Clin Oral Invest DOI 10.1007/s00784-014-1263-9
ORIGINAL ARTICLE
Damage of lithium-disilicate all-ceramic restorations by an experimental self-adhesive resin cement used as core build-ups G. Sterzenbach & G. Karajouli & R. Tunjan & T. Spintig & K. Bitter & M. Naumann
Received: 11 July 2013 / Accepted: 16 May 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract Objectives This in vitro study aimed to predict the potential of fracture initiation after long-term incubation (LTI) of lithiumdisilicate restorations due to a hygroscopic expansion of selfadhesive resin cement (SARC) used as core build-up material. Methods Human maxillary central incisors were divided into four groups (n=10). Teeth were endodontically treated and decoronated. Specimens were restored in a one-stage postand-core procedure using experimental dual-curing SARC. Three application protocols to build up the core were compared as follows: I, auto-polymerisation; II, dual curing including 40 s light-initiated polymerisation; and III, an open matrix technique in a dual-curing mode. In group IV, a chemical-curing composite core build-up material served as control. For all specimens, a 2-mm ferrule design was ensured. Full anatomic lithium-disilicate crowns were adhesively luted. One-year LTI in 0.5 % chloramine solution at 37 °C was performed. Restorations were examined after 3, 6, 9 and 12 month of storage. Survival rates were calculated using log-rank statistics (p=0.05). Results Fifty per cent of lithium-disilicate crowns of groups I and II showed visible crack propagation after 9 months of G. Sterzenbach (*) : G. Karajouli : R. Tunjan : T. Spintig Department of Prosthodontics, Geriatric Dentistry and Craniomandibular Disorders, CharitéCentrum 3, Charité - Universitätsmedizin Berlin, Aßmannshauser Str. 4-6, 14197 Berlin, Germany e-mail: [email protected] K. Bitter Department of Operative Dentistry and Periodontology, CharitéCentrum 3, Charité - Universitätsmedizin Berlin, Aßmannshauser Str. 4-6, 14197 Berlin, Germany M. Naumann Department of Prosthetic Dentistry, Center of Dentistry, University of Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany
incubation, while one crown failed in group III. No failure was observed in group IV. The survival rates differed significantly (p=0.017). Conclusion SARC used to build up the core of severely damaged endodontically treated teeth does have the potential to cause fracture of lithium-disilicate crown restorations. Clinical relevance Hygroscopic expansion of self-adhesive resin cements used as a core build-up material might have an adverse impact on longevity of glass-ceramic crowns. Keywords Hygroscopic expansion . Self-adhesive resin cements . Core material . Lithium disilicate . Full crown
Introduction Within the last few years, significant changes were made in the field of post-endodontic restoration. Chair side protocols using fibre posts and direct core build-ups show promising long-term survival rates [1, 2]. Current developments tend to adhesively restore severely damaged endodontically treated teeth in a one-stage post-and-core procedure [3], whereas core build-up will immediately follow post cementation using the same composite resin material in order to introduce a so-called secondary mono-block [4]. Such a procedure could reduce the technique sensitivity, risk of reduced bond between luting cement and core material and the time necessary to complete post-and-core treatment procedure. In vitro increased push-out bond strengths for self-adhesive resin cements (SARCs) compared to different adhesive approaches or rather adhesive luting materials were shown [5–7]. Furthermore, these luting resins show less fatigue behaviour regarding the bond strength to root canal dentin after thermo-mechanical loading [8]. Recently, promising long-term data in vivo were published [1]. In micro-tensile bond strength tests, SARCs bond to coronal dentin as good as one- or two-step adhesives [9, 10].
