Introduction
The use of prefabricated zirconia crowns for primary teeth has increased significantly in pediatric dentistry due to their superior esthetic properties, biocompatibility, and high mechanical strength compared to traditional stainless steel crowns [1]. Despite differences in fabrication methods between pediatric and permanent tooth zirconia crowns, both share a common critical requirement: achieving reliable and durable adhesion between the crown and resin cement is crucial for clinical success [2,3].
However, to achieve effective adhesion between zirconia and resin cement, appropriate surface treatment is essential [4,5]. Internal surface treatment of zirconia is particularly important because its polycrystalline structure and lack of a glassy phase render it resistant to traditional adhesive bonding techniques [6]. Research has shown that primers containing functional monomers are effective in achieving reliable bond strength to zirconia surfaces [7]. The use of such functional monomer containing primers contributes significantly to the clinical success of zirconia restorations [8-10].
Functional monomers in adhesive systems, particularly 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP) and glycerol phosphate dimethacrylate (GPDM), form chemical bonds with zirconia surfaces through their phosphate groups [8,9]. Building on the effectiveness of these functional monomers, universal adhesives have recently been introduced as innovative solutions to simplify the bonding procedure, while maintaining effective adhesion to various dental materials, including zirconia [9-12]. These universal adhesives incorporate different functional monomers that play a crucial role in establishing durable bonds with zirconia surfaces [9-12]. However, commercially available universal adhesives contain different types of functional monomers, and understanding how these differences affect adhesive performance is important.
Another important consideration in zirconia adhesion is long-term durability. The clinical longevity of zirconia restorations depends primarily on the quality of the adhesive interface between the restoration and the tooth structure [13]. The adhesive interface must withstand various challenging oral conditions, including temperature fluctuations, pH changes, and mechanical stresses from mastication [14]. The durability of universal adhesivezirconia bonds under these intraoral conditions over time continues to be a significant clinical concern [15]. Thermocycling simulates temperature variations in the oral cavity (5°C ‒ 55°C) that create stresses at the adhesive interface, providing valuable information for predicting clinical durability under such challenging environments [15-17].
Despite numerous studies on the bond strength of universal adhesives to permanent tooth zirconia [15,18], few have examined their effectiveness with prefabricated pediatric zirconia crowns [19]. In this context, our study selected two universal adhesives with different primary functional monomers, Scotchbond Universal Plus (SP, 3M ESPE, St. Paul, MN, USA) and Optibond Universal (OB, Kerr, Orange, CA, USA) for comparative research. SP contains 10-MDP as its main functional monomer, while OB contains GPDM. Additionally, dedicated zirconia primers such as Z-Prime Plus (ZP, Bisco, Schaumburg, IL, USA) have been specifically developed for zirconia bonding and are used in various clinical situations.
This study aimed to assess the shear bond strength of two universal adhesives containing different monomers (10-MDP in SP and GPDM in OB) compared with that of a dedicated zirconia primer when bonding to zirconia surfaces designed to replicate the internal surface of prefabricated pediatric zirconia crowns. Through this investigation, we aimed to analyze how differences in molecular structures of various functional monomers influence zirconia bond strength, and to predict longterm clinical performance by evaluating changes in adhesive stability following thermocycling.
Materials and Methods
1. Materials
In this study, two universal adhesives, one zirconia primer and one resin cement were used, following the manufacturer’s protocols. Scotchbond Universal Plus (SP, 3M ESPE), Optibond Universal (OB, Kerr) and Z-Prime Plus (ZP, Bisco) were used as adhesives. RelyXTM Universal Resin Cement (3M ESPE) was used as the resin cement. Detailed information on the research materials is provided in Table 1.
A total of 60 zirconia specimens were prepared for this study. The specimens were divided into three main groups (n = 20 per group) according to the adhesive system used: SP, OB, and ZP. Each main group was further subdivided into two subgroups (n = 10 per subgroup) based on thermocycling application: with and without thermocycling.
