Indeed, luting of zirconia restorations remains a major challenge in clinical situations and many luting protocols discussed in the literature may still be controversial. The aim of this study was to investigate and evaluate the bond strength of zirconia after alumina and glass bead pretreatment using two different primers, a conventional resin cement and an unprimed self-adhesive resin cement containing 10-methacryloyloxydecyl dihydrogen phosphate (MDP). Fully sintered high-transmittance zirconia specimens (n = 160) were divided into 2 pretreatment groups (n = 80): alumina sandblasted (AB) and glass bead blasted (GB). Each group was then divided into 4 subgroups (n = 20 each) depending on the primer and cement used: conventional self-adhesive resin cement, MDP-silane primer, MDP primer and conventional self-adhesive resin cement, and MDP containing cement. The shear bond strength (SBS) was measured after thermal cycling. The failure modes were analyzed using a stereo microscope. The contact angle and surface morphology were investigated using other fully sintered specimens (n = 30) prepared for this purpose and divided into control groups (no pretreatment [unmodified], alumina group, and glass bead blasted group). A two-way ANOVA was performed for the SBS and the failure modes were analyzed. SBS was higher for silane-based MDP primer (p=0.034) and MDP primer (p<0.001) with alumina blasting compared to glass bead pretreatment. While MDP-containing cement showed higher but insignificant SBS with glass bead pretreatment. Alumina blasting and glass bead pretreatment before bonding with conventional resin cements can improve the bond strength of zirconia with primer. In addition, MDP-containing self-adhesive cement as well as surface pretreatment showed the highest bond strength. Conclusion: Both alumina and glass bead treatment can improve the SBS bond to MDP-containing self-adhesive composite cements, thereby reducing the number of clinical steps required in cementing zirconia restorations.
Nowadays, with the rapid development and innovation of digital dentistry, the demand for highly esthetic restorations in daily dental practice is increasing. Zirconia is considered as one of the most commonly used restorative materials in dentistry because it has higher mechanical properties than glass ceramics1. This has expanded the range of indications in the field of fixed and indirect restorations, but early zirconia had low translucency and high opacity, and therefore poor esthetics, since it consists of a dense polycrystalline structure without a glass matrix, and its use was limited to posterior teeth compared to glass ceramics2. To improve optical properties, the yttria content was increased to 5 mol% in yttria-stabilized tetragonal polycrystalline zirconia (5Y-TZP), providing 50% cubic phase compared to the conventional 3 mol% yttria-stabilized tetragonal zirconia3,4,5, thereby improving the overall aesthetics and introducing a new ultra-transparent zirconia.
Compared with silicon-based ceramic materials, the bonding of zirconia to tooth tissue or other synthetic materials is controversial because zirconia is chemically inert and resistant to aggressive chemicals (strong acids, bases, organic and inorganic solvents 6 ). Over a 5-year observation period, the rate of retention loss of zirconia restorations was 4.7% 7 , which was directly caused by bone loss between the various bonding surfaces within the structure 8 .
Many methods have been studied in the literature to improve the bond strength between zirconia ceramics and resin cements, including mechanical, chemical, and/or chemomechanical surface pretreatment methods9,10,11. Two published meta-analyses have demonstrated the importance of combined mechanochemical surface treatment approaches to improve the bond strength between zirconia and resin cements12,13. On the mechanical side, the following methods have been proposed: airborne particle wear, airborne tribochemical particle wear, low melting point porcelain deposition, thermochemical etching solutions, selective infiltration etching, laser irradiation, plasma spraying, and zirconia ceramic powder coating, while zirconia-based primers using 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP) molecules and their salts have been proposed to modify the surface chemistry of zirconia9,14,15,16,17.
Compared with other surface treatment methods, air abrasive machining using alumina particles showed the highest bonding strength18. However, air abrasive machining of alumina may produce surface defects such as cracks, plastic deformation, embedded abrasive alumina and microcracks, which can deteriorate the mechanical properties of zirconia and reduce the fracture strength19. To reduce the surface defects of zirconia, Khanlar et al. in 2022 proposed using glass beads as a softer material than sharp and hard alumina, and then using a 10-MDP-silane primer, which was reported to have ideal bonding properties and did not produce surface microcracks on zirconia20.
There are few reports on the effectiveness of glass bead blasting as a mechanical bonding method, either alone or in combination with different zirconia primer chemistries. Therefore, this in vitro study was conducted to evaluate the effect of alumina and glass bead blasting on the bond strength of highly translucent (5Y-TZP) zirconia, either alone or in combination with different zirconia primer formulations and MDP-containing self-adhesive resin cement (without subsequent primer). The following null hypotheses were tested: 1. Alumina and/or glass bead blasted zirconia will exhibit similar bond strengths; 2. Different MDP primers will not affect the bond strength to zirconia; 3. MDP-containing self-adhesive resin cement will exhibit similar results to the other MDP primers tested.
