Effect of different surface treatments of presintered or sintered zirconia on bond strength to dentine.

Aim: To compare the effect of different zirconia surface treatments on cement bond strength to dentine. Materials and
Methods: Stick-shaped pre-sintered zirconia (N = 128) were prepared and divided into eight groups (n = 16). Three surface treatments (sandblasting, neodymium-doped yttrium aluminum garnet [Nd:YAG] or carbon dioxide laser irradiation) were applied, either before (later to be sintered) or after sintering. The last test group was sintered zirconia coated with feldspathic veneering ceramic. Sintered zirconia without surface treatment was tested as the control group. Zirconia samples were cemented to dentine using Panavia F2 cement. The micro-shear bond strength was measured after 24 h (n = 8) or 10,000 thermocycling (n = 8). Statistical Analysis: Data were analyzed using one-way analysis of variances and Student's t-tests.
Results: Zirconia coated with feldspathic ceramic revealed the highest bond strength (P < 0.001). Presintered zirconia treated with Nd: YAG laser showed a significantly improved bond strength compared to the control group before and after thermocycling. The bond strength after thermocycling was significantly reduced in presintered zirconia treated with Nd: YAG or sandblasting. Adhesive failure at the zirconia-cement interface was the dominant failure type. Conclusions: Surface treatment of presintered zirconia by Nd:YAG laser or coating of the sintered specimens with feldspathic veneering ceramic increased the zirconia-cement bond strength. Copyright:

INTRODUCTION

The recent evolution in restorative dentistry has contributed to the development of novel ceramic systems, including alumina-, silica-, and zirconia-based ceramics; minimizing the use of metal frameworks as well as durable restorations with high levels of esthetics.

Yttrium-stabilized tetragonal zirconia ceramics combined with computer-aided design and computer-aided manufacturing are the novel generation of high-strength ceramics with promising applicability in modern dentistry. Sufficient physical properties, biocompatibility, and satisfactory clinical outcomes[1] make zirconia favorable for prosthodontic treatments. The unique capability of yttrium-stabilized tetragonal zirconia polycrystalline of inhibiting crack propagation due to its tetragonal crystal transformation into a monoclinic crystal with associated volumetric expansion,[2] makes zirconia a very suitable material to tolerate the loading in the oral cavity. Zirconia has a nonreactive surface, which is acid-resistant and nonetchable with limited adhesive luting potential.[3] A robust and durable resin bonding provides high retention, improves marginal adaptation, prevents microleakage, and increases the fracture resistance of the restored tooth and the restoration.[4] Trustworthy adhesion ensures desirable clinical outcomes, especially when the retention mostly relies on chemical adhesion. A strong resin cement bonding necessitates a micromechanical interlocking and adhesive chemical bonding to the ceramic surface. This requires surface roughening for mechanical bonding and surface activation for chemical adhesion. Several methods, including sandblasting with alumina particles,[5] tribochemical procedure followed by salinization,[6] and application of specific primers to the zirconia surface, have been suggested.[7] Resin-based cements with phosphate ester monomers, including 10-methacryloyloxyidecyl-dihyidrogenphosphate (10-MDP), chemically react with zirconium dioxide and promote a water-resistant bond to densely sintered zirconia ceramic.[8] The 10-MDP molecule acts as a silane-coupling agent by its hydrogen groups.[910] Although some studies suggested reliable resin bonding to zirconia is clinically achievable,[11] other researchers have controversially questioned a reliable bond between oxide ceramics, in particular zirconia, and a bonding component.[121314]

Recently, various lasers, including diode lasers, neodymium-doped yttrium aluminum garnet (Nd:YAG), erbium-doped yttrium aluminum garnet laser (Er:YAG), and carbon dioxide (CO2) lasers have been used for the preparation of ceramic surfaces with inconsistent results.[15] Irradiation of Nd:YAG laser in the feldspathic porcelain resulted in an improved bond strength that is comparable with hydrofluoric acid etching.[16] The effectiveness of Nd:YAG laser irradiation in the zirconia surface by the formation of micromechanical retention between zirconia and 10-MDP-based cement has been previously reported.[17] In contrast, Liu et al. showed that Nd:YAG laser irradiation can neither improve the surface properties of zirconia ceramics nor increase the bond strength to the ceramics.[18] The lack of sufficient consistency on the effects of different irradiation methods compared to traditional sandblasting on the bond strength of zirconia, particularly on pre-sintered zirconia, prompts further investigation.