Clin Oral Invest
It would be beneficial if SARCs could also serve as a core build-up material used in a one-stage post-and-core procedure. Core build-up composite resins should have proper mechanical properties to ensure the stabilization of the remaining tooth structure and should provide adequate crown retention. Beside adhesive performance, micro-mechanical properties such as modulus of elasticity, Vickers hardness, creep and elastic-plastic deformation show for some self-adhesive resin cements comparable results to conventional composites [11]. Compared to conventional core build-up composites, selfadhesive resin cements are more hydrophilic since acidic monomers are incorporated. However, water sorption by these polymers reduces the modulus of elasticity and lowers mechanical properties [12]. Of particular interest for core buildup materials is the change of dimension due to hygroscopic expansion. Over-compensated polymerisation shrinkage by hygroscopic expansion in materials that readily take up water was observed up to 24 weeks, causing tooth expansion [13]. Hygroscopic expansion of luting resin cements was held responsible for fracture initiation within glass infiltrated alumina core restorations [14]. Continued expansion of large volume restorations such as core build-ups for severely damaged endodontic treated teeth may create a critical internal strain leading to crack formation within the final glass-ceramic crown restoration [15]. Hence, this in vitro study was conducted to evaluate the potential of crack formation and subsequent fracture of lithium-disilicate glass-ceramic single crown restorations placed to restore endodontically treated maxillary central incisors with no remaining cavity wall using experimental selfadhesive resin cement as a luting material for endodontic post cementation and a core build-up material applied in a onestage post-and-core procedure. The null hypothesis presumes that there is no difference between the experimental self-adhesive resin cement and conventional resin core build-up material regarding the potential to cause crack formation within lithium-disilicate all-ceramic single crowns after 1-year of long-term incubation in 0.5 % chloramine solution.
equally allocated to four groups (n=10). The teeth were decoronated 2 mm above the mesio-approximal point of the CEJ. Root canals were enlarged using the X-Smart (Dentsply Maillefer, Ballaigues, Switzerland) and nickel-titanium files to size F2 (ProTaper Universal Rotary; Dentsply Maillefer) and were intermittently rinsed with 1 % sodium hypochlorite. Root canal filling was performed using gutta-percha (06 tapered; Dentsply Maillefer) and sealer (AH Plus Jet; Dentsply DeTrey, Konstanz, Germany). The crowns were cut 2 mm coronal to the most incisal point of the proximal CEJ. Post-and-core restoration The post cavity within the root canal was prepared with a tapered drill (RelyX Fiber Post Drill Size 2, 3M Espe, Seefeld, Germany) to achieve a post length of 12 mm, leaving at least 4 mm of the root filling in the apical portion. Prior to post placement, the post space was irrigated using 3 ml of 1 % sodium hypochlorite followed by distilled water. Glass-fibrereinforced composite posts (RelyX Post, Size 2 RF; 3M ESPE, LOT: 107200904) were adhesively luted using the experimental SARC (3M ESPE, Lot: AM 595; Table 1) aided by elongation tips (3M ESPE). After initial light curing for 2 s, access material was removed, and final light curing was performed for 40 s (1,200 mW/cm2, Elipar Freeligth 2, 3M ESPE). The core build-up was realized in groups I, II and III immediately following fibre post luting using the same SARC. Group I SARC sc (self-curing, auto-polymerisation) Opaque celluloid crowns (coloured strip crowns, size 113, Frasaco GmbH, Tettnang, Germany) were used as moulds to form the core build-up. No additional dentin surface pretreatment was conducted. The strip crowns were filled with SARC (Lot: AM 595) and manually fixed onto the tooth to allow chemical polymerisation for 6 min at room temperature. Afterwards, the specimens were stored within an incubator in distilled water at 37 °C for at least 10 min to allow for complete setting.
