The research hypothesis, which suggested that combining different primers and cements from various systems would affect the bond strength between zirconia and a dentin surface, was accepted. The results show that not only do these combinations not necessarily decrease bond strength but also enhance it in some cases. Primers from the PV5 cement system demonstrated the highest bond strength to zirconia compared to other primer combinations, regardless of the type of resin cement used. This superior performance may be attributed to the fact that these primers are specifically developed for zirconia, while SU is marketed as a multipurpose primer with a formulation and composition that might differ from the specialized primers for zirconia (11, 14). One significant difference between these cement systems lies in the pH value of the primers for the tooth surface: Scotchbond universal primer is considered an ultra-mild etchant (pH > 2.5), whereas Panavia tooth primer is a mild etchant (pH ≈ 2). Mild primers are preferable to ultra-mild primers, as they have shown better efficacy in removing smear layers and demineralizing the superficial layer of the dentin surface. This increased etching potential, in turn, can enhance bonding effectiveness, as surface smear may interfere the bonding process (11).
All primers used in the present study contained the key component 10-MDP, which has been recommended to achieve a durable bond to zirconia (5, 7, 8). However, the bonding effectiveness is not only dependent on the presence of these monomers but also on parameters such as the concentration and quality (purity) of the 10-MDP in the adhesive material (11). In vitro studies show that impurities of this monomer can inhibit the chemical interaction of 10-MDP with hydroxyapatite of tooth tissue, which may negatively influence the bond strength (9). The Panavia cement system containing the functional monomer 10-MDP has been shown to have a superior purity grade compared to other commercial primers (Yoshihara 2015). Furthermore, the concentration of 10-MDP has been confirmed to significantly enhance the bond strength to zirconia. However, detailed information on the exact concentration and quality of the chemicals used in the present study are not disclosed, as manufacturers commonly consider this information to be a trade secret (6, 11).
Thermocycling was used to stress the bonded interface of the specimens, simulating an intra-oral aging scenario. In the absence of consensus on aging procedures, particularly in the context of evaluating adhesion to oxide ceramics with resin cement in laboratory settings, a minimum of 5,000 thermocycles is considered sufficient (6, 10). Given that the bonded area of the specimens was approximately 1 mm², a more pronounced aging effect, such as hydrolysis of the cement interface, could be expected (6). The hydrolysis effects of thermocycling significantly affect bond strength of resin cement (20). Interestingly, bond strength values obtained using the Panavia adhesive cement systems have been shown to be stable and modestly affected by thermocycling procedures (21–23). One possible explanation for this resilience could be that the monomer in these cement systems is less sensitive to hydrolysis due to the specific structures of their 10-MDP formulation (9). In summary, these factors could explain the superior performance of Panavia cement system as observed in the present study.
The µTBS test was used as it is considered a versatile and standard method for testing bond strength (5, 6, 13, 24). An advantage of this test method is that it focuses on clinically relevant substrates, and it requires less material to produce specimens compared to shear bond strength tests. Furthermore, the test promotes a more homogeneous stress distribution at the bonded interface (25). Unlike the shear bond strength test, failures in this method often originate in the adhesive zone rather than within the bonded substrates (25). In the present study, specimens were prepared using the non-trimmed technique in a rectangular cross-sectional shape, which is considered easier and less technique-sensitive compared to the preparation of cylindrical cross-sectional specimens. The importance of the geometry of these specimens has been emphasized, where cylindrical cross-sectional specimens are favored because they theoretically provide a more uniform stress distribution along the resin–dentin interface, in contrast to the rectangular cross-sectional specimens (13). However, when comparing the performance of these cross-sectional shapes, similar bond strengths have been observed (26, 27). This finding suggests that the non-trimming technique could be a viable, simpler alternative for specimen preparation in an already complex procedure.
Cohesive fractures within the cement were predominant in all groups, regardless of the cement system or cementation procedure. Fracture occurrence within the interface is preferable, as it is considered to accurately represent the actual bond strength of the cement. Reports indicate that in bond strength tests, the modes of failure primarily show cohesive fractures occurring more frequently in resin composite or ceramic materials than at the cement interface (24). In µTBS tests, cohesive failures occur predominantly in the cement layer, contrary to shear bond tests. Therefore, µTBS tests have been considered more appropriate for evaluating the bond strength of resin composite to ceramic (25). The findings might thus more accurately reflect the bond strength in the bonding area rather than the strength of the test materials themselves. No cohesive fractures or adhesive failure in zirconia were observed. This finding is in line with previous studies where pretreatment with airborne abrasion, in combination with a 10-MDP based adhesive cement, leads to durable bonding to zirconia (5, 7, 8, 10). However, several cohesive fractures and adhesive failures were noted on the dentin surface. Establishing a durable bond to enamel has been proven to be reliable, in contrast to dentin, which is acknowledged to be more challenging (11, 13). The variability in dentin structure, such as differences in hydroxyapatite content and humidity, proximity to the pulp, and orientation of tubules may contribute to the lower predictable adhesion to dentin. Furthermore, adhesion could be influenced by other biological and clinical factors. These include the depth and permeability of the dentin, the tooth's location in the mouth, the type of restorative material and procedure used, isolation, and the dentist’s experience (12, 13). In the present study, the ISO/TS 11405:2016 protocol was followed to standardize the laboratory procedures. Human extracted posterior caries-free and unrestored teeth with sound superficial dentin were selected due to their higher permeability of the dentin structure. Furthermore, one single operator performed all cementation procedures according to the manufacturer’s recommendations, a step that might minimize the technique sensitivity of the test materials.
Limitations
In the present study, only two resin cement systems were evaluated. Consequently, the results are limited to these specific products. Ideally, including more resin cement systems would be preferable to determine if any further differences exist. The choice of cements was based on the fact that they are both recommended for use with zirconia, yet they have some interesting differences. The Panavia V5 cement system has specific primers indicated for zirconia, and the manufacturer states that it does not recommend using its primers with other composite cements, even those within its own brand, while the RelyX Ultimate cement system offers a simple, universal primer designed for multipurpose use.
The cementation procedures were performed by a single operator. This aspect might be viewed as a limitation in terms of the study's generalizability, where involving multiple operators would typically be preferred. However, in the case of an experimental in vitro study, it is considered a strength, because the primary study aim was to evaluate the effect of combining different primers on bond strength rather than the technique sensitivity of these materials.
Based on the results in this study, the Panavia V5 cement system is preferable for bonding to zirconia, especially in clinical situations where restorations rely more on adhesive bonding than on macromechanical retention. Moreover, this study highlights the potential effects – both negative and positive – of combining primers and cements from different cement systems, which could lead to less predictable clinical outcomes. However, caution should be taken when interpreting laboratory results, as the bond strength values observed cannot be directly applied to the clinical situations (24).