T, Horsham, PA, USA) and test set-up in accordance to the ISO standard 6872 (2008) [28] were used to determine the load at facture (Newton) of the various ceramic discs. The support area for the discs consisted of three steel balls with a diameter of 3.2 mm each, positioned 120O distance from each other, creating a circle of Ø 10 mm (ISO Standard 6872, 2008) [28]. The load was applied at the center of the specimen with a flat dowel pin with a diameter of 1.6 mm and the crosshead speed was 1 mm/min (ISO standard 6872, 2008) [28]. After the specimens were loaded to fracture a microscope (Leitz UWM-Dig-S, Ernst Leitz GmbH, Wetzlar, Germany) was used to determine the thickness of the specimens. The microscope accuracy was 0.5 µm and the thickness was measured at 20X magnification at 3 selected measuring points; one in the center of the specimen and the other two 1 mm from the border of the specimen at the fractured surface. The mean of the 3 measurements was then used for ‘d ‘ in the formula (1) below. Biaxial flexural strength was determined in accordance to the ISO Standard 6872 (2008) [28] using the equations
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where S is the maximum tensile stress (MPa), P is the load at fracture (N), ( )
[
( ) and is Poisson’s ratio, specimen (mm), and
]( )
( )
is the radius of the support circle,
is the radius of the loaded area (mm),
is the specimen thickness (mm). In the present study = 0.25,
= 5 mm,
is the radius of the
= 0.8 mm,
= 6.5 mm was used.
2.5 Statistical analysis The results were statistically analyzed using ANOVA and LSD (Least Significant Difference) post-hoc tests. The significance level was set to p<0.05. The program used was Statistical Package for the Social Sciences (SPSS) version 23 software (SPSS Inc., Chicago, IL, USA).
13 (35) 3. Results The results and statistical analysis of the biaxial flexural strength test are presented in Figs. 1 and 2 and Tables 1 and 2. The results and statistical analysis of the surface roughness (Ra) are presented in Table 3. The biaxial flexural strength of all the Mg-PSZ (ZirMagnum) specimens was significantly superior (p<0.001) to that of all the glass-ceramic (e.max Cad) specimens (Figs. 1 and 2). Glazing the crystallized 0.4 mm e.max Cad discs significantly (p<0.05) reduced their biaxial flexural strength compared with the crystallized and unglazed 0.8 mm specimens, whereas glazing the 0.8 mm discs did not influence (p>0.05) their strength. Hydrofluoric acid etching did not affect (p>0.05) the biaxial flexural strength of the crystallized and glazed specimens. Comparison between the crystallized and crystallized and autoclaved specimens revealed no significant difference (p>0.05). No significant difference (p>0.05) was seen between the crystallized, glazed and HF-etched and the crystallized, glazed, hydrofluoric acid etched and autoclaved specimens (Fig. 1, Table 1). For ZirMagnum sandblasting with 0.2 MPa or 0.6 MPa did not influence the biaxial flexural strength (p>0.05). Heat treatment reduced (p<0.01) the biaxial flexural strength of ZirMagnum sandblasted with 0.6 MPa, whereas heat treatment did not affect (p>0.05) the biaxial flexural strength when compared with the as-delivered specimens and with the specimens sandblasted with 0.2 MPa. Autoclaving reduced the strength compared with the as-delivered ZirMagnum specimens (p<0.05), whereas autoclaving of the 0.6 MPa sandblasted and previously heat-treated specimens did not influence (p>0.05) biaxial flexural strength (Fig. 2, Table 2). The results and statistical analysis of the surface roughness are presented in Table 3. The mean Ra values of the e.max Cad groups ranged from 0.6-1.6 µm and of ZirMagnum from 0.4-2.3 µm. Glazing of the e.max Cad specimens significantly (p<0.01)
14 (35) increased the Ra value compared with the specimens that were only crystallized. Similarly, the Ra value of e.max Cad increased significantly (p<0.001) after hydrofluoric acid etching compared with the surfaces that were crystallized only. For ZirMagnum the sandblasted specimens exhibited a significantly (p<0.001) higher Ra value than the as-delivered ZirMagnum specimens. The 0.6 MPa sandblasted ZirMagnum exhibited a significantly (p<0.001) higher Ra value than the 0.2 MPa sandblasted ZirMagnum (Table 3).
15 (35) 4. Discussion Comparison between the two types of materials examined in the present study revealed that the biaxial flexural strength of all the groups of ZirMagnum specimens, regardless of thickness, various pretreatments and hydrothermal aging, were significantly (p<0.001) superior to all the e.max Cad specimens (Figs. 1 and 2). In the current study glazing of the crystallized 0.4 mm e.max Cad discs reduced the biaxial flexural strength compared with the crystallized and unglazed 0.8 mm specimens, whereas glazing of the 0.8 mm discs showed no significant effect on the strength (Fig. 1, Table 1). Hydrofluoric acid etching of the lithium di-silicate specimens significantly increased the surface roughness (Table 3) but no effect was seen on the strength (Fig. 1 and Table 1). Similar effects of glazing, surface roughness and etching on the strength of glass ceramics have been reported previously [33-41]. For example, Fairhust et al. (1992) demonstrated that non-glazed feldsparbased ceramic specimens exhibited significantly higher biaxial flexural strength than the glazed-fired specimens [33]. In a previous study of a leucite-reinforced glass-ceramic (IPS Empress Cad) glass firing reduced the strength but did not affect the surface roughness [41]. The paper by Lohbauer et al. (2008) [36] reported that reducing the surface roughness of a glass-ceramic could improve the mechanical properties but it has also been reported [34,38] that surface roughness could have a limited influence on the strength of glass ceramics. Studies on the effect of hydrofluoric acid etching on the strength and surface roughness of three different glass ceramics [37,39,41] revealed that the surface become rougher and the flexural strength was reduced with increased etching time [37,39]. Thus, different outcomes concerning the effects of surface glazing and the surface roughness of glass-ceramics have been reported. Therefore, more studies are needed to elucidate the effects of pretreatments and surface texture on the strength of glass-ceramics.
