## 4 Discussion  
Storage in water and TC of resin-bonded specimens are the most frequently used artificial aging methods [9]. Although TC is an effective accelerated aging protocol for evaluation of bonding durability [23], the extent to which it practically mimics the real clinical situation has been questioned [10]. TC tests cannot discriminate which type of failure causes the degradation of resin bond to ceramic [10]. While there is no definite evidence that thermal stresses cause failures, every TC test should be considered arbitrary [10]. This in vitro study thus evaluated the effectiveness of the MLC aging method, alone or combined with TC, in challenging the resin-zirconia interface. As MLC was applied to the resin materials in a wet environment to simulate intraoral condition (Figure 1), the surface energy parameters, including degree of hydrophobicity/hydrophilicity, as well as the flexural properties of the resin cements bonded to zirconia were studied prior to the SBS test. The flexural property results suggest that the resin materials behave similarly when subjected to MLC in dry condition. The parameter \(\Delta G_{\text{sws}}\) indicates whether the polar attraction of solid to water is greater or smaller than the polar attraction of water molecules to each other and can thus serve as a more appropriate measure than water CA itself [24]. In the context of [PERSON]/[PERSON] acid-base ([PERSON] and [PERSON]) theory, the base components of cements may significantly affect their bonding to zirconia [3]. In this study, notwithstanding a significant difference in the base components between the two resin cements (Table 2), no significant difference in SBS was found between the two materials before aging (Table 3). This may have been due to only a small difference in the base components of the materials. During aging of a resin-bonded zirconia specimen in water, water may diffuse into the resin as well as into the adhesive interface [25]. Water sorption may cause hydrothermal degradation, such as the creation of swelling stresses, of a resin bonded to a zirconia [26]. Thus, the stability of the bonding interface may depend on the hydrophobic/hydrophilic nature of the resins used [25]. When more hydrophobic resins are used, greater stability is supposed, as water would not be able to penetrate the bonding interface so easily [25]. In TC, several factors such as temperature setting, dwell time, and number of cycles can affect the bond strength test results [23]. As regards the number of cycles, ISO 10477 prescribes 5000 cycles [27], which seem insufficient for estimating the long term bonding durability [10]. In this study, the SBS values after TC was not significantly different from those in the NA condition for both resins (Table 3), probably because the specimens were not water-saturated prior to TC and the number of TC was not high enough to age the resin bonds [8; 28]. Unlike TC, the use of MLC, which simulates mastication, applies a direct load to a bonded specimen [29]. A force of 50 N was chosen to simulate an average of constant load found during mastication [21; 29; 30]. For both resin materials, the MLC-containing aging conditions significantly decreased the SBS values when compared to the NA (Table 3). However, the tendency was not the same between the two resin materials. In this study, the only significant difference in SBS between the two resins was found for the MLC-only aging condition. MLC aging of the RelyX U200 exhibited a significantly higher SBS than did the other MLC-containing aging conditions, probably due to the significantly higher hydrophobicity of the resin material (Table 2). The SEM images of the zirconia surfaces after debonding (Figure 3) suggest the application of MLC to the resin-bonded specimens caused crack formation within the resin materials as well as at the resin-zirconia interface. For both cements, thus, the Weibull moduli of the three MLC groups significantly decreased when compared with the NA groups.