Influence of different resin cements and surface treatments on microshear bond strength of zirconia-based ceramics.

AIM: This study aims to evaluate the microshear bond strength of zirconia-based ceramics with different resin cement systems and surface treatments.
MATERIALS AND METHODS: Forty blocks of zirconia-based ceramic were prepared and embedded in polyvinyl chloride (PVC) tubes with acrylic resin. After polishing, the samples were washed in an ultrasonic bath and dried in an oven for 10 min. Half of the samples were subjected to sandblasting with aluminum oxide. Blocks were divided into four groups (n = 10) in which two resin cements were used as follows: (1) RelyX™ U100 with surface-polished zirconia; (2) RelyX™ U100 with surface-blasted zirconia; (3) Multilink with surface-polished zirconia; and 4) Multilink with surface-blasted zirconia. After performing these surface treatments, translucent tubes (n = 30 per group) were placed on the zirconia specimens, and resin cement was injected into them and light cured. The PVC tubes were adapted in a universal testing machine; a stiletto blade, which was bolted to the machine, was positioned on the cementation interface. The microshear test was performed at a speed of 0.5 mm/min. Failure mode was analyzed in an optical microscope and classified as adhesive, cohesive, or mixed.
RESULTS: The null hypothesis of this study was rejected because there was a difference found between the resin cement and the surface treatment. There was a statistical difference (P < 0.005) in RelyX™ U100 with surface-blasted zirconia, in relation to the other three groups. For Multilink groups, there was no statistical difference between them.
CONCLUSION: Self-adhesive resin cement showed a more significant tendency toward bond strength in the ceramic-based zirconium oxide grit-blasted surfaces.

INTRODUCTION

In recent years, there has been an increase in the demand for full ceramic restorations in daily clinical practice, leading to the development of materials with biocompatibility and mechanical properties, such as sintered ceramics made of zirconia and alumina. These high-strength ceramics offer many options for application in the clinic. They can be used as implant abutments, esthetic nuclei, or fixed partial denture infrastructures.[123]

Different aspects should be carefully analyzed for success in treatments involving prostheses, such as mechanical, biological, and esthetic aspects as well as good radiopacity, which facilitates the radiographic evaluation of marginal integrity and recurrent caries. In addition, sealing of the margins is also very important in the success of the prosthesis, as well as avoiding lesions to the pulp and recurrences of cavities. Such restorations may be secured to the prepared tooth using cements capable of promoting chemical interaction, mechanical properties, or a combination thereof. When a cementing agent does not adhere to the tooth chemically, the prosthesis is retained through mechanical bonding.[1456]

Similar to other restorative materials, the successful cementing of a zirconium oxide-based prosthesis is an important factor in its clinical success. According to the manufacturers, because of the high strength of these ceramics, they can be cemented using conventional cements. However, the use of resin cements is advocated due to improved retention, marginal adaptation, reduction in the recurrence of caries, and low solubility in the oral environment. In addition, the use of resin cements can seal possible flaws in the internal structure of the zirconia from the surface-treatment process, such as aluminum oxide blasting, increasing its mechanical strength. To obtain adhesion between the resin cement and the ceramic surface, it is necessary to treat the ceramic surface. The roughness of ceramics made with zirconium oxide is increased by blasting aluminum oxide particles; this strengthens the mechanical bonding of the resin cement on the surface of the ceramics.[146789] Due to the free composition of a glassy phase by resin cement and polycrystalline microstructure, zirconium oxide-based ceramics are acid resistant and chemically stable.[4589]

The longevity of indirect adhesive restorations is directly proportional to the adhesive effectiveness between dental tissues and resin cements. Thus, a durable adhesion at the tooth-restoration interface is critical to the success of adhesive restorations in the long run. Resin cements are the material of choice for cementing indirect adhesive restorations. For such restorations, it is necessary to modify the dental hard tissues, which is sometimes followed by the application of an adhesive system. Resin cements are classified in three ways depending on the treatment done on the substrates. There are resin cements with total acid conditioning (phosphoric acid, followed by a two-step adhesive application), self-conditioning resin cements (using primer acid for conditioning dental tissues), and self-adhesive resin cements (capable of adhesion without prior application of an adhesive). However, there has recently been a conflict over the obtained enamel adhesion strength of these resin cements. Some authors suggest the use of 37% phosphoric acid etching of the marginal enamel before the application of the self-adhesive resin cements; however, this is a practice that generates controversy since generally the presence of aprismatic enamel in the proximal boxes of indirect adhesive restorations makes adhesion difficult.[10]

