The goal of this paper is to undertake a literature search collecting all 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.
A PubMed search was carried out for the years between 1998 and 2009 identifying publications on bond strength measurements of resin composite to dentin using four tests: shear, tensile, microshear and microtensile. The six adhesive resins were selected covering three step systems (OptiBond FL, Scotch Bond Multi-Purpose Plus), two-step (Prime & Bond NT, Single Bond, Clearfil SE Bond) and one step (Adper Prompt L Pop).
Pooling results from 147 references showed an ongoing high scatter in the bond strength data regardless which adhesive and which bond test was used. Coefficients of variation remained high (20–50%) even with the microbond test. The reported modes of failure for all tests still included high number of cohesive failures. The ranking seemed to be dependant on the test used.
The scatter in dentin bond strength data remains regardless which test is used confirming Finite Element Analysis predicting non-uniform stress distributions due to a number of geometrical, loading, material properties and specimens preparation variables. This reopens the question whether, an interfacial fracture mechanics approach to analyze the dentin–adhesive bond is not more appropriate for obtaining better agreement among dentin bond related papers.
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.