To compare the bond strength values and the ranking order of three phosphate monomer containing resin cements using microtensile (μTBS) and microshear (μSBS) bond strength tests.
Materials and methods
Zirconia discs (Procera Zirconia) were bonded to resin composite discs (Filtek Z250) using three different cements (Panavia F 2.0, RelyX UniCem, and Multilink). Two bond strength tests were used to determine zirconia resin bond strength; microtensile bond strength test (μTBS) and microshear bond strength test (μSBS). Ten specimens were tested for each group ( n = 10). Two-way analysis of variance (ANOVA) was used to analyze the data ( α = 0.05).
There were statistical significant differences in bond strength values and in the ranking order obtained using the two test methods. μTBS reported significant differences in bond strength values, whereas μSBS failed to detect such effect. Both Multilink and Panavia demonstrated basically cohesive failure in the resin cement while RelyX UniCem demonstrated interfacial failure.
Based on the findings of this study, the data obtained using either μTBS or μSBS could not be directly compared. μTBS was more sensitive to material differences compared to μSBS which failed to detect such differences.
Adhesive resins used in dentistry have been significantly improved over recent years . Micromechanical entanglements of monomer resins with etched enamel as well as bonding to conditioned dentin by hybridization are considered key factors for achieving successful bonding and an optimal marginal seal. However, the introduction of zirconia to dentistry added an extra challenge as establishing a durable bond with zirconia has been proven to be a difficult task.
Bond strength tests have always gained a lot of interest as a point of research in dental literature . While the aim of these tests is usually to evaluate bond strength of different materials, they also allow establishing of a ranking order. The data obtained from different bond strength tests largely depend on the actual test setup used that may differ between different laboratories. Parameters such as specimen geometry, size of the bonded surface area, the type of material, the loading conditions, operation variability, and more, all have a significant influence on the data obtained. It is, therefore not surprising that bond strength data substantially vary among different studies throughout the world. All these interacting variables make direct comparison between different studies a very difficult task .
Most commonly used bond strength tests rely on subjecting the specimens to tensile or shear stresses in order to evaluate the failure point which is usually referred to as the bond strength of an adhesive system to the substrate material. On the contrary, the specimen may fail by cohesive fracture of either of its components and in that case using this failure load to confer bond strength values may be a little ambiguous. For example, a bond strength value in excess of 20 MPa in a shear test setup would tend to cohesively fracture the specimen meanwhile the bonded interface may remain intact . Therefore, a new test is needed especially for testing specimens with a strongly bonded interface.
Microtensile bond strength test (μTBS) was introduced by Sano and others in 1994 . These authors showed that μTBS values are inversely related to the bonded surface area and that although much higher bond strength values were measured, most failures still occurred at the interface between tooth substrate and adhesive resin. Other advantages of μTBS are that regional bond strength and bonding effectiveness could be applied to small sized specimens as a regional area of a tooth substrate focusing on a carious region or for example a localized area of sclerotic dentin . The major disadvantages of μTBS are that the test is rather labor-intensive, technically demanding, and requires careful handling of the fragile specimens. Special care should be taken to reduce the production of micro-fractures at the interface during specimen preparation which may weaken the bond and thus reduce the actual bond strength .
Microshear bond strength test (μSBS) is used for testing of small areas, which permits precise regional mapping of the surface of interest, e.g. different regions of dentin surface . This method allows for straightforward specimen preparation and gives precise results with relatively small standard deviations . For μTBS test, trimming of the specimen is an indispensable step especially when making small sized specimens which can easily cause micro-cracks in brittle materials like enamel or ceramics. However, when conducting microshear bond test, the resin specimens could be directly bonded on a flat surface without the need of trimming .
A point worth of mentioning is that different bond strength tests result in a characteristic pattern of stress distribution of the applied load in the structure of the specimen. These areas of high stress concentration act as crack initiation sites regardless of the presence of surface or bulk defects in the specimen. Once the stress concentration at a crack tip exceeds the strength of the material, the crack will grow. In this situation, failure of the tested specimen is usually reported as cohesive fracture and the obtained data, in most cases, are used to calculate the mean bond strength value even though that the bonded interface may be totally intact which would directly lead to unjustified high bond strength values. In a previous study, Aboushelib et al. utilized finite element analysis to map the stress distribution in μTBS test setup and have indicated that stresses are concentrated away from the bonded interface between the bonded microbar and the attachment unit. Such condition would imply a higher tendency for cohesive failures in specimens with strong bonded interfaces. In case of cohesive failure, a correction factor of 2.5× has been suggested by the authors to properly report the failure stress in such cases .
