This in-vitro study was the first to evaluate the effect of phosphotungstic acid on bonding of dental zirconia. The results of this study support the proposed hypothesis, because heteropolyacid has effect on bond strength of dental zirconia.
Self-adhesive resin cement was used in this study because it is easy in use and reduces the clinical steps [40]. Also, it promotes wettability of zirconia surfaces and reduces the contact angle between the resin adhesive and the bonding substrate [8]. Self-adhesive resin cement containing MDP has the best bond strength with zirconia ceramics, because of the chemical interaction between functional phosphoric monomer in MDP and hydroxyl group in zirconia [40, 41]. In this study, specimens were subjected to 3500 thermo-cycles simulating one year in the oral cavity [38].
Pyrochemical silica coating (Silano Pen) was reported to improve surface wettability and formation of silica layer on zirconia surface. Silica layer allowed surface to be silaned with 3-methacryloyl oxypropyltrimethyl siloxane silane. Silane is capable of polymerizing with other methacrylic functional groups to form strong bond [18]. Using air-borne particle abrasion before heat treatment allows better surface roughness and utilizes the benefits of mechanical and chemical treatments [16].
Phosphotungistic acid etching solution was chosen according to Devassy et al [27] who stated that phosphotungistic acid made strong acidic area on zirconia after several pilot SEM studies with different concentrations and timing. SEM pilot study indicated that etching of zirconia surface with concentrations more than 1gm/ 50 mL for 1 min 2wt % are effective up to 4 wt%, while 15 wt% of TPA provides the highest recorded acidity. The acidity decreases when the phosphotungstic acid loading exceeds 15 wt% [28]. The corrosive activity of the acid solution leads to increase surface roughness with micro-tensive pits and holes with different sizes.
Concerns have expressed regarding the effect of phosphotungstic acid on zirconia as risk factor for zirconia composition change or phase transformation which may cause spontaneous transformation and affect the mechanical and optical properties of zirconia. Thus, in this study all groups with different surface treatments were subjected to x ray diffraction analysis to detect phase transformation [8]. Only acid treated groups were subjected to FTIR test before and after phosphotungstic acid treatment to detect any change in composition [2, 4].
The results of this study showed a statistically significant difference between Group C (untreated specimens) and the three other groups. Group P2 showed the highest mean value of microtensile bond strength (23.82 ± 5.04 MPa). Group P2 was followed by Group P1 with total mean values of bond strength 17.84 ± 4.79 MPa and Group S 16.88 ± 4.39 MPa with no significant difference between them. Group C showed the lowest total mean value of microtensile bond strength (12.19 ± 4.67 MPa). These results were supported by the study of Gaucci et al [36] who found that specimens etched with acids had higher bond strength than phosphotungstic acid and untreated specimens. Also, Lee et al [33] found that strong acid etching improved the bond strength and increased concentration of acid is directly proportional to bond strength. The current study is in agreement with that of Sakrana et al [35] who found that the specimens treated with strong acid etching recorded the highest bond strength over those treated with air-borne particle abrasion or Silano Pen. Results of Lee et al [8] reported that strong acid etching treatment increased the bond strength, they also revealed that under SEM, the strong acid made a uniform rough surface and removed superficial zirconia layer, although all these previous studies used other types of acids rather than phosphotungistic acid which was firstly used in the dental field in this study.
The results of this study agreed with those of Raeisosadat et al [15] and Shin et al [5] who used silano pen in treating zirconia surface and found that it formed a stronger bond than air-borne particle abrasion and untreated samples. Yenisey et al [37] stated that Silano-Pen device increased the bond strength because silica particles changed the surface chemistry resulting in improving the surface reactivity also the surface has glass-like properties.
The cyclic loading and fatigue have effects on long term durability of Silano Pen technique and uncontrolled firing process from silano-pen could destroy the formed hydroxyl groups at room temperature [18]. Previous studies stated that the minimum clinically acceptable range for bonding was 10–13 MPa [8, 39, 41]. Therefore, the results of this study clearly indicated that, silano pen and phosphotungstic acid etching with resin cements would ensure a durable resin bonding to zirconia ceramic (µTBS 16–23 MPa).
