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INTRODUCTION
Increased interest in an esthetic facial appearance has led to an increase in the number of adult orthodontic patients. Many patients underwent previous dental restorations to treat wear, attrition, missing teeth, and malformed teeth.1 Distinct surface treatments are required to increase the bond strength between orthodontic brackets and the restorations because the bond between bracket and previous restorations is weaker than the bond to natural teeth.2
Esthetic demands and the development of ceramic systems have increased the use of all-ceramic crowns. Zirconia is a widely used core for all-ceramic crowns due to its high strength and superior esthetic appearance. To construct an esthetic prosthesis, a veneer is layered with ceramic powder onto the zirconia core. However, fracturing of the veneer is frequently reported in the posterior teeth, which apply strong masticatory forces,34 prompting increased use of monolithic zirconia crowns without veneers.56 In orthodontic patients with monolithic zirconia crowns, the orthodontic bracket should be bonded directly onto the zirconia surface. Currently, an additional zirconia surface treatment is required to obtain proper bond strength.7
In prosthodontics, various mechanical and chemical surface treatments have been studied to improve the bond strength between zirconia and resin cement. One such technique sandblasting was reported to increases mechanical retention,89 and as a chemical method, many recently developed zirconia primers have been introduced.10111213 However, few studies have examined the bond strength between orthodontic brackets and zirconia prostheses, and the effects of surface treatment with zirconia primers on bonding. Thus, the aim of this study was to compare the shear bond strength (SBS) of orthodontic brackets using three different zirconia primers and one silane primer, and subjected to thermocycling.
MATERIALS AND METHODS
A total 100 blocks of yttrium oxide-stabilized zirconia (NaturaZ α-series; DMAX International, Seoul, Korea) were embedded in acrylic resin (Ortho-Jet™; Lang Dental Manufacturing Co., Inc., Wheeling, IL, USA) polished with 1000-grit silicon carbide paper, and ultrasonically cleaned. All specimens underwent airborne-particle abrasion with 50-µm Al2O3 particles at 40 psi for 10 s at a 10-mm distance perpendicular to the surface. The specimens were randomly divided into five primer groups (n = 20 per group), and each primer was divided into two subgroups (n = 10 each) to examine by thermocycling protocols (Table 1). The composition of the primers used in this study is presented in Table 2.
Before bonding the orthodontic metal brackets (Gemini series™; 3M Unitek Corporation, Monrovia, CA, USA), all specimens were steam-cleaned and air-dried. In the control no-primer (NP) group, Transbond™ XT Primer (3M Unitek Corporation) was applied to the zirconia surface and bracket base. The remaining specimen groups were treated with either Porcelain Conditioner (PC), Z-PRIME Plus (ZP), Monobond Plus (MP), or Zirconia Liner Premium (ZL) primer according to each manufacturer's instructions. All orthodontic brackets were bonded to the specimens using Transbond™ XT Paste and light polymerized for 15 s at 1,100 mW/cm2 (Mr. Light LED curing light; Dent Zar, Tarzana, CA, USA).
After bonding the orthodontic brackets, all specimens were stored at 37℃ in distilled water for 24 hours. The five primer groups were then split into a total 10 subgroups, with one subgroup of specimens from each primer treatment subjected to thermocycling (KD-TCS30; Kwang-duk F.A., Gwangju, Korea) in 5℃ and 55℃ water for 2,000 cycles, yielding groups NPT, PCT, ZPT, MPT, and ZLT. The remaining five subgroups (NP, PC, ZP, MP, and ZL) did not undergo thermocycling.
The SBS was measured in all 10 groups using a universal testing machine (Model 3366; Instron® Co., Norwood, MA, USA) at a 1 mm/min crosshead speed. The failure mode was assessed by examination with a stereomicroscope (SZ61; Olympus, Tokyo, Japan), and the amount of bonding resin remaining on the zirconia surface was scored using the modified adhesive remnant index (ARI)14 (Table 3). One representative specimen from each group was flecked with gold using an ion coater (IB-3; Eiko Co., Tokyo, Japan) and examined via scanning electron microscopy (S-800; Hitachi, Tokyo, Japan) to further evaluate the failure mode.