2. Methods
1) Fabrication of zirconia specimens
To replicate the internal surface of prefabricated zirconia crowns for primary teeth (NuSmile® ZR crown, Houston, TX, USA), zirconia disc specimens were manufactured by HASS (Gangneung, South Korea), a company that produces prefabricated pediatric zirconia crowns in collaboration with NuSmile Pediatric Crowns (Houston, TX, USA). The surfaces of the zirconia discs (HASS) were manufactured to match the internal surfaces of the prefabricated pediatric zirconia crowns, and the similarity was verified using a field emission scanning electron microscope (SEM: JEOL, Boston, MA, USA) (Fig. 1).
Zirconia specimens were fabricated using 3mol% yttria-tetragonal zirconia polycrystal (3Y-TZP), which has the same composition as the prefabricated pediatric zirconia crowns. The test specimens were prepared as discs measuring 8.0 mm in diameter and 3.0 mm in thickness, using pressure molding techniques and subsequent sintering at 1500℃ for 2 hours. The upper punch of the mold used for pressure molding was surface treated using the same pattern as the inner surface of the prefabricated pediatric zirconia crowns to ensure that the zirconia disc specimen surface was similar to the inner surface of the crown.
The zirconia specimens were positioned in a 20.0 × 20.0 × 15.0 mm mold with the specimen surface exposed, and then embedded using self-curing acrylic resin (Ortho-Jet™ Powder: Lang Dental Mfg. Inc., Wheeling, IL, USA).
2) Zirconia specimen bonding with cement
According to the manufacturer’s guidelines (Table 2), zirconia primer and two universal adhesives were applied to the zirconia specimens in each group. Subsequently, RelyXTM Universal Resin Cement (3M ESPE) was incrementally filled in 1.5 mm layers for 20 seconds using a Teflon mold with an internal diameter of 3.0 mm and height of 3.0 mm.
3) Thermocycling
Specimens bonded with the adhesive and RelyXTM Universal Resin Cement (3M ESPE) were divided into two subgroups: a group without thermocycling that was maintained in a humidified environment at 37℃ for 24 hours until shear bond strength testing, and a group with thermocycling that was initially stored at 37℃ for 24 hours, followed by thermocycling (5,500 cycles, alternating between 5℃ and 55℃ water baths with 30 seconds dwell times) using a thermocycling machine (Thermal Cyclic Tester: R&B Inc., Daejeon, South Korea).
4) Shear bond strength test
The evaluation of shear bond strength was conducted with a universal testing machine (Universal Testing Machine: R&B Inc.). Each specimen underwent shear force evaluation with a constant strain rate of 0.5 mm/ min until bonding failure occurred. The maximum force values were documented and normalized to bond strength per unit area (MPa).
5) Analysis of failure modes
Fig. 2 illustrates the three distinct failure modes observed in this study. After shear bond strength testing, specimen surfaces were prepared for microscopic analysis by coating with 10.0 nm platinum particles. Microscopic surface characterization was conducted via scanning electron microscope (SEM: JEOL) at 200× magnification. The failure modes were categorized into three distinct classifications according to the observed fracture morphology: adhesive failure (at the cement-adherend interface), cohesive failure (within either the cement or adherend), and mixed failure (combining both adhesive and cohesive patterns).
6) Statistical analysis
Statistical assessment and analytical procedures were executed utilizing IBM SPSS version 28.0 (SPSS Inc., Chicago, IL, USA). Prior to statistical analysis, the normality of data distribution was evaluated using the Shapiro-Wilk test. The results revealed that the data did not satisfy the assumptions of normal distribution. Therefore, nonparametric tests were employed for further analysis. The Kruskal-Wallis test was conducted to analyze differences between groups. Pairwise comparisons between adhesive categories were systematically evaluated through Bonferroni-adjusted post-hoc analysis to determine mean shear bond strength differentials (p < 0.0167). Comparisons between thermocycled and non-thermocycled specimens within each adhesive group were analyzed utilizing the Mann-Whitney U test (p < 0.05). For the analysis of failure modes, Fisher’s exact test was used to evaluate the distribution differences between groups, with statistical significance set at α = 0.05.
