Evaluation of Effect of Zirconia Surface Treatment with CO₂ and Nd:YAG Lasers on Shear Bond Strength between Zirconia Frameworks and Porcelain Veneers

INTRODUCTION

Esthetics play an important role in patients’ satisfaction in the field of dentistry, and apart from tissue adaptation, nonmetallic ceramic restorations provide a much better appearance than their metal–ceramic counterparts. Thus, there is still an on-going search for an ideal all-ceramic restoration, and in the meanwhile, following features are looked for: the physical characteristics, strength, edge synchronization, and a perfect appearance for the anterior and posterior teeth. In many cases, all-ceramic prosthesis can potentially create a more pleasant look than a metal–ceramic restoration. In the latter, the metal infrastructure creates an opaque appearance, and the metal margins are usually observable. Its other drawbacks include risk for a metal allergy, breaking of the bond, and porcelain’s color change. Reinforced all-ceramic prosthesis include a high-strength core (such as zirconia), which is covered by porcelain.^1–3

Zirconia is a crystalline oxide of zirconium, and its mechanical properties are quite similar to the metals and its color to that of the teeth. In 1975, Garvie showed that it has great mechanical characteristics and called it steel ceramic.⁴ Nowadays, zirconium has been mixed with computer-aided design (CAD)/computer-aided manufacturing (CAM) computer systems and has made a revolution in dentistry. For making a zirconia core, it is necessary to use a proper CAD/CAM system, which can work on zirconia. In comparison with other all-ceramic restorations, their frameworks are the most reliable.⁵ Having said that, their weakest point is chipping or cracking of the veneer. Zirconia restorations also have the mechanical properties for being used in the posterior areas of the mouth.⁶ For reaching a strong bonding, the mechanical properties of the core and porcelain veneer must be adaptable with each other. Having a strong bonding is necessary for making sure of the structural unity of the restoration under functional forces, and also for avoiding delamination and chipping of the veneer.² It has been reported that all-ceramic restorations, which had the veneer ceramic attached to zirconia frame, showed a 13% chipping of the ceramic in 3 years and 15.2% in 5 years while breaking of metal–ceramics were 8–10% in 10 years. As a result, for increasing their long-term clinical success rate, it is important to increase the bonding strength between ceramic veneer and frameworks in zirconia restorations. Stress distribution in a two-phase structure is more complicated than a single-phase one, and since the interface of core and veneer is one of the weakest areas in veneer restorations, thus more factors must be considered in layered restorations.² Bonding mechanism includes chemical bonding, mechanical adaptation, and pressure stress due to difference in heat expansion coefficient between tetragonal zirconia polycrystal and ceramic veneer. However, it must be mentioned that the exact bonding mechanism between zirconia and ceramic veneer has not been fully understood yet. The strength of the mentioned bonding is affected by many factors;^7,8 most important of which are surface finishing in core, chemical bonding strength, mechanical interlocking, type and rate of interface deficiencies, the use of porcelain liner, and wetting features along with pressure stress rate in veneer layer due to difference in heat expansion rate between zirconia and porcelain veneer.

Various methods have been introduced for improving the bond between porcelain veneer and zirconia, some of which are sandblasting, silica coating, liner, abrasion by diamond drills, and thermal preparation of zirconia’s surface.^9–12 The methods that cause roughness in zirconia surface can have devastating effects on its mechanical characteristics, such as flexural strength and durability.^13,14

Due to lack of silica and glass phase in zirconia ceramics, performing acid etching for creating a rough surface by using HF acid does not make a meaningful difference for improving the bond strength.¹⁵ Air abrasion is usually used for creating a rough topography on the surface of zirconia framework in order to increase the contact surface for bond and for increasing the surface energy and also wetting characteristics.^16,17 But it must be mentioned that this method leaves tracks and other deficiencies in zirconia that would be the starting point of broken frames and thus endangering its mechanical features and longevity.¹⁸

Recently, easy-to-use and efficient surface treatment methods have been proposed using laser radiation. The laser beams that are used in dentistry depending on the wavelength, power, and the type of target tissue can either make stimulus, rupture or extracting.¹⁹ Although Maiman used the ruby laser on an extracted tooth for the first time in 1960, its actual use in dentistry started many years later.²⁰ The use of high-strength laser started in 1989 with pulsed Nd:YAG by Terry Myers.²¹ From 1990 onward, the lasers used in surgery, with little modifications, were used in dentistry as well; examples of which include argon, CO₂, Nd:YAG, and Erbium. Recently, certain types of lasers, namely, CO₂, Nd:YAG, and Er:YAG have been used for creating surface roughness in ceramics zirconia.^17,22,23 Using radiation of CO₂ laser is an ideal preparation method for porcelain materials since its beams are almost completely absorbed by the porcelain due to its specific wavelength.