16 (35) For the ZirMagnum specimens, the various specimen thicknesses, i.e. 0.25 mm, 0.4 mm and 1.3 mm, of the as-delivered ZirMagnum specimens did not affect (p≥0.05) the biaxial flexural strength (Fig. 2, Table 2). On the other hand, the strength of the ZirMagnum was affected by sandblasting, heat treatment and hydrothermal aging (Fig. 2, Table 2). This is in agreement with results presented in a previous in vitro study of Mg-PSZ FPD frameworks, in which it was shown that the load at fracture was reduced after heat treatment [5]. Machining was used to produce these Mg-PSZ frameworks and was suggested as possible explanation for the reduced strength, that the mechanical treatment of the framework surfaces had caused t m transformation-toughening and that subsequent heat treatment reversed the phase transformation and caused relaxation of residual compressive stresses introduced on the surface during the grinding process [5]. Magnesia-stabilized zirconia usually contains tetragonal and monoclinic phases in a cubic matrix [42], and it has been stated that heat can cause phase transformation of Mg-PSZ and influences its mechanical properties [20]. That is, although phase transformation toughening is said to be less in Mg-PSZ than in yttrium oxide partiallystabilized zirconia (Y-TZP) [19,23,43] phase transformation could be one explanation for the effects of the mechanical and heat treatments seen on the biaxial flexural strength of the ZirMagnum discs in the current study. There was no difference between the biaxial flexural strength obtained between the two different magnitudes of pressure used for the sandblast treatment of the ZirMagnum specimens. That is, the biaxial flexural strength did not differ significantly (p>0.05) whether 0.2 MPa or 0.6 MPa was used (Fig. 2, Table 2). These findings are interesting since it was expected that higher pressure applied to the surface would initiate increased t m transformation-toughening of the ZirMagnum and strengthen the ceramic [43]. That is, higher pressure should have rendered an increasing extent of monoclin phase and improved the fracture resistance [43]. In the present study the ZirMagnum specimens were sandblasted for 90 sec and it should be noted that sandblasing could induce
17 (35) defects such as flaws in ceramic that reduce its strength [45]. In the previous study by Sundh and Sjögren (2006) it was also stated that excessive sandblasting of zirconia ceramics could reduce the fracture resistance, depending on the pressure and duration of the treatment [5]. Since this could happen more in glass-ceramics than in oxide ceramics the e.max Cad specimens in the present study were not subjected to sandblasting in order to avoid introducing defects, e.g. flaws and/or micro cracks, in the glass-ceramic discs. For surface roughness the Ra value of the e.max Cad ranged from 0.6-1.6 µm and the ZirMagnum specimens from 0.4-2.3 µm (Table 3). Thus, the difference between the highest and lowest mean Ra value determined was 2 µm (Table 3) and there was low correlation (SPSS, Pearson Correlation) between the values obtained for the surface roughness (Ra) and the biaxial flexural strength. For the e.max CAD specimens the Pearson correlation coefficient was -0.081 (p=0.450). Corresponding figures for the ZirMagnum specimen were 0.079 (p=0.393). Although there was a statistically significant difference in the Ra values among a number of the groups (Table 3) the biaxial flexural strength of the ZirMagnum was not apparently affected by the surface roughness. For example, the 0.25 mm ZirMagnum specimens sandblasted with 0.6 MPa exhibited the highest Ra value (2.3 µm) and the highest biaxial flexural strength (879 MPa). One conceivable reason why the Ra value did not seem to substantially reduce the strength of MgPSZ could be that mechanical treatment, such as sandblasting, of the surfaces of zirconia ceramics, induced t m transformation-toughening and strengthened the specimens [20,42,43]. This hypothesis is supported by the fact that subsequent heat-treatment significantly (p<0.01) reduced the biaxial flexural strength (Fig. 2, Table 2). In the paper by Flury et al. (2012) concerning a leucite-reinforced ceramic (IPS Empress Cad) the median Ra value ranged from 0.38-1.57 µm, depending on the grit size of the grinding papers used [38]. That is, values almost similar to the values seen in
18 (35) the current study. In their study Flury et al. (2012) concluded that the correlation between surface roughness and flexural strength of the material studied was moderate and that the surface roughness alone could not have influenced the flexural strength [38]. No paper was found (PubMed) for Mg-PSZ that addressed previous studies of the surface roughness and its influence on the strength of the material. It should, however, be noted that fatigue loading and moisture could influence the effects of surface roughness and defects, such as flaws and micro cracks, in dental ceramics [45-47]. For example, it has been shown that fatigue loading and moisture could accelerate crack propagation and reduce the strength of ceramics [45-47]. More studies are therefore necessary to clarify the clinical effects of surface roughness, flaws, micro cracks and fatigue loading in dental ceramics. 4.1 Effects of hydrothermal aging One common method of simulating aging of materials is autoclave treatment (ISO 13356:2008) and it has been proposed that 1 hour of autoclave treatment at 134 OC and 0.2 MPa pressure corresponds to 3-4 years in vivo at 37 OC [48]. ISO Standard 13356:2008 sets 5 hours at 37 OC in 0.2 MPa for the aging test [28], whereas in the present study the specimens were autoclaved for 10 hours. This means, according to Chevalier et al. (1999), that the hydrothermal aging of the specimens in the present study would correspond to a clinical use of around 30-40 years [48]. In the present study hydrothermally aging by autoclave treatment did not affect (p>0.05) the biaxial flexural strength of the e.max Cad or the heat-treated ZirMagnum specimens (Figs. 1 and 2, Tables 1 and 2). In previous studies it has been shown that
19 (35) magnesia-stabilized zirconia could resist environmental degradation and phase transformation better than yttria-stabilized zirconia [19,24]. However, despite the phase stability of magnesia-stabilized zirconia, phase transformation in Mg-PSZ could be caused, among other things, by heat and could affect its mechanical properties [20] and, as mentioned above, the ZirMagnum specimens in the current study could have been affected by heat treatment and sandblasting. No paper was found, in a survey in database (PubMed) for the lithium di-silicate glass ceramics that addressed their biaxial flexural strength after hydrothermal aging. It is not only the fracture strength of the core that matters for the clinical functioning of all-ceramic restorations. Their success also depends on the properties of the veneering ceramic, the bonding strength between core and veneer and the cementation to the tooth. For example, in most cases glass-ceramics are etched with hydrofluoric acid and adhesively luted which might increase the strength of the restorations and reduce the risk of fracture during function [13]. In the present study the lithium di-silcate glassceramic (IPS e.max Cad) was pretreated using hydrofluoric acid etching. In clinical practice this is followed by adhesive cementation that could improve the fracture strength of the restoration. It should also be noted that in vivo dental materials are not only subjected to heat and moisture but also to wear and dynamic loading, which could affect the aging, strength and survival of ceramics [24]. Values reported in previous studies addressing the biaxial strength of lithium di-silicate glass-ceramics range from 266-416 MPa [49-52]. For e.max Cad the manufacturer states that the biaxial flexural strength of the material is 360 MPa (Ivoclar-Vivadent). A possible explanation for the different values among the studies of lithium di-silicate glass-ceramics could be that the material is sensitive to the machining process, i.e. grinding, milling, slicing and/or polishing. Another reason could be that the crystallization and glazing processes of glass-ceramics were carried out in different laboratories using different furnaces [49-52]. This assumption
20 (35) is based on studies showing that the precision of dental porcelain furnaces varies and deviations were observed during various steps of the firing process between displayed and real temperatures [53]. Only one paper [54], addressing the biaxial flexural strength of Mg-PSZ, was found in a survey in database (PubMed) to compare with the values obtained for ZirMagnum in the current study. In that study the biaxial flexural strength of Mg-PSZ ranged from 808-916 MPa depending on the various pretreatments of the specimens [54]. That is, values almost similar to the findings in the present study. The results of the present study have important clinical implications because nowadays the manufacturing of dental restorations often involves grinding and/or milling from prefabricated ceramic blocks. During these manufacturing processes the prefabricated blocks and the made-up dental restorations are subjected to various treatments, such as grinding/milling, sandblasting, glazing, veneering and heat treatment. The results in the present study reveal that such treatments could significantly affect the materials. The findings also indicate that the properties of dental ceramics could change over time. However, it should be noted that the results in the current study are only valid for the specific ceramics studied. Therefore, application of the current results to other ceramics should be made with caution and after reflection.
