The purpose of this study was to evaluate the cantilever-bending test as a bond-strength measurement method for enamel/composite adhesion.
Materials and methods
The buccal surface of bovine incisors were flattened and subsequently placed with composite Z250 (3M ESPE) incrementally after either self-etching adhesive (SE) or etch-and-rinse adhesive (SB) was applied. Then, they were cut with a low-speed cutter to obtain stick-shape samples. The samples were divided into four groups ( n = 30 each) according to the adhesive and test method: SET, SE-B, SB-T and SB-B, where T denotes the microtensile test and B denotes the cantilever-bending test. The failure mode of each sample was examined under the microscope. Those samples with the fracture surface lying entirely or partially in the adhesive layer were considered to have provided successful measurements. The results from the two test methods were compared.
Results and discussion
The cantilever-bending method produced a comparable success rate of bond strength measurement to that of the microtensile method. The bond strengths of SE and SB from the cantilever-bending test were 58% and 40% higher, respectively, than those measured with the microtensile method. The adhesive SE exhibited almost the same bond strength as SB, irrespective of the test method.
Cantilever bending can be used as an alternative method for bond strength measurement. The ratio of bending to tensile strength is around 1.5.
The bond strength of a dental adhesive is usually assessed with the tensile or shear test method . For example, the micro-tensile test method has been widely used since its introduction in 1994 by Sano et al. , because more specimens can be made with one tooth and the method is capable of measuring regional bond strength on a small area of the tooth. In theory, the tensile test method produces a uniform axial tensile stress in the specimen, as shown in Fig. 1 (a) . While a uniform stress distribution in the specimen is always desirable for strength measurement, this is considered a reason for the large proportion of cohesive failures associated with the test , especially when the adhesive bond strength is comparable to the fracture strength of the substrate materials, i.e. the tooth tissue or composite . The shear test method, with the load applied close to the interface but transverse to the beam axis, does not seem to give better results. In fact, more cohesive failures have been reported in shear tests than in tensile tests . Also, finite element analysis (FEA) has shown that the stress distribution across the interface of a shear test specimen is highly non-uniform , making it difficult to determine a representative bond strength from the failure load. More importantly, the tensile stresses at the interface are actually higher than the shear stresses, suggesting that the former and not the latter are responsible for failure .
If the loading point is some distance away from the bonding interface, the shear test will become the cantilever bending test, which is a flexural strength measuring method. It produces linearly varying stresses along the length of the beam specimen, as shown in Fig. 1 (b), with the highest stress located at the beam root where fracture is most likely to take place. Therefore, by placing the adhesive interface at the beam root, the cantilever bending method can be expected to increase the success rate of bond strength measurement. Another advantage of the cantilever bending method is its simpler sample mounting procedure , which only needs the specimen to be clamped at one end. With the micro-tensile method, on the other hand, both ends of the specimen have to be glued to the loading fixture . The stresses across the interface of a cantilever beam are not uniform though, varying linearly with the beam depth; see Fig. 1 (d). However, a simple equation based on the beam bending theory relates the maximum tensile stress with the load. Finite element analysis (FEA) based on linear-elastic models of the tensile and cantilever bending tests confirms that the actual stress distributions are very close to the theoretical solutions; see Fig. 1 (c) and (d).
The objectives of this study were to compare the adhesive bond strengths at the enamel–composite interface as measured by the cantilever bending and micro-tensile test methods, and to evaluate the effectiveness of cantilever bending as a micro bond-strength measurement technique.
Materials and methods
The bond strengths of two dental adhesives were tested when they were used in the bonding of a composite to bovine enamel. The labial surfaces of bovine incisors were first flattened on 600-grit diamond sand paper. Then, a composite (Filtek™ Z250, 3M ESPE) was incrementally placed and cured on the flattened tooth surface to a thickness of about 5 mm, following the application of either Adper™ Scotchbond™ SE Self-Etch adhesive (SE) or Adper™ Single Bond Plus (SB) adhesive, both from 3M ESPE, according to the manufacturer’s instructions. After 24 h of storage in distilled water at 37 °C, the samples were cut with a low-speed blade to obtain matchstick-shape specimens (0.7 mm × 0.7 mm cross-section). The specimens were then divided into four groups ( n = 30 each) according to the adhesive and test method used, i.e. SE-T, SE-B, SB-T and SB-B, where T denotes the micro-tensile test and B denotes cantilever bending.
The experimental setups of the micro-tensile and cantilever bending tests are shown in Figs. 2 and 3 , respectively. For the micro-tensile test, a cyanoacrilate material (Zapit, Dental Ventures of America, CA, USA) was used to bond the ends of each specimen to the two free-sliding parts of a specially designed holding device called Geraldeli’s jig . The jig is able to transmit to the specimen purely tensile forces, without any torquing or bending component, when it is mounted on a universal loading machine (MTS, USA). The tensile load was applied at a crosshead speed of 0.5 mm/min, until fracture of the specimen occurred. The maximum load recorded was used to calculate the bond strength. For the cantilever bending test, a specially designed loading assembly was used, as shown in Fig. 3 . The enamel end of the specimen was clamped between two L-shape steel plates, 1 and 2, which were coupled together with screws. Screw 1 on the top was used to push Plate 1 against the specimen while Screw 2 held the two steel plates together. The clamping was very gentle to avoid adding undue forces to the specimen. The loading plate, mounted on a sliding track for smooth load application, was also driven by the MTS machine at a loading speed of 0.5 mm/min to bend the specimen upwards from its composite end. The arm length L in Fig. 3 , i.e. the length from the beam root to the loading point, was 2.65 mm. Note that the tooth–composite interface coincided with the left end faces of the clamping plates.
The cross-sectional dimensions at the tooth–composite interface were measured to the nearest 0.01 mm with a digital caliper for the calculation of bond strengths. The bond strength given by the tensile test was simply the failure load divided by the cross-sectional area at the interface. For the bending test, the bond strength, as represented by the maximum stress σ max at the beam root, was calculated with the beam bending equation :
σ = M y I ,
where σ is the direct stress, M is the bending moment, y is the transverse distance from the beam’s neutral axis and I is the second moment of area of the beam’s cross section. Thus, the maximum stress σ max at the beam root is
σ max = 6 F L b h 2 ,