Material and methods Group II SARC dc (dual curing) Specimen pretreatment and distribution Human maxillary incisors were selected and stored at 7 °C in a 0.5 % chloramine solution. To ensure an even distribution of specimen dimension among the experimental groups, mesiodistal (MD) and buccal-lingual (BL) dimensions were measured (accuracy 2 μm) at the level of the cemento-enamel junction (CEJ). A size assessment was calculated from the product of MD × BL (Table 1). Extremely small or large teeth were excluded. According to these data, specimens were
Transparent strip crowns (transparent strip crown, size 113, Frasaco GmbH, Tettnang, Germany) were used as moulds for core build-up procedure. SARC (Lot: AM 595) core was light cured for 40 s from the buccal and palatal aspect, respectively. Group III SARC om (open matrix, dual curing) An open matrix system (Tofflemire Matrize, Kerr GmbH, Raststatt, Germany) was used to build up the core without
Clin Oral Invest Table 1 Composition of test materials, cross-sectional areas of specimens and number of failures during long-term incubation Group
Number Composition
Adhesive technique
Curing mode Cross-sectional area of teeth (mean/SD) At CEJ
SARCa sc SARC dc SARC om
10 10 10
ClearfilCoreb 10
Methacrylated phosphoric acid ester, Self-adhesive dimethacrylates (i.e. TEGDMA), photo-initiator, glass powder, silane-treated silica, calcium hydroxide filler content (60–70 wt%) Bis-GMA, TEGDMA, silica, barium Etch and rinse glass filler, filler content (78 wt%)
Failure during LTI (n)
At dentin-core interface
Self-curing Dual curing Dual curing
45.2/5.3 21.6/2.6 44.8/4.1 23.6/2.6 45.0/4.,5 22.1/3.5
5 5 1
Self-curing
44.7/4.5
0
20.5/2.7
SARC self-adhesive resin cement, TEGDMA triethyleneglycol dimethacrylate, Bis-GMA bisphenol-A-dimethacrylate a
Experimental
b
Adhesive, Clearfil New Bond; TEGDMA and Bis-GMA
any pressure application with SARC (Lot: AM 595). Light curing was performed for 40 s touching the occlusal plane with the LED curing unit. Group IV ClearfilCore The coronal dentin was conditioned according to manufacturer’s instructions using the etch-and-rinse approach and the adhesive Clearfil New Bond adhesive (Kuraray, Okayama, Japan; Lot Catalyst 00968B, Universal 00078B). Similar to group I, the core was built up using strip crowns filled with ClearfilCore (self-curing core build-up composite resin; Kuraray; Lot: 41451). Crown restoration All teeth were prepared using Guide Pin Diamonds (tapered chamfer, round, size 021, grit size coarse 151 μm and fine 46 μm, egg size 023, grit size fine 46 μm; Komet Dental, Gebr. Brasseler GmbH & Co. KG, Lemgo, Germany) with a chamfer design (circumferential depth of 1 mm) to meet all-ceramic crown requirements. The composite core was standardized in height at 3 mm with a convergence angle of 20°. The finishing line was prepared in dentin, 2 mm apical to the core build-up to ensure an appropriate ferrule design. To compare the core build-up dimension between the groups, the specimen dies were scanned (D 800, 3Shape A/S, Copenhagen, Denmark), and the cross-sectional area of the core build-up 2 mm above the finishing line (Fig. 1) was calculated using the software Dental Designer Premium 2013 (3Shape). Temporary crown restorations (Luxatemp, DMG, Hamburg, Germany) were prepared with the aid of Frasaco strip crowns (transparent strip crown, size 113, Frasaco
GmbH, Tettnang, Germany) and cement (TempBond NE, KerrHawe SA, Bioggio, Switzerland). Throughout the fabrication term of all-ceramic crown restorations, the specimens were stored in an incubator. The lithium-disilicate crowns (IPS e.max Press LT A3, Lot: M19072; Ivoclar Vivadent, Schaan, Liechtenstein) were fabricated as full anatomical restorations with similar ceramic wall thickness (1.3–1.5 mm at buccal/ palatal aspect; 1 mm at crown margin; 2 mm at incisal edge). The crowns were manufactured strictly by adhering to the PRESS technique and sintering procedure recommended by the manufacturer. After fitting the restorations on the dies, a final glaze firing was conducted using IPS e.max Ceram Glaze Paste (Ivoclar Vivadent). No additional stain or characterization firing was applied. Twenty-one days after