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5. Conclusions The effects of glazing, heat treatment, aging and mechanical treatment of the materials evaluated should be considered since their strength could be affected.
22 (35) Acknowledgements We would like to express our appreciation to Cad.esthetics AB, Sweden for donating the specimens and for preparation them.
Conflict of Interest Dr. Anders Sundh is employed (part-time) as Manager of Research and Development at Cad.esthetics AB, Skellefteå, Sweden.
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29 (35) Figure legends Fig. 1. Box-plot diagram comprising the biaxial flexural strength (MPa) of the e.max Cad specimens studied. Ten specimens in each test group. Data are presented as medians and 1st and 3rd quartiles. The median is presented by a horizontal line within the box. The maximum and minimum values are illustrated via the upper and lower strokes. A
Glazed surface in compression. marks outliers,
B
Glazed surface in tension.
marks extreme values.
Fig. 2. Box-plot diagram comprising the biaxial flexural strength (MPa) of the ZirMagnum specimens studied. Ten specimens in each test group. Data are presented as medians and 1st and 3rd quartiles. The median is presented by a horizontal line within the box. The maximum and minimum values are illustrated via the upper and lower strokes. All the sandblasted surfaces of the ZirMagnum specimens were placed in tension.
marks outliers,
marks extreme values.
Table
Tabel 1. Results of the statistical analysis of the e.max Cad specimenes. ANOVA supplemented with LSD. Specimen 1. 2. 3. 4. 5. 1. 0.4 mm e.max Cad crystallized and glazed. Glazed surface in compression.
6.
7.
2. 0.4 mm e.max Cad crystallized and glazed. Glazed surface in tension.
0.976
3. 0.4 mm e.max Cad crystallized, glazed & HF-etched. Glazed surface in compression.
0.440
0.458
4. 0.4 mm e.max Cad crystallized & autoclaved.
0.039
0.036
0.005
5. 0.8 mm e.max Cad crystallized.
0.027
0.025
0.003
0.880
6. 0.8 mm e.max Cad crystallized & glazed. Glazed surface in compression.
0.520
0.500
0.158
0.112
0.150
7. 0.8 mm e.max Cad crystallized & glazed. Glazed surface in tension.
0.794
0.770
0.302
0.050
0.070
0.702
8. 0.8 mm e.max Cad crystallized, glazed, HF-etched. Glazed surface in compression.
0.496
0.515
0.927
0.004
0.007
0.187
0.347
9. 0.8 mm e.max Cad crystallized, glazed, HF-etched & 0.338 0.353 0.852 autoclaved. Glazed surface in compression. n.s.; p≥0.05, *; p<0.05, **; p<0.01, ***; p<0.001. The figures of the specimens in Table 1, i.e. 1-9, refer to the figures in Fig. 1.
0.002
0.001
0.111
0.224
8.
0.781
9.
Table
Table 2. Statistical analysis of the biaxial flexural strength test of the ZirMagnum specimens. ANOVA supplemented with LSD. Specimen 10. 11. 12. 13. 14. 15. 16. 17. 18. 10. 0.25 mm ZirMagnum and 0.6 MPa. 11. 0.25 mm ZirMagnum and 0.6 MPa and heat-treated.
0.001
12. 0.4 mm ZirMagnum as delivered.
0.130
0.044
13. 0.4 mm ZirMagnum heat-treated.
0.014
0.286
0.334
14. 0.4 mm ZirMagnum as delivered and autoclaved.
0.000
0.873
0.029
0.221
15. 0.4 mm ZirMagnum and 0.2 MPa.
0.110
0.027
0.934
0.380
0.037
16. 0.4 mm ZirMagnum and 0.2 MPa and heat-treated.
0.023
0.235
0.401
0.903
0.155
0.499
17. 0.4 mm ZirMagnum and 0.6 MPa.
0.926
0.001
0.153
0.018
0.000
0.132
0.025
18. 0.4 mm ZirMagnum and 0.6 MPa and heat-treated.
0.003
0.575
0.141
0.609
0.470
0.166
0.528
0.004
19. 0.4 mm ZirMagnum and 0.6 MPa and heat-treated and autoclaved.
0.001
0.790
0.078
0.420
0.669
0.094
0.356
0.002
0.768
20. 1.3 mm ZirMagnum as delivered.
0.050
0.115
0.650
0.608
0.083
0.712
0.697
0.061
0.306
19.
20.
0.190
21. 1.3 mm ZirMagnum as delivered 0.015 0.281 0.341 0.989 0.214 0.387 0.914 0.018 0.601 0.415 and heat-treated. n.s. p≥0.05; * p<0.05; ** p<0.01; *** p<0.001 ’0.2 MPa’ and ’0.6 MPa’ refer to specimens sandblasted with 0.2 MPa or 0.6 MPa. The figures of the specimens, i.e. 10-21, refer to the figures in Fig. 2.
0.619
21.
Table
Table 3. Surface roughness (Ra) of the various specimens studied. Ten specimens in each group and 10 measurements on each specimen, i.e. 100 measurements in each group. A; Glazed surface in compression. B; Glazed surface in tension. All sandblasted surface in tension. Specimen A (1) 0.4 mm e.max crystallized and glazed. (2) 0.4 mm e.max crystallized and glazed.
Mean ± SD (µm) a, e 0.7 ± 0.2
B
1.3 ± 0.5 0.9 ± 0.3
b, x
(4) 0.4 mm e.max crystallized and autoclaved.
0.6 ± 0.2
c, i
(5) 0.8 mm e.max crystallized.
0.7 ± 0.2
c, d, i
(3) 0.4 mm e.max crystallized, glazed and HF-etched.
A
(6) 0.8 mm e.max crystallized and glazed.
A
0.8 ± 0.2
a, e
(7) 0.8 mm e.max crystallized and glazed.
B
1.6 ± 0.7
f
0.9 ± 0.3
b, g
(8) 0.8 mm e.max crystallized, glazed and HF-etched.
A
(9) 0.8 mm e.max crystallized, glazed, HF-etched and autoclaved.
A
1.2 ± 0.5
(10) 0.25 mm ZirMagnum sandblasted with 0.6 MPa.
2.3 ± 0.3
h
(11) 0.25 mm ZirMagnum sandblasted with 0.6 MPa and heat-treated.
2.3 ± 0.2
h
(12) 0.4 mm ZirMagnum as delivered.
0.6 ± 0.1
c, d, i
(13) 0.4 mm ZirMagnum heat-treated.
0.4 ± 0.2
j
(14) 0.4 mm ZirMagnum as delivered and autoclaved.
0.5 ± 0.1
c, d, i, k, o
(15) 0.4 mm ZirMagnum sandblasted with 0.2 MPa.
0.9 ± 0.2
b, e, g, l
(16) 0.4 mm ZirMagnum sandblasted with 0.2 MPa and heat-treated.
