Bond Strength Stability of Self-adhesive Resin Cement to Etched Vitrified Yttria-stabilized Tetragonal Zirconia Polycrystal Ceramic After Thermomechanical Cycling

Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) ceramics were introduced to replace metallic frameworks in full crowns, implant abutments, and dental bridges in anterior and posterior areas for esthetic oral rehabilitation.¹  In addition to its esthetic appearance, the principal advantage of Y-TZP ceramic is a stress-induced toughening mechanism that is started by the stresses generated at the ceramic surface. As a consequence, a tetragonal to monoclinic phase transformation occurs, along with a volume increase (3%-5%) at the crack origins, thereby developing internal stresses that counterattack crack propagation inside the ceramic bulk and increase the mechanical strength of the ceramic.²  Irrespective of this welcome mechanism, the absence of a vitreous phase, which makes Y-TZP ceramics unsusceptible to hydrofluoric acid etching, still represents a point of weakness for the clinical performance of this class of restorative biomaterial.^3-6 

In an attempt to overcome this limitation, different strategies have been proposed to improve the interaction of resin cements to Y-TZP ceramic surfaces, such as treatment with phosphate adhesive monomers,^7,8  sandblasting with Al₂O₃ and Si-modified Al₂O₃ particles,^9-11  treatment with selective primers,^12,13  Si nanocoating,^14,15  and the application of nonthermal plasmas.^16-18  Although these protocols can increase the immediate bond strength of resin cements to Y-TZP ceramics, a drop in bond strength can occur when submitted to long-term water immersion and thermal or mechanical cycling.^4,9 

Because of a possible chemical interaction at the interfacial grain level between the phosphate groups present in methacrylate phosphoric acid esters and the hydroxyl groups of the passive ZrO₂ coating on the Y-TZP ceramic, self-adhesive resin cements have been advocated for their better adherence to this type of ceramic.⁸  Unfortunately, scientific evidence suggests that this mechanism can also suffer degradation when submitted to aging methods,^9,19  which are clinical matters of concern.

A new approach for optimizing the bonding of resin cements to Y-TZP ceramics has recently presented successful results, even after aging in water and thermocycling. This protocol involves the modification of the ceramic surface with low-fusing glasses or ceramic liners, which is a vitrification process itself. This mechanism creates a silica-rich layer on the Y-TZP ceramic surface that is etchable by hydrofluoric acid, thereby favoring micromechanical interlocking with the resin cement and a chemical interaction with the silane-coupling agent as well.^20-26 

Although these former studies have presented interesting results, it is noteworthy that none of them had submitted the bond interfaces to mechanical loading. From a clinical point of view, the mouth is a dynamic system in which Y-TZP ceramic restorations are subject to changes in temperature and masticatory loading.²⁷  Thus, it seems of some relevance to investigate the effect of thermomechanical cycling on the bond strength of resin cements to a Y-TZP ceramic surface previously submitted to vitrification. Therefore, the purpose of the current study was to evaluate the influence of thermomechanical cycling on the bond strength of self-adhesive resin cement to Y-TZP ceramic submitted to surface vitrification. The tested hypothesis was that thermomechanical cycling would not affect the bond strength of self-adhesive resin cement to vitrified Y-TZP ceramic.

The experimental setup of ceramic-resin composite beam preparation and microtensile bond strength (μTBS) testing is illustrated in Figure 1. Forty presintered blocks of a Y-TZP ceramic (Lava Frame, 3M ESPE, Seefeld, Germany) and 10 blocks of a lithium disilicate glass ceramic (IPS e.MAX CAD, Ivoclar Vivadent AG, Schaan, Liechtenstein) were used in the present study. After sectioning with a diamond disk, at 800 rpm, in a precision sectioning cutter (Isomet 1000 precision saw, Buehler, Lake Bluff, IL, USA), the blocks were sintered and crystallized using the electrical induction furnaces recommended by the manufacturers: Lava Furnace 200 (3M ESPE, St Paul, MN, USA) and Programat P310 (Ivoclar Vivadent AG). After these processes, the final dimensions of all ceramic blocks were 10.0 mm × 10.0 mm × 7 mm. After polishing with 600-grit SiC paper, the blocks were ultrasonically cleaned for five minutes in distilled water and divided into five groups (n=10) according to surface treatment (Table 1).

