Influence of Aluminum Oxide Sandblasting Associated with Nd:YAG or Er:YAG Lasers on Shear Bond Strength of a Feldspathic Ceramic to Resin Cements

Abstract

Objective: This in vitro study evaluated the influence of the surface pretreatment of a feldspathic ceramic on the shear bond strength of two different resin cements. Background Data: Although several conventional surface treatments have been used on feldspathic ceramic, few studies have investigated the effects of an alternative surface treatment, the association of aluminum oxide sandblasting with Nd:YAG and Er: YAG lasers. Methods: Sixty samples made of a feldspathic ceramic were divided into three groups (n = 20) and treated with (1) controlled-air abrasion with Al₂O₃ + 10% hydrofluoric acid (HF), (2) Al₂O₃ + Er:YAG laser, and (3) Al₂O₃ +Nd:YAG laser. Afterward, silane (Dentsply) was applied on each treated surface. Each of the three main groups was divided into two subgroups (n = 10), where a different resin cement was employed for each subgroup. It was built a cylinder with resin cement (RelyX Arc) in subgroup (A) and with self-adhesive cement (RelyX U100) in subgroup (B). After 24 h at 37°C, the prepared specimens were submitted to shear bond strength test and stereoscopic evaluation to determine the type of failure. Results: Bond strength mean values were not statistically significant for the surface treatment methods or resin cements. Conclusion: The null surface treatment proposed with aluminum oxide sandblasting associated with the Er:YAG or Nd:YAG laser and using cementation with self-adhesive cement can be an alternative bonding technique for feldspathic ceramic, since it was as effective as the conventional treatment with aluminum oxide sandblasting and hydrofluoric acid using the conventional resin cement.

Introduction

T he increasing demand for aesthetic restorations has led to the greater use of all-ceramic materials due to their improved biocompatibility and optical properties compared with metal–ceramic restorations.¹ This has been accompanied by further development of adhesive materials, since ceramic restorations are very brittle and, in most of cases, need to be bonded with them. In addition, the increasing improvement of the resin luting cements, has allowed the use of more conservative restorative techniques and provided excellent esthetic appearance and adequate strength.²

The clinical success of an indirect restorative procedure depends, in part, on the cementation technique used to create a link between the restoration and the tooth.³ The increasing interest in the use of resin cements results from their advantageous mechanical and adhesive properties when compared with conventional luting agents.^4,5 However, despite the disadvantages of conventional cements and the advantageous properties of the resin luting materials, their use has not been widespread in all types of procedures. This may be because of the sensitivity of the technique, such as the steps required to produce an adhesive restoration, including etching the dentin and enamel surfaces, application of a bonding agent, and subsequent light activation.⁶

From this, it seems that a resin luting material that eliminates the need for pretreatment can provide advantages to dentists, in terms of simplification of technique, ease of use, and cost. The self-adhesive cements were designed with the intent of overcoming some of the shortcomings of both conventional and resin cements and of combining the favorable characteristics of different cement classes into a single product.³ Additionally, the bond strength between resin cement and ceramic depends, whatever resin cement used, on the composition of the ceramic material, the silane agent used, and the surface treatment that the ceramic received. Numerous options have been suggested, many of which were combinations of various mechanical and chemical conditioning methods used to optimize bond strength at the ceramic/cement interface. Roughening of the surface with aluminum oxide air abrasion or diamond burs is generally regarded as compulsory for achieving reliable bond strength.^7,8

Some studies have demonstrated that etching the ceramic surface with hydrofluoric acid solutions, followed by the application of a silane coupling agent, is the preferred surface treatment method for feldspathic ceramic.^7,9,10 However, when an intraoral ceramic repair is needed, the use of hydrofluoric acid is not recommended because of well-recognized hazardous effects in vivo, since it is a harmful and irritating compound for soft tissues.^11,12

