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CAM-fabricated ZLS molar crowns
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Influence of cementation on in vitro performance, marginal adaptation and fracture resistance of CAD/CAM-fabricated ZLS molar crowns Verena Preis ∗ , Michael Behr, Sebastian Hahnel, Martin Rosentritt Department of Prosthetic Dentistry, Regensburg University Medical Center, Franz-Josef-Strauss-Allee 11, 93042 Regensburg, Germany
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. This study investigated the influence of conventional cementation, self-adhesive
Received 14 June 2015
cementation, and adhesive bonding on the in vitro performance, fracture resistance, and
Accepted 17 August 2015
marginal adaptation of zirconia-reinforced lithium silicate (ZLS) crowns.
Available online xxx
Methods. Human molar teeth (n = 40) were prepared and full-contour crowns of a ZLS ceramic (Celtra Duo, DeguDent, G, n = 32) and a lithium disilicate ceramic (LDS; IPS e.max CAD, Ivoclar-
Keywords:
Vivadent, FL, n = 8) were fabricated and glazed. Four groups of ZLS crowns were defined
Cementation
(n = 8/group) and cemented with different glass-ionomer cements, resin, and resin-modified
Crown
self-adhesive luting materials. The LDS crowns served as reference group with adhesive
Ceramics
bonding.
Zirconia-reinforced lithium silicate Marginal adaptation Fracture resistance
A combined thermal cycling and mechanical loading (TCML: 3000 × 5 ◦ C/3000 × 55 ◦ C; 1.2 × 106 cycles à 50 N) with human antagonists was performed in a chewing simulator. Fracture force of surviving crowns was determined. Marginal adaptation at the cement/tooth and cement/crown interface was investigated by scanning electron microscopy before and after TCML, and the share of perfect margins was determined. Data were statistically analyzed (one-way ANOVA; post hoc Bonferroni, ˛ = 0.05). Results. One crown of the adhesive group failed during TCML (879,000 cycles = 3.7 years). No statistically significant (p = 0.078) differences in fracture resistance were found between different cementations, although highest data in tendency were found for adhesive bonding. Shares of perfect margins at the cement/tooth (93.8 ± 5.6–99.6 ± 0.8%) and cement/crown (84.7 ± 6.6–100.0 ± 0.0%) interfaces did not differ significantly (p > 0.05) between the different cementation groups. Significance. Marginal adaptation and fracture forces of all tested groups are in a range, where no restrictions should be expected for clinical application. © 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗
Corresponding author. Tel.: +49 941 944 6073; fax: +49 941 944 6171. E-mail address: [email protected] (V. Preis).
http://dx.doi.org/10.1016/j.dental.2015.08.154 0109-5641/© 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Preis V, et al. Influence of cementation on in vitro performance, marginal adaptation and fracture resistance of CAD/CAM-fabricated ZLS molar crowns. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.08.154
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1.
Introduction
Natural esthetic appearance and longevity are major aims in restoring teeth with single crowns and fixed partial dentures. With increasing popularity of computer-aided design/computer-aided manufacturing (CAD/CAM), a rising number of machinable esthetic materials have been introduced. Currently, the most popular CAD/CAM ceramics for single crowns are lithium disilicate and zirconia. Both materials may be applied either for monolithic restorations or as substructure with subsequent veneering with porcelain for esthetic improvement. Recently, new classes of polymer-infiltrated ceramic-network materials (e.g. Enamic, Vita), resin-nanoceramics (e.g. Lava Ultimate, 3M Espe), and zirconia-reinforced lithium silicate (ZLS) ceramics (e.g. Celtra Duo, DeguDent) have been developed as alternative materials for CAD/CAM single-tooth restorations. While high translucency and high strength at the same time are mutually exclusive for most types of monolithic ceramics, zirconiareinforced lithium silicate may offer improved esthetics due to increased glass content and a microcrystalline structure, and flexural strength values comparable to lithium disilicate. While common glass ceramics exhibit lower strength values and require adhesive luting to increase the mechanical strength of the restoration [1], zirconia and high-strength lithium disilicate ceramics offer the possibility of conventional cementation, e.g. with glass-ionomer cement. Furthermore, self-adhesive resin cements might be a time-saving and less technique sensitive alternative to multi-step adhesive systems. In vitro studies indicated no significant influence of cementation on fracture load of lithium disilicate crowns [2–4]. Accordingly, clinical data have shown that lithium disilicate crowns were highly successful irrespective of an adhesive or conventional cementation [5]. Cumulative survival rates in clinical studies were reported to be about 97% after 5 years [5,6], and about 87% up to 9 years [7]. However, evidence for medium- or even long-term survival of lithium disilicate crowns is limited and neither clinical nor in vitro data are available about the performance of new ZLS ceramics. Besides esthetics and the resistance to fatigue and fracture, the marginal adaptation contributes to the success of dental crowns. Insufficient marginal adaptation already after incorporation of the prosthetic restoration, or deterioration of the marginal quality during clinical service time might lead to the accumulation of bacterial plaque [8] and can cause gingival inflammation, caries, pulp and periodontal lesions, finally resulting in failure of the restoration [5,9]. Besides manifold influencing factors like the finishing line configuration, the margin location in enamel or dentin, the fabrication process of the crown and the value of the predefined cementing space, the type of cementation is supposed to influence marginal adaptation and quality [10–16]. Prior to routine clinical application, in vitro tests may help to evaluate new materials and restorations by combining reproducible laboratory conditions with basic requirements (occlusal loading, thermocycling) of the clinical situation. In vitro thermal cycling and mechanical loading is supposed to allow a first prediction of the mechanical performance of a
new material. A long-term testing might stimulate fatigue failures and marginal degradation. But even in cases without any catastrophic failures, aging and deterioration effects might occur, thus reducing strength, fracture resistance or marginal adaptation. The hypothesis of this in vitro study was that the type of cementation influences (i) the in vitro performance, (ii) the fracture resistance, and (iii) the marginal adaptation of single molar ZLS crowns during a simulated five-year oral service.
