The Effect of Water Aging on Shear Bond Strength of Pretreated Zirconia-Oxide Ceramics Bonded to Resin Cement – An In Vitro Study

Introduction

There has been a significant growth in interest on zirconia ceramics in recent times, with a major impact in the field of dentistry due to its excellent mechanical properties, biocompatibility and optical properties. Zirconia has been elected as an alternative to metal. ¹

Zirconia exists in three phases: Monoclinic, Tetragonal and Cubic phases at various temperatures. In dentistry, tetragonal phase is preferred as this phase exhibits excellent mechanical properties. Zirconia is stabilized in this phase by the addition of 3 mol% of Yttria (3Y-TZP).1 One of the major limitation for the use of zirconia is the difficulty to adhere to this material.²

Different luting cements have been suggested for zirconia restoration, including GIC cement, conventional resin cement and self-adhesive resin cements. Nevertheless, resin cements are generally preferred for cementation of zirconia to improve retention and to obtain a sealed restoration. Resin bonding to glass matrix ceramics is easily achieved by etching the surface with hydrofluoric acid to partially dissolve the glass phase and create porosities for micromechanical attachments.1 As zirconia does not contain glass phase, hydrofluoric acid etching is less effective on zirconia compared to glass matrix ceramics. The chemical bonding with silane occurs due to the lack of silica in zirconia making conventional silane less form more effective for bonding.² This shall aid in creating a more durable bond with zirconia which is also the most effective clinical protocol; it is however under debate. Various surface treatments such as air abrasion, laser etching, roughening have been developed. Among these, air abrasion and chemical bonding are quite effective. Recently a novel surface roughening technique called Selective Infiltration Etching (SIE) has been explored for zirconia.³

Metals and zirconia have some similarities. For this reason, Hot Etching technology, which was initially used with nickel chromium alloy was explored for zirconia. Hot etching technology provides a corrosion controlled process. The grain structure on the surface of zirconia was less in array and had high energy peripheral atoms deflected during this process. Casucci proved that hot etching could form ideal surface roughness for zirconia.³

The aim of the study was to compare the hot etching technology with the already established bonding using zirconia primer and roughening of the surface with air abrasion. In this study, we have also evaluated the bond strengths of zirconia to resin cement after various surface treatments and also after thermocycling to depict the ageing of zirconia.

Materials and Methods

As per the statistical analysis, sample size of sixty was estimated.

Acrylic block fabrication

A cuboid shaped wax mould measuring 35x30x30 was made using modelling wax. Cold cure powder and liquid were then mixed in a porcelain jar and poured into wax mould. A cold cure acrylic block was made out of it. Putty impression of this acrylic block was taken. Using this putty mould, 12 acrylic blocks were made.

Sample fabrication

a) Twelve sandblasted (air abraded with 50µ Aluminum oxide particles) zirconia discs of dimensions 25 mm diameter and 2.5 mm thickness were taken.

b) Grouping of samples: All the 12 discs were sandblasted.

Group A: Control group - Sandblasted disc (No further treatment was done)

Group B: Hot etching - Zirconia disc specimens were kept in a reaction beaker that was loaded with etching solution made up of methanol (800 mL), 37% HCl (200 mL) and 2 g of ferric chloride. This was constantly heated to a temperature of 100 0 C in a water bath with stirrer. It was kept for an hour (Figure 1).

After the treatment, the specimens were washed with tap water for around 60 seconds, and later were ultrasonically cleaned in a water bath for half an hour and finally air dried.

Group C: Zirconia primer – Zirconia discs were taken and a layer of zirconia primer was applied over the sandblasted surface using an applicator tip.

c) Resin cylinder fabrication: Resin cement (Rely X Unicem-200) was loaded into the plastic matrix of dimension 2.5 mm x 4 mm and then was irradiated for 30 sec using curing light. In this manner, five cylinders were made on one zirconia disc and a total of 20 resin cylinders were prepared for each group. (Figure 2) 

Sub grouping of samples

Two discs from every group were aged via thermocycling, altering the temperature (5°C and 60°C). Six discs were placed in thermocycling unit (Azyme lab). A total 500 cycles were done with a dwell time of three minutes and transfer time of one minute (Figure 3).

All the samples were secured over the acrylic blocks using a thin mix of self-cure acrylic.