For such cases, another promising technique for the surface treatment of ceramic materials is laser irradiation. Li and colleagues¹³ investigated the application of the Nd:YAG laser to feldspathic ceramics, demonstrating that this treatment promotes the formation of irregularities and consequently enhances the adhesion to resin cements. Among the various lasers, the Er:YAG laser is one of the most often recommended types to be used on the dental surface, because its wavelength (2.94 μm) coincides with the main absorption band of water (∼3.0 μm), and it is also well absorbed by OH⁻ groups in hydroxyapatite. Due to its good interaction with dental structures, the Er:YAG laser is a good choice for repair procedures on ceramic materials.¹⁴

In this context, since both the type of surface treatment done on the feldspathic ceramic and the resin cement used may influence the bond strength of indirect all-ceramic restorations, it was necessary to conduct research to evaluate the effects of an alternative surface treatment, such as the association of aluminum oxide sandblasting with Nd:YAG and Er:YAG lasers, that can be used to obtain a reliable bond between the ceramic material and a conventional and self-adhesive resin cement.

Materials and Methods

Sixty feldspathic ceramic discs with a diameter of 5 mm and a thickness of 4 mm (Ceramco 3, Dentsply, York, PA) were fabricated according to the manufacturer`s recommendations. The specimens were embedded in acrylic resin blocks, and the porcelain surface was covered with transparent tape to prevent contamination during embedding. Then the exposed surfaces were flattened under water refrigeration in a polishing machine (EcoMet^® Twin, Buehler, IL, USA) using 600-grit sandpaper and immediately immersed in distilled water for 24 h at 37°C. For this study, only feldspathic ceramic surface was conditioned by the treatment methods. Dentin surface was not used, because we would have two different surfaces to be analyzed (the dental surface and the restorative material surface), which could lead to confounding results and wrong conclusions. Then the specimens were randomly assigned to one of three groups (n = 20) for surface treatments.

Group 1 (control group)

Air abrasion (MicroEtcher; Danville Engineering, San Ramon, CA) with 50-μm aluminum oxide was applied for 10 s at an operating distance of 4 cm from the ceramic surface with 2-bar pressure and etched with 10% hydrofluoric acid (Dentsply Indústria e Comércio Ltda., Petrópolis, RJ, Brazil) for 2 min.

Group 2 (Al₂O₃ + Er:YAG laser)

Air abrasion as described for group 1 was followed by application of a hydroxyapatite paste made from hydroxyapatite powder (Bionnovation Biomedical, Bionnovation Produtos Biomédicos S.A, São Paulo, Brazil/Anvisa no. 10392710010/0.5 cc) and distilled water (0.5 mL) on the ceramic surface and Er:YAG laser (Kavo Key Laser II model, KavoDental GmbH & Co., Biberach, Germany) irradiation with 2.94-μm wavelength. The laser beam was delivered on focused mode at 0.5-mm focal distance, with 500 mJ of energy per pulse at a frequency of 4 Hz, and the entire ceramic area was manually scanned for 20 s with no water spray, in order not to remove the hydroxyapatite paste.

Group 3 (Al₂O₃ + Nd:YAG laser)

Air abrasion as described for group 1 was followed by application of graphite powder (stain) on the ceramic surface and Nd: YAG laser (Pulse Master 1000, American Dental Technology, Southfield, MI, USA) irradiation. The laser optical fiber (300-μm diameter) was kept 1 mm from the surface, and the role ceramic area was manually scanned with no water spray. The laser parameters used were 100 mJ, 20 Hz (pulse/s), 1 W, and 141.54 J/cm².

To simulate oral conditions, sandblasting with aluminum oxide particles and irradiation with Er:YAG and ND:YAG lasers were performed manually by the same operator, who practiced these methods to better promote standardization of the distance from the ceramic surface.¹⁵ All acid-etched specimens (group 1) were subsequently rinsed thoroughly with distilled water for 10 s to remove residual acid and then air-dried.

