Influence of Different Surface Pretreatments of Zirconium Dioxide Reinforced Lithium Disilicate Ceramics on the Shear Bond Strength of Self-Adhesive Resin Cement.

AIM: To analyze the influence of different surface pretreatments of zirconium dioxide reinforced lithium disilicate ceramics on the shear bond strength of self-adhesive resin cement.
MATERIALS AND METHODS: Eighty-four zirconium reinforced lithium disilicate disc Vita suprinity (Vita Zahnfabrick, Bad Säckingen, Germany) 14x12x2 mm specimens were fabricated according to the manufacturer's recommendations. The specimens were embedded in acrylic resin blocks and randomly divided in seven groups (n=12/each) accorrding to the treatment: Group 1- 10% hydrofluoric acid; Group 2- silane; Group 3- hydrofluoric and silane; Group 4- sandblasting with silane; Group 5- Er: YAG laser+ silane; Group 6- Nd: YAG laser + silane; and the control group, in which the specimens were not treated. Round shape composite discs (Filtek Bulk fill, 3M ESPE, St.Paul, Minnesota, USA) with 3.5 mm diameter, were made for shear bond strength testing, and then cemented to the ceramic sample surface using composite cement (RelyX U200 Automix, 3M ESPE, Neuss, Germany). After cementing the composite disc on the sample, the samples were subjected to shear bond strength test of 10 N with a "stress rate" of 1 MPa / s. To determine the nature of the fracture (adhesive, cohesive or adhesive-cohesive), the broken samples were examined under a stereomicroscope. The ANOVA test and the Tukey test were used to compare the values ​​of the bond strength characteristics between different types of materials. All tests were performed with a significance level of α = 0.05.
RESULTS: There was a significant difference in the shear bond strength of self-adhesive cement to dental lithium-disilicate ceramics reinforced with zirconium dioxide after different preparation protocols (p<0, 05). The treatment of lithium disilicate ceramics reinforced with zirconium dioxide by silanization, sandblasting + silanization, Nd: YAG + silanization resulted in significantly higher bond strength compared to the control group. There was statistically higher bond strength of self-adhesive cement after pretreatment of lithium disilicate ceramics Nd: YAG + silanization compared to Er: YAG + silanization (p <0.05). Adhesive fracture dominated in the control group, sandblasting + silanization group, and in the laser groups, while mixed fracture dominated in other groups.
CONCLUSION: Under the limitations of this study, the Nd:YAG irradiation with silanization could be used as pretreatment for providing greater shear bond strength of self-adhesive resin cement to zirconium reinforced lithium disilicate.

Introduction

Due to optical characteristics which are in common with the natural tooth substance and good physical and mechanical properties, dental ceramic materials exhibit chemical stability and excellent biocompatibility with soft tissues, with low plaque adhesion.

Zirconium oxide ceramic materials have excellent mechanical performances, such as high flexural strength (1.0-1.2 GPa) and toughness (7-8 MPa × m0.5). Therefore, they are increasingly used in dental practice, especially in CAD/CAM technology (computer-aided design/computer-aided manufacturing). Zirconium oxide ceramics can be used to make all types of ceramic replacements (crowns, bridges, fixed partial dentures, etc.) The whole procedure, using CAD/CAM technology, is extremely practical and reliable. The zirconia-based core structure is veneered with zirconia veneering ceramics. Both the core and the veneering ceramic have a similar thermal expansion coefficient. In dental practice, clinicians often face a variety of challenges such as choosing the best rehabilitation material when a prosthetic restoration is required on two adjacent teeth — on one zirconium ceramic crown and the other veneer. For esthetic reasons, it would be desirable to use the same type of material, but almost no information is available about zirconia veneering ceramic materials, which have yet to be further explored.