Results
1. Shear bond strength test
Table 3 and Fig. 3 present the shear bond strength results for all tested groups. Without thermocycling, SP showed the highest value at 23.86 ± 3.99 MPa, while OB showed the lowest value at 7.89 ± 1.56 MPa. Statistically significant differences were observed between SP and OB, and between ZP and OB (p < 0.0001). Statistical analysis revealed that SP and ZP showed similar shear bond strength values with no significant difference between them (p= 0.162). With thermocycling, SP showed the highest value at 13.03 ± 2.52 MPa, while OB showed the lowest value at 2.80 ± 0.89 MPa. Both SP and ZP showed significantly higher bond strength than OB (p < 0.0001). The shear bond strength values of SP and ZP were not significantly different (p= 0.037).
For all adhesives and primers (SP, OB, and ZP), the shear bond strength was significantly higher without thermocycling than with thermocycling (p < 0.0001).
2. Analysis of failure modes
Table 4 and Fig. 4 illustrate the failure mode distribution and statistical analysis for all tested groups. Before thermocycling, the SP and ZP groups predominantly exhibited mixed failures (90% and 80%, respectively), while the OB group showed primarily adhesive failures (90%). Statistical analysis revealed significant differences in failure mode distribution among the three adhesive systems before thermocycling (p < 0.0001). Pairwise comparisons showed significant differences between SP and OB groups (p < 0.0001) and between OB and ZP groups (p= 0.002), while no significant difference was found between SP and ZP groups (p= 0.582).
After thermocycling, all groups showed an increased percentage of adhesive failures. The SP group exhibited an equal distribution between adhesive and mixed failures (50% each), while the ZP group showed predominantly adhesive failures (80%). The OB group exhibited exclusively adhesive failures (100%). Significant differences in failure patterns persisted among the three adhesive systems after thermocycling (p= 0.003). The SP group differed significantly from the OB group (p= 0.010), but no significant differences were found between SP and ZP groups (p= 0.349) or between OB and ZP groups (p= 0.474).
When comparing the effect of thermocycling within each adhesive system, both SP and ZP groups showed significant shifts from predominantly mixed failures to adhesive failures after thermocycling (p= 0.033 and p= 0.005, respectively). In contrast, the OB group maintained its predominant adhesive failure pattern regardless of thermocycling, with no statistically significant change after thermal aging (p= 1.000).
Discussion
This study compared the bonding performance of two universal adhesives (SP and OB) against a specific zirconia primer (ZP) in their application to zirconia substrates simulating the internal aspects of prefabricated pediatric zirconia crowns. The results demonstrated that SP and ZP showed similar shear bond strength values with no significant differences between them, both before and after thermocycling. Both adhesive systems showed significantly higher bond strength values than OB. These findings align with those of previous studies on permanent tooth zirconia restorations, in which universal adhesives containing 10-MDP showed superior bonding performance [7,8].
The higher bond strength values observed with SP may be attributed to its composition, which includes both phosphate-based monomers (10-MDP) and carboxylatebased monomers that can form chemical bonds with zirconia surfaces [20]. 10-MDP consists of a polymerizable methacrylate group at one end, a phosphate group at the other, and a long carbon chain spacer between them [7]. The phosphate group forms strong chemical bonds with the oxide layer on the zirconia surface through two primary mechanisms: ionic bonding between electronegative phosphate complexes and electropositive zirconium ions, and hydrogen bonding between the phosphate groups and surface hydroxyl groups [20,21].
ZP showed bond strength values similar to SP, with no statistically significant differences between them, regardless of thermocycling treatment. ZP contains a combination of functional monomers including phosphate monomers and carboxylic acid monomers (BPDM), where the carboxylic groups work in conjunction with 10-MDP to enhance its chemical reactivity with the zirconia surface, resulting in comparable bond strength to that of SP [22].
In contrast, OB demonstrated significantly lower bond strengths compared with those of both SP and ZP, both before (7.89 ± 1.56 MPa) and after (2.80 ± 0.89 MPa) thermocycling. This difference in performance likely relates to the chemical properties and composition of the functional monomers present in each adhesive system. While SP contains 10-MDP as its primary functional monomer, OB contains GPDM. A recent study revealed that although both GPDM and 10-MDP can form chemical bonds with zirconia through their phosphate groups, their bonding mechanisms and effectiveness differ depending on their molecular structures [23].
