The airborne-particle abrasion of zirconia with alumina particle (APA) has been reported to result in the durable bonding of appropriate adhesive luting systems. However, whether a delay between APA and the application of the adhesive luting material might affect the resulting bond strength and its durability is unknown.
A total of 140 disc-shaped zirconia specimens were divided according to the elapsed time between the APA of zirconia and its bonding into 5 test groups (15 min, 1 h, 4 h, 24 h, and 72 h). The specimens were airborne-particle abraded with 50-μm Al 2 O 3 , and then stored at room temperature according to the test group (n = 28/group). Surface free energy (SFE) was measured for 12 specimens per group using a goniometer. For each group 16 Plexiglas tubes filled with composite resin were bonded to the zirconia specimens with an adhesive luting resin (Panavia 21). Tensile bond strength (TBS) was tested for subgroups of 8 specimens after water storage for 3 days and for 150 days with 37,500 thermal cycles.
SFE decreased significantly within 24 h after APA. TBS after 3 days of water storage ranged from 38.3 (1 h) to 28.4 MPa (24 h) and after 150 days with thermocycling from 38.3 (15 min) to 24.8 MPa (24 h).
Based on these results, the time between the APA of zirconia and the application of adhesive materials should be minimized when bonding nonretentive zirconia restorations clinically.
Dental zirconia (ZrO 2 ) restorations have become popular as a result of their good biomechanical and esthetic properties [ , ]. Different types of zirconia materials with different mechanical, physical, and optical properties are commercially available [ , ].
The luting of all-ceramic restorations to tooth structure using adhesive luting cement has been shown to enhance the survivability and fracture strength of the tooth-restoration complex [ , ]. A strong and durable bond between zirconia restorations and tooth structure can be achieved by micromechanical roughening and cleaning of the zirconia ceramic bonding surface through airborne-particle abrasion with alumina (APA) and using an adhesive luting system containing phosphate ester monomers such as 10-methacryloyloxydecyl dihydrogen phosphate (MDP) [ ]. The APA of zirconia at a moderate pressure (0.5–0.25 MPa) has become a standard treatment before bonding [ ].
The surface free energy (SFE) describes the excess energy of a material’s surface compared with that of the bulk of the material [ ]. Contact angles of two different liquids with different surface tensions have been used to calculate the SFE of the solid surface [ ]. However, knowing the SFE of the solid can predict the behavior of any liquid on the solid surface [ ]. Previous studies [ , , ] have reported that a high SFE of the zirconia bonding surface might positively influence the strength and durability of the resin bonding to it.
The influence of the elapsed time between APA and bonding on the microtensile bond strength of computer-aided design/computer-aided manufacturing (CAD/CAM) polymer composite blocks and on bovine dentin has been evaluated [ , ]. The specimens were abraded with 50-μm Al 2 O 3 particles at a pressure of 0.1 and 0.2 MPa and then adhesively bonded with an MDP luting system on the same day as APA or after 7 days. The authors reported that 7 days elapsed time between APA and bonding had a negative effect on microtensile bond strength, as well as on bonding to bovine dentin [ ]. APA of dental alloys results in a substantial decrease in the contact angle of water, indicating an improvement in the SFE. This activation effect was observed over 20 h after APA [ ].
Based on clinical trials [ ] using oxide ceramic restorations placed with a minimally invasive protocol, the recommended procedures include an intraoral try-in of the restoration, followed by cleaning and roughening the zirconia bonding surface by APA with 50-μm Al 2 O 3 particles at a pressure of 0.1 MPa, before luting the restoration with an appropriate adhesive system. However, when an APA device is not available chairside, the zirconia restoration will be airborne-particle abraded in the dental laboratory. Depending on factors such as the location of the dental laboratory, the dental office schedule, and the time of day, the elapsed time between the APA procedure in the dental laboratory and the time of the actual luting procedure in the dental office can vary greatly. During this time, contamination of the bonding surface from air pollution can occur.
To the best of the authors’ knowledge, the influence of the elapsed time between APA and the actual bonding procedure on the bond strength to zirconia ceramic is unknown. Therefore, the purpose of the present study was to investigate the influence of elapsed time between airborne-particle abrasion and bonding on the surface free energy and the tensile bond strength of bonded zirconia. The null hypotheses of this study were that the zirconia surface free energy and tensile bond strength would not be affected by the different elapsed times between APA and the bonding procedure when using an MDP-containing luting resin.
Materials and method
A total of 140 disc-shaped 3Y-TZP zirconia specimens (Cercon, DeguDent, Constance Germany) with a diameter of 6.5 mm and a thickness of 3.4 mm were used in this study. The zirconia specimens were randomly assigned to 5 groups (n = 28) according to the elapsed time between the airborne-particle abrasion and the bonding procedure: 15 min, 1 h, 4 h, 24 h, and 72 h. This resulted in a total of 60 specimens (n = 12) used to measure the surface free energy (SFE) and 80 specimens (n = 16) used to measure the tensile bond strength (TBS). The study design is presented in Table 1 .