The zirconia-reinforced lithium silicate (ZLS) consists of lithium-metasilicate () glass-ceramic and 10% of zirconium dioxide (). Ultimately, the crystallization process leads to the fine-grained microstructure formation (). ZLS is also progressively used in CAD / CAM technology, thanks to its good mechanical properties and excellent esthetics. Good mechanical properties can be attributed to zirconium dioxide, and esthetics to glass-ceramics. ZLS can be etched and cemented with adhesive systems, while the same procedure cannot be applied to zirconia restorations. Researchers have proved that the fracture resistance of adhesively cemented, monolithic CAD/CAM fabricated ceramic crowns is remarkably higher compared to conventional cementation ().

Ceramic restorations have become the “gold standard” for anterior teeth restorations. The shear bond strength of composite cement to previously both etched and silanated porcelain surface surpasses the cohesive porcelain strength. The bonding of composite cement with ceramics is performed in two different ways: mechanically, as a consequence of etching with hydrofluoric acid and the formation of micromechanical retention and chemically, utilizing silane. At the same time, its bonding to enamel is achieved only mechanically. Many researchers encourage porcelain silanization in order to obtain a much stronger connection compared to etching with hydrofluoric acid only, whilst the combination of both is suggested.

The Er: YAG laser acts by thermomechanical ablation vaporizing the water that constitutes the tissues. This vaporization causes an expansion followed by microexplosions, which produces the ejection of both organic and inorganic particles from tissues, promoting the appearance of a dentinal surface with open tubules and without smear layer and an irregular enamel surface, which causes irregularities, thus increasing bond strength (). Previous studies have already suggested that this laser can be used to create irregularities on the surface of ceramics from different types, and enhance the bond strength between these materials and resin cements (, ).

Nd:YAG laser modified the external surface of zirconia ceramics and yielded a smooth surface with irregular small cracks due to laser irradiation. This finding is in line with the previous studies reporting cracks on zirconia surfaces after laser treatment (, ).

This study aimed to analyze the effect of different surface pretreatments of zirconium dioxide reinforced lithium disilicate ceramics on the shear bond strength with self-adhesive resin cement.

The null hypothesis was that the shear bond strength of composite self-etching cement to the surface of ZLS after surface treatment with Nd: YAG and Er: YAG laser would be the same as that of conventional preparation protocol (etching with hydrofluoric acid, silanization, sandblasting).

Materials and methods

Sample preparation

The material used in this research was lithium disilicate glass-ceramic reinforced with zirconium dioxide (Suprinty, Vita Zahnfabrik, Bad Sackingen, Germany). A total of 70 samples were made for the research purpose and cut into 18x12x2 size discs in the Isomet 1000 cutter. After cutting the discs, the material was crystallized according to the manufacturer's instructions in the Programat P300 Furnace (Ivoclar Vivadent AG, Schaan, Liechtenstein).

After crystallization, the discs were prepared for polishing by being immersed in a silicone mold to be stationary when polished with 600 grit sandpaper lasting 1 minute for each cause. Polishing was performed at the Department of Materials of the Faculty of Mechanical Engineering and Naval Architecture, in order to make their surface uniform.

After polishing, the samples were embedded in acrylate to completely fix their position when treating the surface.

Pretreatment protocols of lithium disilicate ceramic

After polishing, the samples were randomly divided into groups depending on the surface treatment method.

In the control group, after polishing with 600 grit sandpaper for a period of 1 minute, the samples were not treated at all.

Group 1- Hydrofluoric acid

In the hydrofluoric acid group, the samples were treated with 9.5% hydrofluoric acid (Bisco Inc., Schaumburg, Illinois, USA) for a period of 90 seconds, whereupon the samples were washed with water and dried according to the manufacturer's instructions.

Group 2- Silanization

In the group subjected to the silanization process, the samples were treated with silane (Monobond, Ivoclar Vivadent AG, Schaan, Liechtenstein) for 60 seconds by rubbing silane with a brush on the surface of the sample, according to the manufacturer's instructions.