GPDM has a shorter carbon chain length and different spatial arrangement than 10-MDP, making it more susceptible to hydrolytic degradation despite possessing two methacrylate groups that could potentially enhance polymerization with resin materials [23,24]. A recent study demonstrated that GPDM primers at 3% and 5% concentrations achieved bond strengths comparable to those with commercial 10-MDP-containing primers, but higher concentrations (8%) showed reduced effectiveness [23]. This finding suggests that the lower performance of OB in our study might not be solely attributed to the use of GPDM itself, but rather to other factors such as the concentration or purity of the functional monomers.
Long-term durability was evaluated through thermocycling, with 5,500 cycles performed to simulate approximately 6 months of clinical service. While 10,000 thermal cycles are suggested to correspond to about one year of clinical service [16], previous studies have demonstrated that shorter thermocycling periods were sufficient to evaluate the reduction in bond strength [17]. The better hydrolytic stability observed in SP and ZP groups may be related to the formation of stable MDP-Zr salt layers at the adhesive interface [25,26]. 10-MDP shows greater resistance to hydrolytic degradation compared to GPDM, which may be attributed to its extended carbonyl chain structure [27,28]. The hydrophobic carbon chains oriented away from the surface not only protect the chemical bonds from degradation but also maintain the structural stability of the adhesive interface over extended time periods [27,29].
Despite these protective mechanisms, thermocycling significantly reduced the bond strength in all groups, aligning with established research outcomes in permanent tooth zirconia restoration studies [9,17,30]. The reduction in bond strength after thermocycling can be attributed to hydrolytic degradation of the adhesive interface and thermal stress-induced fatigue [17,31]. The mixed failure patterns observed in the SP and ZP groups before thermocycling shifted towards more adhesive failures after thermocycling, indicating that the adhesive interface becomes the weakest link after prolonged exposure to thermal and hydrolytic challenges.
The limitations of this study include the use of artificial aging through thermocycling alone, which may not completely simulate the complex intraoral environment including mechanical loading, pH variations, and enzymatic challenges. Furthermore, future research should incorporate multiple aging parameters such as mechanical stress testing and chemical exposure to better simulate the oral environment. A notable limitation was the absence of a control group using only self-adhesive resin cement without any surface conditioning. Such a control group would have provided valuable baseline data to better evaluate the actual enhancement effect of each adhesive system on bond strength between zirconia and resin cement. While the OB group demonstrated significantly lower bond strength compared to the other groups, its chemical interaction might still enhance the bond strength between zirconia and cement compared to using cement alone. This comparison would have further clarified the relative efficacy of different functional monomers in promoting adhesion to zirconia. Moreover, long-term clinical studies in pediatric patients are essential to validate these laboratory findings and assess the durability of these adhesive systems in actual clinical conditions.
This study contributes to the limited research on adhesive bonding to prefabricated pediatric zirconia crowns and provides valuable insights for clinical practice. The findings highlight the fact that MDP-containing universal adhesives offer reliable bonding performance and emphasize the importance of proper adhesive system selection in pediatric dentistry.
Conclusion
10-methacryloyloxydecyl dihydrogen phosphate containing adhesive and zirconia primer (SP and ZP) exhibited higher bond strengths and better hydrolytic stabilities than the GPDM containing adhesive (OB), which could be attributed to 10-MDP’s optimal molecular design and stable chemical interaction with zirconia. Thermocycling significantly reduced the bond strengths in all groups, though 10-MDP-containing adhesives maintained relatively higher bond strengths because of their superior hydrolytic stability and the formation of stable nano-layered structures.
These findings suggest that adhesive systems containing 10-MDP, particularly when used in combination with a compatible resin cement, may provide more predictable and durable bonds for prefabricated pediatric zirconia crowns.
PDF Links
PubReader
ePub Link
Full text via DOI
Download Citation
Print