0.8 ± 0.2
a, e, l
(17) 0.4 mm ZirMagnum sandblasted with 0.6 MPa.
1.7 ± 0.4
m
(18) 0.4 mm ZirMagnum sandblasted with 0.6 MPa and heat-treated.
1.6 ± 0.6
f
(19) 0.4 mm ZirMagnum sandblasted with 0.6 MPa, heat-treated and autoclaved. 1.8 ± 0.1 m (20) 1.3 mm ZirMagnum as delivered.
0.5 ± 0.1
i, j, k, n, o
(21) 1.3 mm ZirMagnum heat-treated.
0.4 ± 0.1
j, k, n
Same lowercase letters (a-s) indicate that the surface roughness (Ra) was not significantly different between groups (p>0.05). (ANOVA supplemented with LSD). Figures within parentheses refer to the figures in Fig. 1.
Figure
Figure
S0300-5712(16)30178-6 http://dx.doi.org/doi:10.1016/j.jdent.2016.09.002 JJOD 2664
To appear in:
Journal of Dentistry
Received date: Revised date: Accepted date:
13-4-2016 20-8-2016 12-9-2016
Please cite this article as: Andr´e Maria, Kou Wen, Sj¨ogren G¨oran, Sundh Anders.Effects of pretreatments and hydrothermal aging on biaxial flexural strength of lithium di-silicate and Mg-PSZ ceramics.Journal of Dentistry http://dx.doi.org/10.1016/j.jdent.2016.09.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1 (35)
Effects of pretreatments and hydrothermal aging on biaxial flexural strength of lithium disilicate and Mg-PSZ ceramics Maria André1, Wen Kou2, Göran Sjögren3, Anders Sundh4
1
DDS, Dental Materials Science, Faculty of Medicine, Umeå University, SE-901 87 Umeå, Sweden.
2
PhD, DDS, Dental Materials Science, Faculty of Medicine, Umeå University, SE-901 87 Umeå, Sweden.
3
PhD, DDS, Professor, Dental Materials Science, Faculty of Medicine, Umeå University, SE-901 87 Umeå, Sweden.
4
PhD, DDS, Manager of Research and Development, Cad.esthetics AB, SE-931 21 Skellefteå, Sweden.
Corresponding author: Professor Göran Sjögren Dental Materials Science Faculty of Medicine Umeå University SE-901 87 Umeå
2 (35) Sweden E-mail: [email protected]
Key words: accelerated aging, ceramic, CAD/CAM, magnesia-stabilized zirconia. Running head; A study of the strength of two different types of ceramic
3 (35)
Effects of pretreatments and hydrothermal aging on biaxial flexural strength of lithium di-silicate and Mg-PSZ ceramics
Key words: accelerated aging, ceramic, CAD/CAM, magnesia-stabilized zirconia.
4 (35) Running title; Biaxial flexural strength of two different types of ceramic
5 (35) Abstract Objectives: To evaluate the effect of specimen thickness, pretreatment and hydrothermal aging on the biaxial flexural strength (BFS) of lithium di-silicate glass (e.max Cad) and magnesia-stabilized zirconia (ZirMagnum) ceramic discs. Methods: The e.max Cad discs was studied: i) crystallized, ii) crystallized and glazed and iii) crystallized, glazed and unglazed side etched with hydrofluoric acid. The ZirMagnum discs were studied: i) as delivered, ii) after sandblasting and iii) after heat treatment similar to veneering. Hydrothermal aging was simulated by autoclave treatment. Results: The BFS of all the ZirMagnum specimens was superior (p<0.001) to all the e.max Cad specimens. Glazing the 0.4 mm e.max Cad discs reduced (p<0.05) their BFS compared with the unglazed 0.8 mm specimens, whereas glazing of 0.8 mm discs had no influence (p>0.05) on the strength. Etching and autoclaving of e.max Cad did not affect (p>0.05) the BFS. For ZirMagnum sandblasting with 0.2 MPa or 0.6 MPa did not influence the biaxial flexural strength (p>0.05), whereas heat treatment reduced (p<0.01) the BFS of 0.6 MPa sandblasted ZirMagnum. Autoclaving reduced the strength (p<0.05) compared with ZirMagnum as delivered, whereas autoclaving of the 0.6 MPa sandblasted and heat-treated specimens did not influence (p>0.05) the BFS. Glazing, etching and sandblasting increased (p<0.05) surface roughness. Conclusions: The effects of glazing, heat treatment, aging and mechanical treatment of the materials evaluated should be considered since their strength could be affected. Clinical significance: Mechanical properties of restorations made from prefabricated ceramic blocks could be affected of various treatments and could change over time.
6 (35) 1. Introduction Ceramics have been an alternative for dental restorations for many years but their brittle nature [1,2] has previously limited their application to the anterior region and to use as veneering material in ceramic-fused-to-metal restorations. The clinical properties of ceramics desirable in dentistry, e.g. esthetics, biocompatibility, chemical resistance and low plaque retention, have encouraged the development of ceramic materials with improved fracture resistance [3]. Today modern manufacturing processes, such as strictly controlled prefabrication of homogenous ceramic blocks and computer-aided design/computer-aided manufacturing (CAD/CAM) techniques, make it possible to handle ceramic materials in a way that was impossible earlier [4-7]. Stronger and tougher dental ceramic materials are nowadays available to be used as all-ceramic single-to-multiple units in anterior and posterior regions [8-11]. Two types of ceramics currently used in dentistry are glass-ceramics and zirconia oxide ceramics [10,12]. Glass-ceramics consist mainly of silica-based glass matrix reinforced with crystals which prevents crack propagation and offers high esthetic quality regarding color and translucency. These have been used in dentistry since the 1950s [1]. Among the advantages of glass-ceramics is that the silica content allows etching and adhesive cementation [13]. During the 1990s another type of glass-ceramic became available for dentistry, the lithium di-silicate glass-ceramic [14,15]. This is a monolithic ceramic, which can be processed using the CAD/CAM technique [14] in selectable colors and finished with painting and glazing. The indications are said to be inlays, onlays, single crowns and three-unit fixed partial dentures (FPDs) [15]. Zirconia oxides offer excellent mechanical properties and are considered among the most fracture resistant dental ceramic materials. Zirconia is mainly used as core material veneered with feldspathic or glass ceramics to improve the esthetics and reduce
7 (35) grinding on antagonists. The fracture resistance of zirconia depends mainly on the crystallographic form and transformation toughening properties. Pure zirconia can exist in three different phases; cubic (c), tetragonal (t) and monoclin (m) [16,17]. The cubic phase occurs at a temperature of >2370OC, the tetragonal phase between 1170 OC – 2370OC and the monoclin phase between 0OC – 1170OC [16,17]. During cooling an increased amount of the t m phase occurs, which unless controlled can be catastrophic for the mechanical properties of the material [17]. In order to control the phase transformation stabilizing oxides such as yttria, magnesia, hafnia or ceria can be added to zirconia. This is known as partially stabilized zirconia (PSZ) [16,17]. The tetragonal phase will then remain at room temperature but transformation to monoclinic phase can still occur, so-called low-temperature degradation (LTD) or aging, among other situations in occasion of stress, could stop crack propagation and thereby improve the fracture resistance of the material [18]. One type of zirconia used as biomaterial is magnesia-stabilized zirconia (Mg-PSZ), used for example in odontology and orthopedics [5,9,10,19]. Mg-PSZ is one of the toughest zirconia-based ceramics [20] and is usually sintered at ~1800OC. It has a grain size ~30-60 µm [16,21,22]. One interesting property of Mg-PSZ as a biomaterial is that it is said to be more resistant to LTD than yttria-partially stabilized zirconia (Y-TZP) [19,23,24]. Since it has been demonstrated that various pretreatments, such as veneering, heat treatment and sandblasting could affect the properties of zirconia ceramics [5,25,26] and it has been stated that Mg-PSZ exhibits good resistance to LTD and aging [23] it was of interest to study the effect on the strength of Mg-PSZ of various pretreatments used in the production of dental restorations. In addition, it has recently been stated that in studies on the effect of various pretreatments and aging on lithium di-silicate glassceramics various testing parameters have been used making direct comparisons between the studies difficult [27]. The aim of the
8 (35) present work was, therefore, to study the effect of various pretreatments and hydrothermal aging on the fracture strength of lithium di-silicate and Mg-PSZ ceramics.
