Indeed, cementation of zirconia restorations remains a major challenge in clinical situations and many cementation options discussed in the literature may remain controversial. The aim of this study was to evaluate the bond strength of zirconia after alumina and glass bead pretreatment using two different primers, as well as a conventional resin cement and an unprimed cement containing 10-methacryloyloxydecane. A self-adhesive resin cement based on dihydrogen phosphate (MDP). Fully sintered high transmissivity zirconia specimens (n = 160) were divided into 2 pretreatment groups (n = 80): alumina blasted (AB) and glass beads (GB). Each group was then divided into 4 subgroups (n = 20 in each group) depending on the primer and cement used: traditional self-adhesive resin cement, MDP-silane primer, MDP-primer with traditional self-adhesive resin cement and cement containing MDP. 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 studied using other fully sintered specimens (n = 30) constructed for this purpose, divided into control groups (no pretreatment [unmodified], alumina group and glass bead blasting group). A two-way ANOVA was performed for SBS and the failure modes were analyzed. SBS was higher for MDP silane primer (p=0.034) and MDP primer (p<0.001) using alumina sandblasting compared to glass bead pretreatment. While MDP containing cement showed higher but not significant SBS with glass bead pretreatment. Alumina sandblasting and glass bead pretreatment improve the bond strength of zirconia when combined with priming before bonding with conventional resin cement. Furthermore, MDP containing self-adhesive cements as well as pretreated surfaces showed the highest bond strength. Conclusion: Alumina sandblasting and glass bead cleaning improve the SBS bond to MDP containing self-adhesive resin cement, thereby reducing the number of clinical steps required in cementation of zirconia restorations.
Today, with the rapid development and innovation of digital dentistry, there is an increasing demand for highly esthetic restorations in daily dental practice. Dental zirconia is considered as one of the most commonly used restorative materials due to its superior mechanical properties compared to glass ceramics1. This has expanded the range of indications in the field of fixed restorations and indirect restorations, however, early zirconia was less transparent and more opaque, thus less esthetic, as it was a dense polycrystalline structure without a glass matrix, unlike glass ceramics, their use is limited to the posterior region2. To improve the optical properties, the yttrium content was increased to 5 mol% yttrium-stabilized polycrystalline tetragonal zirconia (5Y-TZP), which gave 50% cubic phase, thereby improving the overall esthetics and introducing a new ultra-transparent zirconia.
Compared with silicon-based ceramic materials, the bonding of zirconia to dental tissue or other synthetic materials is controversial because zirconia is chemically inert and resistant to aggressive chemicals (strong acids, strong bases, organic and inorganic solvents6). Over a 5-year observation period, loss of retention of zirconia restorations occurred in 4.7%7, which was directly caused by bone delamination between the various junctional boundaries within the structure8.
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 confirmed the importance of combined mechanochemical surface treatment methods to improve the bond strength of zirconia to resin cements12,13. Regarding the mechanical side, the use of airborne particle wear, airborne particle tribochemical wear, the use of low melting point porcelain, thermochemical etching solutions, selective penetrating etching, laser irradiation, plasma spraying, and zirconia ceramic powder coatings have been proposed, as well as the use of 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP) molecule and its salts as zirconia primers to change the surface chemistry of zirconia9,14,15,16,17.
Air abrasive machining using alumina particles showed the highest bond strength compared to other surface treatment methods18. However, air abrasive machining of alumina may produce surface defects such as cracks, plastic deformation, inclusion of abrasive alumina and microcracks, which deteriorate the mechanical properties of zirconia and reduce the fracture strength19. To reduce the defects on the surface of zirconia, Khanlar et al. (2022) proposed using glass beads as a softer material than sharp and hard alumina, followed by the use of 10-MDP and silane primer, which was reported to have ideal bonding performance and did not produce microcracks on the surface of zirconia20.
There are few reports on the effectiveness of glass bead blasting as a mechanical bonding method alone or in combination with different zirconia primer chemistries. Therefore, this in vitro study was conducted to evaluate the effects of alumina sandblasting and glass bead blasting on highly translucent (5Y-TZP) zirconia alone or with different zirconia primer formulations and MDP-containing self-adhesive resin cement (Bonding Effect Without Post-Primer) on bond strength. The null hypotheses of the test were: 1. Zirconia sandblasted with alumina and/or glass bead will show similar bond strength. 2. Different MDP primers will not affect the bond strength to zirconia. 3. Self-adhesive resin cements containing MDP; results are similar to other MDP primers tested.