2.
Materials and methods
Human molars (tooth 46, n = 40) were prepared according to ceramic guidelines. A circular and occlusal anatomical reduction of 1.5 mm was carried out with a preparation angle of 4◦ . The finishing line resulted in a 1 mm deep circular shoulder with rounded inner angles at an isogingival height of the tooth cervix. The resilience of the human periodontium was simulated by coating the roots of the teeth with a 1 mm polyether layer (Impregum, 3 M Espe, G). For achieving a constant layer, the roots were dipped in a wax bath, which was replaced by polyether in a second fabrication process, as previously described [17,18]. Then the teeth were positioned in resin blocks (Palapress Vario, Heraeus-Kulzer, G). The preparations of the teeth were digitalized and 32 full-contour crowns of a zirconia-reinforced lithium silicate ceramic (ZLS; Celtra Duo, DeguDent, G) as well as 8 full-contour crowns of a lithium disilicate ceramic (LDS; IPS e.max CAD, Ivoclar-Vivadent, FL) were milled (Cerec MC XL, Sirona, G), crystallized (only LDS), and glazed with the respective glazing materials according to the manufacturer’s instructions. The circular and occlusal wall thickness was 1.5 mm and the cervical wall thickness was 1 mm. ZLS crowns were randomly divided into four groups (n = 8/group) for cementation with different glass-ionomer cements (GIC), resin, and resinmodified self-adhesive luting materials (Table 1). The LDS crowns served as reference group with adhesive bonding. The inner faces of all crowns were etched with 5% hydrofluoric acid for 20 s (LDS) or 30 s (ZLS) before cementation. For the adhesive luting protocol the crowns were silanized (silane coupling agent Monobond S, 60 s, Ivoclar-Vivadent). The prepared teeth were treated with the bonding system Syntac classic (Syntac Primer/Syntac Adhesive/Heliobond; Ivoclar-Vivadent) according to the instructions of the manufacturer. A combined thermal cycling and mechanical loading (TCML: 3000 × 5 ◦ C/3000 × 55 ◦ C, 2 min each cycle, H2 O dist.; 1.2 × 106 cycles à 50 N, 1.6 Hz) with human molar antagonists was performed in a chewing simulator (EGO, Regensburg, G). The crowns were loaded in three-point-contact. Parameters are based on literature data on zirconia and ceramic restorations expressing that chewing simulation using this parameters might simulate a maximum of five years of oral service [19,20]. During TCML all crowns were controlled for failures. Failed crowns were excluded from further testing and
Please cite this article in press as: Preis V, et al. Influence of cementation on in vitro performance, marginal adaptation and fracture resistance of CAD/CAM-fabricated ZLS molar crowns. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.08.154
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Table 1 – Fracture force, Weibull shape parameter b and marginal quality (share of perfect margins) of the different groups of cementation (identical letters indicate no significant differences between groups; identical numbers indicate no significant differences before-after TCML for each material). Cement
Type
Fracture force Mean ± SD [N] Weibull b
Marginal quality [%] before TCML after TCML Cement-tooth
Syntac classic/Variolink II (Ivoclar-Vivadent, FL) Smart Cem 2 (Dentsply, G)
Adhesive
Aqua Cem (Dentsply, G)
GIC
Ketac Cem (3M Espe, USA)
GIC
IPS e.max CAD; Syntac classic/Variolink II (Ivoclar-Vivadent, FL)
Adhesive (reference)
Self-adhesive
investigated in detail (light-microscope) for failure analysis. After TCML, fracture force of surviving crowns was determined by mechanically loading the crowns to failure in a universal testing machine (Zwick 1446, G). The force was applied on the centre of the crowns using a steel ball (Ø = 12 mm, v = 1 mm/min) with a 1 mm foil (Dentaurum, Ispringen, G) inserted between crown and ball. The failure determination was set to a 10% loss of the maximum loading force or acoustic signal (crack). Marginal adaptation was determined on resin replica (Rencast CW 2215/HY 5162, Huntsman, Switzerland) that were fabricated by making impressions (Impregum, 3 M Espe) before and after TCML. Both interfaces at cement/tooth and cement/crown were investigated with scanning electron microscopy (working distance: 20.4 mm; voltage: 5–10 keV; low vacuum; magnification: 200×; Quanta FEG 400, FEI Company, Hillsboro, USA). SEM pictures of the margins were evaluated and share of perfect margin was determined. Marginal quality was defined using the following criteria: (i) “intact margin” with smooth transition and no interruption of continuity, and (ii) “marginal gap” showing separation of the components due to cohesive or adhesive failure. Power calculation (G*Power 3.1.3, Kiel, G) provided an estimated power of >90% using eight specimens per group. Distribution of the data was controlled with Kolmogorov–Smirnov test. Calculations and statistical analysis were carried out using SPSS 22 (IBM, USA). Mean values and standard deviations (SD) were calculated and analyzed by means of one-way analysis of variance (ANOVA) and the Bonferroni multiple comparison test for post hoc analysis. The level of significance was set to ˛ = 0.05. Weibull analysis was done and shape parameter b was calculated (Visual-XSel 12.1, München, G).
3.