9 (35) 2. Materials and methods 2.1 Preparation of specimens Using computer-aided machining (Cad.esthetics Softwear system, Cad.esthetichs AB, Skellefteå, Sweden) disc-shaped specimens were sliced from prefabricated blocks of a lithium di-silicate glass-ceramic (IPS e.max Cad, Ivoclar-Vivadent, Schaan, Lichtenstein. Batch no N79116) and a magnesia-stabilized zirconia ceramic, Mg-PSZ (ZirMagnum, Cad.esthetichs AB. Batch no SM-0936). The diameter of the discs was 13 mm. The thickness of the e.max Cad discs was 0.4 mm and 0.8 mm and of the ZirMagnum discs 0.25 mm, 0.4 mm and 1.3 mm. The specimens were ultrasonically (Bransonic B221; Bransonic Ultrasonic Co, Danbury, Conn., USA) cleaned for 10 minutes in tap water, rinsed in distilled water and air-dried. To study the effect of glazing and hydrofluoric acid (HFacid) on the strength of the e.max Cad the biaxial flexural fracture strength was determined after the e.max Cad discs were: i) crystallized, ii) crystallized and glazed, iii) crystallized, glazed and the unglazed side etched with hydrofluoric acid gel for 20 seconds (9.5% HF-acid, Ultradent Porcelain Etch, Ultradent Products Inc., South Jordan, Utah, USA. Batch no B3K28). Crystallization and glazing were carried out in accordance with the manufacturers’ instructions (Ivoclar-Vivadent). For glazing IPS e.max Cad Crystal/Glaze (Ivoclar-Vivadent. Lot no N74262) was used. According to the manufacturer’s information the composition (weight%) of the glazing paste was SiO2 60-65 %, K2O 15-19 %, Al2O3 6-10.5 % and other oxides, pigments 5.5-30 %. Crystallization and glaze firing were performed in one step. The firing temperature was 840 OC. To study the effect of glazing on the biaxial flexural fracture strength the e.max Cad specimens were loaded in different directions; one group were placed in the sample holder (The European Standard EN ISO 6872, 2008) [28] with the glazed surface of the specimens facing down (in tension) bearing on the three stainless-steel supporting balls, whereas the other group was placed in
10 (35) the same position but with the unglazed surface facing down, i.e., the glazed surface in compression and the unglazed surface in tension. The specimens etched with hydrofluoric acid were placed in the sample holder with the unglazed and etched surface in tension. To ensure that the thickness of the glazed layer of the e.max Cad specimens did not exceed 30 µm, and considering it to be a mono-layered construction [29-31], the thickness of the specimens was measured before and after glazing using a digital caliper (Solar Absolute Digmatic, Mitutoyo, Japan). The biaxial flexural fracture strength of the ZirMagnum specimens was studied i) as delivered, ii) after sandblasting one of the flat surfaces of the discs and iii) after a heat treatment similar to veneering. The sandblasting of one of the flat surfaces of the discs was done at a pressure of 0.2 MPa or 0.6 MPa using 110 µm Al2O3 applied for 90 sec at a distance of approximately 2 mm between the nozzle and the surface of the disc (Basic Quattro, Renfert GmbH, Hitzingen, Gemany). After sandblasting the ZirMagnum discs were ultrasonically (Bransonic B22) cleaned for 10 minutes in tap water, rinsed in distilled water and air-dried. During the biaxial flexural test the sandblasted specimens were placed in the sample holder with the sandblasted surface face down bearing on the three stainless-steel supporting balls, i.e., the sandblasted surface in tension. For both types of the ceramics, i.e. e.max Cad and ZirMagnum, all the groups examined in the present study comprised 10 specimens. 2.2 Hydrothermal aging Ten of the crystallized glazed and HF-etched e.max Cad discs and 10 of the 0.6 MPa sandblasted and heat-treated ZirMagnum discs were hydrothermally aged at 134OC and 0.2 MPa for 2 x 5 hours (Autoklav Typ GE 446, Getinge AB, Halmstad, Sweden). Between the two autoclave treatments the specimens were stored at room temperature for 16 hours.
11 (35) 2.3 Surface roughness After the specimens were ultrasonically cleaned (Bransonic B221) for 5 minutes in tap water, rinsed in distilled water and airdried the average surface roughness (Ra; µm) of the surface intended to be in tensile stress during the biaxial flexural testing was determined using a measuring profilometer (Taylor/Hobson Precision Form Taylor Surf 50, Taylor Hobson, IL, USA), stylus tip radius 2 μm, cut-off length 0.8 mm, evaluation length 4 mm and stylus speed 0.5 mm/sec. These settings were in accordance with the EN 623-4:2004 standard [32]. Ten parallel scans were made at 0.5 mm intervals resulting in 10 scans for each specimen, giving a total of 100 scans for each test group. 2.4 Biaxial flexural strength test A universal testing machine (Tinius Olsen H10K-T, Horsham, PA, USA) and test set-up in accordance to the ISO standard 6872 (2008) [28] were used to determine the load at facture (Newton) of the various ceramic discs. The support area for the discs consisted of three steel balls with a diameter of 3.2 mm each, positioned 120O distance from each other, creating a circle of Ø 10 mm (ISO Standard 6872, 2008) [28]. The load was applied at the center of the specimen with a flat dowel pin with a diameter of 1.6 mm and the crosshead speed was 1 mm/min (ISO standard 6872, 2008) [28]. After the specimens were loaded to fracture a microscope (Leitz UWM-Dig-S, Ernst Leitz GmbH, Wetzlar, Germany) was used to determine the thickness of the specimens. The microscope accuracy was 0.5 µm and the thickness was measured at 20X magnification at 3 selected measuring points; one in the center of the specimen and the other two 1 mm from the border of the specimen at the fractured surface. The mean of the 3 measurements was then used for ‘d ‘ in the formula (1) below. Biaxial flexural strength was determined in accordance to the ISO Standard 6872 (2008) [28] using the equations
12 (35)
where S is the maximum tensile stress (MPa), P is the load at fracture (N), ( )
[
( ) and is Poisson’s ratio, specimen (mm), and
]( )
( )
is the radius of the support circle,
is the radius of the loaded area (mm),
is the specimen thickness (mm). In the present study = 0.25,
= 5 mm,
is the radius of the
= 0.8 mm,
= 6.5 mm was used.
