An in vitro study conducted at M R Ambedkar Dental College, Bengaluru, Karnataka, India.

For this study, sixty sintered discs of zirconia (10 mm in diameter and 2 mm in thickness) were fabricated using CAD CAM technology [Table/Fig 1(a-g)]. Each zirconia disc were embedded in an aluminium mould (25 mm in length and 15 mm in width) with polymethyl methacrylate autopolymerising acrylic resin. All the sixty samples were divided into three different groups following air abrasion using 110 µm aluminium oxide for 10 seconds at an angle of 90° under 0.28 MPa air pressure, held at a distance of 10 mm from the specimen surface [Table/Fig-2] and all specimens were ultrasonically cleaned in distilled water for 10 minutes and air dried. Based on the surface treatment, all 60 samples were randomly divided into three groups. All three groups were further subdivided into two subgroups based on the resin cement used for bonding. Subgroup A was luted using Rely X Ultimate resin cement while subgroup B was luted using Panavia F resin cement.

[Table/Fig-1]:

Fabrication of Zirconia Discs a) Wax pattern of 10 mm diameter and 2 mm; b) Obtaining master cast of the wax pattern impression; c) Scanning of the pattern using ceramill mind software; d) Scanned pattern nested into zirconia blank for milling; e) Milling of zirconia blank in ceramill motion 2; f) Sintering of zirconia discs in ceramill therm 3. g) sintered 60 zirconia discs.

[Table/Fig-2]:

Sandblasting of zirconia specimens.

Group I-Twenty specimens received no surface treatment following air abrasion with 110 µm aluminium oxide and acted as control.

Group II –Underwent plasma of argon cleaning at 0.5 mb pressure and for a treatment time period of 20 minutes [Table/Fig-3]. [Table/Fig-3]:

Group II samples undergoing plasma of argon cleaning.

GROUP III-Underwent application of silane primer using the silane primer specific for each resin cement used. In subgroup A samples, to be bonded to Rely X Ultimate resin cement, the zirconia specimens were treated using Rely X ceramic primer. Subgroup B samples, to be bonded with Panavia F resin cement, the zirconia specimens were surface treated using Clearfil ceramic primer.

Group I specimens were surface treated as follows: Airborne particle abrasion using 110 µm Al₂O₃ particles was performed for 10 seconds at an angle of 900 under 0.28 MPa air pressure, held at a distance of 10 mm from the specimen surface. Subsequently, all the specimens were ultrasonically cleaned in distilled water for 10 minutes and air dried.

Group II specimens were surface treated as follows: Air abrasion was done using 110 um at 0.28 MPa of air pressure for 10 seconds at an angle of 90° and at a distance of 10 mm from the specimen surface. The specimens were ultrasonically cleaned in distilled water for 10 minutes and air dried. Subsequently, all the specimens underwent plasma of argon cleaning. Plasma treatment was performed using a plasma reactor with argon plasma at 0.5 mb pressure and for a treatment time period of 20 minutes [7].

Group III specimens were surface treated as follows: Air abrasion was done using 110 um Al₂O₃ at 0.28 MPa of air pressure for 10 seconds at an angle of 90° and at a distance of 10 mm from the specimen surface. The specimens were ultrasonically cleaned in distilled water for 10 minutes and air dried. The specimens were treated with silane specific to each resin cement used. In subgroup A which consisted of ten samples, a single coat of Rely X ceramic primer was applied using a disposable applicator tip for 20 seconds on the exposed surface of zirconia disc to be bonded with the Rely X Ultimate resin cement following the manufacturer’s specifications. A gentle stream of air was used over the liquid for 5 seconds and surface of the disc was left to dry at room temperature. In subgroup B which consisted of ten samples, a single coat of Clearfil ceramic primer was applied using an applicator tip for 20 seconds on the exposed surface of zirconia disc to be bonded with the Panavia F resin cement. A gentle stream of air was used over the liquid for 5 seconds and surface of the disc was left to dry at room temperature following the manufacturer’s specifications.