2612 ± 853 2.88 1903 ± 438x 4.43 1848 ± 836x 2.53 1891 ± 593x 3.29 2528 ± 668x 4.07 x
99.6 ± 0.8 99.5 ± 1.4c1 99.0 ± 1.7a 96.3 ± 3.4c 97.5 ± 3.4a 93.8 ± 5.6c 95.2 ± 6.8a2 94.2 ± 5.7c2 96.6 ± 6.0a3 97.2 ± 3.9c3 a1
Cement-crown 99.8 ± 0.3b 97.5 ± 3.0d 97.2 ± 4.3b4 95.2 ± 6.8d4 92.8 ± 9.0b5 96.1 ± 4.0 d5 85.7 ± 10.7b6 84.7 ± 6.6d6 100.0 ± 0.0b7 99.5 ± 1.6d7
significant (p = 0.078) differences in fracture resistance were found between different cementations, although highest values in tendency were found for adhesive bonding of ZLS crowns and the reference LDS crowns (Table 1). Weibull shape parameter b showed values between 2.53 and 4.07, indicating similar reliability for the different groups. Predominant failure modes were fractures of the crowns with simultaneous loosening of the crown fragments (Fig. 2). In case of adhesive bonding the crown fragments partly remained attached to the teeth. Evaluation of the marginal quality at the interface cementtooth showed a share of intact margins between 93.8 ± 5.6% and 99.6 ± 0.8%. No significant (p = 1.000) differences between the different cementation groups were found, neither before nor after TCML. However, for cementation with Smart Cem 2 (p = 0.010) and with Aqua Cem (p = 0.040) marginal quality at the interface cement-tooth decreased significantly by TCML. At the interface cement-crown the share of intact margins varied between 84.7 ± 6.6% and 100.0 ± 0.0% without significant (p > 0.05) differences between the different groups,
Results
One ZLS crown of the adhesive cementation group failed during TCML at 879,000 cycles, corresponding to approximately 3.7 years of oral service time. Detailed failure evaluation showed a fracture in the marginal region due to local overloading (Fig. 1). SEM pictures revealed that this failure was probably caused by fabrication damage. No statistically
Fig. 1 – SEM picture (magnification: 23×) of the failed crown during TCML, showing a fracture in the marginal region.
Please cite this article in press as: Preis V, et al. Influence of cementation on in vitro performance, marginal adaptation and fracture resistance of CAD/CAM-fabricated ZLS molar crowns. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.08.154
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Fig. 2 – Crown fractures after loading to failure.
neither before nor after TCML. Only marginal quality of adhesively luted ZLS crowns decreased significantly (p = 0.004) by TCML. Exemplary SEM pictures of the cervical margins are given in Fig. 3.
4.
Discussion
The three individual parts of the hypothesis that the type of cementation influences (i) the in vitro performance, (ii) fracture resistance, and (iii) marginal adaptation of single molar
ZLS crowns during a simulated five-year oral service were rejected. Irrespective of the cementation type ZLS crowns showed good long-term resistance to fatigue during ageing simulation. The failure of one crown during TCML might not be related to the type of cementation but be rather a random event associated with fabrication defects. The high fracture stability of the ZLS crowns after TCML was independent from cementation and comparable to the LDS reference. Perfect margins were found to be higher than 95% in most cases, indicating good marginal adaptation both
Please cite this article in press as: Preis V, et al. Influence of cementation on in vitro performance, marginal adaptation and fracture resistance of CAD/CAM-fabricated ZLS molar crowns. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.08.154
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Fig. 3 – Exemplary SEM pictures (magnification: 200×) of the cervical margin indicating the criteria for the marginal evaluation at the interface crown-cement: left: perfect margin with smooth transition and no interruption of continuity; right: marginal gap with separation of the components due to cohesive or adhesive failure.
before and after TCML. Marginal adaptation of ZLS crowns did neither differ significantly between the different cementation types nor to the control. The TCML parameters have been chosen congruent to numerous other in vitro studies [21–23]. They are supposed to simulate restoration stress according to a maximum of five years of intraoral use [19,20]. Standardization of testing conditions is wishful as evaluation of crown performance and underlying reasons for clinically observed failures is complicated by individual variables, such as tooth structure, periodontal mobility, occlusal loads, chewing behavior, the oral environment, as well as differences in the preparation design of the abutment teeth. Although human teeth in this study did not allow for complete standardization, they were preferred to artificial abutment teeth that may differ from human teeth in terms of modulus of elasticity and bonding capacity to the cement [24]. Especially for studies investigating adhesive bonding to dentine, the application of human teeth seems to be essential. The influence of resilient support of the abutment teeth on the ageing process and fracture strength of molar crowns should be considered by a polyether interface, which naturally does not stay abreast of the complex human periodontal ligament, but might avoid an overestimation of the strength of ceramic restorations [18]. Particularly with high-strength ceramics, as investigated in the present study, ageing and deterioration often occur without any visible catastrophic failures. In these cases, a subsequent static fracture test could help locate weak points and allow a differentiation between different types of cementation. In previous studies fracture loads of LDS crowns were significantly influenced by thermal and mechanical ageing and partly were further influenced by the type of luting agent [25–27]. Nevertheless, fracture data cannot be directly related to clinical survival but they may provide information on the principal suitability of new ceramics in relation to the requirements of clinically proven systems. As monolithic CAD/CAM-fabricated LDS ceramics have proven their suitability for single tooth restorations both
