Graphical abstract
Highlights
- •
Bond strength of composite cements to dentin was evaluated both by iFT and μTBS.
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μTBS appeared more discriminative than iFT for bonding effectiveness of cements to dentin.
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Correlation between iFT and μTBS was moderate and not significant.
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The etch-and-rinse and ‘universal’ self-etch composite cements performed best in both tests.
Abstract
Objectives
To determine the interfacial fracture toughness (iFT) and micro-tensile strength (μTBS) of composite cements bonded to dentin.
Methods
Fifty feldspar ceramic blocks (Vita Mark II, Vita Zahnfabrik) were luted onto dentin using two self-adhesive (G-CEM LinkAce, GC; SpeedCEM, Ivoclar Vivadent), two self-etch (Multilink Primer & Multilink Automix, Ivoclar Vivadent; Scotchbond Universal & RelyX Ultimate, 3 M ESPE), and one etch-and-rinse (Excite F DSC & Variolink II, Ivoclar Vivadent) composite cement (n = 10). After 48 h in 100% relative humidity at 37 °C, one half of each tooth was sectioned in sticks with a chevron notch at the cement–dentin interface and tested in a 4-point bending test setup (iFT). The remaining half of the tooth was sectioned in micro-specimens and stressed in tension until failure (μTBS). The mode of failure was determined with a stereomicroscope at 50× magnification. Data were submitted to Weibull analysis and Pearson’s correlation ( α = 0.05).
Results
At 10% probability of failure, no significant differences could be found using iFT, while the etch-and-rinse composite cement Variolink II presented a significantly higher μTBS at this level. At 63.2% probability of failure, the self-adhesive composite cement G-CEM LinkAce revealed a significantly lower μTBS and iFT, and the self-etch cement Multilink Automix also revealed a significantly lower μTBS than all other cements. The correlation found between iFT and μTBS was moderate and not significant (r 2 = 0.618, p = 0.11).
Significance
Overall, the etch-and-rinse and ‘universal’ self-etch composite cements performed best. The micro-tensile bond strength and interfacial fracture toughness tests did not correlate well.
1
Introduction
In the mid 90’s, the micro-tensile strength test was introduced by Sano et al. ; they applied this method to measure the ultimate tensile strength and modulus of elasticity of mineralized and demineralized dentin. Concurrently, this method was used to measure bond strength to tooth enamel and dentin (μTBS) . A number of advantages have been attributed to the μTBS approach when compared to conventional tensile and shear bond strength test methods, among which the limited number of teeth needed (although today micro-specimens originating from a single tooth are no longer considered statistically independent), the occurrence of more adhesive than cohesive failures and thus measurement of a bond strength representing more the interfacial adhesive-tooth strength, the μTBS mean and variance that can be calculated per single tooth, the lower probability to incorporate interfacial defects that may falsely lower bond strength, the potential to test different experimental conditions in parallel on a single tooth (hereby enabling statistical comparison on tooth level), the potential to test different cavity configurations, and allowing high-resolution examination of failed specimens using SEM/TEM . Principal variables identified to affect μTBS are specimen size and geometry , dentin region , size and shape of the cross-sectional area , misalignment of the applied load axis , as well as the selected method to handle pre-testing failures to calculate the μTBS . Limitations of μTBS testing include, among others, the labor-intensive and technically demanding specimen preparation, the difficulty to measure low bond strengths (<5 MPa), the potential dehydration of specimens, the risk on specimen damage when removing it from the jig to which it was glued, and the difficulty to fabricate specimens with a consistent geometry without the aid of special equipment such as a Micro-Specimen Former (University of Iowa, Iowa City, IA, USA) .
Today, the μTBS test is the most frequently employed bond-strength test . It however remains criticized, since μTBS data reported in different studies vary highly . This should to a great extent be ascribed to the wide disparity in micro-specimen preparation and actual test parameters employed in the different research centers . Other major criticism concerns the alleged inhomogeneous load distribution within the micro-specimen with the de-bonding stress imposed during tensile loading not necessarily concentrated at the adhesive-tooth bond and uniformly distributed across the actual interface. Previous research using finite element analysis (FEA) to compare the stress-concentration factor (Kt) for stick-shaped homogeneous and bi-material specimens with different notch geometries concluded that dumbbell and stick-shaped specimens are favored for μTBS testing; these micro-specimen geometries were shown to induce uniform stress distribution . There is however no consensus in literature, since another more recent FEA study demonstrated that the main stress is not concentrated at the interface, but is located within the dentin and composite parts near the adhesive interface .
