Abstract
Objectives
This study evaluated the initial and the artificially aged push-out bond strength between ceramic and dentin produced by one of five resin cements.
Methods
Two-hundred direct ceramic restorations (IPS Empress CAD) were luted to standardized Class I cavities in extracted human molars using one of four self-adhesive cements (SpeedCEM, RelyX Unicem Aplicap, SmartCem2 and iCEM) or a reference etch-and-rinse resin cement (Syntac/Variolink II) ( n = 40/cement). Push-out bond strength (PBS) was measured (1) after 24 h water storage (non-aged group; n = 20/cement) or (2) after artificial ageing with 5000 thermal cycles followed by 6 months humid storage (aged group; n = 20/cement). Nonparametrical ANOVA and pairwise Wilcoxon rank-sum tests with Bonferroni–Holm adjustment were applied for statistical analysis. The significance level was set at α = 0.05. In addition, failure mode and fracture pattern were analyzed by stereomicroscope and scanning electron microscopy.
Results
Whereas no statistically significant effect of storage condition was found ( p = 0.441), there was a significant effect of resin cement ( p < 0.0001): RelyX Unicem showed significantly higher PBS than the other cements. Syntac/Variolink II showed significantly higher PBS than SmartCEM2 ( p < 0.001). No significant differences were found between SpeedCEM, SmartCem2, and iCEM.
The predominant failure mode was adhesive failure of cements at the dentin interface except for RelyX Unicem which in most cases showed cohesive failure in ceramic.
Significance
The resin cements showed marked differences in push-out bond strength when used for luting ceramic restorations to dentin. Variolink II with the etch-and-rinse adhesive Syntac did not perform better than three of the four self-adhesive resin cements tested.
1
Introduction
Ceramic restorations such as inlays and onlays allow a wide range of tooth reconstructions with good clinical success . Furthermore, computer-aided design/manufacturing (CAD/CAM) systems such as CEREC simplify fabrication of ceramic reconstructions and are an effective and reliable restorative option . Even restorations on teeth with highly reduced macroretention geometry and only little enamel left have shown acceptable clinical outcome .
However, challenges remain when working with ceramics. Apart from failures due to insufficient ceramic layer thickness, other main reasons for extensive or total failure of ceramic restorations are luting defects or wear of the resin cement between the ceramic restoration and the tooth substance .
The resin cements differ according to the pretreatment of dental tissues prior to cementation and can mainly be divided into three subgroups: (1) conventional “etch-and-rinse” resin cements (cements used after application of an etch-and-rinse adhesive including separate acid etching), (2) “self-etch” resin cements (cements used after application of a self-etch adhesive), and (3) self-adhesive resin cements (“self-adhering” cements used without application of any adhesive system) . Etch-and-rinse resin cements are often time-consuming to use as well as sensitive to handling due to the numerous pretreatment steps involved. With self-adhesive cements, efforts have been made to simplify the luting process while aiming to maintain a reliable as well as durable bond to dental tissues, especially to dentin where obtaining a reliable bond is more challenging than to enamel because of the higher water content of dentin.
Many studies have determined the bond strength of different resin cements by use of microtensile or shear bond strength methods . However, extended sample manipulation such as repeated cutting could influence the bond and the small specimen size of these methods is prone to pretesting failure. There is no consensus so far as how to deal with pretesting failures and whether to include or to exclude these failures in statistical analysis .
In contrast, with the push-out bond strength method less specimen manipulation is needed prior to testing that could influence the bond or lead to pretesting failures. Besides, the push-out method allows a sample preparation that mimics clinical conditions. Despite these advantages, only limited literature exists describing the use of push-out forces for bond strength testing of ceramic restorations luted to dentin . Moreover, information about the performance of self-adhesive cements after storage and artificial ageing is rather sparse .
Therefore, the aim of this study was to compare the push-out bond strength between ceramic and dentin using five different resin cements when measured 24 h after the luting process or when measured after artificial ageing consisting of thermocycling followed by 6 months of humid storage. Failure modes were classified stereomicroscopically and a qualitative analysis of the fracture sites was done by scanning electron microscopy (SEM).
The following hypotheses were tested:
- (1)
Artificial ageing results in a decrease of the initial bond strength.