Regarding SEM images, they supported the results of the current study, as in Group C, the bonding surface showed smooth surface texture. Surface was full of apparent machining scratches made by diamond saw on the surface. These scratches almost disappear in other groups images. Group P1 showed a series of parallel cuts after milling as untreated specimens, with more roughness, irregularities, both irregular and rounded pits and grain coarsening than Group C. For Group P2 images showed more roughness, wide distribution of grooves and grain coarsening. The numerous microspores and channels of different sizes, with extruded crystals or scattered irregular zirconia particles were detected on the etched surfaces which improved micromechanical interlock than group P1 and Group C. The possible explanation is that, increasing the acidity of the solution showed more corrosive activity on zirconia surface. However, for Group S images, they showed local distribution of nano-silica particles with rough surface due to air-borne particle abrasion leading to stratified surface, flaws and micro-cracks. The irregularity of the etched surface was more uniform than that of Group S surface topography and these images were supported by those of Shin et al [5] and Sakrana et al [35].
The failure modes were predominantly adhesive between resin cement and ceramic especially in groups C, S and P1 while in Group P2 cohesive and adhesive failures are equal and the increase in cohesive mode of failure evidenced a greater bond strength [6]. This adhesive failure predominance may be a function of distribution of defects in the material, since a smaller bonded area would probably have less defects than larger specimens especially after stereomicroscope testing of beams before testing. Moreover, the failure of brittle materials usually starts from an existing defect. This meant that a decrease in bonded surface area for the µTBS test would probably decrease the risk of an earlier failure before a cohesive failure was experienced in the substrate. With the smaller bonded area for the µTBS test, cohesive failure of the ceramic was experienced at the margins, indicating that stresses were concentrated at the corners toward the central part of the beam according to Griffith’s theory with constant loading, also may be due to the strong bond strength between resin cement and composite [31].
This in-Vitro study verified with phase transformation analysis by XRD for the tested groups, the main peak was at 30o (2Theta) tetragonal cubic. It should be noted that the cubic and tetragonal structures of zirconia are very similar so many of their diffraction peaks overlap in the 2h < 70 range. At higher 2h Bragg angles, the highest intensity for Group P2 3358.85 cts followed by Group S with intensity 1893.89 cts then Group C with intensity 1455.62 cts and Group P1 with intensity 957.56 cts. Appearance of monoclinic phase with the most intensity at Group S followed by Group C and disappeared in Group P1 and P2. This increase in intensity of monoclinic could produce mechanical stress affecting the bond strength. This increase in stresses might be due to air-borne particle abrasion micro cracks and uncontrolled heat treatment of Silano Pen device.
This study verified the composition changes by FTIR analysis for acid treated groups (P1 and P2). The infrared spectroscopy study of phosphotungistic heteropolyacid with a Keggin structure (H3PW12O40) has FTIR spectra with absorptions at 1081, 984, 891, 795, and 593 cm-1.in the region 1100 − 400 cm-1. The position of these spectra can be dedicated, in response, to the corresponding vibrations related to these bonds P-O, W = O and W-O-W and to P-O bonds are respectively responsible for the in and out vibration movements of the plane [30]. All of these absorptions were absent in FTIR analysis for the two acid concentrations which means that there is no effect on the main composition of dental zirconia with no phosphotungstic acid residuals after vigorous cleaning. While zirconia characteristic bands are in the zone 755–915 cm-1 also there are signals located at 3500 and 1620 cm-1, related with some hydroxyl groups that exist after the calcination process [28].
There were some limitations in this in vitro study, as clinical studies which offers more information about effect of oral environment on bond strength and durability with long term clinical follow up. Other limitations were about the effect of increase of phosphotungstic acid concentration and their effect on bond strength, phase transformation and mechanical properties.