The bond strength of resin cements consists of micromechanical interlocking and chemical adhesion to the ceramic surface.[11] In some situations, high-strength ceramics do not achieve adhesion to the dental structure and can be cemented with conventional cements, which are only dependent on micromechanical retention. Thus, the stability of the restoration do not need adhesion to the dental structure, being determined by the interplay of total occlusal convergence, abutment height, abutment diameter, and finish line design.[12] This highlights the difficulty in modifying the surface of these ceramics. However, adhesion of a resin cement is desirable in many clinical situations, such as in teeth with very short or thin preparations. Thus, a strong chemical adhesion would lead to an increase in fracture and fatigue resistance in the long term.[13]

Surface modification techniques have opened a multitude of surface treatment options.[14] However, these procedures alone are not sufficient to generate a reliable long-term bond between composite cement and zirconia.[14] A combination of pretreatment methods (e.g., tribochemical silica coating, with silanization could attain higher bond strength with bis-GMA-based composite cements) is imperative.[14] Thus, blasting with aluminum oxide has been suggested as the preferred method for the surface treatment of high-strength ceramics, such as ceramics made with zirconium oxide and alumina. This method is usually applied in dental laboratories using aluminum oxide (Al2O3) particles of 30–250 μm particle size and coated with silica.[51516] The blasting pressure results in silica-coated alumina particles being embedded on the ceramic surface, rendering the silica-modified surface chemically more reactive to the resin through silane-coupling agents.[5] The blasting produces uneven roughness on the surface of the substrate, which promotes a mechanical bonding of the resin cement. This surface treatment also increases the adhesion area, surface energy, and wettability, which allows the polymer of the resin cement to flow better through the surface.[14817] In this context, the purpose of this study was to evaluate the influence of surface treatment with aluminum oxide sandblasting of two different resin cement systems on the microshear bond strength of the zirconium oxide-based ceramics.

MATERIALS AND METHODS

Specimen preparation

Forty blocks of zirconia (Lava™ All-Ceramic System, 3M ESPE Dental Products, St. Paul, MN, USA) of 15 mm × 6 × 1 mm were made by means of precision cutting machine (Zircograph, Zirkonzahn GmbH, Gais, Germany) before the sintering process. The sintering process took place in a sintering furnace (Lava™) at a temperature of approximately 1500° C to 1700°C according to the manufacturer's instructions. Afterward, the ceramic surfaces were embedded in polyvinyl chloride (PVC) tube (25 mm in diameter and 40 mm in height) using autopolymerized acrylic resin (Jet, Artigos Odontológicos Clássico, São Paulo, SP, Brazil). The ceramic blocks were further polished on wet #600, 800, and 1000-grit silicon carbide paper in a polishing system (LaboPol-21, Struers A/S, Ballerup, Denmark). After the ceramics were polished, the cleaning was performed with a 10 min ultrasonic bath in distilled water followed by 10 min of drying in an oven at a temperature of 100°C. Next, the specimens were observed in the optical microscope with a ×10 magnification to evaluate the uniformity of the polishing of the ceramic surfaces.

Of the 40 zirconium oxide ceramic blocks, 20 were randomly chosen and then subjected to the sandblast for 12 s with 50 μm of aluminum oxide (Bio-Art Equipamentos Odontológicos Ltda, São Carlos, SP, Brazil) at a distance of 10 mm and a pressure of 2.8 bar.[14] These blocks were randomly divided into the following four groups: (1) RelyX™ U100 polished: Blocks polished (n = 10) and use of autoadhesive resin cement (RelyX™ U100, 3M ESPE); insertion into the silicone tube with a plastic spatula; (2) RelyX™ U100 sandblasted: Blocks sandblasted (n = 10) with aluminum oxide and use of autoadhesive resin cement (RelyX™ U100, 3M ESPE); (3) Multilink polished: Blocks (n = 10) polished and use of dual-curing resin cement (Multilink, Ivoclar Vivadent, Schaan, Lichtenstein) into each matrix; 4) Multilink sandblasted: Blocks (n = 10) sandblasted with aluminum oxide and use of dual-curing resin cement (Multilink, Ivoclar Vivadent). The materials and compositions used in this study are reported in Table 1.