The test substrate was a high transmittance yttria-stabilized polycrystalline tetragonal zirconia sample (5Y-PSZ; Liaoning Upcera Co., Ltd., Liaoning, China). The primer used was MDP-silane (Visalys Repair Primer, Kettenbach GmbH & Co. KG, Eschenborg, Germany) and MDP-BPDM primer (Z-prim, Bisco Inc., Illinois, USA). Conventional resin adhesives (Visalys CemCore, Kettenbach GmbH & Co. KG) and self-adhesive resin adhesives (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 as-sintered) were cut using a low-speed precision saw (Isomet 4000, Buehler, Lake Bluff, IL, USA) under dry conditions without water cutting fluid. In addition, 160 disc-shaped specimens with a diameter of 4 mm and a thickness of 3 mm were bonded together to evaluate the shear bond strength. An additional 30 square specimens of the previously specified dimensions were used to measure the surface roughness and contact angle. All specimens were sintered in a zirconia furnace (Lindberg/Blue M, Asheville, NC, USA) using a 7-step cycle at 1450 °C according to the manufacturer’s instructions. The sintering process parameters are listed in Table 2.
For the shear bond strength analysis, the fully sintered specimens (square and disc-shaped (n = 160 each)) were equally divided into two groups according to the pretreatment scheme (n = 80 each): alumina blasting (AB) group and glass beads (GB) group. Each group was further divided into 4 subgroups (n = 20 each) depending on the bonding scheme: 1. No primer + conventional resin cement, 2. MDF-silane primer + conventional resin cement, 3. MDP-BPDM primer + conventional resin cement and 4. MDP-based cements do not require post-priming. Figure 1 illustrates the method graphically.
Schematic representation of the methods used in this study. (A–F) represent the steps of bond shear strength testing. A: 160 square and 160 disc specimens were prepared. B: Carry out sintering as per Table 2. C: Pre-treated with alumina sand blasting (AB) and glass beads (GB). D: Each group was divided into 4 subgroups according to the bonding scheme (n=20 in each group 1. No primer + normal resin cement, 2. MDF-Silane Primer + normal resin cement, 3. MDP-BPDM Primer +); normal resin cement and 4. Resin cement with MDP, no subsequent primer application. E: After bonding, carry out 10,000 thermal cycles. F. The specimen is attached to the universal testing machine as shown in the diagram to test the shear bond strength. (G–J) represent the surface parameters and contact angle measurements. G: Pre-treated and sintered 30 square specimens according to the steps listed in Table 2. H: The specimens were divided into three groups: control (no pre-treated), alumina sandblasted (AB), and glass beads (GB). I: Use 3D-CLSM to check the surface roughness parameters of the specimen. J: Check the specimen with a contact angle measuring device.
Before carburizing, all specimens were polished with 600 grit silicon carbide paper. Groups AB and GB were sandblasted with 50 μm Al2O3 particles (Kulzer, GmbH, Germany) and 75 μm glass beads, respectively. The sandblasting time for both groups was 20 seconds, 25 psi pressure, 10 mm distance and 90° angle using sandblasting equipment (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.
Bond each large square specimen to the smaller disc-shaped specimen. For the conventional resin cement sets, after individually blasting each set with alumina and glass beads without prior priming, apply dual-cure resin cement (Visalys Cem-Core, Kettenbach GmbH & Co. KG, Eschenborg, Germany). For the MDP silane primer set: apply MDP silane primer (Visalys Restorative Primer, Kettenbach GmbH & Co. KG, Eschenborg, Germany) with a microbrush and allow to dry for 60 seconds, then air dry, then apply the conventional resin cement. For the MDP-BPDM primer set, the specimens were primed with two coats of MDP-BPDM primer (Z-Prime Plus, BISCO Inc., IL, USA) for 5 seconds, followed by a gentle air blast, and then applied with conventional resin cement. In the MDP-containing self-adhesive resin cements group, all specimens were bonded with MDP-containing dual-cure self-adhesive resin cements (TheraCem, Bisco Company, IL, USA) without precoating. During the bonding process, a 12 loading device was used to apply a static standard load of 5 kg to all specimens, and an LED lamp (Eighteen CuringPen, Jiangsu Sifary Medical Technology Co., Ltd.) with a curing intensity of 1500 mW/h. cm2 was used to initiate the curing of the adhesive and continued for 40 s.
All specimens were conditioned using a thermal cycler (SD Mechatronic, Germany) to generate hydrothermal stress 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 residence time of 20 seconds and a dwell time of 10 seconds 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 debonded interface was examined using a stereo microscope (Olympus, Tokyo, Japan) at 20X magnification and the failures were classified into: adhesive failure “A”, cohesive failure “C” and mixed failure “M”.