Fifty blocks of a resin composite (Filtek Z250, 3M ESPE) were built up using a plastic mold with the same dimensions as the ceramic blocks (10.0 × 10.0 × 7.0 mm). The mold was filled with 2.0-mm-thick increments of Z250, which were individually light polymerized for 30 seconds each with an irradiance of 500 mW/cm² (Radii call, SDI, Victoria, Australia). Afterward, the blocks were polished with 600-grit SiC paper to standardize the roughness and were then ultrasonically cleaned for five minutes in distilled water.²⁸ 

A self-adhesive resin cement (RelyX U200, 3M ESPE) was used to bond the resin composite blocks to the ceramic blocks. Equals amounts of base and catalyst pastes of RelyX U200 were mixed with a Teflon spatula on a mixing pad for 10 s. After the resin cement had been applied on the ceramic surface, the resin composite block was placed on the cement and held under a mass of 500 g for two minutes to standardize the resin cement pellicle thickness for all specimens. Afterward, the resin cement was light activated from the four sides of the Y-TZP ceramic-resin composite bonded interface for 40 seconds each with an irradiance of 500 mW/cm² (Radii call, SDI).

After storage in distilled water at 37°C for 24 hours, half of the Y-TZP ceramic-resin composite blocks were attached to a cutting machine (IsoMet 1000, Buehler) and longitudinally sectioned, across the bonded interfaces, in both x- and y-axes, using a water-cooled diamond disc (Isomet Wafering Blade 15LC #114254, 0.3-mm thickness, Buehler) at 800 rpm, thus producing beams with a cross-sectional area of approximately 1.0 mm² and 10.0 mm in length. To avoid premature loss of beams during the ceramic-resin composite block cutting, wash silicone impression material (Speedex, Vigodent-COLTÈNE, RJ, Brazil) was injected between the slices produced during the first cutting to absorb the vibration generated during the second cutting. Afterward, each beam was attached to a μTBS device (ODMT03d, Odeme Dental Research, Luzerna, SC, Brasil) using cyanoacrylate adhesive (SuperBonder, lot No. 15900545, Henkel Ltda, Itapevi, SP, Brazil) and loaded in tension using a universal testing machine (Emic DL 2000, São José dos Pinhais, SP, Brazil), and a 50-N load cell, at a crosshead speed of 0.5 mm/min, until failure occurred. The μTBS (MPa) was obtained by dividing the load at failure (N) by the cross-sectional area of the beam (mm²).

Before the μTBS test, the other half of the blocks were submitted to thermomechanical cycling as follows: first, the blocks were submitted to mechanical cycling in a chewing simulator (ER-37000NG, ERIOS, São Paulo, SP, Brazil), with a load of 98 N and a frequency of 2.5 Hz for 1.2 × 10⁶ cycles. The load was applied using a stainless-steel plate adapted to the piston of the chewing simulator to transmit the mechanical stresses equally to the bond interfaces. After the blocks were sectioned into beams, they were thermocycled (5°C-55°C with a dwell time of 30 seconds and transfer time of two seconds for 10 × 10³ cycles). With the exception of the as-sintered (AS) group, in which all blocks spontaneously debonded during mechanical cycling, 80 beams were produced for each group, with 16 beams from the central area of each ceramic-resin composite block being used for experimentation.

