Aspects of adhesion tests on resin–glass ceramic bonding
Introduction
It is unquestionable that ceramic has become widely employed in dental restorations as its superior mechanical properties and better appearance have improved substantially [1], [2]. However, there remains a major constraint for clinical application of ceramic materials, they are vulnerable when subjected to tensile stresses [3], i.e. ceramics are prone to fracture that is a failure. Despite most restoration failure originating from ceramic fracture, the bonding reliability of the ceramic is a possible explanation for restoration failure and should not be overlooked.
Other than the performance of ceramic or resin cement, adhesion to a ceramic material is the one of the key factors to evaluate the bond durability (adhesion strength)[4], i.e., long term success in clinical application for an adhesion system of an indirect restoration [2]. Current procedures for tooth preparation now aim to preserve as much dental hard tissue as possible [5], hence, restoration retention mainly relies on adhesion to the prepared tooth [3]. Therefore, adhesion is crucial and should be thoroughly evaluated in order to examine and understand bond durability.
To begin with, there are adhesive interfaces. In a traditional indirect restoration adhesive system, there are always two adhesive interfaces: ceramic to resin cement and tooth structure to resin cement [3]. Various studies have evaluated the ceramic to resin [4] and tooth to resin [6], [7] bonds in order to better understand and enhance the adhesive strength [8]. Other than the adhesive interface, the bonding mechanism has also been studied extensively. There are two mechanisms involved in ceramic to resin cement bonding, namely, micromechanical interlocking and chemical bonding [2]. To create a micromechanical interlock, hydrofluoric acid etching [3], [4], [9] or, sometimes, grit-blasting [10], [11], [12] of the ceramic surface are the usual methods. Application of a silane coupling agent on etched glass ceramic surface is mandatory to create durable chemical bonding [6], [8], [13], [14], [15], [16].
As mentioned, to determine whether a restoration is successful, not only needs the individual performance of ceramic and resin cement to be considered, but also the actual bonding mechanisms between the ceramic and resin cement must also be studied. To evaluate durable bonding in a laboratory setting, various bond strength tests have been developed [17], [18]. The strength to hold the adherend components together is denoted as the bond strength (adhesion strength). A typical model for bond strength testing involves either a (pre-)treated tooth or ceramic specimen joined to a resin composite block (specimen) with resin-based luting cement [3]. The bond strength is calculated by dividing the maximum load to break the bonded specimen by the actual bonding area [2]. Bond strength tests can be categorized into two main types: tensile [3], [7], [19], [20], [21], [22], [23], [24], [25] and shear [26], [27], [28], [29], depending on the primary stress applied to the interface. Nevertheless, bearing in mind that there is no genuine shear strength test in existence in dentistry [17], [30].
Debate on bond strength tests has continued, with some researchers challenging the validity of these tests with strong criticism to the experimental methodologies [31], [32]. Numerous studies have been performed, not only to evaluate the bond strength, but also to verify the test methods [32]. Among the various laboratory bond strength test methods, the microtensile bond strength (MTBS, μTBS) test is the most popular technique to test ceramic to resin cement bonding [3]. Before the microtensile bond strength test was developed [18], the tensile bond strength (TBS) test was available. The only difference between the micro- and tensile bond strength test is the bonding area, otherwise the test conditions are almost identical. Indeed, for the microtensile bond strength test, which was first proposed by Sano et al. in 1994, the bonding area was set below 1.0 mm2 so that stress distribution across the bonded interface was suggested to be distributed more evenly [18]. It was also suggested that compared with the traditional tensile bond strength test method, the microtensile bond strength test results in more adhesive failures, i.e., adhesive failures may reveal the ‘true’ bond strength. On the other hand, the microshear bond strength test is the most recently accepted method to test tooth to resin composite cement adhesion [6], e.g., a resin composite component joined to a pre-treated tooth surface. Nevertheless, it seems that the word “micro” is a matter of absolutely arbitrary terminology for the sake of appearance only; it has no real meaning!