in vivo [6,28,29] and in vitro [3,4,30,31], they were chosen as reference in this study. The ZLS ceramic is a new class of ceramic with an inclusion of 10% zirconia dissolved into the glass matrix, resulting in four times smaller lithium silicate crystals. Thus, ZLS shows high flexural strength and high translucency at the same time (Celtra Duo, DeguDent, G). According to the information provided by the manufacturers, flexural strength values of IPS e.max CAD (360 ± 60 MPa) and Celtra Duo after glazing (370 MPa) are comparable. One recent in vitro study has affirmed a higher translucency of polished Celtra Duo compared to IPS e.max CAD [32]. However, neither clinical nor in vitro studies have dealt with the fatigue and fracture resistance of ZLS crowns so far. The present results showed that LDS and ZLS ceramics had comparable resistance against TCML and comparable high fracture values. The cement between crown and tooth transmits loads through the bonded interface. It was reported that the adhesive luting technique improved the fracture resistance of glass ceramic crowns with lower strength values (e.g. feldspathic and leucite-reinforced glass ceramics), while fracture loads of high-strength ceramics like zirconia, alumina or lithium disilicate were not significantly influenced by the mode of cementation (adhesive/conventional) [2,3,33]. In contrast, a study by Borges [26] reported about a significant increase in fracture load for different ceramic crowns (lithium disilicate, leucite-reinforced, and glass-infiltrated alumina ceramics) when they were cemented with a resin cement compared to a resin-modified glass-ionomer cement. In the present study, the type of cementation did not significantly influence fracture values of ZLS crowns and was comparable to the reference group. This is in accordance with findings by a previous in vitro study reporting high fracture values for CAD/CAM-fabricated LDS crowns independent of the mode of cementation [4]. Even though, we found a trend to highest fracture forces for adhesive luting, and lowest values for cementation with glass-ionomer cement. Nevertheless, as the fracture values exceeded maximum chewing forces, which are reported to be up to 900 N [34], all groups of
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differently cemented molar crowns have the potential to withstand occlusal forces applied in the posterior region. However, adhesive luting may be preferred in case of low axial tooth height combined with a conical, low-retentive preparation design to ensure sufficient strength. Failure modes were in all cases fractures of the crowns. While for adhesive groups the fragments partly remained attached to the prepared teeth, they predominately got loosened in case of conventional cementation. Complete crown fractures might be classified as catastrophic failure modes. In clinical studies further failure modes are described that may not necessarily result in replacement of the crown, e.g. ceramic chipping or biological complications (secondary caries, pulp lesion) [5,29]. This underlines that severe fracture testing may only provide limited insight into clinically relevant mechanisms of damage and underlying reasons for failure. Besides resistance to fatigue and fracture, the marginal fit and quality contributes to the clinical success of ceramic crowns. Poor marginal fit and the continuing decrease of marginal adaptation under clinical conditions may culminate in secondary caries or even loss of the restoration due to microleakage [5]. Different methods have been described to evaluate the marginal area of crowns, e.g. microscopic methods, micro-computed tomography (-CT) and dye penetration tests [11,13,35,36]. While the micro-computed tomography might be preferred to measure quantitatively the marginal fit of crowns [11,35], SEM evaluation is an established and reproducible procedure for determining the marginal adaptation [10,13,36]. It is based on comparable conditions and semi-quantitative analysis [36]. Of course potential inaccuracies of taking impressions and fabricating replicas as well as the subjective evaluation of one examiner limit the value of such procedures. Interactions at the cement-tooth as well as at the cementcrown transition may both have an influence marginal quality. Therefore, both transitions were evaluated separately. Bond strength of different LDS and ZLS ceramics to resin adhesives was reported to depend on the surface treatment and on the chemical composition of the glass ceramic [37]. Potential influencing factors for marginal adaptation at the cementcrown transition are the etching process with hydrofluoric acid to create a micro-retentive and activated surface, and the application of silane to establish the chemical bond between ceramic and resin cements. For adhesive luting at the cement-tooth transition, typical dentin pre-treatment includes acid, primer, adhesive, and bonding agent. As the clinical application of adhesive materials is time-consuming and technically sensitive, self-adhesive resin cements that may enable comparable strength of full-coverage crowns are desirable to avoid the complex bonding procedures. In studies investigating zirconia and LDS restorations, self-adhesive cements were reported to provide comparable microleakage values and marginal adaptation than adhesive resin cements [13,38,39]. Accordingly, marginal quality of ZLS crowns did neither differ significantly between the different cementation types nor to the control. The mean share of perfect margins was in most cases higher than 95%, indicating good marginal adaptation both before and after TCML. In tendency lowest share of perfect margins was found for conventional cementation with glass-ionomer cements, partly showing marginal
quality lower than 95%. With a share of perfect margins of about 85% at the cement-crown transition, marginal quality of the Ketac Cem group may still be classified as sufficient. Significant degradation of marginal quality by TCML was only observed in three cases, two times at the cement-tooth transition (Smart Cem 2, Aqua Cem), and once at the cementcrown transition (Variolink II). However, even in these cases share of perfect margins after TCML was higher than 95% in the adhesive and self-adhesive groups, and higher than 90% in the glass-ionomer group. This may indicate only a minor influence of thermal and mechanical ageing processes on the marginal adaptation of ZLS molar crowns, irrespective of the type of cementation. Nevertheless, longer simulation times might have enabled a further differentiation between cementation types. To supplement the superficial SEM evaluation of marginal quality with internal penetration results, microleakage tests with dye solution are suggested in further investigations.
5.
Conclusion
All groups showed high resistance to ageing, high fracture forces, and good to sufficient marginal adaptation. In this regard, ZLS crowns may be at least comparable to clinically proven LDS ceramics, and no restrictions should be expected for clinical application. Both glass-ionomer cements, resin and resin-modified self-adhesive luting materials seem to be suitable for cementation of molar ZLS crowns.