Previous studies have employed different lasers such as CO₂, Nd;YAG, and Er;YAG for the surface treatment of zirconia and reported varying degrees of success.^24,25 So, this study was done to assess the effects of using Nd:YAG and CO₂ lasers and also the role of heat treatment of zirconia’s surface for the bond between zirconia and ceramic veneer.

MATERIALS AND METHODS

RESULTS

Based on the findings in Table 1, the SBS of zirconia for study groups including (C), (CO + S), (S + CO), (Nd + S), and (S + Nd) were 49.8, 47.3, 65.7, 49.7, and 70.8 MPa, respectively.

The results of Table 2 showed a significant difference between the preparation methods (p-value = 0.039) based on the variance analysis. Due to significant difference of SBS between zirconia and porcelain layer in different methods, the follow-up tests were performed by multiple comparisons using the LSD.

Based on the results of Table 3, intergroup comparison showed a significant difference between the SBS using Nd:YAG after sintering with the control group at the meaningful level of 5% (p-value = 0.027). Also, there was a significant difference between the SBS using CO₂ laser before sintering with Nd:YAG and after sintering at the meaningful level of 5% (p-value = 0.014). The SBS using Nd:YAG before sintering in comparison with Nd:YAG after sintering was at the meaningful level of 5% (p-value = 0.026). The SBS of S + Nd group was significantly more than other groups except S + CO₂. The lowest amount of SBS belonged to CO₂ + S and the highest was to S + Nd group. There were no significant differences between other groups.

Regarding Figures 1 to 3, the images were obtained through an SEM, after surface treatment, and before application of porcelain, and finally, photos of post-failure of the samples are shown as well.

DISCUSSION

One of the weak points of using zirconia in restorations is the interface between core and veneer. Since the natural strength of porcelain veneer is less than core and this can cause the failure points in all-ceramic restorations, so a strong bond between ceramic veneer and framework is necessary for making sure of the structural unity of the restoration under functional forces, and also for avoiding delamination and chipping of the veneer.²⁶ This in vitro study was done to evaluate the effect of zirconia surface treatment with different lasers. The SBS test was evaluated for the control group (only sintering) and other groups including (CO₂ + S), (S + CO₂), (Nd + S), and (S + N). The result of the SBS test was analyzed by SPSS16 software. The variance analysis and LSD method were used to compare the means between tested groups and pairwise comparison of means, respectively.

The SBS is the most appropriate examination for porcelain bonds.²⁷ Since the shear stress is the most common type of stress responsible for breaking of the bonding in clinical conditions and SBS is easily and quickly deployable, this was the test of choice in this study as well.²⁸ The important point about stabilizing the samples was a precise parallelism of interface with the surface of mold since any sloping of the mounted sample’s interface, while applying the force, would result in an incorrect measurement of the SBS. For doing so, a different coloring than zirconia core was used for porcelain veneers in the treatment stage so that the prepared surface would be aligned with the jig and parallel to chisel while its sharp edge in the interface would be perpendicular to the horizon.

Based on the results, a significant difference was between the SBS using Nd:YAG after sintering with control group (p-value = 0.027). Also, there was a similar difference between the SBS using CO₂ laser before sintering with Nd:YAG and after sintering (p-value = 0.014). The SBS using Nd:YAG before sintering in comparison with Nd:YAG after sintering was at the meaningful level of 5% (p-value = 0.026). The effect of Nd:YAG laser on the surface of zirconia in order to create roughness to increase SBS was better than CO₂ laser. Thus, the result of this study showed that the type and sequence of laser and sintering application can affect the bond strength of veneering porcelain to zirconia. Similar to this study, there are several other studies that have examined the SBS of several materials and their compatible veneer has been analyzed.^29–32

In this study, the SBS had their lowest figures in CO₂ + S (47.25 MPa) and their highest in S + Nd (70.30 MPa). The differences were not statistically significant except for S + Nd, which showed a significant difference with the control group. The reason for a low number in CO₂ + S, compared to others, could be due to the application of laser on zirconia samples at first and performing the sintering afterward. As shown in SEM pictures (Fig. 1A), CO₂ laser’s penetration in zirconia surface was excessive and created a groovy look.³³ If laser creates deep grooves, it can damage zirconia frame in fixed prostheses since the depth of their frames are 0.3–1 mm.³³