Hence, a fracture mechanics approach is considered more appropriate to assess bonding effectiveness . This approach considers flaw size and features, component geometry, loading conditions and fracture toughness to predict fracture resistance at a flawed site . By definition, fracture toughness is a property that describes the ability of a material containing a crack to resist fracture. Interfacial fracture toughness (iFT) has been proposed as an alternative method to measure bonding effectiveness in the laboratory. Having been applied in various forms, such as ‘short rod chevron notch’ , ‘notchless triangular prism’ , ‘chevron notch beam’ (CNB) , the tests generally appeared more accurate and reproducible; they were less test-dependent and revealed the interfacial bonding properties better . The correlation between μTBS and iFT has been investigated in previous researches showing controversial results. While a moderate and non-significant correlation was found when bonding systems of different adhesive approaches were compared immediately , the same correlation tested after aging revealed a strong and highly significant correlation . Our group recently miniaturized iFT to a so-called mini-iFT . A significant and strong positive correlation was found between mini-iFT and μTBS. The new mini-iFT test appeared more discriminative and valid than the μTBS test to assess bonding effectiveness.
Fracture toughness tests have been widely used to investigate ceramics, composites, glass-ionomers, as well as enamel- and dentin–composite adhesive interfaces . However, up to the date, no studies employed this approach to assess bonding effectiveness of composite cements to dentin. Therefore, the aim of this study was to assess iFT of composite cements bonded to dentin and to correlate the iFT data with μTBS data that were gathered in parallel for the same cements. The null hypotheses tested were that (1) there is no difference in interfacial bond strength among the composite cements tested and (2) iFT and μTBS of the cement–dentin interface do not correlate.
2
Methods
2.1
Tooth preparation
Fifty non-carious human third molars (gathered following informed consent approved by the Commission for Medical Ethics of KULeuven under the file S57622) were stored in 0.5% chloramine solution at 4 °C no longer than 6 months after extraction. The teeth were embedded in gypsum blocks and the occlusal third of the crowns was removed with a diamond saw (Isomet 1000, Buehler, Lake Bluff, IL, USA), thereby exposing a flat mid-coronal dentin surface. The surfaces were checked for remaining enamel and exposed pulp tissue using a stereomicroscope (Wild M5A, Heerbrugg, Switzerland). The specimens were excluded in case the pulp chamber was exposed and if enamel was observed, it was promptly eliminated with a diamond bur.
A standardized bur-cut smear layer was produced by removing a thin layer of the dentin surface using a Micro-Specimen Former (University of Iowa, Iowa City, IA, USA) equipped with a high-speed cylindrical regular-grit (107 μm) diamond bur (842, Komet, Lemgo, Germany).
2.2
Ceramic block preparation
Fine-structured feldspar ceramic (Vitablocks Mark II/3D Master, VITA Zahnfabrik, Bad Säckingen, Germany) was used in 10 × 12 × 15 mm blocks. The top surface of the ceramic block was polished with a #600 SiC paper in a polishing machine (Buehler Beta & Vector Grinder-Polisher, Buehler, Lake Buff, IL, USA). Next, the surface was etched with 5% hydrofluoric acid (IPS Ceramic Kit, Ivoclar Vivadent, Schaan, Liechtenstein) for 60 s, rinsed with water spray for 60 s and dried with oil-free air for 20 s. Then, silane (Monobond Plus, Ivoclar Vivadent) was applied for 60 s and air-dried for 5 s. In order to improve resin infiltration into the ceramic surface, an unfilled bonding resin (Heliobond, Ivoclar Vivadent) was applied and light-cured for 10 s using the ‘Bluephase 20i’ light-curing unit (Ivoclar Vivadent) in ‘high-mode’ (1200 mW/cm 2 ).
2.3
Specimen preparation
Fig. 1 shows a schematic drawing of the specimens’ preparation. The ceramic blocks were cemented to the prepared dentin surfaces using different adhesive/composite cement combinations (n = 10 per group). Two self-adhesive, 2 self-etch and 1 etch-and-rinse composite cement were used according to the manufacturers’ recommendations in a dual-cure mode. The detailed description of materials, composition and application mode is presented in Table 1 .