After the surface treatments, a silane agent (Silano, Coupling Agent, Dentsply Indústria e Comércio Ltda., Petrópolis, RJ, Brazil) was applied according to the manufacturer's recommendations. Each of the three main groups was divided into two subgroups (n = 10) that employed two different resin cements. In one subgroup (A) a resin cement (RelyX^TM ARC, 3M ESPE, Dental products, Seefeld, Germany) was applied, and in the other subgroup (B) a self-adhesive resin cement (RelyX^TM U100, 3M ESPE, Dental products, Seefeld, Germany) was applied according to manufacturer's recommendations. Samples were then inserted into rubber-ring matrices (3 mm in diameter and 3 mm thick) and positioned firmly onto the center of the ceramic disks. The resin cement was then inserted into the matrix with a syringe and initially polymerized with a light-polymerizing unit (Elipar Freelight 2, 3M ESPE, St Paul, MN, USA), with the light directed perpendicularly to the resin cement for 40 s to subgroup A and 20 s to subgroup B. The rubber matrix was then carefully removed and the polymerization was completed with 20-s polymerization sequences per surface around the circumference of the cement cylinder, resulting in a total time of 100 s for the specimens of subgroup A, and 40-s polymerization sequences per surface, resulting in a total time of 200 s for the specimens of subgroup B, following manufacturer's recommendations. After the bonding procedures, light-intensity output was measured with a radiometer (Demetron Curing Radiometer, model 100, Kerr Demetron, Danbury, CT, USA), maintaining 600 mW/cm².

The specimens were stored in distilled water for 24 h at 37°C. The samples were then submitted to a shear bond strength test (Mini-Instron 4442, Instron Corp., Norwood, MA, USA), with a load cell of 50 kgf and a crosshead speed of 0.5 mm/min.

After the shear bond strength test, ceramic bonding areas were analyzed under a stereoscopic optical magnifier (Nikon 88286, 40X, Nikon, Kawasaki, Kanagawa, Japan) to assess the type of failure. From this analysis, three types of failures were defined; adhesive failure at the ceramic–resin cement interface; cohesive failure in the resin cement or ceramic, with no damage to the interface; and mixed failure that was defined involved both the interface and the material.

The means of each group were analyzed by two-way analysis of variance (ANOVA) to determine significant differences between the conditioning methods, the resin cements, and the interaction between them, at the significance level of p < 0.05.

Results

Mean bond strengths for each group, with minimum and maximum values and standard deviations, are shown in Table 1. ANOVA showed no significant influence of the surface treatment methods or the resin cements on the bond-strength values (p = 0.55). With respect to mode of failure, all specimens failed cohesively in the body of the feldspathic ceramic.

Table 1.
Mean Shear Bond Strength (X), Minimum (Min), Maximum (Max) Values, and Standard Deviations (SD) for Each Group

Group Subgroup X Min Max SD

1 A 10.35 7.40 14.86 2.37

B 10.71 5.39 16.51 3.17

2 A 11.12 7.90 14.69 2.20

B 11.18 7.85 16.20 3.61

3 A 9.78 7.23 15.01 2.40

B 9.71 7.30 13.55 2.09

Discussion

The properties of a luting agent and the preparation technique for ceramic surfaces prior to cement application play a major role in the clinical success of many indirect ceramic restorations. The intention of this study was to investigate the influence of the surface pretreatment of a feldspathic ceramic on the shear bond strength of two different resin cements. It was of special interest to test whether simplifications of the pretreatment process would give results comparable to the recommended procedures.

When considering the luting agents, some recently developed resin-based cements claim to be “self-adhesive,” in that conventional surface pretreatments are not required (such as one-step dentin bonding agents), with the ease of handling significantly improved over contemporary materials. This is the case for RelyX^TM U100 self-adhesive, dual-polymerizing universal resin cement, which presented bond values similar to conventional resin cement (RelyX^TM ARC). Some studies^16

–19 have already observed that a comparable self-adhesive cement, RelyX Unicem, presented similar or even superior bond values to conventional resin cements.