- (2)
The bond strengths of self-adhesive resin cements are significantly lower than that of the reference etch-and-rinse resin cement.
2
Materials and methods
2.1
Standardized ceramic restoration preparation
The crown of a caries-free, extracted human molar was ground flat (Struers LaboPol-21/Struers Silicon Carbide paper (SiC) grit #320, diameter 200 mm, Ballerup, Denmark) until the entire surface was in coronal dentin. An elliptic occlusal cavity (diameter: 4 mm × 5 mm, depth: 3 mm) was prepared with a 6° conical diamond bur (40 μm grit; Intensiv FG A10, Grancia, Switzerland). The cavity was inspected under a microscope at 15× magnification (Leica ZOOM 2000, Leica, Buffalo, NY, USA) to ensure the absence of residual fissures, cracks, or partial exposure of the dental pulp. The tooth was then coated with IPS Contrast Spray Chairside (Ivoclar Vivadent AG, Schaan, Liechtenstein) and an optical impression was taken with the camera of the Sirona CEREC 3 acquisition center (Sirona, Bensheim, Germany). The shape of the future restoration was adjusted on screen with the CEREC 3D Software (Version 3.10, Sirona).
With the CEREC 3 CAD/CAM milling unit (Sirona) two-hundred standardized ceramic restorations were milled from IPS Empress CAD blocks (HT A3, size I8, LOT-Nr: M19049, Ivoclar Vivadent AG). The restorations were inspected under a magnifying glass and the milling stub was removed with a carbide bur and an OptraFine F finishing rubber point (Ivoclar Vivadent AG). The restorations were then sonicated for 3 min in ethanol (Telsonic TUC-150, Bronschhofen, Switzerland) and air-dried. The tooth-sided surfaces of the restorations were etched for 60 s with IPS Ceramic Etching (<5% hydrofluoric acid, LOT-Nr: M13217, Ivoclar Vivadent AG), cleaned with water spray, sonicated again for 1 min in ethanol and air-dried. Monobond S silane coupling agent (LOT-Nr: J27895, Ivoclar Vivadent AG) was applied for 60 s and air-dried.
2.2
Standardized tooth cavity preparation
The ground human molar with the elliptic occlusal cavity was cleaned from IPS Contrast Spray Chairside and a socket including a mounting for CELAY Plus (Mikrona Technologie AG, Spreitenbach, Switzerland & Ismaning, Germany) was waxed-up. The waxed-up tooth was embedded in Biotan Vest MG material (Schütz Dental GmbH, Rosbach, Germany) and the cast was effused with Biotan Titan 1 (LOT-Nr: 2006000638, Schütz Dental GmbH) in a DOR-A-MATIC Argon-Vacuum/High-pressure apparatus (Labtech, Schütz Dental GmbH). The obtained titanium model of the tooth with the elliptic occlusal cavity was mounted in the CELAY Plus copy-grinding device (Mikrona Technologie AG).
A final amount of two-hundred extracted and sound human third molars was used in this study. The teeth were cleaned under tap water with a scaler and a hard toothbrush to remove calculus and debris, stored in 1% chloramine solution and kept at 5 °C until needed. On each day prior to the luting process of one group ( n = 20), 20 randomly selected teeth were ground flat as previously explained. With the CELAY Plus device, elliptic occlusal cavities were copy-drilled from the titanium tooth model into coronal dentin of the human molars with a 40 μm grit diamond bur (CELAY ZY-54S, Mikrona Technologie AG) under water-cooling. The teeth were then rinsed under tap water and briefly air-dried. The standardized cavities were inspected under a microscope at 15× magnification (Leica ZOOM 2000). Teeth showing residual fissures or cracks, parts of cavity walls in enamel, or exposure of the pulp were discarded and replaced. The group of molars with standardized cavities was then stored in an incubator (Incubat, Melag, Berlin, Germany) for at least 2 h at 37 °C and 100% humidity to assure intraoral temperature before the luting procedure.