| Composite cement Company LOT number |
Type | Composition a | Application in dual-curing mode b |
|---|---|---|---|
| G-CEM LinkAce GC, Tokyo, Japan 1302041/2015-2 |
Self-adhesive | Cement: 4-MET, phosphoric acid ester monomer, water, UDMA, dimethacrylate, silica, fluoroaluminosilicate glass, initiator, pigment, initiator, stabilizer (mean filler size: 4 μm; 71.4 wt%) | Coat the internal surface of the restoration; seat immediately; apply moderate pressure; cure for 2–4 s; remove excess; light-cure all margins for 20 s each; let the material set for 4 min |
| SpeedCEM Ivoclar Vivadent, Schaan, Liechtenstein S07441/2014-09 |
Self-adhesive | Cement: dimethacrylates, methacrylated phosphoric ester monomer, barium glass filler, ytterbium trifluoride, silicon dioxide, co-polymer, initiators, stabilizers, pigments (particle size: 0.1–7 μm; mean filler size: 5 μm; 40 vol%) | Apply the cement paste to the restoration; seat the restoration and hold in place with uniform pressure; tack-cure for 1 s per quarter surface; remove excess with a scaler; light-cure all segments for 20 s |
| Multilink Automix Ivoclar Vivadent S09080/2014-09 |
Self-etch | Multilink Primer A: aqueous solution of initiators Multilink Primer B: HEMA, phosphonic acid monomer, methacrylate monomers Cement: dimethacrylate, HEMA, barium glass, ytterbium trifluoride, spheroid mixed oxide (particle size: 0.25–3.0 μm; mean filler size: 0.9 μm; 40 vol%) |
Mix Primer A and B in a 1:1 ratio; apply the mixed primers onto the bonding surface and scrub it in for 15 s; blow with air until the mobile liquid film is no longer visible; no light-curing is necessary; apply the cement directly to the inner surface of the restoration; seat the restoration in place and cure for 2–4 s; remove excess; light-cure all margins for 20 s |
| RelyX Ultimate 3M ESPE, Seefeld, Germany 512866/2014-10 |
Self-etch | Scotchbond Universal: 10-MDP, Vitrebond copolymer, HEMA, BisGMA, dimethacrylates Base: methacrylate monomers, radiopaque and silanated filler, initiator components, stabilizers, rheological additives Catalyst: methacrylate monomers, alkaline filler, initiator components, stabilizers, rheological additives, pigments, dark cure activator for Scotchbond Universal |
Apply Scotchbond Universal to the tooth surface for 20 s while rubbing; air-thin for 5–10 s; light-cure for 10 s; apply a thin layer of cement to the restoration; seat the restoration firmly; clean up excess; light-cure each surface for 20 s |
| Variolink II Ivoclar Vivadent 514260/2015-4 |
Etch-and-rinse | Excite F DSC: HEMA, dimethacrylate, phosphonic acid methacrylate, silicone dioxide, initiators, stabilizers, potassium fluoride in alcohol Cement: bis-GMA, UDMA, TEGDMA, barium glass, ytterbium trifluoride, Ba-Al-fluorosilicate glass, spheroid mixed oxide, catalysts, stabilizers, pigments (particle size: 0.04–3 μm; mean filler size: 0.7 μm; base: 46.7 vol%, low viscosity catalyst: 43.6 vol%) |
Etch dentin for 10–15 s; rinse it with water spray for at least 5 s; leave the dentin glossy; rub the adhesive for at least 10 s; apply a weak stream of air; apply the cement on the inner surface of the restoration; seat it and maintain a slight pressure; remove excess; light-cure for at least 40 s per segment |
a According to technical information provided by the respective manufacturer.
After the cement was applied to the ceramic and positioned on the prepared dentin surface, a constant load of 1 kg was placed on the top of the ceramic block using a custom-made loading device. Excess material was removed with a scaler and the specimens were light-cured for 40 s per surface (Bluephase 20i, Ivoclar Vivadent; high mode). The load was removed and the specimens were additionally light-cured from the top during 40 s (200 s light-curing in total).
The roots of the teeth were removed 3 mm below the composite cement–dentin interface and another ceramic block (12 mm high) was cemented at the root side of the specimen. The top surface of the ceramic block was prepared using the same surface treatment (polishing + 5% HF + silane + bonding agent) and cemented to the dentin using Scotchbond Universal and RelyX Ultimate (3M ESPE, Seefeld, Germany), both used according to the manufacturer’s instructions following a self-etching and dual-curing mode, respectively.
After 48 h storage in 100% relative humidity at 37 °C, the specimens were sectioned perpendicularly to the interface using an automated precision water-cooled diamond saw (Accutom-50, Struers, Ballerup, Denmark). The specimen was first cut in the middle, perpendicular to the interface, dividing it in two equal halves. After serial cuts along the X and Y axis, 1–2 rectangular sticks (3.0 × 4.0 × 25–30 mm) were obtained for the interfacial fracture toughness test (iFT) and 7–9 sticks (1.0 × 1.0 × 12–15 mm) were obtained for the micro-tensile bond strength test (μTBS) ( Fig. 1 ).