Table 1

Materials and compositions in this study

After surface treatment, translucent tygon tubes (Tygon Medical Tubing, Saint-Gobain; Akron, OH, USA) with an internal diameter of 1.0 mm and a height of 1.0 mm were used as matrices. The resin cement used in each group was handled according to the manufacturer's instructions and inserted with a plastic spatula into the tygon tube positioned on the ceramic. Three tygon tubes were cemented onto each ceramic block, giving rise to three specimens per ceramic block, totaling 120 specimens divided into four groups. The excesses were removed with a spatula and photopolymerized for 60 s with Poly Wireless® (Kavo, Joinville, SC, Brazil) at 10 mm and a power of 1000 mW/cm2.

After 30 min at room temperature, the silicone tubes were carefully removed, yielding resin cement cylinders that adhered to the ceramic surface. In the groups where the resin cement used was Multilink (Ivoclar Vivadent), one coat of zirconia primer (Metal/Zirconia Primer, Ivoclar Vivadent) was applied with microbrush, left to react for 180 s, and dried with water- and oil-free air. One drop of multilink primer A and one drop of multilink primer B were mixed. The mixture was scrubbed on the surface of the ceramic with a microbrush, bounded by a label with holes that were 2 mm in diameter and equidistant. The excess of the adhesive system was removed by a light stream of air. Subsequently, the silicone cement tubes were positioned on the surface of the zirconium oxide ceramic block.

Microshear bond strength test

The PVC tubes containing the ceramic cylinders that adhered to the ceramic surface were fitted into a fabricated metal device. This device was screwed into a universal testing machine (EMIC DL-1000, EMIC, São José dos Pinhais, Brazil). A stiletto blade screwed to the universal test machine (EMIC) was adjusted to the cementing interface of the resin cement cylinders with the ceramic surface. The microshear test was then performed at a speed of 0.5 mm/minutes until fracture occurred.

Statistical analysis

The results obtained from the adhesive resistance were analyzed by the two-way ANOVA test. For the multiple comparisons, the Games–Howell test was used for all groups (α = 5%). Analyses were performed using the SPSS 15 Program (SPSS Inc., Chicago, IL, USA) and Statistica 9.0 (Statisoft Inc., Tulsa, OK, USA).

Failure analysis

After the microshear tests were performed, all specimens were analyzed by Olympus XC30 microscope with an Olympus UC30 digital camera (Olympus America Inc., Center Valley, PA, USA) with a 10-fold increase to verify the type of fracture that occurred in each of the tests. The fractures were classified according to the following criteria: Type 1, adhesive failure between resin cement and ceramics; Type 2, cohesive failure of resinous cement; Type 3, mixed failure, where the adhesive failure of part of the resin cement occurred in relation to the ceramic surface.

RESULTS

Figure 1 shows mean microshear bond strengths (MPa) for ceramic bonded to composite groups. The means and standard deviations (MPa) of the experimental groups are depicted in Table 2. The two-way ANOVA test presented a statistical difference for the cement variable and for the cement and treatment interaction (P < 0.001). Games–Howell's multiple comparison test was used to compare the results between the groups. The resin cement RelyX™ U100 showed the highest value of bond strength (24.02 ± 6.41) in relation to the polished treatment. Multilink cement presented a statistical difference between the polished and blast treatment, presenting a value of 16.11 ± 4.97, which was higher than the blasted treatment (P < 0.001). For the polished treatment, there was no statistical difference (P = 0.38). For the blasted treatment, the RelyX™ U100 presented the highest value of bond strength (24.02 ± 6.41) when compared to the Multilink resin cement (P < 0.001). Figure 2 shows failure pattern distribution and Figure 3 shows representative SEM micrographs of the failure modes.