Acknowledgements We would like to thank the manufacturers for providing the materials.
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
Influence of cementation on in vitro performance, marginal adaptation and fracture resistance of CAD/CAM-fabricated ZLS molar crowns Verena Preis ∗ , Michael Behr, Sebastian Hahnel, Martin Rosentritt Department of Prosthetic Dentistry, Regensburg University Medical Center, Franz-Josef-Strauss-Allee 11, 93042 Regensburg, Germany
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. This study investigated the influence of conventional cementation, self-adhesive
Received 14 June 2015
cementation, and adhesive bonding on the in vitro performance, fracture resistance, and
Accepted 17 August 2015
marginal adaptation of zirconia-reinforced lithium silicate (ZLS) crowns.
Available online xxx
Methods. Human molar teeth (n = 40) were prepared and full-contour crowns of a ZLS ceramic (Celtra Duo, DeguDent, G, n = 32) and a lithium disilicate ceramic (LDS; IPS e.max CAD, Ivoclar-
Keywords:
Vivadent, FL, n = 8) were fabricated and glazed. Four groups of ZLS crowns were defined
Cementation
(n = 8/group) and cemented with different glass-ionomer cements, resin, and resin-modified
Crown
self-adhesive luting materials. The LDS crowns served as reference group with adhesive
Ceramics
bonding.
Zirconia-reinforced lithium silicate Marginal adaptation Fracture resistance
A combined thermal cycling and mechanical loading (TCML: 3000 × 5 ◦ C/3000 × 55 ◦ C; 1.2 × 106 cycles à 50 N) with human antagonists was performed in a chewing simulator. Fracture force of surviving crowns was determined. Marginal adaptation at the cement/tooth and cement/crown interface was investigated by scanning electron microscopy before and after TCML, and the share of perfect margins was determined. Data were statistically analyzed (one-way ANOVA; post hoc Bonferroni, ˛ = 0.05). Results. One crown of the adhesive group failed during TCML (879,000 cycles = 3.7 years). No statistically significant (p = 0.078) differences in fracture resistance were found between different cementations, although highest data in tendency were found for adhesive bonding. Shares of perfect margins at the cement/tooth (93.8 ± 5.6–99.6 ± 0.8%) and cement/crown (84.7 ± 6.6–100.0 ± 0.0%) interfaces did not differ significantly (p > 0.05) between the different cementation groups. Significance. Marginal adaptation and fracture forces of all tested groups are in a range, where no restrictions should be expected for clinical application. © 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗
Corresponding author. Tel.: +49 941 944 6073; fax: +49 941 944 6171. E-mail address: [email protected] (V. Preis).
http://dx.doi.org/10.1016/j.dental.2015.08.154 0109-5641/© 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Preis V, et al. Influence of cementation on in vitro performance, marginal adaptation and fracture resistance of CAD/CAM-fabricated ZLS molar crowns. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.08.154
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1.
Introduction
Natural esthetic appearance and longevity are major aims in restoring teeth with single crowns and fixed partial dentures. With increasing popularity of computer-aided design/computer-aided manufacturing (CAD/CAM), a rising number of machinable esthetic materials have been introduced. Currently, the most popular CAD/CAM ceramics for single crowns are lithium disilicate and zirconia. Both materials may be applied either for monolithic restorations or as substructure with subsequent veneering with porcelain for esthetic improvement. Recently, new classes of polymer-infiltrated ceramic-network materials (e.g. Enamic, Vita), resin-nanoceramics (e.g. Lava Ultimate, 3M Espe), and zirconia-reinforced lithium silicate (ZLS) ceramics (e.g. Celtra Duo, DeguDent) have been developed as alternative materials for CAD/CAM single-tooth restorations. While high translucency and high strength at the same time are mutually exclusive for most types of monolithic ceramics, zirconiareinforced lithium silicate may offer improved esthetics due to increased glass content and a microcrystalline structure, and flexural strength values comparable to lithium disilicate. While common glass ceramics exhibit lower strength values and require adhesive luting to increase the mechanical strength of the restoration [1], zirconia and high-strength lithium disilicate ceramics offer the possibility of conventional cementation, e.g. with glass-ionomer cement. Furthermore, self-adhesive resin cements might be a time-saving and less technique sensitive alternative to multi-step adhesive systems. In vitro studies indicated no significant influence of cementation on fracture load of lithium disilicate crowns [2–4]. Accordingly, clinical data have shown that lithium disilicate crowns were highly successful irrespective of an adhesive or conventional cementation [5]. Cumulative survival rates in clinical studies were reported to be about 97% after 5 years [5,6], and about 87% up to 9 years [7]. However, evidence for medium- or even long-term survival of lithium disilicate crowns is limited and neither clinical nor in vitro data are available about the performance of new ZLS ceramics. Besides esthetics and the resistance to fatigue and fracture, the marginal adaptation contributes to the success of dental crowns. Insufficient marginal adaptation already after incorporation of the prosthetic restoration, or deterioration of the marginal quality during clinical service time might lead to the accumulation of bacterial plaque [8] and can cause gingival inflammation, caries, pulp and periodontal lesions, finally resulting in failure of the restoration [5,9]. Besides manifold influencing factors like the finishing line configuration, the margin location in enamel or dentin, the fabrication process of the crown and the value of the predefined cementing space, the type of cementation is supposed to influence marginal adaptation and quality [10–16]. Prior to routine clinical application, in vitro tests may help to evaluate new materials and restorations by combining reproducible laboratory conditions with basic requirements (occlusal loading, thermocycling) of the clinical situation. In vitro thermal cycling and mechanical loading is supposed to allow a first prediction of the mechanical performance of a
new material. A long-term testing might stimulate fatigue failures and marginal degradation. But even in cases without any catastrophic failures, aging and deterioration effects might occur, thus reducing strength, fracture resistance or marginal adaptation. The hypothesis of this in vitro study was that the type of cementation influences (i) the in vitro performance, (ii) the fracture resistance, and (iii) the marginal adaptation of single molar ZLS crowns during a simulated five-year oral service.