A number of studies have been performed to investigate the bonding ability of adhesive systems to tooth structures or core materials , including tensile and shear bond strength tests. Additionally, these tests have also been used to evaluate the bond strength between different components in one structure as the bond between the veneer ceramic and the underlying framework material or the bond between composite luting cement and zirconia ceramic surfaces . However, a small number of investigations have compared the results of μTBS and μSBS which need to be properly evaluated to establish a reliable comparative method and thus enable ranking of different adhesives.
Up to date, the combination of airborne particle abrasion and 10-methacryloyloxydecyl dihydrogenphosphate (MDP) monomer is the recommended method of bonding to zirconia frameworks. This method has proven to produce a good bond strength and bond durability after thermo-cycling and long term water storage . In previous parts of this study, MDP monomer of Panavia F 2.0 resin cement was successful in establishing good bond strength with zirconia, which was not influenced by 90 days of water storage. The performance of this bond was enhanced by using new types of adhesion promoters designed to enhance wetting and bonding to ceramic substrates .
Due to patent rights about the structure of MDP monomer ( Fig. 1 a ), manufacturers have produced new phosphate monomers designed not only to bond to zirconia but have also cross-linking branches for bonding the resin matrix as well ( Fig. 1 ). Unfortunately, not much data are available about the performance of these new adhesive resins. One of the recently developed phosphate monomers (RelyX Unicem) has a characteristic of self-etching phosphorylated methacrylates that is designed to bond directly to both enamel and dentin. With two phosphate groups and at least two double bonded carbon atoms, a good bond strength to zirconia plus adequate cross-linking to the resin matrix is achieved ( Fig. 1 b). Another new self-etch phosphate monomer (Multilink Automix) characterized by hydrolytic stability has one phosphate terminal and at least two sites capable of bonding to resin matrix through oxygen bond. This molecule has a terminal hydroxyl group as a substituent that gives the monomer stability under water and in acidic conditions ( Fig. 1 c).
The aim of this study was to determine zirconia resin bond strength and the ranking order of three phosphate monomer containing adhesive resins by means of μTBS and μSBS tests. Fracture surface analysis of the broken specimens was used to classify failure pattern. The null hypothesis predicted no significant differences between the two tested methodologies.
Materials and methods
Microtensile bond strength (μTBS) test
Fully sintered yttria tetragonal zirconia polycrystal (Y-TZP) discs (11.8 mm in diameter × 3.0 mm thick) were prepared by cutting zirconia milling blocks (Procera zirconia; NobelBiocare, Göteborg, Sweden) using a precision cutting instrument (Isomet 1000; Buehler, Lake Bluff, IL) and a diamond-coated cutting disc (Diamond Wafering Blade, No 11-4276; Buehler). The location of the cuts was controlled using a traveling stage and a horizontally displaced digital micrometer (ID-C1508; Mitutoyo Corp, Utsunomiya, Japan). The sintered discs were polished using a rotating metallographic polishing device (Ecomet; Buehler) under a fixed load (300 g) and water cooling. The specimens were airborne particle abraded with 50 μm aluminum oxide particles (P-G 400; Harnisch & Rieth, Winterbach, Germany) at 0.35 MPa pressure (S-U-Alustral; Schuler-Dental, Ulm, Germany) at a distance of 1 cm followed by ultrasonic cleaning in distilled water for 10 min.
Composite resin (Filtek Z250, shade A2; 3 M ESPE, St. Paul, MN, USA) discs (11.8 mm in diameter × 3 mm thick) were prepared in a plastic mold and light polymerized at 4 different locations, 60 s each (Elipar FreeLight 2; 3M ESPE). Light intensity, 800 mW/cm 2 , was frequently monitored to ensure adequate polymerization of all specimens (Demetron 100; Demetron Research Corp, Danbury, CT).