Each failed beam was evaluated using a stereomicroscope at 40× magnification (SZ40, Olympus, Tokyo, Japan), and the failure mode was classified as adhesive (failures at the adhesive interfaces), cohesive (failures occurring mainly within resin composite), or mixed (mixture of adhesive and cohesive failure within the same fractured surface). In addition, beams presenting different failure modes and with the value of μTBS close to the mean of each group were viewed using scanning electron microscopy (SEM). The beams were mounted in a charge reduction sample holder and observed under SEM (PhenomProX, PhenomWorld, Eindhoven, the Netherlands) operating in the backscattered mode, in a low-vacuum environment. The SEM images were taken at a magnification of 270×.

Two ceramic surfaces submitted to each treatment were analyzed using SEM and three-dimensional (3D) profilometry. First, the ceramic specimens were observed under SEM (Phenom ProX, Phenom World) operating in the backscattered mode under low vacuum. The images were taken with 10 kV, at magnifications of 1000×. Afterward, the topographic analysis was performed using a 3D profilometer (Form Talysurf 60i, Taylor Hobson, Leicester, UK). For each specimen, an area of 1 mm² was scanned with a 20-nm z-resolution, employing 4000 steps in the x-axis and a spacing of 2 μm in the y-axis. The reconstruction of 3D images was made according to the parameter Sa (μm) using the following formula:

where (M, N) are the number of points in (x, y), and z is the height of measured points at x and y coordinates.

The obtained data were analyzed using Statgraphics Centurion XVI software (STATPOINT Technologies Inc, Warrenton, VA, USA). The sample size for the μTBS test was calculated based on the mean and standard deviation values obtained in the pilot study and considering a statistical test power of 0.87. As all blocks in the AS group spontaneously debonded during mechanical cycling (μTBS = 0 MPa), these data were not included in the statistical analysis.^25,29  The prematurely failed beams³⁰  were included in the statistical analysis by assigning a value that corresponded to half of the minimum μTBS for their experimental group.³¹  The normal distribution of errors and the homogeneity of variances were checked using Shapiro-Wilk's and Levene's test, respectively. Based on these preliminary analyses, the μTBS data after 24 hours and after thermomechanical cycling (TMC) were analyzed separately using one-way analysis of variance and Tukey's honest significant difference post hoc test. The effect of TMC in each group was evaluated using the Student t-test. The analyses were performed at a significance of α=0.05.

The means and standard deviations of μTBS (MPa) are depicted in Table 2. After 24 hours, etching with 10% hydrofluoric acid for 20 seconds (EM; 37.3±4.4) and vitrification with a low-fusing glaze and etching with 10% hydrofluoric acid (LG; 30.6±4.2) presented the highest μTBS values, with no statistical differences from each other, followed by vitrification with a ceramic liner and etching with 10% hydrofluoric acid (HC; 23.0±2.6) and sandblasting with 50-μm Al₂O₃ particles (SB; 18.1±2.0; p<0.05). The lowest μTBS was presented by AS (p<0.05). After TMC, EM (34.8±4.7) presented the highest μTBS (p<0.05), followed by LG (24.1±3.3), whereas SB (12.3±2.7) and HC (11.6±1.4) presented statistically similar μTBS (p>0.05). All specimens of AS spontaneously debonded during mechanical cycling. Only EM and LG maintained the stability of μTBS after TMC.

Table 3 summarizes the number of specimens that failed prematurely and the number of specimens that were submitted to μTBS testing. All blocks in AS-TMC spontaneously debonded during mechanical cycling. The number of prematurely failed beams in the other groups was as follows: AS-24 > SB-24 > SB-TMC > HC-TMC > HC-24 > LG-24. The groups LG-TMC, EM-24, and EM-TMC presented no prematurely failed beams.