According to the surface roughness, 30 square fully sintered specimens were divided into 3 groups (n = 10 in each group) depending on the type of pretreatment, namely, control group (sand blasting without alumina or glass beads), alumina blasting (AB) and glass beads (GB). The specimens were analyzed using a 3D confocal laser scanning microscope (CLSM, Keyence VK-X100, Keyence, Japan) with a magnification of 50× (scanning area 205 × 273.3 μm) and MultiFile Analyser software (V.1.3.1.120, Keyence) to analyze the obtained scanning results. The values of arithmetic mean height (Sa), expanded 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 using the sessile drop method of deionized water using a contact angle measuring device (DSA25B, Krüss GmbH, Germany). For each sample, record 3 readings for 3 different drops and consider the average value as the average value for each test sample. All average values are given for statistical analysis.
The sample size was calculated based on the data obtained from Khanlar et al.202220. When α = 0.05, at least 20 samples per group are sufficient for 95% detection power. The mean of the control group is 9.2 and that of 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 examined using the Shapiro-Wilk test. The tested pretreatments and primers/cements were compared using a two-way ANOVA followed by pairwise comparison using Tukey’s 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 rates). For surface roughness parameters and contact angle measurements, test groups were compared using one-way ANOVA followed by pairwise comparisons using the Tukey HSD test. (α = 0.05). Statistical analysis was performed using IBM SPSS Statistics for Windows version 26.0. Armonk, NY: IBM Corporation.
The pretreatment groups and the primer/cement groups 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 show that alumina sandblasting (AB) exhibits higher shear strength than glass beads (GB) for the MDP-silane group (p = 0.034) and the MDP-BPDM group (p < 0.001). However, for the conventional resin cements and the self-adhesive resin cements containing MDP group, there was no significant difference 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 there was no significant difference between the MDP-silane primer, the MDP-BPDM primer, and the MDP-containing cement. For the GB pretreatment group, the conventional resin cement showed the lowest significant shear bond strength, while the MDP-containing resin cement showed the highest shear bond strength compared to all other groups. 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 Weibull analysis are shown in Table 5 and Figure 2. The characteristic Weibull strength of conventional polymer cement is significantly lower than all other AB and GB primer/cement groups and is not significantly different from each other. For AB pretreatment, the difference in characteristic Weibull strength between MDP-silane + conventional cement, MDP-BPDM + conventional cement and MDP-containing resin cement was not significant. For GB pretreatment, MDP-containing resin cement clearly showed the highest characteristic Weibull strength compared with all other groups. AB + MDP-BPDM + conventional cement and GB + MDP-containing resin cement showed the highest Weibull modulus. Both ANOVA and Weibull analysis showed similar results except that there was no significant difference between AB and GB MDP-silane primer + conventional cement in Weibull analysis.
Weibull survival plot for shear bond strength (MPa) of the test groups. The horizontal dotted line at 63.2% failure probability helps compare the strengths of the features. The vertical control dotted lines at 20 MPa and 40 MPa are used to compare the survival curves of the test groups. The cement containing MDP resin showed the highest characteristic strength compared to all other primer/cement treatments.
With glass bead (GB) pretreatment, all test groups showed 100% adhesive failure. While with aluminum oxide (AB) pretreatment, all groups showed mixed failure and adhesion failure. No group showed a cohesive failure of both GB and AB. The failure mode analysis is shown in Figure 3 and representative failure mode images are shown in the supplementary file.
The histograms show 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 control group (0.82 ± 0.04) and GB (0.84 ± 0.07) had the lowest Sa values, p < 0.001, compared to AB (0.97 ± 0.05). While for Sdr, the Sdr values were highest 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 significance value (p = 0.038) compared to AB (44.94 ± 2.12). The contact angle measurements of GB (47.60 ± 2.34) were not significantly different from those of the control and AB.
The diagrams show the surface topography parameters [arithmetic mean height (Sa), surface 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 zirconia has always been a major challenge as most bonding solutions rely on bonding to glass matrices, which can be attacked by strong acids. This is not the case with zirconia as it is polycrystalline in nature and does not have a glass matrix.
This study investigated the bond strength of zirconia after alumina and glass bead sandblasting using two different MDP primers in combination with conventional resin cement and MDP-containing self-adhesive resin cement. The null hypothesis of the test was rejected because alumina and glass bead sandblasting, priming before bonding, and using MDP-containing cement alone showed higher bond strength.
The strength of the bond between zirconia 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 repair materials, and cracks can extend to the adhesive interface, resulting in failure of repair21.