2.
Materials and methods
Human molars (tooth 46, n = 40) were prepared according to ceramic guidelines. A circular and occlusal anatomical reduction of 1.5 mm was carried out with a preparation angle of 4◦ . The finishing line resulted in a 1 mm deep circular shoulder with rounded inner angles at an isogingival height of the tooth cervix. The resilience of the human periodontium was simulated by coating the roots of the teeth with a 1 mm polyether layer (Impregum, 3 M Espe, G). For achieving a constant layer, the roots were dipped in a wax bath, which was replaced by polyether in a second fabrication process, as previously described [17,18]. Then the teeth were positioned in resin blocks (Palapress Vario, Heraeus-Kulzer, G). The preparations of the teeth were digitalized and 32 full-contour crowns of a zirconia-reinforced lithium silicate ceramic (ZLS; Celtra Duo, DeguDent, G) as well as 8 full-contour crowns of a lithium disilicate ceramic (LDS; IPS e.max CAD, Ivoclar-Vivadent, FL) were milled (Cerec MC XL, Sirona, G), crystallized (only LDS), and glazed with the respective glazing materials according to the manufacturer’s instructions. The circular and occlusal wall thickness was 1.5 mm and the cervical wall thickness was 1 mm. ZLS crowns were randomly divided into four groups (n = 8/group) for cementation with different glass-ionomer cements (GIC), resin, and resinmodified self-adhesive luting materials (Table 1). The LDS crowns served as reference group with adhesive bonding. The inner faces of all crowns were etched with 5% hydrofluoric acid for 20 s (LDS) or 30 s (ZLS) before cementation. For the adhesive luting protocol the crowns were silanized (silane coupling agent Monobond S, 60 s, Ivoclar-Vivadent). The prepared teeth were treated with the bonding system Syntac classic (Syntac Primer/Syntac Adhesive/Heliobond; Ivoclar-Vivadent) according to the instructions of the manufacturer. A combined thermal cycling and mechanical loading (TCML: 3000 × 5 ◦ C/3000 × 55 ◦ C, 2 min each cycle, H2 O dist.; 1.2 × 106 cycles à 50 N, 1.6 Hz) with human molar antagonists was performed in a chewing simulator (EGO, Regensburg, G). The crowns were loaded in three-point-contact. Parameters are based on literature data on zirconia and ceramic restorations expressing that chewing simulation using this parameters might simulate a maximum of five years of oral service [19,20]. During TCML all crowns were controlled for failures. Failed crowns were excluded from further testing and
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Table 1 – Fracture force, Weibull shape parameter b and marginal quality (share of perfect margins) of the different groups of cementation (identical letters indicate no significant differences between groups; identical numbers indicate no significant differences before-after TCML for each material). Cement
Type
Fracture force Mean ± SD [N] Weibull b
Marginal quality [%] before TCML after TCML Cement-tooth
Syntac classic/Variolink II (Ivoclar-Vivadent, FL) Smart Cem 2 (Dentsply, G)
Adhesive
Aqua Cem (Dentsply, G)
GIC
Ketac Cem (3M Espe, USA)
GIC
IPS e.max CAD; Syntac classic/Variolink II (Ivoclar-Vivadent, FL)
Adhesive (reference)
Self-adhesive
investigated in detail (light-microscope) for failure analysis. After TCML, fracture force of surviving crowns was determined by mechanically loading the crowns to failure in a universal testing machine (Zwick 1446, G). The force was applied on the centre of the crowns using a steel ball (Ø = 12 mm, v = 1 mm/min) with a 1 mm foil (Dentaurum, Ispringen, G) inserted between crown and ball. The failure determination was set to a 10% loss of the maximum loading force or acoustic signal (crack). Marginal adaptation was determined on resin replica (Rencast CW 2215/HY 5162, Huntsman, Switzerland) that were fabricated by making impressions (Impregum, 3 M Espe) before and after TCML. Both interfaces at cement/tooth and cement/crown were investigated with scanning electron microscopy (working distance: 20.4 mm; voltage: 5–10 keV; low vacuum; magnification: 200×; Quanta FEG 400, FEI Company, Hillsboro, USA). SEM pictures of the margins were evaluated and share of perfect margin was determined. Marginal quality was defined using the following criteria: (i) “intact margin” with smooth transition and no interruption of continuity, and (ii) “marginal gap” showing separation of the components due to cohesive or adhesive failure. Power calculation (G*Power 3.1.3, Kiel, G) provided an estimated power of >90% using eight specimens per group. Distribution of the data was controlled with Kolmogorov–Smirnov test. Calculations and statistical analysis were carried out using SPSS 22 (IBM, USA). Mean values and standard deviations (SD) were calculated and analyzed by means of one-way analysis of variance (ANOVA) and the Bonferroni multiple comparison test for post hoc analysis. The level of significance was set to ˛ = 0.05. Weibull analysis was done and shape parameter b was calculated (Visual-XSel 12.1, München, G).
3.