|140 zirconia discs|
|Polished with rotating carbide paper down to 600 grit|
|Airborne-particle abrasion with Al 2 O 3 50 μm at pressure of 1 bar|
|Cleaning with 99% Isopropanol in ultrasonic path for 3 min|
|Specimens were assigned into 5 groups according to elapsed time between airborne-particle abrasion and bonding|
|Bonding after 15 min n = 28||Bonding after 1 h n = 28||Bonding after 4 h n = 28||Bonding after 24 h n = 28||Bonding after 72 h n = 28|
|SFE measurement of 12 specimens from each group|
|Plexiglas tubes were filled with self-curing restorative composite resin (Clearfil FII New Bond)|
|Bonding of the zirconia discs of each group with:|
|Panavia 21 TC luting resin n = 16/subgroup|
|After bonding, each subgroup n = 16 was divided into|
|8 specimens from each subgroup were stored in a distilled water bath at 37 °C for 3 days without thermal cycling||8 specimens from each subgroup were stored in a distilled water bath at 37 °C for 3 days interrupted by thermal cycling between 5 and 55 °C in distilled water with a dwell time of 30 s for 37,500 cycles|
|Debonding of specimens in the universal testing machine (Zwick Z010; ZwickRoell Group) at cross-head speed of 2 mm/min|
|Light microscopic evaluation of the debonded area and assigning adhesive or cohesive failure modes|
|Selected specimens were examined by a scanning electron microscopy (SEM)|
The zirconia specimens were polished with rotating silicon carbide papers down to 600 grit under water cooling, followed by ultrasonic cleaning in 99% isopropanol for 3 min and dried with a flow of oil-free air. The zirconia specimens were airborne-particle abraded with 50-μm Al 2 O 3 at a pressure of 0.1 MPa for 15 s at a distance of 10 mm, and the abrasion procedure was applied to the bonding surface of the specimens in a horizontal and vertical direction to ensure that the complete surface was abraded [ ]. Then, all specimens were ultrasonically cleaned in 99% isopropanol for 3 min. Finally, the airborne-particle abraded zirconia specimens were stored in closed plastic boxes in a dark dry place at room temperature (20 °C) for different times according to the 5 groups tested.
Surface free energy (SFE)
The SFE measurements were carried out at room temperature by using a goniometer (Drop shape analyzer DSA25E, Krüss, Hamburg, Germany) and the sessile drop technique. Two liquids with different surface tensions were used to measure the SFE. A 2-μL drop of deionized water or diiodomethane was dripped onto the abraded surface of a zirconia specimen, and the contact angles of both solutions were measured and calculated with a software program (Advance 184.108.40.206). The method of Owen, Wendt, Rabel and Kaelble (OWRK) was used to obtain the dispersive and polar energy of the tested zirconia, where the sum of the dispersive and polar energy represent the SFE of the investigated zirconia specimen.
The measurement of the SFE using two liquids with different surface tensions is typically performed on the same specimen. In this study, the surface area of a single specimen was not large enough for two separate drops of water and diiodomethane. Therefore, 40 × 40 × 10-mm square Plexiglas jigs were used to hold two specimens together side by side to be conditioned and treated as a single specimen, whereas one specimen was used to measure the contact angle of deionized water and the other to measure the contact angle of diiodomethane.
Tensile bond strength (TBS)
The bonding procedure of the 5 groups was carried out after the determined elapsed time from the APA. Plexiglas tubes with a standard diameter of 3.2 mm [ , ] were filled with autopolymerizing restorative composite resin (Clearfil FII New Bond, Kuraray Medical, Osaka, Japan) and left undisturbed to polymerize for 10 min before bonding them to the zirconia specimens (n = 16). The filled Plexiglas tubes were then bonded to the conditioned zirconia specimens using an autopolymerizing luting resin (Panavia 21 TC, Kuraray Medical). To ensure that the tube axis was perpendicular to the zirconia bonded surface, an alignment apparatus under a load of 7.4 N was used [ ]. Excess cement was removed with a sponge pellet, and an air-blocking gel (Oxyguard II, Kuraray Medical) was applied around the bonding margins to prevent the formation of an oxygen-inhibited unpolymerized layer.
Eight specimens from each group were stored in distilled water at 37 °C for 3 days without thermal cycling to evaluate the initial bond strength, while the remaining 8 specimens were stored in distilled water at between 5 and 55 °C for 150 days with 37,500 thermal cycles and a dwell time of 30 s to test their hydrolytic durability by aging the bonded specimens [ ].
The TBS was evaluated with a universal testing machine (Zwick Z010, Zwick Roell Group, Ulm, Germany) at a crosshead speed of 2 mm/min using a self-aligning chain loop attachment [ , ].
After TBS testing, the debonded zirconia ceramic specimens were examined under a light microscope (Wild M 420, Wild Heerbrugg, Germany) at a 30× magnification. Failures were assigned to the adhesive failure mode if the bond failed at the zirconia surface or to the cohesive failure mode if the failure occurred in the luting resin or in the tube filling composite resin. The areas of each failure mode were calculated and expressed as a percentage of the total bonding surface area for each tested group. Representative debonded zirconia specimens were sputtered using a gold alloy conductive layer of approximately 15 nm and examined by a scanning electron microscope (SEM, XL 30 CP, Philips, Kassel, Germany) with an acceleration voltage of 15 kV and a working distance of 10 mm to evaluate the failure modes and to compare them with the light microscope observations.
The normal distribution of TBS and SFE data was explored by the Shapiro-Wilk test for all groups. The SFE data were normally distributed. As a result, the ANOVA and Tukey HSD tests were used for statistical analysis. Dispersive and polar energy data were analyzed descriptively. As the TBS data for some groups were not normally distributed, the statistical analysis of TBS was performed with the Kruskal-Wallis test followed by the multiple pairwise comparison of groups with the Mann-Whitney U tests. Significance levels were adjusted for multiple testing with the Bonferroni-Holm correction. Statistical analysis was performed using a statistical software program (IBM SPSS Statistics for Windows, Version 20.0, IBM Corp).
Surface free energy
The results of the surface free energy (SFE) test showed that the longer the elapsed time between APA and bonding, the lower the SFE of the specimens. The SFE results of the 5 test groups are shown in Table 2 and Fig. 1 .