Four-point bending test indeed is one of the newest methods to access bond strength, such that only few studies were published to date [33], [34], [35]. Although interfacial tension test has been advocated for measuring bond strength, the stress distribution at interface is indeed complex and the specimen preparation as well as the alignment is not as simple as that of the other methods [33], [36]. More importantly, four-point bending test has the maximum tensile stress on the convex surface [33] and removed the stress concentration at the surface of adhesive [30], which deemed to be more clinical relevant than direct tension test. However, different from the tensile bond strength test, the four-point bend test requires the specimen to be placed horizontally with the adhesive joint placed centrally where the stress is placed at the adhesive joint and the specimen supported at both ends using a fixed distance. The load leads to bending of the specimen and creates a combination of stresses, namely tension and compression, and therefore, interpretation can be more difficult. At the same time, it is achievable to have most jointed specimens fail adhesively but no further research has been undertaken to better understand this method and its applicability to clinical outcomes. This said, direct comparison between four-point bending and tensile bond strength tests are impossible, since the test conditions, e.g. the stress application along the adhesive interface and generated outcomes are different. Certain test configuration and conditions are necessary for bending test, such as the ratio between span width and specimen depth should be greater than 10 (based on the requirement for testing dental ceramics [33]), in order to prevent shear stresses within the adhesive joint [36], otherwise correction is necessary.
Furthermore, another factor of a bond test that has been often overlooked is the failure mode. It is a crucial parameter to judge whether a bond strength test method is reliable. An adhesive failure represents failure at the bonded interface while cohesive failures in the parts of the test set-up represent failure found outside the bonded interface [2], [24], i.e., the failure originates from either adherend components or adhesive. It is universally believed that a true bond strength can only be determined from an adhesive failure mode [2], [24]. Therefore, it is not a surprise that the microtensile bond strength is claimed to be reliable as most joints fail adhesively [18]. In fact, study [35] has shown that it is achievable to have adhesive failure in the majority of specimens with a four-point bending test, i.e., similar outcomes as with the microtensile bond strength test can be achieved. This raises the question that four-point bend tests may also be a very reliable test method.
Nowadays, researchers still attempt to optimize bond strength tests [3], [17], [30], and so far, no wide agreement has been reached. Some claim the microtensile bond strength test method is the most reliable, others insist on using a microshear bond strength test method [9]. Therefore, the aim of this laboratory study was to compare and contrast the two bond test methods: the four-point bending test and the tensile bond test. In this research, the interest of the bond strength test only focuses on ceramic to resin cement bonding. Given this, it is suggested to establish a test model with two ceramic components being joined with a resin composite cement [3], so that fracture can only be generated at the bonded interface or cohesively from the ceramic component or resin composite cement. In addition, the influences of HF etching time and storage conditions were studied.
Section snippets
Materials and Methods
The materials used, together with the batch numbers and manufacturers are listed in Table 1.
Tensile bond strength
The sample size, means and standard deviations of each test group for the tensile bond test are listed in Table 2. Two-way AoV (Table 3(a)) revealed HF etching time had no significant effect on the tensile bond strengths (p ∼ 0.059 > 0.05), and marginally showed the etching time have significant effect on tensile bond strength (p ∼ 0.045 < 0.05) Box plots show the data spread together with mean and median in Fig. 1.
Flexural bond strength
The sample size, flexural bond strength means and standard deviations of each test group
Discussion
In this study, there was a significant difference determined in terms of the Weibull parameter, η and B10 indicating the four-point bend test shows a higher consistency than the tensile bond test in evaluating the bond strengths. The Relative Standard Deviation (RSD, also named coefficient of variation, CV) values also confirm that the tensile bond strength test (50.37–85.19) has a lower precision and reliability compared to the four-point bend test (16.90–37.46). This coincides with the
Conclusion
Four-point bending showed a similar outcome to previous research, namely, etching time could significantly weaken the flexural bond strength, while water aging enhanced the flexural bond strength. However, the tensile bond test failed to contrast any differences. Accordingly, the four-point bend test seems to provide a better approach to evaluate bond strengths (adhesion strength) of ceramics luted to ceramic materials.
Acknowledgements
This work was done in partial fulfillment of the requirements of the degree of MSc (DMS) for the first author at the Faculty of Dentistry, The University of Hong Kong. The ceramic material and resin cement were generously supplied by the manufacturer Ivoclar Vivadent AG, Schaan, Liechtenstein.
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