A highly translucent polycrystalline yttria-stabilized tetragonal zirconia sample (5Y-PSZ; Liaoning Upcera Co., Ltd., Liaoning, China) was used as the test substrate. MDP-Silane primer (Visallys repair primer, Kettenbach GmbH & Co. KG, Eschenburg, Germany) and MDP-BPDM primer (Z-prim, Bisco Inc., Illinois, USA) were used. A conventional resin adhesive (Visalys CemCore, Kettenbach GmbH & Co. KG) and a self-adhesive resin adhesive (Z-prim, Bisco Inc.) were used for bonding in this study. The complete composition of the materials used is given in Table 1.
A total of 160 square 5Y-PSZ specimens (10 × 10 × 3 mm before sintering) were cut using a low-speed precision saw (Isomet 4000, Buehler, Lake Bluff, IL, USA) under dry conditions without water cooling. In addition, 160 disc-shaped specimens with a diameter of 4 mm and a thickness of 3 mm were bonded together to evaluate the bond shear strength. An additional 30 square specimens with the previously mentioned dimensions were used to measure the surface roughness and contact angle. All specimens were sintered at 1450 °C using a 7-step cycle in a zirconia furnace (Lindberg/Blue M, Asheville, NC, USA) according to the manufacturer’s instructions. The sintering process parameters are listed in Table 2.
For shear bond strength analysis, fully sintered specimens [square and disc (n = 160 each)] were equally divided into two groups according to the pretreatment schemes (n = 80 each): alumina sandblasted (AB) group and glass bead (GB) group. Each group was further divided into four subgroups (n = 20 each) according to the bonding scheme: 1. no primer + conventional resin cement, 2. MDP-silane primer + conventional resin cement, 3. MDP-BPDM primer + conventional resin cement, and 4. resin cement with MDP without subsequent primer. Figure 1 clearly illustrates this approach.
Schematic representation of the methodology used in the current study. (A–F) represent the steps of the shear bond strength test. A: 160 square and 160 disc-shaped specimens were prepared. B: Sintering was carried out according to Table 2. C: Pre-treated with alumina sandblasting (AB) and glass beads (GB). D: Each group was divided into 4 subgroups (n = 20 each) according to the bonding scheme: 1. no primer + conventional resin cement, 2. MDP-silane primer + conventional resin cement, 3. MDP-BPDM primer + conventional resin cement, and 4. resin cement with MDP without primer. E: After bonding, 10,000 thermal cycles were performed. F. The specimens are mounted on the universal testing machine as shown in the diagram for shear bond strength testing. (G–J) Presentation of surface parameters and contact angle measurements. G: 30 square specimens were pretreated and sintered according to the steps listed in Table 2. H: The specimens were divided into three groups: control (no pretreatment), alumina sandblasted (AB), and glass beads (GB). I: Surface roughness parameters of the specimens tested by 3D-CLSM. J: Test the specimen with a contact angle measuring instrument.
Before bonding, all specimens were polished with 600 grit silicon carbide paper. Groups AB and GB were sandblasted using 50 μm Al2O3 particles (Kulzer, GmbH, Germany) and 75 μm glass beads, respectively. The blasting time for both groups was 20 s, pressure 25 psi, distance 10 mm, angle 90° using a blasting device (Microetcher IIA, Danville Materials, San Ramon, CA, USA). Afterwards, all specimens were cleaned using an ultrasonic cleaner (Easyclean MD, Renfert, GmbH, Germany) filled with distilled water for two minutes and then completely dried with oil-free air.
Each large square specimen was bonded to a smaller disc-shaped specimen. For the conventional resin cement groups, dual-cure resin cement (Visalys Cem-Core, Kettenbach GmbH & Co. KG, Eschenburg, Germany) was applied without prior priming after treatment with aluminum oxide and glass beads for each group, respectively. For the MDP-silane primer group, MDP-silane primer (Visalys Restorative Primer, Kettenbach GmbH & Co. KG, Eschenburg, Germany) was applied with a microbrush and allowed to dry for 60 s, then air-dried, after which conventional resin cement was applied. For the MDP-BPDM primer group, the specimens were primed with two coats of MDP-BPDM primer (Z-Prime Plus, BISCO Inc., IL, USA) for 5 seconds, followed by gentle air spraying, and finally, conventional resin cement was applied. In the MDP-containing self-adhesive resin cement group, all specimens were bonded with MDP-containing dual-cure self-adhesive resin cement (TheraCem, Bisco, IL, USA) without prior priming. During the bonding process, a static standard load of 5 kg was applied to all specimens using a 12-gauge loader, and an LED lamp with a curing intensity of 1500 mW/cm2 (Eighteen CuringPen, Jiangsu Sifary Medical Technology Co., Ltd.) was used to initiate the curing of the adhesive for 40 s.