2.4
Interfacial fracture toughness testing (iFT)
After 4 additional days in 100% relative humidity at 37 °C, a chevron notch was prepared in the FT bars using an ultra-thin 150-μm diamond blade (M1DO8, Struers). The tip of the chevron notch was located exactly at the cement/adhesive interface. Immediately after the notch preparation, the specimens were transferred to a universal testing machine (Instron 5848 Micro Tester, Instron, Norwood, MA, USA) and tested in a 4-point bend test setup (20 mm outer and 10 mm inner span) with a cross-head speed of 0.05 mm/min. After failure, the exact dimensions of the specimen were measured using a measuring optical microscope (400-NRC; Leitz, Wetzlar, Germany) at 250× magnification. The fracture toughness was calculated in MPa m 1/2 .
2.5
Micro-tensile bond strength testing (μTBS)
The μTBS specimens were also stored for 4 additional days in 100% humidity at 37 °C and fixed to a BIOMAT jig with cyanoacrylate glue (Model Repair II Blue, Dentsply-Sankin, Tochigi, Japan) and stressed in tension at a cross-head speed of 1 mm/min using a universal testing machine (Lloyd LXR, Lloyd West Sussex, UK). The μTBS was calculated by dividing the imposed force at time of fracture (F) by the bonded area (mm 2 ), which was checked with a digital caliper. For those specimens that failed before the actual test, a bond strength value of 0 MPa was attributed.
2.6
Failure mode analysis
The failure mode was determined with a stereomicroscope at 50× magnification (Wild M5A, Heerbrugg, Switzerland) and recorded as either ‘cohesive failure in dentin’, ‘cohesive failure in ceramic’, ‘cohesive failure in cement’, ‘adhesive failure in the cement–dentin interface’, ‘adhesive failure at the cement–ceramic interface’ or ‘mixed failure’.
2.7
Statistical analysis
Weibull analyses were performed to highlight differences between the cements for the different methods and pivotal 95% confidence bounds were calculated using Monte Carlo simulation . Groups were compared at the 10% probability of failure level and at characteristic strength (63.2% probability of failure). As Weibull cannot operate with zero values, pre-testing failures were replaced by a random value between zero and the lowest value measured in the respective group. To ensure repeatability, this procedure was performed 1000 times and the obtained Weibull parameters, along with their 95% confidence intervals, were averaged. Pearson’s correlation on both methods was also performed on the respective means of each group and by tooth. All tests were carried out at a significance level of α = 0.05 using a statistical package (R2.12 and Weibull-toolkit 2.2, R Foundation for Statistical Computing, Vienna, Austria).
2
Methods
2.1
Tooth preparation
Fifty non-carious human third molars (gathered following informed consent approved by the Commission for Medical Ethics of KULeuven under the file S57622) were stored in 0.5% chloramine solution at 4 °C no longer than 6 months after extraction. The teeth were embedded in gypsum blocks and the occlusal third of the crowns was removed with a diamond saw (Isomet 1000, Buehler, Lake Bluff, IL, USA), thereby exposing a flat mid-coronal dentin surface. The surfaces were checked for remaining enamel and exposed pulp tissue using a stereomicroscope (Wild M5A, Heerbrugg, Switzerland). The specimens were excluded in case the pulp chamber was exposed and if enamel was observed, it was promptly eliminated with a diamond bur.
A standardized bur-cut smear layer was produced by removing a thin layer of the dentin surface using a Micro-Specimen Former (University of Iowa, Iowa City, IA, USA) equipped with a high-speed cylindrical regular-grit (107 μm) diamond bur (842, Komet, Lemgo, Germany).
2.2
Ceramic block preparation
Fine-structured feldspar ceramic (Vitablocks Mark II/3D Master, VITA Zahnfabrik, Bad Säckingen, Germany) was used in 10 × 12 × 15 mm blocks. The top surface of the ceramic block was polished with a #600 SiC paper in a polishing machine (Buehler Beta & Vector Grinder-Polisher, Buehler, Lake Buff, IL, USA). Next, the surface was etched with 5% hydrofluoric acid (IPS Ceramic Kit, Ivoclar Vivadent, Schaan, Liechtenstein) for 60 s, rinsed with water spray for 60 s and dried with oil-free air for 20 s. Then, silane (Monobond Plus, Ivoclar Vivadent) was applied for 60 s and air-dried for 5 s. In order to improve resin infiltration into the ceramic surface, an unfilled bonding resin (Heliobond, Ivoclar Vivadent) was applied and light-cured for 10 s using the ‘Bluephase 20i’ light-curing unit (Ivoclar Vivadent) in ‘high-mode’ (1200 mW/cm 2 ).