Besides the determination of adhesion values, the failure modes were also examined in this study to obtain further information about the probable success of a pretreatment method under clinical conditions. In general, a cohesive failure mode within the ceramics indicates that the adhesion of the resin material to the ceramic is higher than the shear strength of the ceramic. The performance of Ceramco blocks is well established in the literature,^20
–22 so the observation of the totality of cohesive failure may lead to the conclusion that the adhesion of both resin cements to the ceramic, regardless of its surface treatment, is sufficient within the limits of this study.

Nevertheless, the occurrence of cohesive failures can also be attributed to the bond test employed. Many studies that used conventional shear bond tests observed that this methodology often produces fracture away from the adhesion zone.^14,22–23 Such failures of the substrate prevent measurement of interfacial bond strength and limit further improvements in bonding systems.

Several studies have identified nonuniform stress distributions along bonded interfaces.^24
–26 The nonuniform interfacial stress distribution generated for conventional tensile and shear bond strength tests initiates fractures from flaws at the interface or in the substrate in areas of high stress concentration.²⁷ Therefore, analyzing the results of the present study, one might consider that the surface treatments applied to the ceramic could have caused these possible flaws within the body of the ceramic material and, consequently, the occurrence of all cohesive failures.

When analyzing the surface treatment for the ceramic, many researchers^7,28
–30 recommend the use of a combination of airborne particle abrasion (50-μm aluminium oxide), HF acid etching, and application of a silane coupling agent to aid the adhesion of conventional silica-based ceramics. Many studies have verified that neither the use of only one of the methods cited above^14,31–32 nor the use of any combination of two methods from them (Al₂O₃ + HF, HF + silane, or Al₂O₃ + silane)^27,32 achieved better bond-strength values than the combination of the three methods. This is the reason why, in this study, this combination was considered the control group.

Among the mechanical treatments of ceramic surfaces, aluminum oxide sandblasting has been widely used to promote micromechanical retention in several types of ceramics.^{1,23

–34} In this context, in this study aluminum oxide sandblasting was applied before laser irradiation to simulate the clinical situation, since it is the conventional procedure performed by commercial laboratories.

On the other hand, hydrofluoric acid attacks the glass phase of ceramics, partially dissolving it and creating microporous retention by exposing areas of crystals that make up the crystalline phase of the materials.³⁵ Microporosity increases the surface area and makes micromechanical interlocking of resin possible. Various acid solutions can be used for this purpose; however, hydrofluoric acid has been shown to be the most effective.³⁶ After mechanical treatment and acid etching, the use of a silane improves bond strength,^37,38 since silane molecules, after being hydrolyzed to silanol, can form a polysiloxane network or hydroxyl groups to cover the silica surface. Monomeric ends of silane molecules react with the methacrylate groups of the adhesive resins by free- radical polymerization.³⁹

The results of the present study showed that the control group was not significantly different from the groups irradiated with Er:YAG or Nd:YAG lasers, so one might prefer these alternative conditioning techniques because of HF acid's potential danger. HF acid is well recognized to have hazardous effects in vivo; it is a harmful and irritating compound for soft tissues,^11,12 so it is not a practical method to use in dentistry, particularly for intraoral ceramic repairs.

When using the Nd:YAG laser, Li and colleagues¹³ reported that a feldspathic ceramic irradiated with 0.9- and 1.2-W laser potency had the same shear bond strength as the ceramic treated with HF acid. Further, the scanning electron microscope (SEM) images showed that the laser-irradiated surface was favorable to mechanical retention between ceramic and resin cement.

In the present study, the methodology for Nd:YAG laser application on ceramic surfaces (2 W, 20 Hz, 100 mJ, and 141.54 J/cm²) was based on a study performed by Silveira and colleagues,⁴⁰ who defined this protocol after a series of pilot studies on In-Ceram Alumina and observed that the Nd:YAG laser was the most effective surface treatment, followed by Rocatec and Al₂O₃ sandblasting.