Therefore, the aim of this study was to evaluate the bond strength of presintered (later to be sintered) and sintered zirconia to dentine with various surface treatments (Nd:YAG laser, CO2 laser, sandblasting, and feldspathic veneering porcelain coating) before and after thermocycling. The null hypothesis was that the different zirconia surface treatments do not influence the bond strength of zirconia to dentine after cementation with 10-MDP-based resin cement.

MATERIALS AND METHODS

Specimen preparation

Sixty-four human molars, extracted due to periodontal disease, were collected for this study. The teeth were disinfected in 0.5% chloramine T for 1 day and thereafter stored in normal saline. Ethical approval was not required because the study was performed in anonymous biological materials (i.e., teeth), already removed for dental treatment purposes. Each tooth was sectioned longitudinally in mesiodistal direction with a diamond disk (TRICRAFT, Hebei Province, China) under constant water coolant (N = 128). Two presintered zirconia blocks (Cercon Zirconia, Degodent, Germany) were cut by high-speed handpiece to obtain stick-shaped zirconia specimens with a dimension of 1 mm × 1 mm × 10 mm on the intact surface. The dimension of each zirconia stick at the outmost top of the intact surface was checked using a digital caliper (Syntek, Deqing Syntek Electronic Technology CO., LTD, Deqing County, Zhejiang China). Each zirconia stick was checked under stereomicroscope at ×25 and samples with deformities or chipped-off edges on the intact zirconia surface were discarded. Included zirconia sticks (N = 128) were cleaned in isopropanol in an ultrasonic bath for 5 min followed by air-drying. The zirconia samples were randomly divided into eight groups (n = 16) corresponding to the treatment on intact zirconia surface as:

Group 1: Control group with no surface treatment on sintered zirconia

Group 2: Presintered zirconia irradiated with pulsed Nd:YAG laser

Group 3: Sintered zirconia irradiated with pulsed Nd:YAG laser

Group 4: Presintered zirconia irradiated with CO2 laser

Group 5: Sintered zirconia irradiated with CO2 laser

Group 6: Presintered zirconia sandblasted with alumina particles

Group 7: Sintered zirconia sandblasted with alumina particles

Group 8: Sintered zirconia coated with feldspathic veneering ceramic

In Groups 2 and 3, Nd:YAG laser with a wavelength of 1064 nm, 10-watt power, frequency of 2 kHz, and 230 ns pulse width equipped with Galvo Scanner and f-e lens (Model P150 INLC, Iranian National Laser Center, Tehran, Iran) was used. The applied laser pulse energy to the target was 5 mJ with 90 μm laser beam spot diameter and energy density (fluency) of 78 J/Cm2. The target was ablated on a rectangular pattern by laser beam, moved with a scanner, located at the laser head, and focused on the workpiece by a 100 mm focal length f-theta lens. The scanning speed was adjusted to 1.5 cm/s at the target surface so that the pulses with applied frequency could strike successively to the target with constant overlapping, to maintain the target surface at the focal point and to prevent the probable defocusing. The surface treatment using Nd:YAG laser had the same pattern and procedure in Groups 2 and 3.

In Groups 4 and 5, noncontact CO2 laser (Deka, Florence, Italy) was applied in focused mode with the power of 1 Watt, 1-pulsed energy, and 10.6-μm wavelength with the power density of 127.39 J/Cm2. The surface treatment using CO2 laser had the same pattern and procedure in Groups 4 and 5. In Groups 6 and 7, we sandblasted the zirconia surface using 50 μm alumina particles, at 2.5 bar pressure with approximately 10 mm distance for 20 s.