tooth preparation and impression taking (Provil novo putty and medium, Heraeus Kulzer, Hanau, Germany), the restorations were adhesively luted with SARC (Lot: AM 595). Prior to luting procedure, the crowns were acid etched with 5 % hydrofluoric acid for 60 s. Monobond Plus (Ivoclar Vivadent) was applied and left for 1 min. After final positioning with slight finger pressure, excess material was removed, Airblock (Dentsply DeTrey, Konstanz, Germany) was applied, and light curing was performed for 45 s from each specimen aspect. Long-term incubation The restored specimens were stored for 1 year in 0.5 % chloramine solution at 37°. According to Sindel et al. [15], the all-ceramic crowns were examined for cracks after 3, 6, 9 and 12 months. After air-drying, the specimens were examined using loupe glasses with 2.5-fold magnification. To accentuate possible cracks, diaphanoscopy was performed with the aid of LED curing unit. After LTI, the crowns were sectioned (IsoMet Low Speed Saw; Buehler, Düsseldorf, Germany) into two 1.5-mm-thick
Clin Oral Invest Fig. 1 a Three-dimensional CAD model of prepared specimen shows cemento-enamel junction (CEJ), finishing line of crown preparation (FL) and the calculated cross-sectional area at dentin-core interface (DCI). b Three-dimensional CAD model with mesio-approximal view of the specimen
serial slices for qualitative failure analysis. According to the long tooth axis, the roots were embedded in acrylic resin (Technovit 4004, Kulzer, Wehrheim, Germany). The sectioning was performed perpendicular to the tooth axis and coronal to the cross-sectional area at the dentin-core interface (DCI) (Fig. 1a, b) to ensure that the slice originates from the portion of the crown containing the core build-up. The slices were examined using a stereomicroscope (Stemi SV11; Carl Zeiss Jena, Jena, Germany) at ×50 magnification. Statistical analysis The cross-sectional areas at the level of CEJ and at DCI of the experimental groups were compared by one-way ANOVA. Kaplan-Meier survival plots were constructed. The numbers of mechanical cycles until failure of the experimental groups were compared using log-rank test
(SPSS Statistics 19; SPSS, Inc., Chicago, IL). The level of significance was set at p
ORIGINAL ARTICLE
Damage of lithium-disilicate all-ceramic restorations by an experimental self-adhesive resin cement used as core build-ups G. Sterzenbach & G. Karajouli & R. Tunjan & T. Spintig & K. Bitter & M. Naumann
Received: 11 July 2013 / Accepted: 16 May 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract Objectives This in vitro study aimed to predict the potential of fracture initiation after long-term incubation (LTI) of lithiumdisilicate restorations due to a hygroscopic expansion of selfadhesive resin cement (SARC) used as core build-up material. Methods Human maxillary central incisors were divided into four groups (n=10). Teeth were endodontically treated and decoronated. Specimens were restored in a one-stage postand-core procedure using experimental dual-curing SARC. Three application protocols to build up the core were compared as follows: I, auto-polymerisation; II, dual curing including 40 s light-initiated polymerisation; and III, an open matrix technique in a dual-curing mode. In group IV, a chemical-curing composite core build-up material served as control. For all specimens, a 2-mm ferrule design was ensured. Full anatomic lithium-disilicate crowns were adhesively luted. One-year LTI in 0.5 % chloramine solution at 37 °C was performed. Restorations were examined after 3, 6, 9 and 12 month of storage. Survival rates were calculated using log-rank statistics (p=0.05). Results Fifty per cent of lithium-disilicate crowns of groups I and II showed visible crack propagation after 9 months of G. Sterzenbach (*) : G. Karajouli : R. Tunjan : T. Spintig Department of Prosthodontics, Geriatric Dentistry and Craniomandibular Disorders, CharitéCentrum 3, Charité - Universitätsmedizin Berlin, Aßmannshauser Str. 4-6, 14197 Berlin, Germany e-mail: [email protected] K. Bitter Department of Operative Dentistry and Periodontology, CharitéCentrum 3, Charité - Universitätsmedizin Berlin, Aßmannshauser Str. 4-6, 14197 Berlin, Germany M. Naumann Department of Prosthetic Dentistry, Center of Dentistry, University of Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany
incubation, while one crown failed in group III. No failure was observed in group IV. The survival rates differed significantly (p=0.017). Conclusion SARC used to build up the core of severely damaged endodontically treated teeth does have the potential to cause fracture of lithium-disilicate crown restorations. Clinical relevance Hygroscopic expansion of self-adhesive resin cements used as a core build-up material might have an adverse impact on longevity of glass-ceramic crowns. Keywords Hygroscopic expansion . Self-adhesive resin cements . Core material . Lithium disilicate . Full crown
Introduction Within the last few years, significant changes were made in the field of post-endodontic restoration. Chair side protocols using fibre posts and direct core build-ups show promising long-term survival rates [1, 2]. Current developments tend to adhesively restore severely damaged endodontically treated teeth in a one-stage post-and-core procedure [3], whereas core build-up will immediately follow post cementation using the same composite resin material in order to introduce a so-called secondary mono-block [4]. Such a procedure could reduce the technique sensitivity, risk of reduced bond between luting cement and core material and the time necessary to complete post-and-core treatment procedure. In vitro increased push-out bond strengths for self-adhesive resin cements (SARCs) compared to different adhesive approaches or rather adhesive luting materials were shown [5–7]. Furthermore, these luting resins show less fatigue behaviour regarding the bond strength to root canal dentin after thermo-mechanical loading [8]. Recently, promising long-term data in vivo were published [1]. In micro-tensile bond strength tests, SARCs bond to coronal dentin as good as one- or two-step adhesives [9, 10].
Clin Oral Invest
It would be beneficial if SARCs could also serve as a core build-up material used in a one-stage post-and-core procedure. Core build-up composite resins should have proper mechanical properties to ensure the stabilization of the remaining tooth structure and should provide adequate crown retention. Beside adhesive performance, micro-mechanical properties such as modulus of elasticity, Vickers hardness, creep and elastic-plastic deformation show for some self-adhesive resin cements comparable results to conventional composites [11]. Compared to conventional core build-up composites, selfadhesive resin cements are more hydrophilic since acidic monomers are incorporated. However, water sorption by these polymers reduces the modulus of elasticity and lowers mechanical properties [12]. Of particular interest for core buildup materials is the change of dimension due to hygroscopic expansion. Over-compensated polymerisation shrinkage by hygroscopic expansion in materials that readily take up water was observed up to 24 weeks, causing tooth expansion [13]. Hygroscopic expansion of luting resin cements was held responsible for fracture initiation within glass infiltrated alumina core restorations [14]. Continued expansion of large volume restorations such as core build-ups for severely damaged endodontic treated teeth may create a critical internal strain leading to crack formation within the final glass-ceramic crown restoration [15]. Hence, this in vitro study was conducted to evaluate the potential of crack formation and subsequent fracture of lithium-disilicate glass-ceramic single crown restorations placed to restore endodontically treated maxillary central incisors with no remaining cavity wall using experimental selfadhesive resin cement as a luting material for endodontic post cementation and a core build-up material applied in a onestage post-and-core procedure. The null hypothesis presumes that there is no difference between the experimental self-adhesive resin cement and conventional resin core build-up material regarding the potential to cause crack formation within lithium-disilicate all-ceramic single crowns after 1-year of long-term incubation in 0.5 % chloramine solution.