A custom- made metal split matrix (5 mm internal diameter and 2 thickness) was placed on the center of the zirconia disc. The cement was dispensed according to manufacturer’s instructions and inserted into the matrix. In subgroup A, Rely X Ultimate cement was dispensed from the clicker dispenser onto the mixing pad. The base and catalyst was mixed for 2 minutes and 30 seconds following which it was applied onto the zirconia disc and light curing was performed for 20 seconds using light wave of 400-500 nm. In subgroup B, the Panavia F base and catalyst paste was dispensed onto a mixing pad and mixed for 20 seconds. The cement was then applied over zirconia disc. A glass slab and a weight exerting 500 g was placed on top of the cement to permit overflow of a slight excess of material and light curing was done for 20 seconds. The specimens were left undisturbed for 15 minutes and the excess cement was removed. All the specimens were stored in distilled water at 37°C for 24 hours [Table/Fig-4].

[Table/Fig-4]:

Specimens stored in distilled water.

All the samples were thermocycled 5000 times in water between 5°C and 55°C. The dwell time at each temperature was 30 seconds with a transfer time of 15 seconds between baths. The thermocycling conditions were based on the estimation that 5000 thermocycles would simulate the situation during 5 years in the oral cavity [9]. After thermocycling, the specimens were tempered to room temperature in a water bath prior to measuring SBS.

The specimens were then subjected to shear load of 1 KN with a universal testing machine (3345 model; instroncorp). A knife edge chisel apparatus (5 mm blade length) running at a cross head speed of 0.5 mm/minute was used to direct a parallel shearing load at the resin cement/zirconia interface. SBS values were recorded in megapascal (MPa). Following SBS testing, after debonding of the samples, the failure type was assessed and classified according to the failure origin by using a stereomicroscope (Lawrence & Mayo; Nsz 606, Lynx) at 10X magnification.

The collected data were analyzed by 3- way Analysis of Variance test (ANOVA). When significant differences were found between the groups, Bonferroni test (post-hoc test) was applied. The level of significance was set at p-value ≤0.05.

The SBS values were evaluated for each group in comparison with their respective subgroups [Table/Fig-5]. The mean SBS and standard deviation for surface treatment methods and bonding resin cements was calculated [Table/Fig-6,7].

ANOVA test was performed to compare the mean SBS values in different groups [Table/Fig-8]. Highest mean SBS was recorded in Group III (11.81± 2.09 MPa) followed by Group II (8.20 ±1.24 MPa) and control Group I (4.14 ±78 MPa) [Table/Fig-6]. The difference in mean shear bond strength among the groups was found to be statistically significant (p<0.001). In order to find out among which pair of groups there exist a significant difference, Bonferroni test (post hoc test) was applied [Table/Fig-9].

Among the air abraded specimens, a consistent amount of adhesive fractures (between zirconia and cement) was found [Table/Fig-10]. Controversially among the silane treated and plasma treated groups, most detachments were pure cohesive (within the cement) or mixed with part of cement remaining attached to the zirconia.

The results in this study concluded that among the various surface treatments done, the highest mean SBS was recorded in Group III (11.81±2.09 MPa) followed by Group II (8.20±1.24 MPa) and control Group I (4.14±0.78 MPa). Among the two subgroups of resin cements used, subgroup A had the highest SBS (8.72±3.69 MPa) and subgroup B showed lower bond strength value (7.38±3.17 MPa)

The Group III specimens which were abraded with 110 µm Al₂O₃ particles followed by treatment with silane primer of the respective resin cements used, produced the highest SBS values. Air abrasion with 110 µm Al₂O₃ particle roughened the surface which increased its surface area for bonding. The application of silane on this surface hence resulted in improved wettability of the resin cement. Also, the stereomicroscopic results showed the absence of pure adhesive failure in the silane treated group, suggesting that the increase of the SBS in silane treated group is due to a stronger zirconia-cement interface bonding. Combining air abrasion with silane results in association of micromechanical with the chemical retention via siloxane bonds. Silane also has the property of increasing the wettability of cement, thus improving the ability of the zirconia to adhere to the resin cement [10,11].