Following surface treatment in Groups 2, 4, and 6, the zirconia samples were sintered according to the manufacturer's instruction (VITA Vacumat, Zahnfabrik, Bad Säckingen, Germany). In Group 8, we applied three layers of feldspathic veneering ceramic (Cercon® Ceram kiss DeguDent, Dentsply, Germany) to the surface of sintered zirconia. Afterward, the zirconia-feldspathic ceramic specimens were fired at VITA Vacumat (VITA Zahnfabrik, Bad Säckingen, Germany) according to the firing schedules defined by the manufacturers. After the firing, veneered feldspathic ceramic was etched for 90 s with 9.5% hydrofluoric acid (Porcelain Etch, South Jordan, UT, USA). The veneered feldspathic surface was then cleaned with distilled water, following 37% phosphoric acid gel application (Ultradent, South Jordan, UT, USA) for 30 s to remove porcelain salts.[19] The specimens were immersed in isopropanol in an ultrasonic bath for 5 min. Finally, the feldspathic veneer was silanized (Ultradent, South Jordan, UT, USA) and dried with oil-free airflow.

We used 10-MDP-containing resin cement (Panavia F2, Kuraray Medical. Inc, Tokyo, Japan) to cement the zirconia samples to the prepared dentine sections. Briefly, a putty index (before the cementation) was made to correctly place and cement the zirconia to the dentine section. ED Primer A and B Panavia F2 were mixed according to the manufacturer's instruction, applied to dentine, and air-dried gently for 30 s. Equal amounts of A and B pastes Panavia F2 were mixed for 20 s and applied onto the intact surface of the zirconia stick using a putty index to secure a perpendicular assembly. The zirconia stick was cemented mid-dentine on the crown. Then, the sample was tack-cured for 2 to 3 s mesially and distally with dental LED light-curing unit (Woodpecker Med. Instrument, Guilin, China) with 1000 mv/Cm2 intensity. The excess cement was gently removed and the specimen was light-cured for 20 s.

The specimens for short-term assessment (n = 8 per group) were stored in 37°C distilled water for 24 h, while the specimens for long-term aging were immersed in distilled water for 1 week at room temperature (n = 8 per group). Subsequently, long-term specimens underwent 10,000 thermocycles between 5°C and 55°C, with 30 s dwelling time in each water bath with 5 s transfer time.

Micro-shear bond testing

Each zirconia-cement-dentine specimen was fixed to a micro tensile tester (BISCO, Inc., Schaumburg, IL, USA) with cyanoacrylate glue (SuperGlue, Loctite, Henkel Loctite, Hertfordshire, UK). A stainless-still wire (0.8-mm diameter) was looped around the zirconia stick, and the shear force was applied at a crosshead speed of 0.5 mm/min until debonding. The maximum shear force required for breaking the bonding was recorded and converted to Mega Pascal by dividing the maximum shear force to the specimen bonded area (1 mm2). Following the shear test, the failure types were evaluated using a stereomicroscope at ×25.

Scanning electron microscopy

One sample from each group was gold-sputtered for surface topographic evaluation using scanning electron microscopy (SEM) after surface treatment (Philips XL 30, North Billerica, USA).

Statistical analyses

The normality of the data was checked using the quantile-quantile plots. Data were analyzed using one-way analysis of variances (ANOVA) followed by Bonferroni post hoc comparison to evaluate the difference between the specimens before and after thermocycling, separately. The effect of aging on bond strength was analyzed using Student's t-tests comparing the values before and after thermocycling in each group at the significance level of 0.05 using STATA 11.0 software (StataCorp, Texas, USA).

RESULTS

Figure 1 shows the means and standard deviations of bond strength values of zirconia to dentine for in different groups before and after thermocycling. The ANOVA revealed a statistically significant difference in bond strength between the groups both before and after thermocycling (P < 0.001). In post hoc comparisons, the bond strength of zirconia samples coated with feldspathic veneering porcelain was significantly higher than other groups both before and after thermocycling (P < 0.001). Only presintered zirconia samples treated with Nd:YAG laser (Group 2) showed a significant increase in bond strength compared to the control group before (P = 0.012) and after (P = 0.002) thermocycling. Although the bond strength in all groups declined after thermocycling, the reduction was only statistically significant in Group 2 (P < 0.001) and Group 6 (P = 0.028). No statistically significant difference was observed between the surface treatments performed on presintered or sintered zirconia samples for any of the treatment groups. Adhesive failure at the zirconia-cement interface was the most dominant failure. We observed only one dentine cohesive failure in the feldspathic veneered zirconia group.