2612 ± 853 2.88 1903 ± 438x 4.43 1848 ± 836x 2.53 1891 ± 593x 3.29 2528 ± 668x 4.07 x
99.6 ± 0.8 99.5 ± 1.4c1 99.0 ± 1.7a 96.3 ± 3.4c 97.5 ± 3.4a 93.8 ± 5.6c 95.2 ± 6.8a2 94.2 ± 5.7c2 96.6 ± 6.0a3 97.2 ± 3.9c3 a1
Cement-crown 99.8 ± 0.3b 97.5 ± 3.0d 97.2 ± 4.3b4 95.2 ± 6.8d4 92.8 ± 9.0b5 96.1 ± 4.0 d5 85.7 ± 10.7b6 84.7 ± 6.6d6 100.0 ± 0.0b7 99.5 ± 1.6d7
significant (p = 0.078) differences in fracture resistance were found between different cementations, although highest values in tendency were found for adhesive bonding of ZLS crowns and the reference LDS crowns (Table 1). Weibull shape parameter b showed values between 2.53 and 4.07, indicating similar reliability for the different groups. Predominant failure modes were fractures of the crowns with simultaneous loosening of the crown fragments (Fig. 2). In case of adhesive bonding the crown fragments partly remained attached to the teeth. Evaluation of the marginal quality at the interface cementtooth showed a share of intact margins between 93.8 ± 5.6% and 99.6 ± 0.8%. No significant (p = 1.000) differences between the different cementation groups were found, neither before nor after TCML. However, for cementation with Smart Cem 2 (p = 0.010) and with Aqua Cem (p = 0.040) marginal quality at the interface cement-tooth decreased significantly by TCML. At the interface cement-crown the share of intact margins varied between 84.7 ± 6.6% and 100.0 ± 0.0% without significant (p > 0.05) differences between the different groups,
Results
One ZLS crown of the adhesive cementation group failed during TCML at 879,000 cycles, corresponding to approximately 3.7 years of oral service time. Detailed failure evaluation showed a fracture in the marginal region due to local overloading (Fig. 1). SEM pictures revealed that this failure was probably caused by fabrication damage. No statistically
Fig. 1 – SEM picture (magnification: 23×) of the failed crown during TCML, showing a fracture in the marginal region.
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Fig. 2 – Crown fractures after loading to failure.
neither before nor after TCML. Only marginal quality of adhesively luted ZLS crowns decreased significantly (p = 0.004) by TCML. Exemplary SEM pictures of the cervical margins are given in Fig. 3.
4.
Discussion
The three individual parts of the hypothesis that the type of cementation influences (i) the in vitro performance, (ii) fracture resistance, and (iii) marginal adaptation of single molar
ZLS crowns during a simulated five-year oral service were rejected. Irrespective of the cementation type ZLS crowns showed good long-term resistance to fatigue during ageing simulation. The failure of one crown during TCML might not be related to the type of cementation but be rather a random event associated with fabrication defects. The high fracture stability of the ZLS crowns after TCML was independent from cementation and comparable to the LDS reference. Perfect margins were found to be higher than 95% in most cases, indicating good marginal adaptation both
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Fig. 3 – Exemplary SEM pictures (magnification: 200×) of the cervical margin indicating the criteria for the marginal evaluation at the interface crown-cement: left: perfect margin with smooth transition and no interruption of continuity; right: marginal gap with separation of the components due to cohesive or adhesive failure.
before and after TCML. Marginal adaptation of ZLS crowns did neither differ significantly between the different cementation types nor to the control. The TCML parameters have been chosen congruent to numerous other in vitro studies [21–23]. They are supposed to simulate restoration stress according to a maximum of five years of intraoral use [19,20]. Standardization of testing conditions is wishful as evaluation of crown performance and underlying reasons for clinically observed failures is complicated by individual variables, such as tooth structure, periodontal mobility, occlusal loads, chewing behavior, the oral environment, as well as differences in the preparation design of the abutment teeth. Although human teeth in this study did not allow for complete standardization, they were preferred to artificial abutment teeth that may differ from human teeth in terms of modulus of elasticity and bonding capacity to the cement [24]. Especially for studies investigating adhesive bonding to dentine, the application of human teeth seems to be essential. The influence of resilient support of the abutment teeth on the ageing process and fracture strength of molar crowns should be considered by a polyether interface, which naturally does not stay abreast of the complex human periodontal ligament, but might avoid an overestimation of the strength of ceramic restorations [18]. Particularly with high-strength ceramics, as investigated in the present study, ageing and deterioration often occur without any visible catastrophic failures. In these cases, a subsequent static fracture test could help locate weak points and allow a differentiation between different types of cementation. In previous studies fracture loads of LDS crowns were significantly influenced by thermal and mechanical ageing and partly were further influenced by the type of luting agent [25–27]. Nevertheless, fracture data cannot be directly related to clinical survival but they may provide information on the principal suitability of new ceramics in relation to the requirements of clinically proven systems. As monolithic CAD/CAM-fabricated LDS ceramics have proven their suitability for single tooth restorations both
in vivo [6,28,29] and in vitro [3,4,30,31], they were chosen as reference in this study. The ZLS ceramic is a new class of ceramic with an inclusion of 10% zirconia dissolved into the glass matrix, resulting in four times smaller lithium silicate crystals. Thus, ZLS shows high flexural strength and high translucency at the same time (Celtra Duo, DeguDent, G). According to the information provided by the manufacturers, flexural strength values of IPS e.max CAD (360 ± 60 MPa) and Celtra Duo after glazing (370 MPa) are comparable. One recent in vitro study has affirmed a higher translucency of polished Celtra Duo compared to IPS e.max CAD [32]. However, neither clinical nor in vitro studies have dealt with the fatigue and fracture resistance of ZLS crowns so far. The present results showed that LDS and ZLS ceramics had comparable resistance against TCML and comparable high fracture values. The cement between crown and tooth transmits loads through the bonded interface. It was reported