Research PaperFatigue of the resin–enamel bonded interface and the mechanisms of failure☆
Introduction
In the field of restorative dentistry, bond strength testing has been used to characterize the adhesion of restorative materials to dentin and enamel since the invention of acid etching (Buonocore, 1955). Micro-tensile and micro-shear tests are the primary approaches for investigating the potential performance of new dental materials and/or the bonding procedures (Pashley et al., 1999, Spencer et al., 2010, De Munck et al., 2012). There are several variations of these two methods, and a number of reviews have been presented to discuss their qualities and the advantages of particular configurations (e.g. Armstrong et al., 2010, Braga et al., 2010, Scherrer et al., 2010). The bond strength obtained using these techniques has been adopted as a metric of performance, and high strength is considered to be indicative of longevity in the oral environment. But there are concerns regarding the applicability of these methods, and their relevance toward understanding the true causes of clinical failures (Van Meerbeek et al., 2003, Kelly et al., 2012, Roulet, 2012, Söderholm, 2012). Perhaps the prevailing concern is that the results of in vitro experiments do not reflect the reality of failures in vivo, and there is little correlation to clinical behavior (Ferracane, 2013).
A load-bearing restoration bonded to dentin and enamel must resist damage over many years of function. Cyclic stresses transmitted across the resin adhesive and hybrid layers may cause degradation of the interface by fatigue (Spencer et al., 2010, Pashley et al., 2011). Yet, in comparison to quasi-static loading, the influence of cyclic loading to the adhesive interface has received limited attention. Ruse et al. (1995) reported one of the earliest studies to evaluate the strength of resin–enamel bonds in response to cyclic loading. Since then, surprisingly few studies have experimentally examined fatigue of the resin–enamel interface (De Munck et al., 2005, Erickson et al., 2006, Erickson et al., 2008, Erickson et al., 2009a, Barkmeier et al., 2009). Reported studies on fatigue properties of the resin–dentin interface are equally scant (Drummond et al., 1996, Frankenberger et al., 1999, Frankenberger et al., 2003, Frankenberger et al., 2005, De Munck et al., 2005, Soappman et al., 2007, Staninec et al., 2008a, Staninec et al., 2008b, Belli et al., 2010). In fact, a quick survey of the literature shows that more manuscripts are published on micro-tensile testing in one month than the total number of studies reported on fatigue degradation of the bonded interface overall!
Two qualities summarize the findings of prior studies on the fatigue strength of adhesive bonds to dentin and enamel. Firstly, the fatigue strength of the interface is lower than that found in monotonic testing, with ratios of fatigue to static strength ranging from roughly 0.35 to 0.6. Secondly, adhesive bonds to dentin appear to be more durable than those to enamel (De Munck et al., 2005). One potential limitation of the reported studies on enamel bonding is that cyclic loading has been limited to 1×105 cycles or lower. Considering that there are several thousands cycles of mastication per day (Anusavice, 2003), the aforementioned duration of previous fatigue evaluations is much less than one year of oral function. In addition, previous studies on enamel bonding have been conducted primarily using a cyclic shear configuration, whereas the bonded interface appears least durable in cyclic tension (Staninec et al., 2008a).
A recent study on the durability of resin–dentin adhesive interfaces adopted the use of a novel twin bonded interface specimen subjected to cyclic flexure loading (Mutluay et al., 2013a). Fatigue failures occurred by cyclic tension and the ratio of the apparent endurance limit to the ultimate strength of this interface was 0.26. That estimate is essentially half that reported for cyclic shear loading of enamel. If cyclic tensile stresses are indeed most detrimental (Staninec et al., 2008a), then the durability of enamel bonds should be assessed under that mode of loading. Therefore, the present study adopted a novel approach to evaluate the strength of resin–enamel adhesive bonds under both monotonic and cyclic flexure loading. The objectives were to characterize the interface strength under the two modes of loading, distinguish the mechanisms of degradation and failure, and compare the results with those achieved for dentin bonding.
Section snippets
Materials and methods
Bonded interface specimens were prepared from the enamel of caries-free human third molars. All extracted teeth were obtained with record of patient age (18≤age≤30) and gender from participating clinics in Maryland according to a protocol approved by the University of Maryland Baltimore County (#Y04DA23151). The teeth were stored in Hank's Balanced Salt Solution (HBSS) for less than 1 month, and then sectioned using a computer-controlled grinder (Chevalier Smart-H818II, Chevalier Machinery,
Results
The strength distributions of the resin–enamel interface, resin composite control and resin–dentin bonded interface specimens obtained from monotonic loading to failure are shown in Fig. 4. All three responses are shown in terms of the average and standard deviation and were obtained using the exact same specimen configuration and loading conditions. According to the one-way ANOVA, the average flexural strength of the resin–enamel interface (19.9±3.9 MPa) was significantly lower (p≤0.000) than
Discussion
An experimental evaluation on the strength and durability of the resin–enamel bonded interface was conducted. Although restored teeth undergo cyclic loading during mastication, few studies have assessed the fatigue behavior of enamel bonds (De Munck et al., 2005, Erickson et al., 2006, Erickson et al., 2008, Erickson et al., 2009a, Barkmeier et al., 2009). Moreover, previous evaluations on this topic have been limited to 1×10^5 cycles of loading and less. With the average daily routine
Conclusion
An experimental evaluation of the strength and durability of resin–enamel bonds was conducted under monotonic and cyclic normal stresses using a specially designed bonded interface specimen. The apparent endurance limit of the resin–enamel interface resulting from bonding with a commercial self-etch adhesive and resin composite was less than 20% that of the resin composite, and just over 60% of the value achieved in dentin bonding when evaluated in the same manner. Failure of the bonded
Acknowledgments
This study was supported in part by a seed Grant from the University of Maryland Baltimore County (D. Arola) and an award from the National Institutes of Health (NIDCR R01 DE016904; Arola). The authors also gratefully acknowledge Kuraray America for their generous donation of bonding supplies and resin composite, and Prof. Frederick Rueggeberg of the Georgia Health Sciences University for kindly supplying and calibrating the light curing unit.
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Cited by (0)
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Support for the following investigation was provided in part by the National Institutes of Dental and Craniofacial Research (DE016904).
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Co-first authors.