2.5 Statistical analysis The results were statistically analyzed using ANOVA and LSD (Least Significant Difference) post-hoc tests. The significance level was set to p<0.05. The program used was Statistical Package for the Social Sciences (SPSS) version 23 software (SPSS Inc., Chicago, IL, USA).
13 (35) 3. Results The results and statistical analysis of the biaxial flexural strength test are presented in Figs. 1 and 2 and Tables 1 and 2. The results and statistical analysis of the surface roughness (Ra) are presented in Table 3. The biaxial flexural strength of all the Mg-PSZ (ZirMagnum) specimens was significantly superior (p<0.001) to that of all the glass-ceramic (e.max Cad) specimens (Figs. 1 and 2). Glazing the crystallized 0.4 mm e.max Cad discs significantly (p<0.05) reduced their biaxial flexural strength compared with the crystallized and unglazed 0.8 mm specimens, whereas glazing the 0.8 mm discs did not influence (p>0.05) their strength. Hydrofluoric acid etching did not affect (p>0.05) the biaxial flexural strength of the crystallized and glazed specimens. Comparison between the crystallized and crystallized and autoclaved specimens revealed no significant difference (p>0.05). No significant difference (p>0.05) was seen between the crystallized, glazed and HF-etched and the crystallized, glazed, hydrofluoric acid etched and autoclaved specimens (Fig. 1, Table 1). For ZirMagnum sandblasting with 0.2 MPa or 0.6 MPa did not influence the biaxial flexural strength (p>0.05). Heat treatment reduced (p<0.01) the biaxial flexural strength of ZirMagnum sandblasted with 0.6 MPa, whereas heat treatment did not affect (p>0.05) the biaxial flexural strength when compared with the as-delivered specimens and with the specimens sandblasted with 0.2 MPa. Autoclaving reduced the strength compared with the as-delivered ZirMagnum specimens (p<0.05), whereas autoclaving of the 0.6 MPa sandblasted and previously heat-treated specimens did not influence (p>0.05) biaxial flexural strength (Fig. 2, Table 2). The results and statistical analysis of the surface roughness are presented in Table 3. The mean Ra values of the e.max Cad groups ranged from 0.6-1.6 µm and of ZirMagnum from 0.4-2.3 µm. Glazing of the e.max Cad specimens significantly (p<0.01)
14 (35) increased the Ra value compared with the specimens that were only crystallized. Similarly, the Ra value of e.max Cad increased significantly (p<0.001) after hydrofluoric acid etching compared with the surfaces that were crystallized only. For ZirMagnum the sandblasted specimens exhibited a significantly (p<0.001) higher Ra value than the as-delivered ZirMagnum specimens. The 0.6 MPa sandblasted ZirMagnum exhibited a significantly (p<0.001) higher Ra value than the 0.2 MPa sandblasted ZirMagnum (Table 3).
15 (35) 4. Discussion Comparison between the two types of materials examined in the present study revealed that the biaxial flexural strength of all the groups of ZirMagnum specimens, regardless of thickness, various pretreatments and hydrothermal aging, were significantly (p<0.001) superior to all the e.max Cad specimens (Figs. 1 and 2). In the current study glazing of the crystallized 0.4 mm e.max Cad discs reduced the biaxial flexural strength compared with the crystallized and unglazed 0.8 mm specimens, whereas glazing of the 0.8 mm discs showed no significant effect on the strength (Fig. 1, Table 1). Hydrofluoric acid etching of the lithium di-silicate specimens significantly increased the surface roughness (Table 3) but no effect was seen on the strength (Fig. 1 and Table 1). Similar effects of glazing, surface roughness and etching on the strength of glass ceramics have been reported previously [33-41]. For example, Fairhust et al. (1992) demonstrated that non-glazed feldsparbased ceramic specimens exhibited significantly higher biaxial flexural strength than the glazed-fired specimens [33]. In a previous study of a leucite-reinforced glass-ceramic (IPS Empress Cad) glass firing reduced the strength but did not affect the surface roughness [41]. The paper by Lohbauer et al. (2008) [36] reported that reducing the surface roughness of a glass-ceramic could improve the mechanical properties but it has also been reported [34,38] that surface roughness could have a limited influence on the strength of glass ceramics. Studies on the effect of hydrofluoric acid etching on the strength and surface roughness of three different glass ceramics [37,39,41] revealed that the surface become rougher and the flexural strength was reduced with increased etching time [37,39]. Thus, different outcomes concerning the effects of surface glazing and the surface roughness of glass-ceramics have been reported. Therefore, more studies are needed to elucidate the effects of pretreatments and surface texture on the strength of glass-ceramics.