equally allocated to four groups (n=10). The teeth were decoronated 2 mm above the mesio-approximal point of the CEJ. Root canals were enlarged using the X-Smart (Dentsply Maillefer, Ballaigues, Switzerland) and nickel-titanium files to size F2 (ProTaper Universal Rotary; Dentsply Maillefer) and were intermittently rinsed with 1 % sodium hypochlorite. Root canal filling was performed using gutta-percha (06 tapered; Dentsply Maillefer) and sealer (AH Plus Jet; Dentsply DeTrey, Konstanz, Germany). The crowns were cut 2 mm coronal to the most incisal point of the proximal CEJ. Post-and-core restoration The post cavity within the root canal was prepared with a tapered drill (RelyX Fiber Post Drill Size 2, 3M Espe, Seefeld, Germany) to achieve a post length of 12 mm, leaving at least 4 mm of the root filling in the apical portion. Prior to post placement, the post space was irrigated using 3 ml of 1 % sodium hypochlorite followed by distilled water. Glass-fibrereinforced composite posts (RelyX Post, Size 2 RF; 3M ESPE, LOT: 107200904) were adhesively luted using the experimental SARC (3M ESPE, Lot: AM 595; Table 1) aided by elongation tips (3M ESPE). After initial light curing for 2 s, access material was removed, and final light curing was performed for 40 s (1,200 mW/cm2, Elipar Freeligth 2, 3M ESPE). The core build-up was realized in groups I, II and III immediately following fibre post luting using the same SARC. Group I SARC sc (self-curing, auto-polymerisation) Opaque celluloid crowns (coloured strip crowns, size 113, Frasaco GmbH, Tettnang, Germany) were used as moulds to form the core build-up. No additional dentin surface pretreatment was conducted. The strip crowns were filled with SARC (Lot: AM 595) and manually fixed onto the tooth to allow chemical polymerisation for 6 min at room temperature. Afterwards, the specimens were stored within an incubator in distilled water at 37 °C for at least 10 min to allow for complete setting.
Material and methods Group II SARC dc (dual curing) Specimen pretreatment and distribution Human maxillary incisors were selected and stored at 7 °C in a 0.5 % chloramine solution. To ensure an even distribution of specimen dimension among the experimental groups, mesiodistal (MD) and buccal-lingual (BL) dimensions were measured (accuracy 2 μm) at the level of the cemento-enamel junction (CEJ). A size assessment was calculated from the product of MD × BL (Table 1). Extremely small or large teeth were excluded. According to these data, specimens were
Transparent strip crowns (transparent strip crown, size 113, Frasaco GmbH, Tettnang, Germany) were used as moulds for core build-up procedure. SARC (Lot: AM 595) core was light cured for 40 s from the buccal and palatal aspect, respectively. Group III SARC om (open matrix, dual curing) An open matrix system (Tofflemire Matrize, Kerr GmbH, Raststatt, Germany) was used to build up the core without
Clin Oral Invest Table 1 Composition of test materials, cross-sectional areas of specimens and number of failures during long-term incubation Group
Number Composition
Adhesive technique
Curing mode Cross-sectional area of teeth (mean/SD) At CEJ
SARCa sc SARC dc SARC om
10 10 10
ClearfilCoreb 10
Methacrylated phosphoric acid ester, Self-adhesive dimethacrylates (i.e. TEGDMA), photo-initiator, glass powder, silane-treated silica, calcium hydroxide filler content (60–70 wt%) Bis-GMA, TEGDMA, silica, barium Etch and rinse glass filler, filler content (78 wt%)
Failure during LTI (n)
At dentin-core interface
Self-curing Dual curing Dual curing
45.2/5.3 21.6/2.6 44.8/4.1 23.6/2.6 45.0/4.,5 22.1/3.5
5 5 1
Self-curing
44.7/4.5
0
20.5/2.7
SARC self-adhesive resin cement, TEGDMA triethyleneglycol dimethacrylate, Bis-GMA bisphenol-A-dimethacrylate a
Experimental
b
Adhesive, Clearfil New Bond; TEGDMA and Bis-GMA