All specimens were aged using a thermal cycler (SD Mechatronic, Germany) to create thermohydraulic stresses at the interface of the test material and cement. The bonded specimens were immersed in a distilled water bath at 5 °C and 55 °C with a hold time of 20 s and a dwell time of 10 s for 10,000 cycles to simulate one year of clinical use in the oral cavity. 13 The shear bond strength values were measured and recorded using a universal testing machine (Instron, UK, model 3345) with a crosshead speed of 1 mm/min. For failure mode analysis, the delaminated interface was examined at 20× magnification using a stereo microscope (Olympus, Tokyo, Japan), and the failures were classified as: adhesive failure “A”, cohesive failure “C”, and mixed failure “M”.
To determine the surface roughness, 30 square fully sintered specimens were divided into three groups (n = 10 in each group) depending on the type of pretreatment, namely, control group (without alumina or glass beads treatment), with alumina (AB) and glass beads (GB) treatment. The specimens were analyzed using a 3D confocal laser scanning microscope (CLSM, Keyence VK-X100, Keyence, Japan) at a magnification of 50× (scanning area 205 × 273.3 μm), and the obtained scans were analyzed using MultiFile Analyzer software (V.1.3.1.120, Keyence). The values of arithmetic mean height (Sa), developed interface area ratio (Sdr) and texture aspect ratio (Str) were recorded and analyzed.
After measuring the surface roughness parameters, the same samples were used to measure the surface wettability by the sessile drop method with deionized water using a contact angle measuring device (DSA25B, Krüss GmbH, Germany). For each sample, 3 readings from 3 different drops were recorded and the average value was considered as the average reading for each sample tested. All average values were submitted for statistical analysis.
The sample size was calculated based on the data obtained from Khanlar et al.202220. At α = 0.05, at least 20 samples in each group are sufficient to have 95% power to detect. The mean for the control group is 9.2 and for the AB group is 11.7, with a standard deviation of 2, so the effect size (d) is 1.2. The sample size was calculated using G*Power version 3.1.9. The normality of the data was tested using the Shapiro-Wilk test. The tested pretreatments and primer/cement were compared using a two-way analysis of variance (ANOVA), followed by pairwise comparisons using the Tukey HSD test. In addition, the shear bond strength data were analyzed using Weibull analysis (R4, R Foundation for Statistical Computing, Vienna, Austria). Weibull parameters were calculated using the Wald estimator, and 95% confidence intervals were calculated using Monte Carlo simulation. The characteristic strengths of the different groups were compared (63.2% and 10% failure probabilities). For surface roughness and contact angle parameters, test groups were compared using one-way analysis of variance (ANOVA), followed by pairwise comparisons using Tukey’s HSD test. (α = 0.05). Statistical analysis was performed using IBM SPSS Statistics for Windows version 26.0. Armonk, NY: IBM Corporation.
The pretreatment group and the primer/cement group had a significant effect on the shear bond strength, p = 0.003 and p < 0.001, respectively. As shown in Table 3, the interaction between the two variables had a significant effect on the shear bond strength, p = 0.002. The results of the pairwise comparisons showed that alumina sandblasting (AB) showed higher shear bond strength than glass beads (GB) for both the MDP-silane group (p = 0.034) and the MDP-BPDM group (p < 0.001). However, for the conventional resin cement and MDP-containing self-adhesive resin cement group, no significant difference was found between AB and GB pretreatment. For the AB pretreatment group, the conventional resin cement showed the lowest significant shear bond strength compared to all other groups, while no significant difference was found between the MDP-silane primer, MDP-BPDM primer, and MDP-containing cement. For the GB pretreatment group, the conventional resin cement showed the lowest significant shear bond strength compared to all other groups, while the MDP-containing resin cement showed the highest shear bond strength. The difference between the MDP-silane primer and the MDP-BPDM primer was not significant. The shear bond strength values for the different test groups are shown in Table 4.