The percentage of failure modes are described in Figure 2A. After 24 hours, EM and LG presented the highest percentage of cohesive failures, whereas the HC group showed equilibrium between adhesive and mixed failures. On the other hand, AS and SB presented a predominance of adhesive failures. After TMC, SB presented a predominance of adhesive failures, whereas LG, HC, and EM showed a greater percentage of mixed failures. Representative SEM images of debonded specimens are presented in Figure 2 (B, C, D, E): (B) adhesive failure of an HC specimen showing the Y-TZP ceramic surface free of composite (c) and small remnants of resin cement (rc), (C) an EM specimen showing a cohesive failure in the bulk of composite (c), (D) mixed failure in an LG specimen, and (E) mixed failure in an SB specimen showing residual resin cement (rc) and composite (c) on the Y-TZP ceramic surface.

Figure 3 shows representative SEM images of an IPS e.max CAD specimen etched with 10% hydrofluoric acid for 20 seconds (A) and Y-TZP ceramic surfaces submitted to the following treatments: (B) as sintered, (C) sandblasted with 50-μm Al₂O₃ particles for 20 seconds, (D) vitrified with low-fusing ceramic glaze and etched with 10% hydrofluoric acid for one minute, and (E) vitrified with ceramic liner and etched with 10% hydrofluoric acid for one minute. The surfaces of the IPS e.max CAD specimen and the Y-TZP ceramic vitrified with low-fusing glaze presented a similar appearance, with disperse porous characteristics of partially dissolved glass ceramics (pointers). Typical grooves are clear in the sandblasted Y-TZP ceramic surface (asterisks). A more irregular and porous surface, with several gaps (arrows), was demonstrated by the Y-TZP ceramic vitrified with a ceramic liner.

Representative 4D images of EM, LG, and HC surfaces before (A, B, and C) and after hydrofluoric acid etching (A′, B′, and C′) are depicted in Figure 4. After acid etching, EM (A′) presented deep craters, whereas LG (B′) presented several peaks, characterizing a deep and homogeneous effect of the hydrofluoric acid. On the other hand, HC showed a more irregular topography, with areas suggesting a deeper action of hydrofluoric acid along with areas that were poorly modified by this acid.

Among other aspects, the establishment of a stable bond between a resin cement and the ceramic surface is crucial for achieving clinical longevity of any kind of Y-TZP ceramic restoration. The principal goal of the present study was to test the hypothesis that bonding of a self-adhesive resin cement to vitrified Y-TZP ceramic (LG and HC groups) could withstand TMC. The other tested groups were included for specific reasons: as-sintered Y-TZP ceramic (AS) was used as a negative control because the literature clearly demonstrates that bonding to this substrate is extremely poor.^9,25  Contrarily, sandblasting (SB) was chosen as a positive control because this is one of the most reliable clinical protocols for preparing Y-TZP ceramic restorations for cementation.^4,32  IPS e.max CAD (EM), a vitreous ceramic susceptible to hydrofluoric acid etching, was used to produce results intended to support the discussion about those obtained from the vitrified Y-TZP ceramic. The parameters used to perform the TMC in the present study were chosen to simulate approximately one year of clinical service.^33,34 

The LG group maintained the bond strength stability after TMC. On the other hand, this did not occur with the HC group. Therefore, the tested hypothesis of the present study was partially accepted. The mechanism involved in the Y-TZP ceramic vitrification processes used here could be described through the melting of the components (SiO₂, Al₂O₃, K₂O, Na₂O, CeO₂) present in both the glaze and liner, which were bonded to each other, creating a vitreous and Si-rich layer, capable of interacting with the Y-TZP ceramic surface through electrostatic and van der Waals forces.²¹  As a result, these vitrified Y-TZP ceramic surfaces were susceptible to hydrofluoric acid etching, thereby favoring the micro-retention of the resin cement,²⁰  a typical behavior of vitreous ceramics.

After 24 hours of water storage, AS presented the worst μTBS (Table 2), reinforcing the difficulty of establishing a suitable interaction with the nonreactive Y-TZP ceramic surface, even when using self-adhesive resin cements.⁹  Alternatively, the three treatments tested here increased the immediate bond strength to Y-TZP ceramic (Table 2). In percentage terms, this increase was 154.9% for SB, 330.9% for LG, and 223.9% for HC. Moreover, LG was the only group that presented μTBS (30.6 MPa) statistically similar to that of EM (37.3 MPa). This result suggests that vitrification using a low-fusing glaze could be a good alternative to traditional sandblasting for improving the bond strength of resin cements to Y-TZP ceramics.