Primary studies that used the SBS test reported a range of the SBS between 9.4³⁴ and 31 mega pascal³⁵ between Y-TZP and ceramic veneer. In the current study, the figures are higher and the reason for such difference was believed to be due to the lack of any standard for sample preparation, study conditions, and analyzing methods. On the other hand, some researchers believe that the shear test results are greatly affected by the shapes and designs of the samples, and this makes comparing the results of various studies complicated.³⁶

The results of this research is in agreement with other studies and they showed that CO₂, compared to other surface treatment methods such as sandblasting, Er:YAG laser, and Nd:YAG laser, was not effective for increasing the SBS and even reduced it.^37,38 In CO₂ + S group, since sintering is performed after CO₂ procedure, it can be conceived that sintering would decrease the roughness in zirconia surface because of phase transformation of zirconia and as a result affects the SBS. SEM pictures are a confirmation to this notion.

In S + Nd group, first, the sintering was done. The sintered zirconia would probably resist better toward the undesirable changes of laser and minute explosion happening at the time of laser procedure. Those explosions leave debris behind which can attach to the melted surface of the ceramic and thus unbalance the bond.³⁹ Also, sintering increases the t-phase in zirconia, and the heat expansion coefficient between zirconia framework and porcelain veneer causes the strengthening of the bond.⁴⁰ Thus, laser procedures should be done after sintering. The pictures of SEM after the failure mode approve the point that the SBS in S + Nd had been the highest since the SEM picture of the random sample shows the failure to be completely cohesive in porcelain layer (Fig. 3A) while in other groups a range of failures was seen. In the microscopic pictures after the failure, backscatter mode was used for a better diagnosis and analysis of the layers which caused an easier differentiation between zirconia and porcelain.

Under the conditions of performing laser after sintering, the CO₂ laser group showed a lower SBS compared to Nd:YAG (Table 1). The rate and pattern of the surface roughness created by Nd:YAG explain this difference. SEM pictures showed that the tracks caused by CO₂ were wider compared to Nd:YAG. The probable cause of the tracks can be attributed to higher energy of CO₂ laser and making a higher temperature and a subsequent cooling stress.^38,41 Moreover, CO₂ laser beam has a bigger diameter compared to Nd:YAG (0.5 mm vs 320 micron), which can transfer more energy and damage the surface layer more.³⁸ The present study shows similar results to those reporting a higher bond strength for Nd:YAG compared to air-borne and silica coating.^42,43

Similar to the results of the present study, Paranhos et al.³⁸ and Henriques et al.⁴⁴ reported Nd:YAG laser to be meaningfully effective on the SBS, and compared to CO₂ laser, significantly increased the bond strength (14 vs 7.9 MPa).

Da Silveira et al. and Spohr et al.^42,43 also reported results similar to the study at hand. Those two studies had laser settings similar to the present study but used graphite powder for increasing laser energy absorption since alumina-based ceramics do not absorb the 1064 nm frequency emitted by Nd:YAG laser. Zirconia ceramic can absorb the laser energy and does not need any initiators, such as graphite.

Akyil et al.¹⁷ announced that Nd:YAG and ER:YAG lasers had increased the irregularities on the zirconia surface. In contrast with the current results, they showed that Nd:YAG laser had decreased the SBS compared to control group.

Kasraei et al.⁴⁵ showed that surface treatment by CO₂ laser makes a stronger bond compared to Er:YAG laser and the reason can be attributed to the amount and type of created surface irregularities on the zirconia surface.

In accordance with this research, Bitencourt et al.⁴⁶ reported that the laser treatment is capable of improving the roughness of the zirconia surface and, consequently, the bond strength with resin cement and veneering ceramics. Er:YAG laser, however, did not present a favorable tendency toward bond strength, which was not investigated in the present research

CONCLUSION

With the limitations of this study in mind, following points can be concluded: Surface treatment can affect the strength of porcelain veneer bond to zirconia; also, the type of laser, the sequence of laser procedures, and the temperature can affect the bond strength. Moreover, for creating surface roughness in order to increase the SBS, Nd:YAG laser on zirconia surface acts more effectively than CO₂. Thus, for increasing the mentioned strength, using laser after sintering is recommended.

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