Figure 1

Mean microshear bond strengths (MPa) for ceramic bonded to composite groups. Values are plotted with standard deviation error bars

Table 2

Mean (standard deviation) microtensile bond AQ6 strength (MPa) of resin cement/zirconia combinations with different treatments

Figure 2

Failure pattern distribution of the different groups tested: cohesive failure (within the resin cement; almost all the fracture surface was covered with resin cement); adhesive failure (at the resin cement/zirconia interface); mixed failure (a partial zirconia surface and a partial resin cement cover were visible)

Figure 3

Representative photomicrographs (×10) of failure modes. (a) Type 1 (adhesive failure); (b) Type 2 (cohesive failure); (c) Type 3 (mixed failure)

The tested null hypothesis evaluated in this study was rejected because there was a difference between the resin cements and the surface treatment of zirconium oxide ceramics.

DISCUSSION

Cementation in ceramics made with zirconium oxide, or silica-free ceramics, requires an alternative method to those traditionally used, such as the use of hydrofluoric acid and silane. Methods usually recommended for this procedure include the use of silica sandblasting and silanization or the use of cements containing a phosphate monomer (10-methacryloyloxydecyl dihydrogen phosphate [MDP]). Both methods provide a treatment of the ceramic surface before cementation.[1918] Chemically or mechanically modifying the surface of ceramics made with zirconium oxide are acceptable methods for reaching a lasting and reliable adhesion.[6]

The microshear bond strength test was chosen based on its advantages such as the reduced test area, the possibility of using several test areas per specimen, and the uniform distribution of stress caused by the adhesive interface. This makes the results more realistic because the use of a single cylinder cemented over an unfavorable area of the ceramic has a small effect on the results obtained.[141920]

The present study corroborates the literature regarding the increase in the adhesive strength of the resin cement (RelyX™ U100) used in conjunction with an aluminum oxide blasting before cementation. The failure pattern, when the specimens were analyzed under an optical microscope, is also in agreement with these studies for resin cement (RelyX™ U100), most of which are cohesive failures, followed by mixed faults. However, this study does not agree with these authors that all resin cements obtain better performance when the surface of the zirconia is sandblasted with aluminum oxide. This did not occur when Multilink was used, which reached higher values in the microshear test when the surface of zirconia-based ceramics was polished.[315] In the present study, resin cement (RelyX™ U100) also obtained better results when used in conjunction with aluminum oxide blasting when compared to the polished surface. This result was probably achieved due to the microrugosities that the aluminum oxide blasting promoted on the surface of zirconium oxide ceramics.[21]

As seen in prior studies, the results found here demonstrate that resin cement (RelyX™ U100) reached a low value in the microshear test when applied on a polished zirconium oxide ceramic surface. However, the values in the test increased when the surface of the zirconium oxide ceramics was sandblasted with aluminum oxide. This probably occurred, as previous studies show, due to a chemical interaction of the phosphate monomers present in the RelyX™ U100 and the hydroxyl groups present in the ceramics.[1422] The increase in the values obtained in the RelyX™ U100 group can also be explained by two factors: a higher surface roughness and surface energy of zirconium oxide-based ceramics caused by blasting with aluminum oxide. This probably causes the cement to achieve better flow through the ceramic surface producing better micromechanical retention between the interface. The silica-modified aluminum oxide blasting may also have produced hydroxyl groups (OH-), causing a chemical interaction with the PO4 groups of the resin cement (RelyX™ U100).[1422] This finding could be attributed to the increase in surface roughness and surface energy of the zirconia surface, which enabled micromechanical interlocking between the resin cements within the zirconia surface, thanks to airborne particle abrasion with 50-μm Al2O3.[1423]

This chemical reaction between the phosphate monomer (MDP) present in the RelyX™ U100 resin cement and the oxide layer present on the zirconium oxide ceramic surface has also been reported as responsible for improving adhesion between resin cements and ceramics made with zirconium oxide,[14] corroborating the thesis that sandblasting with aluminum oxide is essential for the success of resin cements that have this phosphate monomer in its composition. It is during the process of a surface treatment with aluminum oxide blasting that this layer of oxides is produced on the surface of zirconium oxide-based ceramics.[1]