Because ceramics do not effectively absorb the 1064-nm wavelength energy emitted by an Nd:YAG laser, a graphite powder was applied to cover the ceramic surface, thus increasing energy absorption.^15,40 Microexplosions occurred due to energy discharge on the coated surface and removed the graphite in the process, which was visible when the surface was scanned by the optical fiber tip. However, since In-Ceram Alumina has a higher percentage of alumina and a smaller amount of silica than feldspathic ceramics, the use of high laser parameters is needed. It is then possible that the use of intermediate parameters to irradiate feldspathic ceramics between those employed by Li and colleagues¹³ and Silveira and colleagues⁴⁰ may give superior results than those found by the present research.

On the other hand, the clinical use of the Er:YAG laser on dental substrate has been well described in the literature since it acts on these tissues by thermomechanical ablation, vaporizing its water contents, which causes expansion followed by microexplosions that produce the ejection of both organic and inorganic tissue particles, providing a surface with open dentinal tubules and no smear layer.^41,42

However, regarding restorative materials, Burnett and colleagues⁴² verified that indirect composite surface treatment with the Er:YAG laser (λ = 2.94 μm) at 200 mJ and 10 Hz enhances single-bond tensile-bond strength compared with air abrasion and fluoridic acid. Nevertheless, in a previous study,¹⁴ even when using higher-energy intensity (500 mJ), the Er:YAG laser was not able to promote an adequate roughness on the feldspathic ceramic's surface, probably due to the different composition of ceramic material and its reflectance.

Based on the Shiu and colleagues¹⁴ study, a novel aspect of the present study was the use of a hydroxyapatite paste, which was applied to stain the ceramic surface to increase energy absorption and, consequently, create a micromechanical retention pattern more favorable to bonding, because ceramics also do not effectively absorb the 2.94-μm wavelength energy emitted by the Er:YAG laser. This technique allowed the achievement of bond values similar to the control group, a fact that was not possible in the Shiu and colleagues¹⁴ study.

The extent of the superficial changes on the ceramic surface depends on both the energy density of the laser radiation and the type of irradiated ceramic. So, to achieve a better clinical performance from both the Er:YAG and Nd:YAG laser for the superficial treatment of dental ceramics, further studies that test different parameters of such lasers will be needed.

Conclusion

The null surface treatment proposed with aluminum oxide sandblasting associated with the Er:YAG or Nd:YAG laser and the cementation with the self-adhesive cement can be an alternative bonding technique to feldspathic ceramic, because it was as effective as the conventional treatment with aluminum oxide sandblasting and hydrofluoric acid using the conventional resin cement.

Group	Subgroup	X	Min	Max	SD
1	A	10.35	7.40	14.86	2.37
	B	10.71	5.39	16.51	3.17
2	A	11.12	7.90	14.69	2.20
	B	11.18	7.85	16.20	3.61
3	A	9.78	7.23	15.01	2.40
	B	9.71	7.30	13.55	2.09

Footnotes

Acknowledgment

We thank Laboratório Júlio for the feldspathic ceramic discs used in this study.

Author Disclosure Statement

No competing financial interests exist.

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Influence of Aluminum Oxide Sandblasting Associated with Nd:YAG or Er:YAG Lasers on Shear Bond Strength of a Feldspathic Ceramic to Resin Cements

Abstract

Introduction

Materials and Methods

Group 1 (control group)

Group 2 (Al2O3 + Er:YAG laser)

Group 3 (Al2O3 + Nd:YAG laser)

Results

Discussion

Conclusion

Footnotes

Acknowledgment

Author Disclosure Statement

References

Group 2 (Al₂O₃ + Er:YAG laser)

Group 3 (Al₂O₃ + Nd:YAG laser)