The results of this study indicate that alumina or glass bead sandblasting combined with chemical surface treatment (primer) can significantly improve the bond shear strength of 5Y-PSZ zirconia. Previous studies have found that alumina particle blasting is the most preferred and reliable surface treatment method for high-strength ceramics because it increases the surface roughness, thereby increasing the surface energy, improving wettability and potentially cleaning the bonding surfaces 22, 23 . As for glass beads, the increase in bond strength may be attributed to the addition of silica to the bonding surface, which results in the formation of stable chemical bonds between the hydroxyl groups (OH) of silica on the glass surface and the primer/resin cement 24 . Another study evaluated the effects of different siliconizing schemes (glass beads and tribochemical coatings) with different silane treatments on the bond strength of translucent zirconia and found that both alumina and glass bead blast cleaning improved the bond strength. Their findings were supported by energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) analyses, which confirmed that high levels of silica were deposited on the surface of the cemented specimens even when they were not cleaned with ethanol or ultrasonic waves10.
The surface roughness of sintered zirconia may play a role in bonding because it can increase the surface area of the sintered substrate. The surface roughness after alumina sandblasting was higher compared to the surface treated with glass beads. 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. Furthermore, their SEM/EDS study showed that alumina formed grooves and protrusions on the surface, while glass beads caused the precipitation of silica particles without affecting the surface roughness20. 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 three different types of zirconia, which may be due to the sandblasting of alumina surface caused by increased roughness.
Alumina sandblasting showed the lowest significant contact angle and the highest surface roughness compared to glass beads and controls, two factors that we found to result in high bond strength. Translucent zirconia has a large grain size, which makes the grains easily pulled out during alumina sandblasting, causing surface defects and thus increasing the surface roughness of the zirconia.26 In addition, alumina sandblasting results in the formation of micromechanical features, which manifests itself as increased roughness, increased surface energy, and increased resin flow in these microresidues, thereby improving bond strength.27 Previous studies have suggested that sandblasting may be responsible for the formation of hydroxyl groups on the zirconia surface, which leads to an increase in the reactivity of zirconia toward phosphate monomers in MDP, thereby affecting the bonding strength. 28, 29 This is not the case for glass beads, since the low hardness of glass beads cannot change the surface roughness of zirconia. On the contrary, its effect is limited because 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 bonding effect. 10, 20
The results of this study show that the self-adhesive resin cement containing MDP has the highest bond strength regardless of the pretreatment method used, followed by MDP-BPDM primer and then MDP-silane primer for air-abrasive alumina. For the glass bead pretreated group, the primers containing MDP-silane and MDP-BPDM had similar bond strength values. Pure MDP has the best adhesion to the zirconia surface, and adding silane to the same bottle containing MDP will reduce the adhesion16. However, pure MDP primers are not commercially available, and the commercially available primers are a combination of primers that are 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 “bicoordinate” or “monocoordinate” 30 . Primed MDP has hydrophobic phosphate groups that react with the hydroxyl groups on the translucent zirconia surface, thereby enhancing the bond strength 31 . In addition, MDP prevents water from penetrating between the hydrophobic phosphate layer and the zirconium oxide layer due to the role of the decyl group in MDP 32 .
According to the literature, MDP in resin-based adhesives can form stable bonds with pre-treated zirconia in air even after thermal cycling33. This can be explained by the fact that MDP contains both polymerizable methacrylate ends (which adhere to the resin) and hydrophilic phosphate ends (which chemically adhere to the zirconia), thereby increasing the bond strength twenty-one.
According to the results of previous studies, the use of primers containing MDP silane improved the bond strength between resin cement and zirconia ceramics15,34,35. In addition, for the GB group, higher bond strength values were indicated by 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 silanol content in the primer containing MDP silane, which may lead to a decrease in the bond strength between MDP surfaces and zirconia11,36.
Previous studies have shown that MDP-BPDM containing primers have a positive effect on the bond strength of zirconia37,38,39. Previous studies were not consistent with our findings, suggesting that the reason was that the presence of carboxylic acid monomers in BPDM may have a critical effect on the bond between this primer and self-adhesive polymer cement methacrylate40.
In summary, according to this study, the use of air abrasion of aluminum oxide has always been considered the gold standard for bonding to zirconia. It is worth noting that glass beads may be a promising alternative to improve adhesion to zirconia combinations, whether in combination with zirconia primer or MDP cement. Due to 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. In addition, further in vivo studies are needed to evaluate the effectiveness of the proposed adhesion scheme.
Within the framework of this study, based on the results obtained, the following conclusions can be drawn:
1. Alumina sandblasting and glass bead pre-treatment improve the bond strength of zirconia using MDP primer or MDP-containing self-adhesive resin cement alone without primer pre-treatment.
2. MDPs containing self-adhesive resins that do not require pre-priming can be used as a successful cementation solution and reduce the number of clinical steps required for successful zirconia cementation.
3. Sandblasting should be combined with appropriate chemical treatment to improve the bond strength with zirconium dioxide.
Post time: Jan-20-2025