In fact, previous studies have already shown that Y-TZP ceramics vitrified with low-fusing glazes presented stronger and more stable bond strengths than when submitted to traditional protocols (sandblasting or tribochemical coating). After vitrification with two layers of a low-fusing glaze and etching with hydrofluoric acid, Cheung and others²⁵  showed that a Y-TZP ceramic presented bond strength stability after 3 weeks of water storage and thermocycling (6000×/5°C-55°C), behavior that was not presented by the groups sandblasted with Al₂O₃ particles or tribochemically coated with Si-coated Al₂O₃ particles. Also, after thermocycling (1800×/5°C and 55°C), Everson and others²⁶  showed that a Y-TZP ceramic treated with five different low-fusing glazes and hydrofluoric acid etching presented a significantly higher and more stable bond strength when compared with the tribochemically coated group. Thus, when considering that the resin cement–Y-TZP ceramic interfaces in the current study were submitted to mechanical and thermal cycling, it is reasonable to consider the results obtained here as relevant, since they show the performance of the tested bonding protocols in a worst-case scenario.³⁵ 

Although the LG group maintained bond stability after TMC, it still presented significantly lower μTBS than the EM group (Table 2). Figure 3 (A and D) demonstrates that both surfaces present typical aspects of vitreous ceramics etched with hydrofluoric acid.^23,26  However, the 3D images (Figure 4) present remarkable differences in the action of hydrofluoric acid in these groups; while the EM specimen presented several craters (Figure 4A′), these structures were less visible on the LG surface, which presented picks protruding from it (Figure 4B′). Thus, it is possible that more effective micromechanical interlocking with the self-adhesive resin cement took place in the EM group due to the presence of these craters, thereby creating a better interaction between both materials. Even though the relationship between in vitro results and the clinical performance of adhesive protocols is questionable, as evidenced by the values of bond strength presented in previous studies,^7,9,19,29  it is reasonable to consider the bond strength presented by the LG group after TMC (24.1 MPa) as indicative of good performance for indirect Y-TZP ceramic restorations submitted to this luting protocol.

Although the immediate μTBS of HC was statistically higher than that obtained for the as-sintered Y-TZP ceramic surfaces and similar to that of SB, the traditional gold standard, the μTBS of HC was lower than that of LG (Table 2). As both VITA Akzent Plus and HeraCeram Zirkonia are vitreous materials with similar composition (Table 1), this finding was somewhat surprising. However, the features depicted in Figures 3 and 4 show some possibilities for explaining this result. From the comparison of Figures 3D and 3E, the surface vitrified with HeraCeram Zirkonia (HC group) is discontinuous, with rougher grains, and presents several gaps, which are not present on the surface vitrified with VITA Akzent Plus. In addition, the Figure 4C′ clearly shows that the HC surface is more irregular than the LG surface (Figure 4B′), with areas suggesting the absence of surface vitrification. Thus, one can suppose that the pattern produced by the brushing with HeraCeram Zirkonia (Figure 4C) could have favored the penetration of hydrofluoric acid through its vitreous layer, which could have partially disrupted the interaction between the liner and the Y-TZP ceramic, thereby weakening the adhesive interface. The fact that this group did not maintain bonding stability suggests that this effect was amplified after TMC.