Figure 1

Means and standard deviations of zirconia bond strength to dentine in different groups. * indicates a statistically significant difference between the two groups. § and ¥ denotes a statistically significant difference with all other groups before and after thermocycling, respectively

SEM analysis showed that intact presintered zirconia surface [Figure 2a] got more homogenous surface after sintering [Figure 2b]. Globular structure of porcelain deposition on the zirconia surface was observed in the feldspathic veneered zirconia surface after the application of hydrofluoric acid [Figure 2c]. We observed the application of Nd:YAG laser caused harmonious distinct striped surface irregularities over presintered zirconia samples [Figure 3a]. Interestingly, the surface irregularities were still observable after sintering [Figure 3b]. However, Nd:YAG laser did not cause an obvious topographical alteration on the sintered zirconia surface [Figure 3c]. Surface treatment of presintered zirconia using CO2 laser [Figure 4a] caused a minor visual topographical change compared to presintered zirconia with no treatment [Figure 2a]. Surface irregularities with no specific pattern were observable after the sintering of presintered zirconia with CO2 laser [Figure 4b]. CO2 laser irradiation was not able to alter the sintered zirconia [Figure 4c] compared to sintered zirconia without any surface treatment [Figure 2b]. The traces of physical topographic changes caused by alumina particles during sandblasting in presintered zirconia were detectable [Figure 5a], which became less evident following sintering of sandblasted presintered zirconia [Figure 5b]. Localized spot trace of alumina particles after sandblasting of sintered zirconia was observable [Figure 5c].

Figure 2

Scanning electron microscopy micrographs of intact zirconia surface with no treatment before (a) and after sintering (b) as the control group. Sintered zirconia surface coated with feldspathic porcelain after etching with hydrofluoric acid (c)

Figure 3

Scanning electron microscopy micrographs of presintered zirconia surface treated with neodymium-doped yttrium aluminium garnet laser before (a) and after sintering (b). Sintered zirconia surface treated with neodymium-doped yttrium aluminium garnet laser (c)

Figure 4

Scanning electron microscopy micrographs of pre-sintered zirconia surface treated with carbon dioxide laser before (a) and after sintering (b). Sintered zirconia surface treated with carbon dioxide laser (c)

Figure 5

Scanning electron microscopy micrographs of pre-sintered zirconia surface sandblasted before (a) and after sintering (b). Sintered zirconia surface treated with sandblasting (c). * and # represent the sandblasting alumina particles trace in pre-sintered zirconia and sintered zirconia surface, respectively

DISCUSSION

The results of the current study suggested that the irradiation of presintered zirconia using Nd: YAG laser or surface coating with feldspathic porcelain may improve the bond strength of zirconia to dentine. In contrast, sandblasting or CO2 laser as surface treatment of presintered and sintered zirconia samples did not improve the bond strength. Therefore, the null hypothesis was rejected (P < 0.001).

Shear and tensile bond strength tests are the most common methods to evaluate bond strength. These interface evaluation methods most often assess either the cement-dentine interface or the cement-zirconia interface. However, in the clinical situation, both interfaces are present. Therefore, we performed the tests in a complex of the dentine-cement-zirconia in an attempt to mimic the clinical situation. In general, for a micro-shear or micro-tensile bond strength testing, the specimens are sectioned after the application of the resin-based cement on the treated zirconia surface. This method might influence the bonding results due to implemented interface stress during sectioning, which may negatively affect the bond interface.[20] Therefore, we sectioned the presintered zirconia samples before cementation to obtain test specimens with minimal interface stress induced by sample sectioning. However, this method of specimen preparation was essentially more time-consuming. Another limitation of dentine-cement-zirconia complex testing was the difficulty to standardize the cement thickness in all samples, which may increase the variation in results caused by varying cement thickness. However, the observation of adhesive failure at the zirconia-cement interface as the dominant failure type and no cohesive failure within the cement in this study indirectly indicates a minor effect caused by cement thickness.