The results of the Weibull analysis are shown in Table 5 and Figure 2. The characteristic Weibull strength of the conventional resin cement was significantly lower than all other primer/cement groups AB and GB, which were not significantly different from each other. For the AB pretreatment, the differences in characteristic Weibull strength between MDP-Silane + conventional cement, MDP-BPDM + conventional cement and MDP-containing resin cement were not significant. For the GB pretreatment, the MDP-containing resin cement showed significantly higher Weibull characteristic intensity compared to all other groups. The highest Weibull modulus was shown by AB + MDP-BPDM + conventional cement and GB + MDP-containing resin cement. Both ANOVA and Weibull analyses showed similar results except that the Weibull analysis showed no significant differences in AB and GB between MDP-Silane Primer and conventional cement.
Weibull plot of shear bond strength (MPa) for the test groups. The horizontal dotted line at 63.2% failure probability helps compare the characteristic strengths. The vertical control dotted lines at 20 MPa and 40 MPa were used to compare the survival curves of the test groups. The resin cement containing MDP showed the highest characteristic strength compared to all other primer/cement schemes.
In the glass bead (GB) pretreatment, 100% bond failure was observed in all test groups. While in the alumina sandblasting (AB) pretreatment, mixed failures and adhesion failures were observed in all groups. None of the groups showed cohesive failure of both GB and AB. The failure mode analysis is shown in Figure 3 and representative failure mode images are provided in the supplementary file.
The histogram shows the failure modes for the different test groups. A) adhesive failure, (M) mixed failure, C) cohesive failure.
The results of surface roughness and contact angle parameters are shown in Figure 4. For Sa, the lowest Sa values were found in the control group (0.82 ± 0.04) and GB (0.84 ± 0.07) compared to AB (0.97 ± 0.05), p < 0.001. As for Sdr, the highest Sdr values were found in the control group (1.20 ± 0.11) and AB (1.36 ± 0.70) compared to GB (0.74 ± 0.14), p < 0.001. For Str, the control group (0.85 ± 0.03) had the lowest str value compared to AB (0.77 ± 0.10), p = 0.044. The Str value of GB (0.79 ± 0.07) was not significantly different from that of the control group and AB. The contact angle measurements showed that the control group (50.53 ± 4.29) had the highest significant value (p = 0.038) compared to AB (44.94 ± 2.12). There was no significant difference in the contact angle measurements between GB (47.60 ± 2.34) and the control and AB groups.
Box plots show surface topography parameters [arithmetic mean height (Sa), developed interface area ratio (Sdr), and texture aspect ratio (Str)] and contact angle (CA) measurements for the different test groups.
Over the past few decades, zirconia has been introduced into dentistry as a stronger alternative to weaker silicon-based ceramics and has expanded its range of indications as a restorative alternative. However, even with the development of more translucent monolithic alternatives, bonding of zirconia has remained a major challenge as most bonding schemes rely on bonding to a glass matrix, which can be attacked by strong acids, whereas zirconia is not as susceptible due to its polycrystalline nature and lack of a glass matrix.
This study investigated the bond strength of zirconia treated with alumina and glass beads using two different MDP primers in combination with conventional resin cement and self-adhesive resin cement containing MDP. The null hypothesis of the test was rejected because sandblasted with alumina and glass beads, primer before bonding and cement containing MDP alone showed higher bond strength compared to the sandblasted group bonded with conventional resin cement (without primer).
The bond strength between zirconium oxide and adhesive cement plays an important role in the success and longevity of dental restorations. Poor adhesion can be a major cause of cracks in the repair material, which can extend to the adhesive interface and lead to repair failure.21
The results of this study indicate that alumina or glass bead blasting combined with chemical surface treatment (primerization) can significantly improve the shear bond strength of 5Y-PSZ zirconia. Previous studies have found that air blasting using alumina particles is the most preferred and reliable surface treatment method for high-strength ceramics because it can increase the surface roughness, thereby increasing the surface energy, improving wettability and potentially cleaning the bond surface.22,23 As for glass beads, the increase in bond strength may be due to the addition of silica to the bonding surface, which results in the formation of stable chemical bonds between the hydroxyl (OH) groups of silica on the glass surface and the primer/resin cement.24 Another study evaluated the effects of different silicification protocols (glass beads and tribochemical coating) with different silane treatments on the bond strength of translucent zirconia and found that both alumina and glass bead air cleaning improved the bond strength. Their findings were supported by energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) analyses, which confirmed that large amounts of silica were deposited on the surface of the cemented specimens even when they were not cleaned with ethanol or ultrasound.10
The surface roughness of sintered zirconia may play a role in bonding because it can increase the surface area of the sintered substrate. Compared with the glass bead-blasted surface, the surface roughness after alumina blasting is higher. The results of this study are consistent with those of Khanlar et al. 20, who found that air abrasion of alumina particles increased the surface roughness, while glass beads had no effect. Moreover, their SEM/EDS studies showed that alumina resulted in groove formation and surface roughness, while glass beads resulted in the deposition of silicon particles without affecting the surface roughness 20. On the other hand, Mehari et al. 25 found that alumina increased the bond strength while glass beads and untreated zirconia showed almost the same results for the three different types of zirconia, which may be due to the increased surface roughness of alumina blasted zirconia.