The results observed for the SB are in agreement with earlier studies.^36,37  The immediate μTBS for SB was 154.9% higher than that obtained for the as-sintered surfaces, which can be explained by the increase in the micromechanical interlocking between the self-adhesive resin cement and the grooves produced by air abrasion (Figure 3C) and by the chemical interaction between the PO₄ groups present in the self-adhesive resin cement and the hydroxyl groups produced by air abrasion itself.⁸  On the other hand, the μTBS of the SB suffered a significant drop of 32% after TMC (Table 2). Although we did not find any previous studies that used a similar experimental protocol, other studies that evaluated air abrasion and resin cements containing phosphoric acid ester methacrylate monomers could be used to support this finding. When using the MDP-containing Panavia, Bromicke and others⁴  and Quaas and others³⁸  observed a reduction in tensile bond strength from 31.86 MPa to 4.93 MPa (84%) and from 23.6 MPa to 12.1 MPa (51%), respectively, after 150 days of water storage and 37,500 thermocycles. Moreover, when submitting specimens to fatigue, Mirmohammadi and others³⁹  found a reduction from 43.9 MPa to 22.0 MPa (50%) for the flexural bond strength of Panavia bonded to a Y-TZP ceramic. In agreement with the present study, the analysis of all these results suggest that air abrasion may not be the most effective protocol for creating reliable bonding to Y-TZP ceramics.

In addition to the values of μTBS itself, the analysis of the number of premature debonded specimens also provides important information regarding the behavior of resin cement-ceramic interfaces. It is well known that during TMC, water diffusion, thermal transfer, and the stress generated at the adhesive interfaces may accelerate the hydrolytic degradation of the adhesive polymer structures, thereby decreasing the bonding of resin cements to Y-TZP ceramics.^25,40,41  In the current study, all of the resin composite-Y-TZP ceramic blocks from AS did not survive mechanical cycling. This finding is noteworthy because it means that the chemical interaction produced by the self-adhesive resin cement may not be strong enough to resist the loading generated during chewing. In addition, after 24 hours of water storage, 80% of the beams from AS prematurely failed during the μTBs test (Table 3), while those that were tested presented a predominance (93.75%) of adhesive failures (Figure 2A). These aspects reflect the lowest μTBS found for this group and again reinforce the poor chemical interaction between the phosphate groups present in self-adhesive resin cements and the passive ZrO₂ layer on the Y-TZP ceramic surface.⁹ 

The specimens from SB also presented a great number of premature failures, 49% and 40%, after 24 hours and TMC, respectively. This finding fits well with its intermediary values of μTBS. A similar behavior was observed for HC. Contrarily, LG behaved similarly to EM, with only 16% and 0% of premature failures after 24 hours and TMC, respectively. Undoubtedly, this finding, associated with the maintenance of the bond strength stability observed in this group, allows advocating for vitrification with low-fusing glaze as a suitable protocol to luting Y-TZP restorations. Of course, this assumption falls within the limits of the present investigation. Furthermore, previous studies have shown that the impact of Al₂O₃ particles during sandblasting may produce subcritical microcracks and t-m phase transformation in Y-TZP ceramics, which may affect their mechanical properties.^42-44  This is another factor defending the vitrification of Y-TZP ceramics as a more suitable protocol for improving the bond strength to this class of ceramic material.

Although the present study adds new information regarding the bonding stability to vitrified Y-TZP ceramics, it still presents some limitations, such as the use of only one resin cement and Y-TZP ceramic, as well as the absence of the impact of this approach on the dentin-resin cement adhesive interface. Moreover, the development of clinical trials could provide evidence for considering the present outcome more clinically relevant. These aspects should be addressed in future investigations.

Within the limitations of the present investigation, the vitrification of a Y-TZP ceramic surface with a low-fusing glaze, followed by hydrofluoric acid etching, allowed the adhesive interface to better withstand TMC. In addition, this protocol showed better results than the traditional sandblasting and similar results to that observed with an IPS e.max CAD vitreous ceramic etched with hydrofluoric acid. Thus, it can be concluded that this protocol might be useful for improving the adhesion between Y-TZP ceramics and self-adhesive resin cements.

The authors certify that they have no proprietary, financial, or other personal interest of any nature or kind in any product, service, and/or company presented in this article.