Comparative evaluation of 3 microbond strength tests using 4 adhesive systems: Mechanical, finite element, and failure analysis

Abstract

Statement of problem. Bond strength (BS) values from in vitro studies are useful when dentists are selecting an adhesive system, but there is no ideal measuring method. Purpose. The purpose of this in vitro study was to investigate the influence of the evaluation method in the BS between dentin and composite resin. Material and methods. Molars with exposed superficial dentin (N=240) were divided into 3 groups according to the test: microtensile (mu TBS), microshear (mu SBS), and micropush-out (mu PBS). Each one was subdivided into 4 groups according to the adhesive system: total etch, 3- and 2-step; and self-etch, 2- and 1-step). For the mu PBS test, a conical cavity was prepared and restored with composite resin. An occlusal slice (1.5 mm in thickness) was obtained from each tooth. For the SBS test, a composite resin cylinder (1 mm in diameter) was built on the dentin surface of each tooth. For the mu TBS test, a 2-increment composite resin cylinder was built on the dentin surface, and beams with a sectional area of 0.5 mm(2) were obtained. Each subgroup was divided into 2 (n=10) as the specimens were tested after 7 days and 1 year of water storage. The specimens were submitted to load, and the failure recorded in units of megapascals. Original BS values from the mu TBS and mu SBS tests were normalized for the area from mu PBS specimens. Original and normalized results were submitted to a 3-way ANOVA (alpha=.05). The correlation among mechanical results, stress distribution, and failure pattern was investigated. Results. Significant differences (P<.05) were found among the adhesive systems and methods within both the original and normalized data but not between the storage times (P>.05). Within the 7 days of storage, the original BS values from mu TBS were significantly higher (P<.001) than those from mu PBS and mu SBS. After 1 year, mu SBS presented significantly lower results (P<.001). However, after the normalization for area, the BS values of the mu TBS and mu PBS tests were similar, and both were higher (P<.001) than that of mu SBS in both storage times. In the mu SBS and mu TBS specimens, cohesive and adhesive failures were observed, whereas mu PBS presented 100% of adhesive failures. The failure modes were compatible with the stress distribution. Conclusions. The storage time did not affect the results, but differences were found among the adhesives and methods. For comparisons of bond strength from tests with different bonding areas, the normalization for area seemed essential. The microshear bond test should not be used for bond strength evaluation, and the microtensile test needs improvement to enable reliable results regarding stress concentration and failure mode. The micropush-out test may be considered more reliable than the microtensile in the bond strength investigation, as demonstrated by the uniform stress concentration and adhesive failure pattern.