16 (35) For the ZirMagnum specimens, the various specimen thicknesses, i.e. 0.25 mm, 0.4 mm and 1.3 mm, of the as-delivered ZirMagnum specimens did not affect (p≥0.05) the biaxial flexural strength (Fig. 2, Table 2). On the other hand, the strength of the ZirMagnum was affected by sandblasting, heat treatment and hydrothermal aging (Fig. 2, Table 2). This is in agreement with results presented in a previous in vitro study of Mg-PSZ FPD frameworks, in which it was shown that the load at fracture was reduced after heat treatment [5]. Machining was used to produce these Mg-PSZ frameworks and was suggested as possible explanation for the reduced strength, that the mechanical treatment of the framework surfaces had caused t m transformation-toughening and that subsequent heat treatment reversed the phase transformation and caused relaxation of residual compressive stresses introduced on the surface during the grinding process [5]. Magnesia-stabilized zirconia usually contains tetragonal and monoclinic phases in a cubic matrix [42], and it has been stated that heat can cause phase transformation of Mg-PSZ and influences its mechanical properties [20]. That is, although phase transformation toughening is said to be less in Mg-PSZ than in yttrium oxide partiallystabilized zirconia (Y-TZP) [19,23,43] phase transformation could be one explanation for the effects of the mechanical and heat treatments seen on the biaxial flexural strength of the ZirMagnum discs in the current study. There was no difference between the biaxial flexural strength obtained between the two different magnitudes of pressure used for the sandblast treatment of the ZirMagnum specimens. That is, the biaxial flexural strength did not differ significantly (p>0.05) whether 0.2 MPa or 0.6 MPa was used (Fig. 2, Table 2). These findings are interesting since it was expected that higher pressure applied to the surface would initiate increased t m transformation-toughening of the ZirMagnum and strengthen the ceramic [43]. That is, higher pressure should have rendered an increasing extent of monoclin phase and improved the fracture resistance [43]. In the present study the ZirMagnum specimens were sandblasted for 90 sec and it should be noted that sandblasing could induce
17 (35) defects such as flaws in ceramic that reduce its strength [45]. In the previous study by Sundh and Sjögren (2006) it was also stated that excessive sandblasting of zirconia ceramics could reduce the fracture resistance, depending on the pressure and duration of the treatment [5]. Since this could happen more in glass-ceramics than in oxide ceramics the e.max Cad specimens in the present study were not subjected to sandblasting in order to avoid introducing defects, e.g. flaws and/or micro cracks, in the glass-ceramic discs. For surface roughness the Ra value of the e.max Cad ranged from 0.6-1.6 µm and the ZirMagnum specimens from 0.4-2.3 µm (Table 3). Thus, the difference between the highest and lowest mean Ra value determined was 2 µm (Table 3) and there was low correlation (SPSS, Pearson Correlation) between the values obtained for the surface roughness (Ra) and the biaxial flexural strength. For the e.max CAD specimens the Pearson correlation coefficient was -0.081 (p=0.450). Corresponding figures for the ZirMagnum specimen were 0.079 (p=0.393). Although there was a statistically significant difference in the Ra values among a number of the groups (Table 3) the biaxial flexural strength of the ZirMagnum was not apparently affected by the surface roughness. For example, the 0.25 mm ZirMagnum specimens sandblasted with 0.6 MPa exhibited the highest Ra value (2.3 µm) and the highest biaxial flexural strength (879 MPa). One conceivable reason why the Ra value did not seem to substantially reduce the strength of MgPSZ could be that mechanical treatment, such as sandblasting, of the surfaces of zirconia ceramics, induced t m transformation-toughening and strengthened the specimens [20,42,43]. This hypothesis is supported by the fact that subsequent heat-treatment significantly (p<0.01) reduced the biaxial flexural strength (Fig. 2, Table 2). In the paper by Flury et al. (2012) concerning a leucite-reinforced ceramic (IPS Empress Cad) the median Ra value ranged from 0.38-1.57 µm, depending on the grit size of the grinding papers used [38]. That is, values almost similar to the values seen in
18 (35) the current study. In their study Flury et al. (2012) concluded that the correlation between surface roughness and flexural strength of the material studied was moderate and that the surface roughness alone could not have influenced the flexural strength [38]. No paper was found (PubMed) for Mg-PSZ that addressed previous studies of the surface roughness and its influence on the strength of the material. It should, however, be noted that fatigue loading and moisture could influence the effects of surface roughness and defects, such as flaws and micro cracks, in dental ceramics [45-47]. For example, it has been shown that fatigue loading and moisture could accelerate crack propagation and reduce the strength of ceramics [45-47]. More studies are therefore necessary to clarify the clinical effects of surface roughness, flaws, micro cracks and fatigue loading in dental ceramics. 4.1 Effects of hydrothermal aging One common method of simulating aging of materials is autoclave treatment (ISO 13356:2008) and it has been proposed that 1 hour of autoclave treatment at 134 OC and 0.2 MPa pressure corresponds to 3-4 years in vivo at 37 OC [48]. ISO Standard 13356:2008 sets 5 hours at 37 OC in 0.2 MPa for the aging test [28], whereas in the present study the specimens were autoclaved for 10 hours. This means, according to Chevalier et al. (1999), that the hydrothermal aging of the specimens in the present study would correspond to a clinical use of around 30-40 years [48]. In the present study hydrothermally aging by autoclave treatment did not affect (p>0.05) the biaxial flexural strength of the e.max Cad or the heat-treated ZirMagnum specimens (Figs. 1 and 2, Tables 1 and 2). In previous studies it has been shown that
19 (35) magnesia-stabilized zirconia could resist environmental degradation and phase transformation better than yttria-stabilized zirconia [19,24]. However, despite the phase stability of magnesia-stabilized zirconia, phase transformation in Mg-PSZ could be caused, among other things, by heat and could affect its mechanical properties [20] and, as mentioned above, the ZirMagnum specimens in the current study could have been affected by heat treatment and sandblasting. No paper was found, in a survey in database (PubMed) for the lithium di-silicate glass ceramics that addressed their biaxial flexural strength after hydrothermal aging. It is not only the fracture strength of the core that matters for the clinical functioning of all-ceramic restorations. Their success also depends on the properties of the veneering ceramic, the bonding strength between core and veneer and the cementation to the tooth. For example, in most cases glass-ceramics are etched with hydrofluoric acid and adhesively luted which might increase the strength of the restorations and reduce the risk of fracture during function [13]. In the present study the lithium di-silcate glassceramic (IPS e.max Cad) was pretreated using hydrofluoric acid etching. In clinical practice this is followed by adhesive cementation that could improve the fracture strength of the restoration. It should also be noted that in vivo dental materials are not only subjected to heat and moisture but also to wear and dynamic loading, which could affect the aging, strength and survival of ceramics [24]. Values reported in previous studies addressing the biaxial strength of lithium di-silicate glass-ceramics range from 266-416 MPa [49-52]. For e.max Cad the manufacturer states that the biaxial flexural strength of the material is 360 MPa (Ivoclar-Vivadent). A possible explanation for the different values among the studies of lithium di-silicate glass-ceramics could be that the material is sensitive to the machining process, i.e. grinding, milling, slicing and/or polishing. Another reason could be that the crystallization and glazing processes of glass-ceramics were carried out in different laboratories using different furnaces [49-52]. This assumption
20 (35) is based on studies showing that the precision of dental porcelain furnaces varies and deviations were observed during various steps of the firing process between displayed and real temperatures [53]. Only one paper [54], addressing the biaxial flexural strength of Mg-PSZ, was found in a survey in database (PubMed) to compare with the values obtained for ZirMagnum in the current study. In that study the biaxial flexural strength of Mg-PSZ ranged from 808-916 MPa depending on the various pretreatments of the specimens [54]. That is, values almost similar to the findings in the present study. The results of the present study have important clinical implications because nowadays the manufacturing of dental restorations often involves grinding and/or milling from prefabricated ceramic blocks. During these manufacturing processes the prefabricated blocks and the made-up dental restorations are subjected to various treatments, such as grinding/milling, sandblasting, glazing, veneering and heat treatment. The results in the present study reveal that such treatments could significantly affect the materials. The findings also indicate that the properties of dental ceramics could change over time. However, it should be noted that the results in the current study are only valid for the specific ceramics studied. Therefore, application of the current results to other ceramics should be made with caution and after reflection.
21 (35)
5. Conclusions The effects of glazing, heat treatment, aging and mechanical treatment of the materials evaluated should be considered since their strength could be affected.
22 (35) Acknowledgements We would like to express our appreciation to Cad.esthetics AB, Sweden for donating the specimens and for preparation them.
Conflict of Interest Dr. Anders Sundh is employed (part-time) as Manager of Research and Development at Cad.esthetics AB, Skellefteå, Sweden.