Statistical analysis
Statistical analysis was performed with IBM SPSS Statistics version 20.0 (IBM Co., Armonk, NY, USA). Two-way analysis of variance (ANOVA) was applied to detect significant differences in the SBS between the groups. The significance for all statistical tests was set at p < 0.05, and Tukey's honest significant difference post-hoc test was used to detect pairwise differences between the groups (Table 4).
RESULTS
The SBS values for all groups are presented in Table 5. The variables and the interactions between the variables exerted significant effects on the SBS (p < 0.05; Table 6). 2015-07-28In all five surface treatment groups, the SBS decreased after thermocycling (Figure 1). Among the groups that did not undergo thermocycling, group NP exhibited a significantly lower SBS than the other groups (PC, ZP, MP, and ZL; p < 0.05). Group PC, which was treated with a silane primer, showed significantly lower SBS than groups ZP and ZL (p < 0.05). However, groups PC and MP did not significantly differ between each other (p > 0.05), and no significant differences were detected among the groups treated with zirconia primers (p > 0.05; Figure 1).
In the thermocycling groups, there was no significant difference between groups NPT and PCT (p > 0.05). Groups ZPT, MPT, and ZLT, which used zirconia primers, were associated with a significantly higher SBS than the groups NPT and PCT (p < 0.05; Figure 1). There were no significant differences between groups ZPT and MPT or between groups MPT and ZLT (p > 0.05); however, groups ZPT and ZLT differed significantly between each other (p < 0.05; Figure 1).
DISCUSSION
In this study, the effect of zirconia primers on the SBS of orthodontic metal brackets was determined. Kern and Wegner15 reported that sandblasting treatment increased the roughness of the zirconia surface, but compared to other alloys, it also decreased the number of undercuts and failed to maintain bond stability after water invasion. Similarly, the SBS of group NPT was 49% of the bond strength for group NP.
The control groups that lacked primer showed the lowest SBS both with and without thermocycling (Table 5). Reynolds16 indicated that the minimum SBS able to resist normal orthodontic forces is 5.9-7.9 MPa, and McCarthy and Hondrum17 reported a minimum value of 7 MPa. Based on these prior studies, groups NPT (5.68 MPa) and PCT (6.12 MPa) lack proper SBS. Group PC, which used a silane primer, exhibited significantly higher SBS than group NP (p < 0.05). The SBS of group PCT (which underwent thermocycling) was only 26.1% of the bond strength of group PC, and no significant difference was detected between PCT and NPT (p > 0.05).
Z-PRIME Plus contains biphenyl dimethacrylate and hydroxyethyl methacrylate. Monobond Plus largely comprises silane methacrylate, 10-methacryloyloxydecyl dihydrogen phosphate (MDP), and sulfide methacrylate. Zirconia Liner Premium is similar to Monobond Plus, but does not contain sulfide methacrylate. Z-PRIME Plus is a mixture of phosphate and carboxylic acid monomers. Phosphate monomer can co-polymerize with resin monomers because its functional group bonds with the metal oxide of the substrate via its phosphoric acid group. Another monomer, carboxylic acid, helps form the bond to the substrate.1218 However, Lorenzoni et al.19 reported that Z-PRIME Plus displayed lower bond strength than MDP-containing primers because the carboxylic acid monomer of Z-PRIME Plus weakened the bonding with the methacrylate group of the resin cement. Many studies have examined the efficacy of MDP monomer in chemical bonding with zirconium oxide on a zirconia surface152021 and found that MDP-containing primers maintain bonding stability both before and after thermocycling of a zirconia surface treated with sandblasting.15172223 In the current study, MDP-containing primers also showed less strength reduction than observed in the other groups (39.2% MDP in groups MP and MPT; 39.6% MDP in groups ZL and ZLT).
Koizumi et al.24 reported that the sulfide methacrylate monomer in Monobond Plus affects bond strength, but in our study, there was no significant difference in SBS between the MDP-containing primers (Monobond Plus and Zirconia Liner Premium) (p > 0.05).
CONCLUSION
Following the results of this study, regardless of thermocycling application, the groups without primer (NP and NPT) showed lower SBS and adhesive failure but the groups using zirconia primers (ZP, ZPT, MP, MPT, ZL, and ZLT) showed higher SBS and mixed failure. Thus, surface treatment with zirconia primer contributes to an increased in SBS compared with no-primer and silane primer application for orthodontic bracket bonding to zirconia prostheses.
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