Group II which underwent plasma of argon cleaning following air abrasion with Al₂O₃ showed bond strength values lower than the silane treated group. The values were significantly higher than the control group which was only air abraded. This signifies the importance of combining air abrasion with other surface treatment modalities, plasma argon cleaning effectively cleaned the air abraded surface of the zirconia, thus reducing the surface contamination and increased the surface energy. Studies have also shown thatplasma argon cleaning reduced the carbon based contaminants and eventually created super hydrophilic surfaces [12,13]. Since MDP present in both the resin cements used directly interacts with zirconia, a deeply cleaned surface with increased wettability from plasma cleaning treatment might have allowed for superior primer behavior. The results are in agreement with similar study which concluded that plasma argon cleaning is effective in increasing the adhesion between zirconia and composite cement [10].

In Group I, samples which were only airborne particle abraded using 110 µm Al₂O_3, the bond strength values were found to be the least. The results are in agreement with a study which concluded that zirconia with surface abrasion only and no chemical bonding showed low bond strength and ultimately resulted in adhesive failures [14].

Among the two subgroups in all the three groups, highest bond strength values were obtained in subgroup A which was bonded using Rely X Ultimate resin cement in comparison with subgroup B which was bonded using Panavia F resin cement. Though, both the cements contained acidic phosphate group, MDP studies have revealed higher bond strength values for Rely X cement when compared with Panavia F cement [8]. Hence, the results of this study was in agreement with various other researchers [8,15].

The significant improvement in SBS was because of combined mechanical treatment using airborne particle abrasion and chemical treatment using silane primer. Therefore, it can be concluded that silane treatment of zirconia is a prerequisite to ensure a stronger and more durable bond between zirconia and resin cement from this study.

The results of this study rejected the null hypothesis, since the surface treatment produced SBS values significantly greater than the control group.

In day to day clinical practice, zirconia crowns are most often luted with resin cements without prior application of primers available with the resin cement. This study has proven the significance of using a primer containing silane that can effectively improve the bonding of resin cement. Application of silane primers to zirconia must be employed in day to day clinical practice. The use of air abrasion of the zirconia crowns before cementation will improve the microroughness and ensure better wettability of the primers used. Hence, air abrasion is a prerequisite for bonding and must be used in combination with primer application for a better chemical and mechanical bonding.

SBS of resin cement to zirconia was the only factor used for assessing the strength of the bond. Bond strength will be significantly affected based on the oral environment which could not be simulated in the study being an in vitro study and long term longevity could not be evaluated. Future studies taking into consideration the effect of intraoral conditions need to be done.

The techniques that can be employed in clinical practice to increase the SBS at the resin–zirconia interface were investigated. Air abrasion with alumina particles provided greater surface area for bonding and increased the micromechanical retention of the resin cement to zirconia. Plasma treatment with argon plasma effectively enhanced and cleaned the zirconia surface considerably removing the smear layer which led to increase in SBS values.

The silane primer promoted a significant increase in the adhesive bonding of resin cement to zirconia. Highest SBS values were yielded by Group III specimens (silane treated group) followed by Group II (plasma of argon cleaning) and Group I specimens (control). The SBS values for Groups III and Group II were more than 5MPa which satisfied the ISO requirements.

Highest SBS values were obtained with Rely X Ultimate resincement when compared to Panavia F resin cement. Lowest SBS values were yielded by control Group I (only air borne particle abraded) followed by Group II (plasma argon cleaning). The SBS values for control group were less than 5MPa and hence was not suitable for clinical situations. Stereomicroscopic analysis showed cohesive failure among the silane treated group signifying a stronger resin cement zirconia bonding with use of silane treatment.