2.3
Luting procedures and artificial ageing
The self-adhesive cements used were SpeedCEM, RelyX Unicem Aplicap, SmartCem2 and iCEM. The etch-and-rinse resin cement Variolink II with Syntac Adhesive served as reference. The resin cements were stored according to manufacturers’ instructions. Cements that needed to be stored in the refrigerator were taken out at least 4 h before use. Detailed information about the resin cements is listed in Table 1 .
| Variolink II Ivoclar Vivadent AG, Schaan, Liechtenstein | LOT-Nr: M03806 (Base)/L51132 (Catalyst, high viscosity) Paste/Paste |
|---|---|
| Type | Etch-and-rinse adhesive resin cement |
| Base (% weight) | Catalyst (% weight) | |
|---|---|---|
| Methacrylates | 26.3% | 22% |
| Filler | 73.4% | 77.2% |
| Filler particle size | 0.04–3 μm (mean 0.7 μm) | |
| Initiators, stabilizers and pigments | 0.4% | 0.9% |
| SpeedCEM Ivoclar Vivadent AG, Schaan, Liechtenstein | LOT-Nr: M32347 Paste/Paste (Automix) |
|---|---|
| Type | Universal dual-curing, self-adhesive resin cement |
| Base (% weight) | Catalyst (% weight) | |
|---|---|---|
| Methacrylates | 23.3% | 26% |
| Filler | 75% | 2.2% |
| Filler particle size | 0.1–7 μm (mean: 5 μm) | |
| Initiators, stabilizers and pigments | 1.7% | 0.9% |
| RelyX Unicem Aplicap 3M ESPE, St. Paul, MN, USA | LOT-Nr: 356306 Powder/Liquid (Capsule) |
|---|---|
| Type | Universal dual-curing, self-adhesive resin cement |
| Powder (% weight) | Liquid (% weight) | |
|---|---|---|
| Methacrylates | – | ∼25% |
| Filler | ∼70% | – |
| Filler particle size | 90% of filler <12.5 μm | |
| Initiators, stabilizers and pigments | total: 5% additives (powder & liquid) | |
| SmartCem2 DENTSPLY Caulk, Milford, DE, USA | LOT-Nr: 0904281 Paste/Paste (Automix) |
|---|---|
| Type | Universal dual-curing, self-adhesive resin cement |
| Base (% weight) | Catalyst (% weight) | |
|---|---|---|
| Methacrylates | n.a. | n.a. |
| Filler | Total: 69% | |
| Filler particle size | Glass (mean): 3.8 μm/Aerosil (mean): 16 nm | |
| Initiators, stabilizers and pigments | n.a. | n.a. |
| iCEM Heraeus Kulzer, Hanau, Germany | LOT-Nr: 305322 Paste/Paste (Automix) |
|---|---|
| Type | Universal dual-curing, self-adhesive resin cement |
| Base (% weight) | Catalyst (% weight) | |
|---|---|---|
| Methacrylates | n.a. | n.a. |
| Filler | Total: 49% | |
| Filler particle size | n.a. (sub-micron and micron sized particles) | |
| Initiators, stabilizers and pigments | n.a. | n.a. |
For a constant baseline temperature of the teeth before cementation and to mimic an intraoral condition, only one of the 20 prepared molars was taken out of the incubator at a time. Immediately after, the resin cement was applied in the tooth cavity and the pretreated ceramic restoration was inserted. A custom-made weight with a cone point was used to seat the restoration and ensured a constant force of 5 N. Excessive cement was removed with disposable soft brushes (Ivoclar Vivadent AG). The restoration was then light-cured with a LED light-curing unit (Bluephase Polywave, Ivoclar Vivadent AG) for 60 s in the “High”-power mode. Light power density was verified to be at least 1200 mW/cm 2 with a radiometer (Bluephase Meter, Ivoclar Vivadent AG) at the beginning and end of each luting procedure. The five luting procedures are described in Table 2 .
