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Background and introduction

Many researchers would agree that bond strength testing and interpreting outcomes of such tests can be frustrating, in large part due to the typically high variance of the data and the discrepancy between results reported in the literature by different laboratories. One could argue that this situation has not changed substantially in decades. There is as yet no agreed upon protocol to follow when testing adhesion in dentistry. This is especially critical when bonding to tooth structure which presents a more variable bonding surface than many other dental materials. And despite the fact that a testing specification exists (ISO/TS 11405: Dental materials – testing of adhesion to tooth structure. Switzerland, 2003), it is often not followed, perhaps because it is not easily identified by a search of the literature. Thus, the majority of published studies fail to report information about testing conditions that are crucial for comparing and interpreting results. Furthermore, the limitations of certain testing modes and conditions are typically not discussed, so the real value of the results and their interpretation is often questionable.

It is interesting to look back at the numerous symposia and conferences on this topic that have been held in the dental materials community over the years. Many similar conclusions have been drawn by speakers at these conferences. For example, at a symposium entitled “Adhesion – Its Theory and Practice In Restorative Dentistry,” sponsored by the Dental Materials Group of the International Association for Dental Research and Imperial Chemical Industries (ICI) Dental held in Chicago, IL, USA on March 10, 1987, Dr. Kevin Kendall (ICI) in his talk entitled “The Need for Adhesion,” stated that “Measuring adhesion is a problem because many practitioners use adhesive strength. The only way to resolve this issue is to use fracture mechanics to interpret the cracking of adhesive bonds. Adhesive fracture energy is the key parameter and not adhesive strength.” In the 1994 Academy of Dental Materials Annual Meeting entitled “Lifetime Prediction and Failure analysis of Restorative Materials,” held in Southampton, Bermuda, November 10–12, Dr. Paul Dehoff (University of North Carolina – Charlotte) presented a paper entitled “A Critique of Bond Testing: What Are We Measuring?” He concluded “Because of the presence of stress concentration effects at the loading site … it is unlikely that a mechanical strength test will yield quantitative information about adhesion. However, bond strength tests may still provide useful information on procedural changes although the actual bond strength value may have little meaning.” But despite these and similar statements made at other meetings and in certain publications, the field appears not to have moved significantly forward in terms of describing the true significance of bond strength testing to the overall performance of adhesive systems used in dentistry.

With these issues in mind, an argument can be made for a comprehensive critique of the bond strength testing modalities used in dentistry, along with a treatment of the association between these tests and other in vitro and clinical outcomes. Ideally from this effort a manuscript would be generated containing the necessary information to include in studies and subsequent reports of adhesion of dental materials to any substrate. Such a consensus document is lacking in the literature, though many excellent reviews have been written about the subject and are cited in the manuscripts that follow this introduction.

The literature addressing the testing of bond strength of dental materials to various surfaces, and in particular adhesion to enamel and/or dentin, is voluminous. A quick search using PubMed and the search terms “dentistry bond strength testing” yielded 3197 articles. When “dentistry bond strength testing teeth” was entered, 1563 articles were returned. Searching on the words “dentistry bond strength testing dentin” returned 1667, and “dentistry bond strength testing enamel” returned 932. When the date of publication for the 3197 articles was queried in five-year increments, the figure generated showed a relatively consistent publication of roughly 1000 articles per year over the last 15 years. Reviewing the bond strength testing literature is therefore somewhat intimidating, but this was the task requested of nine leading dental researchers in order to address the titled topic of the 2009 Academy of Dental Materials meeting.

There are many variables to consider when performing bond strength tests, and some of these are summarized in the table below.

It is important for any literature on this topic to include a description of these variables in order to accurately assess the results and to enable comparisons between studies. However, often this is not done, and making general statements about a specific material or condition from this literature becomes even more difficult. This is particularly so when trying to correlate the results of bond strength tests to other measured properties, as well as to the clinical performance of specific materials. Therefore, it was appropriate to review these considerations based on the current literature, and to discuss them with a wide audience.

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Goal of the conference

The overall goal of this three half-days conference was to provide a critical assessment of the various test methods used primarily for dentin and enamel bonding studies in order to identify general trends, important variables to be considered, and their relation to outcomes. It was thought that the results of this conference might serve as a basis for a future meeting with the goal of reaching consensus about what is important to include in bond strength testing and which method may be most accurate, reliable and preferred.

With this in mind, the agenda for the 2009 ADM meeting was set with three invited speakers per day as outlined below. The presenters were charged with reviewing and presenting a critical analysis of the current literature pertaining to their topic, and making recommendations for further study and consideration where appropriate. At the end of each day, a short summary was presented based in part on the content of the presentation and the manuscripts received beforehand and appearing in the transactions for the meeting.