Shear bond strength testing

Immediate shear bond strength (after 24 hours) and Late shear bond strength (after 500 thermocycles) testing of samples was done using universal testing machine. Samples were tested under a load of 50 KN and testing speed was 0.1 mm/min. (Figure 4)

Results

On comparison of mean immediate shear bond strength between the three groups, the mean immediate shear bond strength for sandblasting group was observed as 14.18 ± 1.81, for hot etching group was 28.31 ± 1.19 and for zirconia primer group was 37.57 ± 1.81. This mean difference in the shear bond strength between three groups was statistically significant at p <0.001.

On multiple comparison of mean differences in immediate shear bond strength between three groups, the test results showed that zirconia primer group demonstrated significantly higher shear bond strength as compared to hot etching and sand blasting groups with p <0.001. This was then followed by hot etching group demonstrating a significantly higher mean shear bond strength as compared to sandblasting group (p <0.001). This infers that the immediate shear bond strength was significantly highest in zirconia primer group, followed by hot etching group and was least with sandblasting group.

The test results demonstrated that the mean shear bond strength after ageing for sandblasting group was 8.49 ± 0.74, for hot etching group was 21.86 ± 1.28 and for zirconia primer group was 28.27 ± 1.31. This mean difference in the shear bond strength between the three groups was statistically significant at p <0.001.

The test results showed that zirconia primer group showed significantly highest shear bond strength as compared to hot etching and sand blasting groups (p <0.001). This was then followed by hot etching group demonstrating a significantly higher mean shear bond strength as compared to sandblasting group at p <0.001. This infers that the shear bond strength after ageing was significantly highest in zirconia primer group, followed by hot etching group and was least with sandblasting group.

Table 1 illustrates the comparison of mean shear bond strength between immediate and after ageing time intervals in each study group. The test results showed that the mean shear bond strength after ageing time interval in sandblasting group, hot etching group and zirconia primer group [8.49 ± 0.74, 21.86 ± 1.28 & 28.27 ± 1.31] was significantly reduced as compared to immediate time interval [14.18 ± 1.81, 28.31 ± 1.19 & 37.57 ± 1.81] respectively. These differences in the mean shear bond strength between immediate and after ageing time intervals in each study group was statistically significant at p <0.001. (Figure 5)

Discussion

Strong and durable adhesion to dental zirconia has eluded researchers for more than ten years. Cementation technique selection represents a critical factor when dealing with high-strength core ceramics. Various surface treatments have been mentioned in the literature such as sandblasting with alumina particles, tribochemical silica coating, zirconia primer, hydrofluoric acid etching (HF), deposition of silica nanofilm (NF) etc. Recently hot etching has been introduced.

In the present study, three types of surface treatments were compared. The control group included sandblasting, the second group was subjected to hot etching using methanol, HCl and ferric chloride while the third group included the application of zirconia primer containing methacryloyloyloxydecyl dihydrogen phosphate. Also shear bond strength was compared after thermocycling.

The test results demonstrated the mean immediate shear bond strength for sandblasting to be 14.18 ± 1.81. Sandblasting was carried out on all zirconia discs using 110 µm Aluminum oxide particles (Al2 O3 ) when applied perpendicular to the surface, using an airborne particle-abrasive device. This aids in mechanical retention by increasing the surface roughness. ^4,11

Magne P, Paranhos MPG reported that the original roughness that is present as a result of milling during fabrication is insufficient to promote adhesion. However, the roughening and activating the surface are also important to achieve durable resin bond to densely sintered zirconia ceramic.¹

The shear bond strength between zirconia and resin after surface treatment with hot etching was 28.31 ± 1.19. This result is in accordance with the study conducted by Pin Lv, Xin Yang and Ting Jiang who demonstrated that hot-etching significantly improved the shear bond strength of resin cement to zirconia as opposed to no treatment or treatment with airborne-particle-abrasion for both immediate testing and testing after thermal cycling (p <0.05). The cellular structure post etching was reported to be favorable for micromechanical interlocking either with the resins or human enamel (after three-step etchand-rinse cycle). These findings contradict the findings of El-Korashy who reported that deep grooves that occur after hot-etching would lower the bond strength of composite cement. The effect of hot-etching treatment had raised the overall shear bond strength between the zirconia and the resin cement as opposed to the modified SIE treatment. ^3,12

The hot acid solution etches the grain structure on the zirconia surface, enlarging the grain boundaries via removal of less-ordered outer high-energy atoms. The acid solution had methanol acting as a solvent, hydrochloric acid that rendered hydrogen ions, and ferric chloride which was the prime corrosive agent. ³