| Resin cement | Treatment steps (according to manufacturer instructions) | Time |
|---|---|---|
| Syntac/Variolink II ( n = 40) | Total etching gel (37% phosphoric acid, LOT-Nr.: M30960) | 15 s (dentin only) |
| Water spray | >10 s + air dry | |
| Syntac primer (LOT-Nr.: M34555) | 15 s + air dry | |
| Syntac adhesive (LOT-Nr.: M19675) | 10 s + air dry | |
| Heliobond (LOT-Nr.: M08772) | (no light-curing) | |
| Variolink II (base/catalyst 1:1) application | ||
| SpeedCEM ( n = 40) | Water spray | >10 s + air dry |
| SpeedCEM application | ||
| RelyX Unicem Aplicap ( n = 40) | Water spray | >10 s + air dry |
| Capsule activation | >2 s | |
| Capsule mixing (CapMix, 3M ESPE) | 15 s | |
| RelyX Unicem Aplicap application | ||
| SmartCem2 ( n = 40) | Water spray | >10 s + air dry |
| SmartCem2 application | ||
| iCEM ( n = 40) | Water spray | >10 s + air dry |
| iCEM application | ||
Twenty teeth per cement were produced for the group without artificial ageing and these were stored for 24 h in tap water at 37 °C before determination of the push-out bond strength (non-aged group). Another 20 teeth of every cement underwent artificial ageing (aged group) immediately after the luting procedure. Artificial ageing consisted of 5000 thermal cycles (5 °C/55 °C, dwell time: 30 s, transfer time 5 s) followed by storage for 6 months at 37 °C and 100% humidity in an incubator (Memmert UM 500, Schwabach, Germany).
2.4
Push-out bond strength
Before the push-out test, every tooth was completely embedded in self-curing acrylic resin (Paladur, Heraeus Kulzer GmbH, Hanau, Germany) using cylindrical molds. The coronal surfaces of the specimens were then polished with #500 and #1000 SiC paper (Struers) to remove remnants of Paladur or cement. The coronal dentin/restoration part of the embedded teeth was then marked with a felt pen, and discs of 2 mm (1.8–2.3 mm) were cut using a diamond blade low-speed saw (Isomet, Lake Bluff, IL, USA). Discs were stored in tap water after their thickness had been measured with a digital caliper (Mitutoyo IP 65, Kawasaki, Japan) to calculate the bonding surface (BSU (mm 2 )): as the lateral area of the ceramic restorations represented a trapeziform surface, the bonding surface was calculated as A trapezoid (mm 2 ) = height (mm) × center line (mm). The mean center line of 10 restorations was measured and set to be 14.75 mm. Consequently, the formula was: BSU (mm 2 ) = thickness (mm) × 14.75 mm.
For the push-out test, the embedded dentin/restoration discs were placed in a Zwick Z010 universal testing machine (Zwick GmbH & Co, Ulm, Germany) fitted with a custom-made jig (M. E. Mueller Institute, Bern, Switzerland) and loaded at a cross-head speed of 1.0 mm/min. Push-out force was applied on the restorations from the apical side because of their conicity. The maximum force ( F max (N)) was recorded (testXpert software, V9.0, Zwick GmbH) and the push-out bond strength value (PBS (MPa)) was calculated according to the formula: PBS (MPa) = F max (N)/BSU (mm 2 ).
2.5
Failure mode and SEM documentation
Additionally, failure modes were analyzed visually with a stereomicroscope at 40× magnification (Leica ZOOM 2000) and classified into 4 categories: (1) cohesive failure in dentin (failure with dentin involvement), (2) adhesive failure of cement at the dentin interface, (3) adhesive failure of cement at the ceramic interface, (4) cohesive failure in ceramic (failure with ceramic involvement).
For a qualitative documentation of fracture sites with SEM, three specimens per failure mode were mounted on aluminum stubs and sputter coated with gold/palladium for 100 s at 50 mA (Balzers SCD 050, Balzers, Liechtenstein). SEM was performed with a Stereoscan S360 scanning electron microscope at 20 kV (Cambridge Instruments, Cambridge, UK). Digital SEM micrographs (of 100× and 1000× magnification, respectively) were taken (Digital Image Processing System, version 2.3.1.0, point electronic GmbH, Halle, Germany).
2.6
Statistical analysis
The sample size used in the study was determined on the basis of a series of preliminary tests after the level of significance had been set at α = 0.05 (NCSS/PASS 2005, NCSS, Kaysville, UT, USA).
The PBS results were statistically analyzed with SAS 9.1.3 (SAS Institute Inc., Cary, NC, USA). A nonparametrical ANOVA model for two fixed factors after Brunner and Munzel was used to analyze PBS values with regard to any influence of resin cement and of storage condition.
Pairwise Wilcoxon rank-sum tests with Bonferroni-Holm adjustment for multiple testing were used to analyze differences in PBS among the cements. The level of significance was set at α = 0.05.