that the adhesive luting technique improved the fracture resistance of glass ceramic crowns with lower strength values (e.g. feldspathic and leucite-reinforced glass ceramics), while fracture loads of high-strength ceramics like zirconia, alumina or lithium disilicate were not significantly influenced by the mode of cementation (adhesive/conventional) [2,3,33]. In contrast, a study by Borges [26] reported about a significant increase in fracture load for different ceramic crowns (lithium disilicate, leucite-reinforced, and glass-infiltrated alumina ceramics) when they were cemented with a resin cement compared to a resin-modified glass-ionomer cement. In the present study, the type of cementation did not significantly influence fracture values of ZLS crowns and was comparable to the reference group. This is in accordance with findings by a previous in vitro study reporting high fracture values for CAD/CAM-fabricated LDS crowns independent of the mode of cementation [4]. Even though, we found a trend to highest fracture forces for adhesive luting, and lowest values for cementation with glass-ionomer cement. Nevertheless, as the fracture values exceeded maximum chewing forces, which are reported to be up to 900 N [34], all groups of
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differently cemented molar crowns have the potential to withstand occlusal forces applied in the posterior region. However, adhesive luting may be preferred in case of low axial tooth height combined with a conical, low-retentive preparation design to ensure sufficient strength. Failure modes were in all cases fractures of the crowns. While for adhesive groups the fragments partly remained attached to the prepared teeth, they predominately got loosened in case of conventional cementation. Complete crown fractures might be classified as catastrophic failure modes. In clinical studies further failure modes are described that may not necessarily result in replacement of the crown, e.g. ceramic chipping or biological complications (secondary caries, pulp lesion) [5,29]. This underlines that severe fracture testing may only provide limited insight into clinically relevant mechanisms of damage and underlying reasons for failure. Besides resistance to fatigue and fracture, the marginal fit and quality contributes to the clinical success of ceramic crowns. Poor marginal fit and the continuing decrease of marginal adaptation under clinical conditions may culminate in secondary caries or even loss of the restoration due to microleakage [5]. Different methods have been described to evaluate the marginal area of crowns, e.g. microscopic methods, micro-computed tomography (-CT) and dye penetration tests [11,13,35,36]. While the micro-computed tomography might be preferred to measure quantitatively the marginal fit of crowns [11,35], SEM evaluation is an established and reproducible procedure for determining the marginal adaptation [10,13,36]. It is based on comparable conditions and semi-quantitative analysis [36]. Of course potential inaccuracies of taking impressions and fabricating replicas as well as the subjective evaluation of one examiner limit the value of such procedures. Interactions at the cement-tooth as well as at the cementcrown transition may both have an influence marginal quality. Therefore, both transitions were evaluated separately. Bond strength of different LDS and ZLS ceramics to resin adhesives was reported to depend on the surface treatment and on the chemical composition of the glass ceramic [37]. Potential influencing factors for marginal adaptation at the cementcrown transition are the etching process with hydrofluoric acid to create a micro-retentive and activated surface, and the application of silane to establish the chemical bond between ceramic and resin cements. For adhesive luting at the cement-tooth transition, typical dentin pre-treatment includes acid, primer, adhesive, and bonding agent. As the clinical application of adhesive materials is time-consuming and technically sensitive, self-adhesive resin cements that may enable comparable strength of full-coverage crowns are desirable to avoid the complex bonding procedures. In studies investigating zirconia and LDS restorations, self-adhesive cements were reported to provide comparable microleakage values and marginal adaptation than adhesive resin cements [13,38,39]. Accordingly, marginal quality of ZLS crowns did neither differ significantly between the different cementation types nor to the control. The mean share of perfect margins was in most cases higher than 95%, indicating good marginal adaptation both before and after TCML. In tendency lowest share of perfect margins was found for conventional cementation with glass-ionomer cements, partly showing marginal
quality lower than 95%. With a share of perfect margins of about 85% at the cement-crown transition, marginal quality of the Ketac Cem group may still be classified as sufficient. Significant degradation of marginal quality by TCML was only observed in three cases, two times at the cement-tooth transition (Smart Cem 2, Aqua Cem), and once at the cementcrown transition (Variolink II). However, even in these cases share of perfect margins after TCML was higher than 95% in the adhesive and self-adhesive groups, and higher than 90% in the glass-ionomer group. This may indicate only a minor influence of thermal and mechanical ageing processes on the marginal adaptation of ZLS molar crowns, irrespective of the type of cementation. Nevertheless, longer simulation times might have enabled a further differentiation between cementation types. To supplement the superficial SEM evaluation of marginal quality with internal penetration results, microleakage tests with dye solution are suggested in further investigations.
5.
Conclusion
All groups showed high resistance to ageing, high fracture forces, and good to sufficient marginal adaptation. In this regard, ZLS crowns may be at least comparable to clinically proven LDS ceramics, and no restrictions should be expected for clinical application. Both glass-ionomer cements, resin and resin-modified self-adhesive luting materials seem to be suitable for cementation of molar ZLS crowns.
Acknowledgements We would like to thank the manufacturers for providing the materials.
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Please cite this article in press as: Preis V, et al. Influence of cementation on in vitro performance, marginal adaptation and fracture resistance of CAD/CAM-fabricated ZLS molar crowns. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.08.154