Three different adhesive resin cement systems, Panavia F 2.0 (Kuraray Co. Ltd., Tokyo, Japan), RelyX UniCem (3 M ESPE), and Multilink Automix (Ivoclar-Vivadent, Schaan, Liechtenstein) were used to lute the composite resin discs to the airborne particle abraded zirconia discs, according to the manufacturers’ instructions. These bonding agents were selected because each contains a different phosphate based monomer which is known to enhance bond strength to zirconia based materials. Following manufacturer recommendations, the zirconia surface was prepared and each resin cement was mixed and applied on the surface of the composite resin disc which was seated on top of the zirconia disc and loaded with 50 N for 60 s using a special loading gig, excess cement was wiped off, and the specimen was light polymerized with the same unit at 4 different locations for 60 s each. Properties of all used materials are listed in Table 1 .
|Multilink Automix, Ivoclar-Vivadent, Schaan, Liechtenstein||The monomer matrix is composed of DMA, HEMA, Ba-glass fillers, ytterbium fluoride, spheroid mixed oxide||K54378|
|A primer: aqueous solution of initiator|
|B primer: HEMA and phosphoric acid and acrylic acid monomers|
|Metal/zirconia primer: phosphoric acid acrylate and methacrylate cross-linking agents in an organic solution|
|Panavia F 2.0, Kuraray Co. Ltd., Osaka, Japan||A paste: silica, dimethacrylate monomer, functional acid MDP, photo initiator, accelerator||41233|
|B paste: brown color, barium glass, sodium fluoride, dimethacrylate (DMA) monomer|
|RelyX UniCem, 3M ESPE, St. Paul, Mn, USA||Powder: glass powder, initiator, silica sil., pyrimidine, calcium hydroxide, peroxy compound, pigment||288018|
|Liquid: Methacrylated phosphoric ester, DMA, acetate, stabilizer, initiator|
Each bonded specimen was sectioned with a 0.3 mm thick diamond-coated cutting disc (Diamond Wafering Blade, No 11-4254; Buehler) into at least 20 microbars (6 mm × 1 mm × 1 mm). The microbars were examined under a stereomicroscope (SZ; Olympus, Tokyo, Japan) and only intact microbars were selected ( Fig. 2 ).
For every group ( n = 20), the microbars were tested after 24 h water storage (demineralized water at 37 °C). Each microbar was bonded to the attachment unit using a light-polymerized adhesive resin (Clearfil SE; Kuraray Co. Ltd.), taking care to center the zirconia–resin interface at the free space between the two plates of the attachment unit . The zirconia–resin microtensile bond strength (MPa) was measured by applying tensile load to the bonded interface using a universal testing machine (Instron 6022; Instron Corp, High Wycombe, England) at a crosshead speed of 0.5 mm/min .
Microshear bond strength (μSBS) test
Twelve fully sintered zirconia discs (22.2 mm diameter, 0.8 mm height), were prepared using the same materials and methods as previously described. Composite resin (Filtek Z250, shade A2; 3 M ESPE, St. Paul, MN) discs (0.9 mm in diameter × 0.7 mm thick) were prepared by injecting the resin composite into a plastic tube which was held between two glass slides and then was light polymerized for 20 s form the top and for 20 s from the bottom side (Elipar FreeLight 2; 3M ESPE). The specimens were then stored in distilled water at 37 °C for 24 h prior to removal from the tubing. The same previously described adhesives and bonding method were used to lute the composite cylinders to the zirconia discs. Excess cement was removed by using compressed air and micro-brushes, nevertheless the small size of the bonded discs required careful handling.
The bonded specimens were gently fixed between two steel plates using a traveling stage micrometer (Mitutoyo Corp, Utsunomiya, Japan). Shear force was applied to the bonded interface at a crosshead speed of 0.5 mm/min until failure occurred taking care to properly align the specimens so that the bonded interface was parallel to the direction of load application ( Fig. 3 ).
Analysis of failure mode
The fractured zirconia surface after μTBS or μSBS was examined under an optical microscope at 30× magnification then at higher magnifications using a scanning electron microscopy (XL 20; Philips, Eindhoven, The Netherlands). Failure mode was classified either as interfacial failure where the crack traveled at the zirconia–resin cement interface with consideration of the area of crack origin or a cohesive failure in the resin cement where the crack originated outside the bonded interface. This classification allowed accurate and easier interpretation of failure mode as from a clinical point of view a debonding failure will originate at the interfacial region .
Two-way analysis of variance (ANOVA) with one between group variable (2 bond strength tests) and one within group variable (3 resin adhesives) were used to analyze the data ( α = 0.05). Statistical analysis was carried out using computer software (SigmaStat Version 3.0, SPSS, Inc., Chicago, USA).