Our bond strength observations revealed that treatment of the zirconia surface before sintering might result in improved bond strength, although nonsignificant, compared to after sintering. As also observed in SEM micrographs, the topographical alterations, in terms of surface irregularity, were more evident in presintered zirconia, especially in Group 2 [Figure 3a], which was still detectable after the sintering process [Figure 3b]. Increased surface hardness and mechanical properties of zirconia following sintering may decrease the possibility of surface topographic alteration with the different treatments used in this study. Similarly, another study exhibited that the topographic changes are more likely to be generated in presintered zirconia than sintered zirconia.[21]

We used a resin-based cement containing the adhesive phosphoric 10-MDP monomer. Studies have shown 10-MDP-containing cements provide a long-term durable resin bond to zirconium-oxide ceramic.[822] However, our findings indicated that the bond strength reduces after aging with thermocycling. This is in agreement with other studies, which reported that artificial aging decreases the shear bond strength.[2324]

The results of this study suggested that feldspathic porcelain-coated zirconia, etched with hydrofluoric acid and primed by silane, results in the highest bond strength compared to the other treatment groups. This finding was consistent with studies conducted by Yamaguchi et al.,[5] and Kitayama et al.,[25] illustrating the effectiveness of porcelain-coated zirconia to obtain a better bond strength. However, the difficulty to control the thickness of the porcelain layers and risk of fracture of the feldspathic layer stand as a drawback of the method in the clinical scenario. Further, the crown fit, cement space, accuracy of zirconia-based restorations will be influenced by using this method.

Our study revealed that surface treatment of presintered zirconia using Nd:YAG laser would increase the bond strength compared to the control group. SEM analysis indicated more surface irregularities in this group, which may enhance the interlocking of resin cement and, eventually, increasing the bond strength. In line with our findings, Usumez et al. reported that Nd:YAG laser irradiation can lead to an increase in surface roughness and bond strength to resin cement.[26] We observed that the treatment of presintered or sintered zirconia surfaces using CO2 laser or sandblasting did not significantly improve the bond strength. It has been previously shown that the application of Er:YAG and Nd:YAG lasers may lead to a superior bond strength of zirconia compared to those of sandblasting or CO2 laser.[5] This is in contrast to the finding of Ural et al. that showed CO2 laser is a suitable method for the preparation of zirconia to improve the zirconia-cement bond strength compared to sandblasting.[27] Akyil et al. have argued that CO2, Nd:YAG, or Er:YAG laser irradiation alone or in combination with air abrasion may be used as alternative methods for strengthening the bond between resin cement and Yttria-stabilized tetragonal polycrystals.[28] The discrepancies between the studies could be explained by methodological variations. Obviously, extending the exposure time or raising the laser energy level may cause more surface irregularities to potentially improve the zirconia-cement bonding, but it might also damage the zirconia structure.

Sandblasting is known to increase the surface roughness of zirconia ceramics, which leads to micromechanical interlocks of luting agents over the zirconia surface.[29] Some studies suggest that the bonding of cement to zirconia improves when surfaces are air-abraded.[3031] However, others concluded that sandblasting does not effectively enhance the bonding of resin cement to a zirconia surface.[3132] Our results indicated that sandblasting of neither presintered nor sintered zirconia did significantly improve the bond strength compared to the control group. However, 10-MDP-containing cements and sandblasting have clinically been recommended to improve the bond strength compared to other self-adhesive cement.[33]

In the oral cavity, the restorations are under the combination of different mechanical forces and various other stresses, including temperature changes, moisture, acidity, and bacterial plaque that is difficult to simulate in vitro. Therefore, the results of an in vitro study should always be interpreted with caution. Randomized controlled clinical trials to evaluate the long-term performance of zirconia restorations with different surface treatment modalities are recommended.

In this study, we observed a dominant frequency of adhesive failure mode accompanied by no remnants of adhesive cement left on the zirconia surface, regardless of the surface treatment group. This observation suggests that the cement-zirconia interface is still more prone to debonding than the cement-dentine interface.

CONCLUSION

Within the limitation of the study, the feldspathic coating of the zirconia surface resulted in the highest bond strength. Application of Nd:YAG laser to presintered zirconia samples significantly improved the bond strength compared to the control group. Thermocycling significantly reduced the bond strength in the presintered sandblast and Nd:YAG laser groups.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.