Other studies reported that the blasting of the ceramic surface has been viewed with some care.[215] In these studies, when observing the specimens in the microscope, the presence of microcracks was verified in the ceramic surface that could have been caused by this treatment. This may lead to a decrease in the mechanical strength of zirconium oxide-based ceramics. The current paper demonstrated that cement achieved better results when the zirconium oxide ceramic surface was polished. However, the aluminum oxide particles used were larger and under lower pressure (120 μm and 0.35 bar),[20] thus achieving a higher mechanical roughness and bonding. However, the failure pattern found in the blasted and polished groups was typical of the adhesive type.[20]

This work contradicts a study[21] who found better results for Multilink cement when the ceramic surface was subjected to blasting with aluminum oxide particles. However, the test used was of microtensile bond strength. Furthermore, points out that the results were mainly due to the resinous cement having 2-hydroxyethyl methacrylate, dimethacrylate, and silica filler particles in its composition.[22] These substances are responsible for increasing the flexural strength of resin cement and do not necessarily increase the adhesion of zirconium oxide-based ceramics. Adhesion would be improved if a phosphate monomer was present in the resin cement used in the study, such as in RelyX™ U100.[22]

According to previous works, a statistically significant difference was found in the groups in which the zirconium oxide ceramic surfaces were subjected to blasting with aluminum oxide in relation to the groups with the polished surface. These results were not found in this study since in the groups where the Multilink resin cement was used, the group that used aluminum oxide surface-treatment blasting did not obtain better results when compared to the group that used a polished surface for the same resin cement.[2]

Although this work evaluates the performance of RelyX™ U100 and Multilink resin cements on zirconium oxide-based ceramic surfaces, it is worth noting that other studies have found results regarding the evaluation of the significantly better adhesive strength of RelyX™ U100 resin cement in dental enamel. The failure types found were also mostly cohesive for RelyX™ U100 and adhesive for Multilink resin cement.[10]

When it comes to analysis of the failure type that occurs in the microshear test, cohesive and mixed failure are associated with better results in the work. Therefore, adhesive failure is usually associated with a low adhesion of the resin cement.[19] In analyzing the failure type after performing the microshear test, this study found that the higher the values obtained in the test, the greater the percentage of cohesive failures in the specimens. This type of failure agrees with the studies analyzed so far.[15]

After the analysis of optical microscope failures, a study found a predominance of mixed faults for all groups, followed by adhesive failures and cohesive failures in their minority.[22] These results are compatible with those found in the present study, demonstrating that cements that achieve chemical bonding and micromechanical retention with zirconium oxide-based ceramics are more effective.[22]

The results obtained in the present study were similar to those of previous studies in which the authors found a predominance of adhesive failures after shear tests.[22122] Thus, based on the failure pattern found, this study is in agreement with other works with respect to the Multilink resin cement, where the predominance of defects was of the type of adhesive for zirconium oxide-based ceramics polished and sandblasted with aluminum oxide. However, Lee et al.[24] observed a predominance of mixed failures for this cement. According to most of the articles reviewed, the zonation of ceramics made with zirconium oxide showed better adhesive strength results, regardless of the resin cement system used. Therefore, this is a valid method to improve the adhesive strength of the resin cements to the ceramics in evaluation.[6172025]

CONCLUSION

Within the limitations of this study, the following conclusions were drawn:

There was no statistically significant difference on microshear bond strength between the following groups of surface treatments: Multilink polished, Multilink sandblasted, RelyX U100 polished

The group RelyX U100 sandblasted presented the higher bond strength on microshear bond strength tests

These findings are probably as the result by the action of the MDP monomer present on the RelyX U100 formula and a higher surface roughness and surface energy of zirconium oxide-based ceramics caused by sandblasting.

Clinical relevance

The best treatment to promote greater bond strength to zirconia is to associate tribochemical treatment with the self-adhesive luting resin cement containing a functional phosphate monomer.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.