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29 (35) Figure legends Fig. 1. Box-plot diagram comprising the biaxial flexural strength (MPa) of the e.max Cad specimens studied. Ten specimens in each test group. Data are presented as medians and 1st and 3rd quartiles. The median is presented by a horizontal line within the box. The maximum and minimum values are illustrated via the upper and lower strokes. A
Glazed surface in compression. marks outliers,
B
Glazed surface in tension.
marks extreme values.
Fig. 2. Box-plot diagram comprising the biaxial flexural strength (MPa) of the ZirMagnum specimens studied. Ten specimens in each test group. Data are presented as medians and 1st and 3rd quartiles. The median is presented by a horizontal line within the box. The maximum and minimum values are illustrated via the upper and lower strokes. All the sandblasted surfaces of the ZirMagnum specimens were placed in tension.
marks outliers,
marks extreme values.
Table
Tabel 1. Results of the statistical analysis of the e.max Cad specimenes. ANOVA supplemented with LSD. Specimen 1. 2. 3. 4. 5. 1. 0.4 mm e.max Cad crystallized and glazed. Glazed surface in compression.
6.
7.
2. 0.4 mm e.max Cad crystallized and glazed. Glazed surface in tension.
0.976
3. 0.4 mm e.max Cad crystallized, glazed & HF-etched. Glazed surface in compression.
0.440
0.458
4. 0.4 mm e.max Cad crystallized & autoclaved.
0.039
0.036
0.005
5. 0.8 mm e.max Cad crystallized.
0.027
0.025
0.003
0.880
6. 0.8 mm e.max Cad crystallized & glazed. Glazed surface in compression.
0.520
0.500
0.158
0.112
0.150
7. 0.8 mm e.max Cad crystallized & glazed. Glazed surface in tension.
0.794
0.770
0.302
0.050
0.070
0.702
8. 0.8 mm e.max Cad crystallized, glazed, HF-etched. Glazed surface in compression.
0.496
0.515
0.927
0.004
0.007
0.187
0.347
9. 0.8 mm e.max Cad crystallized, glazed, HF-etched & 0.338 0.353 0.852 autoclaved. Glazed surface in compression. n.s.; p≥0.05, *; p<0.05, **; p<0.01, ***; p<0.001. The figures of the specimens in Table 1, i.e. 1-9, refer to the figures in Fig. 1.
0.002
0.001
0.111
0.224
8.
0.781
9.
Table
Table 2. Statistical analysis of the biaxial flexural strength test of the ZirMagnum specimens. ANOVA supplemented with LSD. Specimen 10. 11. 12. 13. 14. 15. 16. 17. 18. 10. 0.25 mm ZirMagnum and 0.6 MPa. 11. 0.25 mm ZirMagnum and 0.6 MPa and heat-treated.
0.001
12. 0.4 mm ZirMagnum as delivered.
0.130
0.044
13. 0.4 mm ZirMagnum heat-treated.
0.014
0.286
0.334
14. 0.4 mm ZirMagnum as delivered and autoclaved.
0.000
0.873
0.029
0.221
15. 0.4 mm ZirMagnum and 0.2 MPa.
0.110
0.027
0.934
0.380
0.037
16. 0.4 mm ZirMagnum and 0.2 MPa and heat-treated.
0.023
0.235
0.401
0.903
0.155
0.499
17. 0.4 mm ZirMagnum and 0.6 MPa.
0.926
0.001
0.153
0.018
0.000
0.132
0.025
18. 0.4 mm ZirMagnum and 0.6 MPa and heat-treated.
0.003
0.575
0.141
0.609
0.470
0.166
0.528
0.004
19. 0.4 mm ZirMagnum and 0.6 MPa and heat-treated and autoclaved.
0.001
0.790
0.078
0.420
0.669
0.094
0.356
0.002
0.768
20. 1.3 mm ZirMagnum as delivered.
0.050
0.115
0.650
0.608
0.083
0.712
0.697
0.061
0.306
19.
20.
0.190
21. 1.3 mm ZirMagnum as delivered 0.015 0.281 0.341 0.989 0.214 0.387 0.914 0.018 0.601 0.415 and heat-treated. n.s. p≥0.05; * p<0.05; ** p<0.01; *** p<0.001 ’0.2 MPa’ and ’0.6 MPa’ refer to specimens sandblasted with 0.2 MPa or 0.6 MPa. The figures of the specimens, i.e. 10-21, refer to the figures in Fig. 2.
0.619
21.
Table
Table 3. Surface roughness (Ra) of the various specimens studied. Ten specimens in each group and 10 measurements on each specimen, i.e. 100 measurements in each group. A; Glazed surface in compression. B; Glazed surface in tension. All sandblasted surface in tension. Specimen A (1) 0.4 mm e.max crystallized and glazed. (2) 0.4 mm e.max crystallized and glazed.
Mean ± SD (µm) a, e 0.7 ± 0.2
B
1.3 ± 0.5 0.9 ± 0.3
b, x
(4) 0.4 mm e.max crystallized and autoclaved.
0.6 ± 0.2
c, i
(5) 0.8 mm e.max crystallized.
0.7 ± 0.2
c, d, i
(3) 0.4 mm e.max crystallized, glazed and HF-etched.
A
(6) 0.8 mm e.max crystallized and glazed.
A
0.8 ± 0.2
a, e
(7) 0.8 mm e.max crystallized and glazed.
B
1.6 ± 0.7
f
0.9 ± 0.3
b, g
(8) 0.8 mm e.max crystallized, glazed and HF-etched.
A
(9) 0.8 mm e.max crystallized, glazed, HF-etched and autoclaved.
A
1.2 ± 0.5
(10) 0.25 mm ZirMagnum sandblasted with 0.6 MPa.
2.3 ± 0.3
h
(11) 0.25 mm ZirMagnum sandblasted with 0.6 MPa and heat-treated.
2.3 ± 0.2
h
(12) 0.4 mm ZirMagnum as delivered.
0.6 ± 0.1
c, d, i
(13) 0.4 mm ZirMagnum heat-treated.
0.4 ± 0.2
j
(14) 0.4 mm ZirMagnum as delivered and autoclaved.
0.5 ± 0.1
c, d, i, k, o
(15) 0.4 mm ZirMagnum sandblasted with 0.2 MPa.
0.9 ± 0.2
b, e, g, l
(16) 0.4 mm ZirMagnum sandblasted with 0.2 MPa and heat-treated.
0.8 ± 0.2
a, e, l
(17) 0.4 mm ZirMagnum sandblasted with 0.6 MPa.
1.7 ± 0.4
m
(18) 0.4 mm ZirMagnum sandblasted with 0.6 MPa and heat-treated.
1.6 ± 0.6
f
(19) 0.4 mm ZirMagnum sandblasted with 0.6 MPa, heat-treated and autoclaved. 1.8 ± 0.1 m (20) 1.3 mm ZirMagnum as delivered.
0.5 ± 0.1
i, j, k, n, o
(21) 1.3 mm ZirMagnum heat-treated.
0.4 ± 0.1
j, k, n
Same lowercase letters (a-s) indicate that the surface roughness (Ra) was not significantly different between groups (p>0.05). (ANOVA supplemented with LSD). Figures within parentheses refer to the figures in Fig. 1.
Figure
Figure