Fracture toughness versusmicro-tensile bond strength testing of adhesive–dentin interfaces

Abstract

Objective

To assess interfacial fracture toughness of different adhesive approaches and compare to a standard micro-tensile bond-strength (μTBS) test.

Methods

Chevron-notched beam fracture toughness (CNB) was measured following a modified ISO 24370 standard. Composite bars with dimensions of 3.0 × 4.0 × 25 mm were prepared, with the adhesive–dentin interface in the middle. At the adhesive–dentin interface, a chevron notch was prepared using a 0.15 mm thin diamond blade mounted in a water-cooled diamond saw. Each specimen was loaded until failure in a 4-point bend test setup and the fracture toughness was calculated according to the ISO specifications. Similarly, adhesive–dentin micro-specimens (1.0 × 1.0 × 8–10 mm) were stressed in tensile until failure to determine the μTBS.

Results

A positive correlation ( r 2 = 0.64) was observed between CNB and μTBS, which however was only nearly statistically significant, mainly due to the dissimilar outcome of Scotchbond Universal (3M ESPE). While few μTBS specimens failed at the adhesive–dentin interface, almost all CNB specimens failed interfacially at the notch tip. Weibull moduli for interfacial fracture toughness were much higher than for μTBS (3.8–11.5 versus 2.7–4.8, respectively), especially relevant with regard to early failures.

Significance

Although the ranking of the adhesives on their bonding effectiveness tested using CNB and μTBS corresponded well, the outcome of CNB appeared more reliable and less variable. Fracture toughness measurement is however more laborious and requires specific equipment. The μTBS nevertheless appeared to remain a valid method to assess bonding effectiveness in a versatile way.

Introduction

Bond strength tests are the most common method to evaluate the bonding effectiveness to enamel and dentin. However, 47 years after these tests were introduced in dentistry , concerns are still raised regarding the validity of bond strength testing . Nevertheless bond strength tests are widely used at university institutes and in dental industry . Besides, bond strength tests do not measure or quantify specific material properties, as the obtained results are dependent on many and even minor set-up variables , including specimen size and geometry, load application, inherent material properties of the different components of the adhesive assembly, and flaws located within and/or between the different interfaces. Up to now, no consensus has been reached which test suits best to assess bonding effectiveness. The micro-tensile bond strength (μTBS) test appears most versatile, reliable and discriminative , most likely why it is today most used . Nevertheless, as any bond strength test should be regarded as flawed , a fracture toughness test, that better concentrates the de-bonding stress at the actual interface, might be more suitable to assess mechanical bonding effectiveness of dental adhesives bonded to dentin (and enamel) . However, currently only few studies have measured fracture toughness, thereby using varying methodologies .

Therefore, the purpose of this study was to assess the fracture toughness of adhesives bonded to dentin using a modified fracture toughness ISO standard for ceramics, as compared to their micro-tensile bond strength. Both parameters were comparatively determined for different adhesive approaches, representing a conventional 3-step etch-and-rinse, a 2-step self-etch, a simplified 1-step self-etch and a self-adhesive (no separate adhesive) approach. The hypotheses tested were that (1) there is no difference in interfacial fracture toughness among different adhesive approaches; and that (2) the fracture toughness data do not correlate to μTBS data.