any pressure application with SARC (Lot: AM 595). Light curing was performed for 40 s touching the occlusal plane with the LED curing unit. Group IV ClearfilCore The coronal dentin was conditioned according to manufacturer’s instructions using the etch-and-rinse approach and the adhesive Clearfil New Bond adhesive (Kuraray, Okayama, Japan; Lot Catalyst 00968B, Universal 00078B). Similar to group I, the core was built up using strip crowns filled with ClearfilCore (self-curing core build-up composite resin; Kuraray; Lot: 41451). Crown restoration All teeth were prepared using Guide Pin Diamonds (tapered chamfer, round, size 021, grit size coarse 151 μm and fine 46 μm, egg size 023, grit size fine 46 μm; Komet Dental, Gebr. Brasseler GmbH & Co. KG, Lemgo, Germany) with a chamfer design (circumferential depth of 1 mm) to meet all-ceramic crown requirements. The composite core was standardized in height at 3 mm with a convergence angle of 20°. The finishing line was prepared in dentin, 2 mm apical to the core build-up to ensure an appropriate ferrule design. To compare the core build-up dimension between the groups, the specimen dies were scanned (D 800, 3Shape A/S, Copenhagen, Denmark), and the cross-sectional area of the core build-up 2 mm above the finishing line (Fig. 1) was calculated using the software Dental Designer Premium 2013 (3Shape). Temporary crown restorations (Luxatemp, DMG, Hamburg, Germany) were prepared with the aid of Frasaco strip crowns (transparent strip crown, size 113, Frasaco
GmbH, Tettnang, Germany) and cement (TempBond NE, KerrHawe SA, Bioggio, Switzerland). Throughout the fabrication term of all-ceramic crown restorations, the specimens were stored in an incubator. The lithium-disilicate crowns (IPS e.max Press LT A3, Lot: M19072; Ivoclar Vivadent, Schaan, Liechtenstein) were fabricated as full anatomical restorations with similar ceramic wall thickness (1.3–1.5 mm at buccal/ palatal aspect; 1 mm at crown margin; 2 mm at incisal edge). The crowns were manufactured strictly by adhering to the PRESS technique and sintering procedure recommended by the manufacturer. After fitting the restorations on the dies, a final glaze firing was conducted using IPS e.max Ceram Glaze Paste (Ivoclar Vivadent). No additional stain or characterization firing was applied. Twenty-one days after tooth preparation and impression taking (Provil novo putty and medium, Heraeus Kulzer, Hanau, Germany), the restorations were adhesively luted with SARC (Lot: AM 595). Prior to luting procedure, the crowns were acid etched with 5 % hydrofluoric acid for 60 s. Monobond Plus (Ivoclar Vivadent) was applied and left for 1 min. After final positioning with slight finger pressure, excess material was removed, Airblock (Dentsply DeTrey, Konstanz, Germany) was applied, and light curing was performed for 45 s from each specimen aspect. Long-term incubation The restored specimens were stored for 1 year in 0.5 % chloramine solution at 37°. According to Sindel et al. [15], the all-ceramic crowns were examined for cracks after 3, 6, 9 and 12 months. After air-drying, the specimens were examined using loupe glasses with 2.5-fold magnification. To accentuate possible cracks, diaphanoscopy was performed with the aid of LED curing unit. After LTI, the crowns were sectioned (IsoMet Low Speed Saw; Buehler, Düsseldorf, Germany) into two 1.5-mm-thick
Clin Oral Invest Fig. 1 a Three-dimensional CAD model of prepared specimen shows cemento-enamel junction (CEJ), finishing line of crown preparation (FL) and the calculated cross-sectional area at dentin-core interface (DCI). b Three-dimensional CAD model with mesio-approximal view of the specimen
serial slices for qualitative failure analysis. According to the long tooth axis, the roots were embedded in acrylic resin (Technovit 4004, Kulzer, Wehrheim, Germany). The sectioning was performed perpendicular to the tooth axis and coronal to the cross-sectional area at the dentin-core interface (DCI) (Fig. 1a, b) to ensure that the slice originates from the portion of the crown containing the core build-up. The slices were examined using a stereomicroscope (Stemi SV11; Carl Zeiss Jena, Jena, Germany) at ×50 magnification. Statistical analysis The cross-sectional areas at the level of CEJ and at DCI of the experimental groups were compared by one-way ANOVA. Kaplan-Meier survival plots were constructed. The numbers of mechanical cycles until failure of the experimental groups were compared using log-rank test
(SPSS Statistics 19; SPSS, Inc., Chicago, IL). The level of significance was set at p