2
Materials and methods
2.1
Standardized ceramic restoration preparation
The crown of a caries-free, extracted human molar was ground flat (Struers LaboPol-21/Struers Silicon Carbide paper (SiC) grit #320, diameter 200 mm, Ballerup, Denmark) until the entire surface was in coronal dentin. An elliptic occlusal cavity (diameter: 4 mm × 5 mm, depth: 3 mm) was prepared with a 6° conical diamond bur (40 μm grit; Intensiv FG A10, Grancia, Switzerland). The cavity was inspected under a microscope at 15× magnification (Leica ZOOM 2000, Leica, Buffalo, NY, USA) to ensure the absence of residual fissures, cracks, or partial exposure of the dental pulp. The tooth was then coated with IPS Contrast Spray Chairside (Ivoclar Vivadent AG, Schaan, Liechtenstein) and an optical impression was taken with the camera of the Sirona CEREC 3 acquisition center (Sirona, Bensheim, Germany). The shape of the future restoration was adjusted on screen with the CEREC 3D Software (Version 3.10, Sirona).
With the CEREC 3 CAD/CAM milling unit (Sirona) two-hundred standardized ceramic restorations were milled from IPS Empress CAD blocks (HT A3, size I8, LOT-Nr: M19049, Ivoclar Vivadent AG). The restorations were inspected under a magnifying glass and the milling stub was removed with a carbide bur and an OptraFine F finishing rubber point (Ivoclar Vivadent AG). The restorations were then sonicated for 3 min in ethanol (Telsonic TUC-150, Bronschhofen, Switzerland) and air-dried. The tooth-sided surfaces of the restorations were etched for 60 s with IPS Ceramic Etching (<5% hydrofluoric acid, LOT-Nr: M13217, Ivoclar Vivadent AG), cleaned with water spray, sonicated again for 1 min in ethanol and air-dried. Monobond S silane coupling agent (LOT-Nr: J27895, Ivoclar Vivadent AG) was applied for 60 s and air-dried.
2.2
Standardized tooth cavity preparation
The ground human molar with the elliptic occlusal cavity was cleaned from IPS Contrast Spray Chairside and a socket including a mounting for CELAY Plus (Mikrona Technologie AG, Spreitenbach, Switzerland & Ismaning, Germany) was waxed-up. The waxed-up tooth was embedded in Biotan Vest MG material (Schütz Dental GmbH, Rosbach, Germany) and the cast was effused with Biotan Titan 1 (LOT-Nr: 2006000638, Schütz Dental GmbH) in a DOR-A-MATIC Argon-Vacuum/High-pressure apparatus (Labtech, Schütz Dental GmbH). The obtained titanium model of the tooth with the elliptic occlusal cavity was mounted in the CELAY Plus copy-grinding device (Mikrona Technologie AG).
A final amount of two-hundred extracted and sound human third molars was used in this study. The teeth were cleaned under tap water with a scaler and a hard toothbrush to remove calculus and debris, stored in 1% chloramine solution and kept at 5 °C until needed. On each day prior to the luting process of one group ( n = 20), 20 randomly selected teeth were ground flat as previously explained. With the CELAY Plus device, elliptic occlusal cavities were copy-drilled from the titanium tooth model into coronal dentin of the human molars with a 40 μm grit diamond bur (CELAY ZY-54S, Mikrona Technologie AG) under water-cooling. The teeth were then rinsed under tap water and briefly air-dried. The standardized cavities were inspected under a microscope at 15× magnification (Leica ZOOM 2000). Teeth showing residual fissures or cracks, parts of cavity walls in enamel, or exposure of the pulp were discarded and replaced. The group of molars with standardized cavities was then stored in an incubator (Incubat, Melag, Berlin, Germany) for at least 2 h at 37 °C and 100% humidity to assure intraoral temperature before the luting procedure.
2.3
Luting procedures and artificial ageing
The self-adhesive cements used were SpeedCEM, RelyX Unicem Aplicap, SmartCem2 and iCEM. The etch-and-rinse resin cement Variolink II with Syntac Adhesive served as reference. The resin cements were stored according to manufacturers’ instructions. Cements that needed to be stored in the refrigerator were taken out at least 4 h before use. Detailed information about the resin cements is listed in Table 1 .