Alumina shot peening showed the smallest significant contact angle and the largest surface roughness compared with glass beads and the control group, and according to our results, these two factors led to high bond strength. Transparent zirconia has a larger grain size, which makes the grains easily pulled out during alumina sand blasting, which leads to surface defects and increases the surface roughness of zirconia26. In addition, alumina sand blasting leads to the formation of micromechanical characteristics, such as increased roughness, increased surface energy, and increased resin flow in these micro-retention characteristics, thereby improving the bond strength27. Previous studies have suggested that sandblasting may cause the formation of hydroxyl groups on the zirconia surface, which leads to an increase in the reactivity of zirconia with the phosphate monomers in the MDP, thereby affecting the bond strength28,29. This is not the case for glass beads, since their low hardness cannot change the surface roughness of zirconia. On the contrary, its effect is limited, since the incorporation of silica particles into the zirconia surface leads to a decrease in the contact angle and an increase in the surface energy, thereby improving the adhesion10,20.
The results of this study showed that the self-adhesive resin cement containing MDP had the highest bond strength regardless of the pretreatment method, followed by MDP-BPDM primer and then MDP-silane primer for alumina air abrasion. For the glass bead pretreated group, the primers containing MDP-silane and MDP-BPDM had similar bond strength values. Pure MDP showed the best adhesion to the zirconia surface, and the addition of silane in a single bottle containing MDP decreased the adhesion16. However, pure MDP primers are not available in the market, and the primers available in the market are a combination of several primers, making them versatile and compatible with various substrates.
The phosphate groups in the MDP molecule can theoretically react with one or two zirconium atoms to form two bond configurations, namely “bidentate” or “monodentate”30. The primer-containing MDP has hydrophobic phosphate groups that react with the hydroxyl groups on the translucent zirconia surface, thereby increasing the bond strength31. In addition, MDP prevents water from penetrating between the hydrophobic phosphate layer and the zirconium oxide layer through the action of the decyl groups in MDP32.
According to literature, MDP in resin adhesive can form a stable bond with pre-treated zirconia in air even after thermal cycling33. This may be due to the fact that MDP contains both polymerizable methacrylate ends (which can adhere to the resin) and hydrophilic phosphate ends (which can chemically adhere to zirconia), thereby enhancing the bond strength21.
According to the results of previous studies, the use of primers containing MDP silane can improve the bond strength between resin cement and zirconia ceramics15,34,35. In addition, for the GB group, higher bond strength values were demonstrated due to the chemical interaction between silane and silica in the glass beads remaining on the zirconia surface20. However, the bond strength was lower than that of the self-adhesive resin cement containing MDP. This may be due to the silanols in the primer containing MDP silane, which may lead to a decrease in the bond strength between MDP and the zirconia surface11,36.
Previous studies have shown that MDP-BPDM containing primers have a positive effect on the bond strength to zirconia37,38,39. Previous studies disagreed with our findings and suggested that the reason is that the presence of carboxylic acid monomers in BPDM may have a critical effect on the adhesion between the primer and the self-adhesive resin cement-methacrylate40.
In conclusion, according to this study, the use of aluminum oxide air abrasion has always been considered the gold standard for bonding to zirconia, and it is worth noting that glass beads may be a promising alternative to improve bonding to zirconia, whether used in combination with primer or MDP cement. Considering the limitations of this study, masticatory forces and anatomical designs of fixed partial dentures should be evaluated to simulate oral conditions rather than the simplified designs implanted in the current work. Furthermore, further in vivo studies are needed to evaluate the efficacy of the proposed bonding protocol.
Within the limitations of this study, the following conclusions can be drawn from the results obtained:
1. Aluminum oxide sandblasting and glass bead pre-treatment can improve the bond strength of zirconium using MDP primer or self-adhesive resin cement containing MDP only without pre-primer.
2. MDP containing self-adhesive resins without pre-primer can be used as a successful cementing solution and shortens the clinical steps for successful cementation of zirconia.
3. Sandblasting should be combined with appropriate chemical treatment strategy to enhance the bond strength to zirconia.
Post time: Feb-07-2025