Materials and methods

Fracture toughness

The fracture toughness of adhesive–dentin interfaces was determined using a chevron-notched beam (CNB) test, adapted from the modified ISO 24370 standard to measure fracture toughness of ceramics . The specimen preparation procedure is graphically presented in Fig. 1 . Non-carious human third molars (gathered following informed consent approved by the Commission for Medical Ethics of KU Leuven) were stored in 0.5% chloramine solution at 4 °C and were used within 6 months after extraction. To expose a flat mid-coronal dentin surface, the occlusal third of the crown was removed with a diamond saw (Isomet 1000, Buehler, Lake Bluff, IL, USA). A standardized bur-cut smear layer was produced by removing a thin layer of the surface using a Micro-Specimen Former (University of Iowa, Iowa City, IA, USA), equipped with a high-speed regular-grit (100 μm) diamond bur (842, Komet, Lemgo, Germany). All dentin surfaces were carefully examined for absence of enamel and/or pulp tissue using a stereo-microscope (Wild M5A, Heerbrugg, Switzerland). Without removing the tooth from the lathe, an adhesive or a thin layer of the self-adhesive composite was applied ( Table 1 ) and light-cured with a high-power (>1000 mW/cm 2 ) LED polymerization unit (Bluephase 16i, Ivoclar Vivadent, Schaan, Liechtenstein), followed by a layer of composite (Herculite XRV Ultra) that was flattened using a microscope glass slide. A 10 × 12 × 15 mm composite block (Herculite XRV Ultra), pre-made in a silicon mold was next co-polymerized to the surface using the same composite resin. The top of the composite side was then cut using the Micro-Specimen Former to maintain perfect parallelism to the resin–dentin interface. The root of the tooth was removed 3 mm below the resin–dentin interface and a similar composite build up was made at the root side of the specimen using the self-etch adhesive Clearfil SE Bond and the same composite (Herculite XRV Ultra). After 48 h of water storage at 37 °C, the specimens were sectioned perpendicular to the interface using an automated precision water-cooled diamond saw (Accutom-50, Struers A/S, Ballerup, Denmark) to obtain four rectangular sticks (3.0 × 4.0 mm wide; 25–30 mm long). After 8 additional days of water storage, a chevron notch was prepared using an ultra-thin diamond blade (150 μm, M1DO8, Struers A/S, Ballerup, Denmark), with the tip of the chevron located at the adhesive–dentin interface in the mid-coronal dentin part of the specimen. Immediately after preparation of the chevron notch, the specimen was transferred to the universal testing machine (Instron 5848 Micro Tester, Instron, Norwood, MA, USA) and tested in a 4-point bend test setup with a crosshead speed of 0.05 mm/min. The outer and inner span was 20 and 10 mm, respectively. Each test record was carefully examined for stable fracture from its load-extension graph ( Fig. 2 ) that should exhibit a smooth, nonlinear transition to the maximum force prior to final fracture. Next, the exact dimensions of the chevron notch were measured using a traveling microscope, from which the minimum stress intensity factor coefficient ( Y min ) was calculated for each specimen individually, according to the ISO standard . Using this Y min , the interfacial fracture toughness was calculated in MPa m 1/2 . All fractured surfaces were processed for scanning electron microscopy evaluation (SEM, JSM-6610LV, JEOL, Tokyo, Japan) using common preparation procedures, including fixation, dehydration and gold-sputter coating, to determine fracture location, crack propagation and possible imperfections.

Fig. 1
Schematic explaining the experimental setup. (a) A flat mid-coronal dentin surface was prepared from non-carious human third molars. After application of the respective adhesive, a 10 × 12 × 15 mm composite built-up was made on either side of the dentin slab. (b) Using an automated diamond saw rectangular sticks of 3.0 × 4.0 mm wide and 25–30 mm long were cut. (c) A chevron notch was prepared using an ultra-thin diamond blade. (d) The specimen was tested in a 4-point bend test set-up with a crosshead speed of 0.05 mm/min.

Table 1
Materials tested.
Adhesive Class Application Manufacturer (LOT numbers)
Optibond FL 3-Step E&R Application of gel etchant on dentin surface for 15 s, followed by rinsing for 15 s and gentle air-drying; Application of the primer for 15 s (light scrubbing), 5 s air-drying, application of the adhesive, 20 s light-curing Kerr, Orange, CA, USA
Gel etchant: 3754618
Primer: 3457744
Adhesive: 3461592
Clearfil SE Bond 2-Step SE Application for 20 s (rubbing), 5 s air-drying, application of the adhesive, 10 s light-curing Kuraray, Tokyo, Japan
Primer: 00964B
Bond: 01429 A
G-aenial Bond 1-Step SE Application for 10 s, 5 s thorough air-drying, 10 s light-curing GC, Tokyo, Japan
111012181
Scotchbond Universal (single dose) 1-Step SE Mix well for 5 s, apply and rub for 20 s, gently air-drying for 5 s, 10 s light-curing 3 M ESPE, St Paul, MN, USA
453637
All-bond Universal 1-Step SE Application of 1 st layer, scrub for 10–15 s, evaporate thoroughly for 10 s; application of 2nd layer, repeat step 2 and 3, 10 s light-curing Bisco, Schaumburg, IL, USA
NB-679-193a
Vertise Flow 0-Step SE Apply a thin layer (<0.5 mm thick), using an applicator and brushing for 15–20 s. 20 s light-curing Kerr, Orange, CA, USA
3686291
Herculite HRV ultra
Shade A2 Enamel
Composite Kerr
2991218, 3668986, 3686171, 4144165
E&R = etch&rinse adhesive; SE = self-etch adhesive; 0-step SE = self-adhesive composite.