2.3
Specimen preparation
Fig. 1 shows a schematic drawing of the specimens’ preparation. The ceramic blocks were cemented to the prepared dentin surfaces using different adhesive/composite cement combinations (n = 10 per group). Two self-adhesive, 2 self-etch and 1 etch-and-rinse composite cement were used according to the manufacturers’ recommendations in a dual-cure mode. The detailed description of materials, composition and application mode is presented in Table 1 .
| Composite cement Company LOT number |
Type | Composition a | Application in dual-curing mode b |
|---|---|---|---|
| G-CEM LinkAce GC, Tokyo, Japan 1302041/2015-2 |
Self-adhesive | Cement: 4-MET, phosphoric acid ester monomer, water, UDMA, dimethacrylate, silica, fluoroaluminosilicate glass, initiator, pigment, initiator, stabilizer (mean filler size: 4 μm; 71.4 wt%) | Coat the internal surface of the restoration; seat immediately; apply moderate pressure; cure for 2–4 s; remove excess; light-cure all margins for 20 s each; let the material set for 4 min |
| SpeedCEM Ivoclar Vivadent, Schaan, Liechtenstein S07441/2014-09 |
Self-adhesive | Cement: dimethacrylates, methacrylated phosphoric ester monomer, barium glass filler, ytterbium trifluoride, silicon dioxide, co-polymer, initiators, stabilizers, pigments (particle size: 0.1–7 μm; mean filler size: 5 μm; 40 vol%) | Apply the cement paste to the restoration; seat the restoration and hold in place with uniform pressure; tack-cure for 1 s per quarter surface; remove excess with a scaler; light-cure all segments for 20 s |
| Multilink Automix Ivoclar Vivadent S09080/2014-09 |
Self-etch | Multilink Primer A: aqueous solution of initiators Multilink Primer B: HEMA, phosphonic acid monomer, methacrylate monomers Cement: dimethacrylate, HEMA, barium glass, ytterbium trifluoride, spheroid mixed oxide (particle size: 0.25–3.0 μm; mean filler size: 0.9 μm; 40 vol%) |
Mix Primer A and B in a 1:1 ratio; apply the mixed primers onto the bonding surface and scrub it in for 15 s; blow with air until the mobile liquid film is no longer visible; no light-curing is necessary; apply the cement directly to the inner surface of the restoration; seat the restoration in place and cure for 2–4 s; remove excess; light-cure all margins for 20 s |
| RelyX Ultimate 3M ESPE, Seefeld, Germany 512866/2014-10 |
Self-etch | Scotchbond Universal: 10-MDP, Vitrebond copolymer, HEMA, BisGMA, dimethacrylates Base: methacrylate monomers, radiopaque and silanated filler, initiator components, stabilizers, rheological additives Catalyst: methacrylate monomers, alkaline filler, initiator components, stabilizers, rheological additives, pigments, dark cure activator for Scotchbond Universal |
Apply Scotchbond Universal to the tooth surface for 20 s while rubbing; air-thin for 5–10 s; light-cure for 10 s; apply a thin layer of cement to the restoration; seat the restoration firmly; clean up excess; light-cure each surface for 20 s |
| Variolink II Ivoclar Vivadent 514260/2015-4 |
Etch-and-rinse | Excite F DSC: HEMA, dimethacrylate, phosphonic acid methacrylate, silicone dioxide, initiators, stabilizers, potassium fluoride in alcohol Cement: bis-GMA, UDMA, TEGDMA, barium glass, ytterbium trifluoride, Ba-Al-fluorosilicate glass, spheroid mixed oxide, catalysts, stabilizers, pigments (particle size: 0.04–3 μm; mean filler size: 0.7 μm; base: 46.7 vol%, low viscosity catalyst: 43.6 vol%) |
Etch dentin for 10–15 s; rinse it with water spray for at least 5 s; leave the dentin glossy; rub the adhesive for at least 10 s; apply a weak stream of air; apply the cement on the inner surface of the restoration; seat it and maintain a slight pressure; remove excess; light-cure for at least 40 s per segment |
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