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Introduction to the review

When an assignment for a literature review involves the subject of “dentin bonding” an instantaneous shiver runs down the backbone as the amount of data is a knock out (over 6000 papers just with the Keywords: “bonding” AND “dentin”) and when combined with the sorting through testing parameters and variables, it becomes a nightmare. The reasons for such a popular topic of research is of course the rapid development of bonding adhesives to dentin and the fact that the product screening test methods such as shear, tensile and microtensile are inexpensive testing routines in most dental school or research laboratories. Dentin bonding however has been notorious for high spread in the results, whether within the same laboratory or between laboratories using the same tests, and even 15–20 years ago suggestions were raised to improve the standardization of bond strength testing after reviewing possible reasons for such variability . The introduction in 1994 of the microtensile bond strength test allowing measurements of the tensile bond strength on very small surfaces (∼1 mm ² ) opened the research to regional differences within dentin, and had the advantage of producing many specimens from the same extracted tooth. It was thought that with this test, the characteristic high bond strength spread obtained in conventional shear and tensile tests would diminish due to a better stress distribution over a very small surface during loading, generating more interface failures (i.e. fewer cohesive failures) in dentin . In 1999, Pashley et al. reviewed positively the microtensile bond test and summarized its versatile usage providing new insights into strength of adhesion of restorative materials to clinically relevant sites and substrates and advocated this test as a means for evaluating the long-term durability of resin-hard-tissue bonds. Since then, the microtensile bond strength test has become the most used dentin bond test.

The philosophical question however to be asked here is what is the final goal of measuring bond strength? Is it to measure interfacial bond strength? Is it to distinguish product A from product B? Is it to rank the products according to their results? Is it to understand localized degradation within a bonded surface? Is it a test to indicate reliability of the bonding? Which test should be used? Are the results dubious? Well, considering the information gathered from Finite Element Analyses (FEA) generated for shear , microshear , tensile and microtensile , almost every possible testing variable (i.e. specimen’s geometry, loading condition, film thickness, modulus of elasticity of the materials involved) has a significant influence on the stress state and thus on the bond strength values.

Therefore, the goal of this paper is to perform a 10 year literature search collecting dentin bond strength data obtained for six adhesives with four tests (shear, microshear, tensile and microtensile) and to critically analyze the results with respect to average bond strength, coefficient of variation, mode of failure and product ranking. No attempt is made to undertake additional statistical analysis, as the major focus of this paper is to point out limitations of the most popular methods for evaluating adhesion. Alternative approaches to bond strength evaluation including the use of Weibull statistics and fracture mechanics will be discussed.