The shear bond strength between zirconia and resin after application of zirconia primer was 37.57 ± 1.81. This result is in line with the results reported by Inokoshi et al, who had reported that combined mechanical and chemical surface pretreatment of zirconia improved the bond durability of both composite cements bonding to zirconia. Likewise, another study conducted by Attia and Kern investigated the durability of bond strength of adhesive luting cement to zirconia ceramic after the application of different ceramic primers. They concluded that after storage for 150 days in water without thermal cycling, a new universal primer provided significantly better long-term resin bonding to zirconia ceramic than did a conventional silane. They also concluded that cleaning methods had little effect on long-term resin bonding to zirconia ceramic. They also reported that a combined mechanical and chemical pretreatment was the best option for a durable bond to zirconia. ^5,15

The chemical bond strategies on the other hand involved the use of adhesive monomers (seen in luting agents or zirconia/metal primers). The new primer used in the current study had a mixture of organophosphate with arboxylic acid monomers. Organophosphate monomers with a methacrylate group can be easily co-polymerized with the monomers of a composite. The phosphate monomers can develop the bond with the metal oxides in the substrate. ⁶

Another drawback of zirconia is “ Ageing” i.e. low temperature degradation. Hence, we have tested the shear bond strength (SBS) after thermocycling to check the changes in SBS. Thermocycling can simulate the oral environment, and be used as a reference for the artificial aging process. There is a large variation in the number of cycles and in the temperature extremes. According to ISO protocol, a thermocycling regimen comprising 500 cycles in water between 5 °C and 60 °C is an appropriate artificial ageing test, and the same was followed in the present study. Ceramics (brittle material) fail owing to microscopic errors that could occur during the fabrication or service of these materials. Also, they are susceptible to slow crack growth at the tips of surface imperfections when exposed to a moist environment due to hydrolysis of the internal silicate bonds. ^7,13

Studies by White et al., have shown that immersion of ceramics in water decreases their static strength and increases the crack velocity. Sherill and O’Brien, Fairhurst et al., and Myers et al., had reported a fall in flexural strength of porcelains (aluminous and feldspathic) when tested in water.

With the increase in use of monolithic complete contour crowns and fixed partial dentures, the effect of the long term aging should be determined. Similar reaserch was done by Beuer et al (2001).

The contact of zirconia with humidity causes the transformation of the tetragonal phase to monoclinic one which occurs on the surface of zirconia and negatively affects its mechanical properties. The samples were aged by subjecting them to thermocycling. The aging due to thermocycling can be induced by repetitive contraction or expansion stresses generated by materials with different coefficient of thermal expansions or by hydrolysis of the interfacial components. ^8,14

The mean shear bond strength after ageing for sandblasting was 8.49 ± 0.74, for hot etching group was 21.86 ± 1.28 and for zirconia primer group was 28.27 ± 1.31. Results after aging showed that the Control group: Sandblasting group was the most affected one with significant decrease in the shear bond strength. This result is in line with that reported by Raquel Osorio who showed that degrading is faster in zirconia which was not treated when compared to the surface treated one. ^9,16

The results of this study showed that primer coated zirconia ceramics gave rise to the highest shear bond strength compared to the other surface treatments tested, even with thermocycling exposure. There was a statistically significant difference between the primer coated subgroups and the other tested subgroups. This may be attributed to the ceramic blasting with Al₂ 0₃ and primer combining the micromechanical retention produced by airborne-particle abrasion and chemical bonding within. ^10,17,18

Limitations of the study

Only one brand of zirconia and resin cement were tested.

Another limitation was the specimen design. Ideally, crown-shaped specimens are cemented on natural teeth.

Slight variations in angulations of resin cylinders when bonded on zirconia disc.

Thermocycling provides artificial oral environment, and not the natural oral niche.

Sample size was less.

Conclusion

Within the limitations of the present study, it can be concluded that the resin ceramic interfacial longevity can be altered by surface treatment. Chemical treatments (primer application) affect the surface topography and physico-chemistry (roughness, wettability, and surface free energy) of yttrium stabilized Tetragonal Zirconia (Y-TZP). This study showed that the use of a 10-methacryloxydecyldihydrogen phosphate (MDP) containing primer is an efficient treatment method and is capable of promoting a stable bond to Y-TZP. Water aging played an important role in resin cement - zirconia bond degradation and due to thermal aging, there was an alteration in the strength of resin- zirconia bond. Zirconia primer has significantly improved SBS of zirconia cemented to resin cement, especially under thermocycling aging conditions, than did the sandblasting and hot etching group.

Conflicts of Interest

Nil