Fig. 2
Force as a function of time curves of representative specimens of each group. Note the unstable pop-in fracture (arrow) that was followed by a stable fracture for OptiBond FL. For G-aenial Bond and All-bond Universal some smaller pop-ins were recorded as well.

Micro-tensile bond strength (μTBS)

The bond strength to dentin was determined using a standardized micro-tensile bond strength protocol . The occlusal crown third of human third molars was removed with a diamond saw (Isomet 1000), thereby exposing a flat mid-coronal dentin surface. A bur-cut smear layer was produced as described above. Next, the dentin adhesive or self-adhesive composite was applied ( Table 1 ) and a composite build-up was made (Herculite XRV Ultra) in three to four layers to a height of 5–6 mm. To obtain rectangular sticks (1.0 × 1.0 mm wide; 8–9 mm long), the teeth were sectioned perpendicular to the interface using an automated precision water-cooled diamond saw (Accutom-50) after 7 days of storage in distilled water (37 °C). Only the eight central dentin sticks were used to reduce substrate regional variability . The specimens were fixed to a BIOMAT jig with cyanoacrylate glue (Model Repair II Blue, Dentsply-Sankin, Tochigi, Japan) and stressed in tension at a crosshead speed of 1 mm/min using a universal testing device (Instron 5848 Micro Tester). The μTBS was calculated by dividing the imposed force at the time of fracture by the bond area (mm 2 ). Specimens that failed before actual testing (pre-testing failure or ptf) were excluded from further analyses and not taken into account for the calculation of the mean μTBS. The actual number of ptf was explicitly noted as well. The mode of failure (interfacial, or cohesive within dentin or composite) was determined with a stereomicroscope at 50× magnification.

Study setup and statistical analysis

Both the interfacial fracture toughness and μTBS of 5 adhesives and one self-adhesive composite was measured ( Table 1 ). For G-aenial Bond and Scotchbond Universal the adhesive was used solely in a self-etch mode (1-step self-etch). For the CNB fracture toughness tests, 6 teeth per group were used, yielding each 4 sticks, of which 2 were used for the current study and 2 are subjected to long-term water storage. For the μTBS test 5 teeth per group were used, each yielding about 8 sticks, of which 4 were used for the current study and 4 are subjected to long-term water storage. The interfacial fracture toughness and μTBS data were statistically analyzed by Weibull analysis; pivotal 95% confidence bounds were calculated using Monte Carlo simulation . The different groups were compared at the 10% probability of failure level and at the characteristic strength (63.2% probability of failure). For Weibull analyses, ptf’s were excluded, as Weibull can not operate with zero values. To compare the CNB interfacial fracture toughness and μTBS, a correlation analysis on the respective means was performed. All tests were performed at a significance level of α = 0.05 using a software package (R2.12 and weibulltoolkit 2.2, R Foundation for Statistical Computing, Vienna, Austria).

Materials and methods

Fracture toughness

The fracture toughness of adhesive–dentin interfaces was determined using a chevron-notched beam (CNB) test, adapted from the modified ISO 24370 standard to measure fracture toughness of ceramics . The specimen preparation procedure is graphically presented in Fig. 1 . Non-carious human third molars (gathered following informed consent approved by the Commission for Medical Ethics of KU Leuven) were stored in 0.5% chloramine solution at 4 °C and were used within 6 months after extraction. To expose a flat mid-coronal dentin surface, the occlusal third of the crown was removed with a diamond saw (Isomet 1000, Buehler, Lake Bluff, IL, USA). A standardized bur-cut smear layer was produced by removing a thin layer of the surface using a Micro-Specimen Former (University of Iowa, Iowa City, IA, USA), equipped with a high-speed regular-grit (100 μm) diamond bur (842, Komet, Lemgo, Germany). All dentin surfaces were carefully examined for absence of enamel and/or pulp tissue using a stereo-microscope (Wild M5A, Heerbrugg, Switzerland). Without removing the tooth from the lathe, an adhesive or a thin layer of the self-adhesive composite was applied ( Table 1 ) and light-cured with a high-power (>1000 mW/cm 2 ) LED polymerization unit (Bluephase 16i, Ivoclar Vivadent, Schaan, Liechtenstein), followed by a layer of composite (Herculite XRV Ultra) that was flattened using a microscope glass slide. A 10 × 12 × 15 mm composite block (Herculite XRV Ultra), pre-made in a silicon mold was next co-polymerized to the surface using the same composite resin. The top of the composite side was then cut using the Micro-Specimen Former to maintain perfect parallelism to the resin–dentin interface. The root of the tooth was removed 3 mm below the resin–dentin interface and a similar composite build up was made at the root side of the specimen using the self-etch adhesive Clearfil SE Bond and the same composite (Herculite XRV Ultra). After 48 h of water storage at 37 °C, the specimens were sectioned perpendicular to the interface using an automated precision water-cooled diamond saw (Accutom-50, Struers A/S, Ballerup, Denmark) to obtain four rectangular sticks (3.0 × 4.0 mm wide; 25–30 mm long). After 8 additional days of water storage, a chevron notch was prepared using an ultra-thin diamond blade (150 μm, M1DO8, Struers A/S, Ballerup, Denmark), with the tip of the chevron located at the adhesive–dentin interface in the mid-coronal dentin part of the specimen. Immediately after preparation of the chevron notch, the specimen was transferred to the universal testing machine (Instron 5848 Micro Tester, Instron, Norwood, MA, USA) and tested in a 4-point bend test setup with a crosshead speed of 0.05 mm/min. The outer and inner span was 20 and 10 mm, respectively. Each test record was carefully examined for stable fracture from its load-extension graph ( Fig. 2 ) that should exhibit a smooth, nonlinear transition to the maximum force prior to final fracture. Next, the exact dimensions of the chevron notch were measured using a traveling microscope, from which the minimum stress intensity factor coefficient ( Y min ) was calculated for each specimen individually, according to the ISO standard . Using this Y min , the interfacial fracture toughness was calculated in MPa m 1/2 . All fractured surfaces were processed for scanning electron microscopy evaluation (SEM, JSM-6610LV, JEOL, Tokyo, Japan) using common preparation procedures, including fixation, dehydration and gold-sputter coating, to determine fracture location, crack propagation and possible imperfections.

Fig. 1
Schematic explaining the experimental setup. (a) A flat mid-coronal dentin surface was prepared from non-carious human third molars. After application of the respective adhesive, a 10 × 12 × 15 mm composite built-up was made on either side of the dentin slab. (b) Using an automated diamond saw rectangular sticks of 3.0 × 4.0 mm wide and 25–30 mm long were cut. (c) A chevron notch was prepared using an ultra-thin diamond blade. (d) The specimen was tested in a 4-point bend test set-up with a crosshead speed of 0.05 mm/min.

Table 1
Materials tested.
Adhesive Class Application Manufacturer (LOT numbers)
Optibond FL 3-Step E&R Application of gel etchant on dentin surface for 15 s, followed by rinsing for 15 s and gentle air-drying; Application of the primer for 15 s (light scrubbing), 5 s air-drying, application of the adhesive, 20 s light-curing Kerr, Orange, CA, USA
Gel etchant: 3754618
Primer: 3457744
Adhesive: 3461592
Clearfil SE Bond 2-Step SE Application for 20 s (rubbing), 5 s air-drying, application of the adhesive, 10 s light-curing Kuraray, Tokyo, Japan
Primer: 00964B
Bond: 01429 A
G-aenial Bond 1-Step SE Application for 10 s, 5 s thorough air-drying, 10 s light-curing GC, Tokyo, Japan
111012181
Scotchbond Universal (single dose) 1-Step SE Mix well for 5 s, apply and rub for 20 s, gently air-drying for 5 s, 10 s light-curing 3 M ESPE, St Paul, MN, USA
453637
All-bond Universal 1-Step SE Application of 1 st layer, scrub for 10–15 s, evaporate thoroughly for 10 s; application of 2nd layer, repeat step 2 and 3, 10 s light-curing Bisco, Schaumburg, IL, USA
NB-679-193a
Vertise Flow 0-Step SE Apply a thin layer (<0.5 mm thick), using an applicator and brushing for 15–20 s. 20 s light-curing Kerr, Orange, CA, USA
3686291
Herculite HRV ultra
Shade A2 Enamel
Composite Kerr
2991218, 3668986, 3686171, 4144165
E&R = etch&rinse adhesive; SE = self-etch adhesive; 0-step SE = self-adhesive composite.

Nov 25, 2017 | Posted by in Dental Materials | Comments Off on Fracture toughness versusmicro-tensile bond strength testing of adhesive–dentin interfaces
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