Group 3- Hydrofluoric acid + Silanization

In the group subjected to a combination of hydrofluoric acid and silane, the samples were treated with 9.5% hydrofluoric acid (Bisco Inc., Schaumburg, Illinois, USA) for 90 seconds, whereupon the samples were washed with water and dried according to the manufacturer's instructions, and then treated with silane (Monobond, Ivoclar Vivadent AG, Schaan, Liechtenstein) in the duration of 60 seconds in a way that silane was rubbed with a brush on the surface of the sample, according to the manufacturer's instructions.

Group 4- Sandblasting + silanization

In the sandblasting group, the samples were sandblasted with 30 µm size Al2O3 particles (CoJet Sand, 3M ESPE, Neuss, Germany) at a pressure of 2.7 atm, and from sandblaster's vertical distance of 1 cm from the sample in duration of 15 seconds. After sandblasting, the samples were washed under water, dried, and blown off to remove residual particles. The samples were then treated with silane (Monobond, Ivoclar Vivadent AG, Schaan, Liechtenstein) for 60 seconds. Silane was rubbed with a brush on the surface of the sample, according to the manufacturer's instructions.

Group 5- Er: YAG irradiation + silanization

In the group treated with Er: YAG laser, the samples were treated with a laser (LightWalker, Fotona, Slovenia) of certain parameters: pulse energy of 500 mJ with a power of 10 W and a frequency of 4 Hz for 20 seconds. After laser treatment, the samples were treated with silane (Monobond, Ivoclar Vivadent AG, Schaan, Liechtenstein) for a period of 60 seconds. Silane was rubbed with a brush on the surface of the sample, according to the manufacturer's instructions.

Group 6- Nd: YAG irradiation + silanization

The group treated with Nd: YAG laser was treated with a laser (LightWalker, Fotona, Slovenia) of the following parameters: 100 mJ pulse duration, frequency 20 Hz using a power of 1W, after which the samples were treated with silane (Monobond, Ivoclar Vivadent AG, Schaan, Liechtenstein) for 60 seconds in a way that silane was rubbed with a brush on the surface of the sample, according to the manufacturer's instructions.

Shear bond strength test

Before shear bond strength testing, it was necessary to make round shape composite disks (Filtek Bulk fill, 3M ESPE, St.Paul, Minnesota, USA) with a diameter of 3.5 mm, and cement them on the surface of the sample using composite cement (RelyX U200 Automix, 3M ESPE, Neuss, Germany).

After cementing the composite disc on the sample, the samples were subjected to a shear bond strength test at the School of Dental Medicine, University of Zagreb on a testing machine (type LRX with built-in Nexygen programme, Lloyd Instruments, Fareham, United Kingdom) at a test speed of 1 mm/min. All tests were performed by the same person using force on the sample of 10 N with a "stress rate" of 1 MPa / s. Specimens were fixed in a testing jig. The load required to debond the specimen was recorded and expressed in MPa by dividing the load by the surface area of the bonded specimen, and the mean shear bond strength for each study group was calculated.

Fracture analysis of the sample surface in order to determine whether it was a cohesive, an adhesive, or a combined fracture were performed at the Department of Materials, Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb using a stereomicroscope Mantis Elite-Cam HD (Vision Engineering Ltd, Woking, Great Britain), 20x optical magnification.

Surface roughness measurements were tested on the first, third and fifth samples within the same group, and on each of these samples the measurement was performed on six roughness profiles. A Gaussian filter was used for filtration, the limit value was set to λc = 0.8 mm, the probe radius (r) of 5 µm, the grading length (ln) of 4.0 mm and the measuring force (F) of 1.3 mN.

The effects of the laser irradiation on a surface of the zirconium reinforced lithium disilicate were examined by scanning electron microscopy (SEM). The SEM analysis was performed at the Department of Materials, Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb. A Tescan Vega TS 5136 MM scanning microscope (TESCAN, Brno, Czech Republic) was used for topographic analysis. The analysis was performed by the same person at 300 x and 1000 x magnifications.

Statistical analysis

The ANOVA test and the Tukey test were used to compare the values ​​of the bond strength characteristics between different types of materials. The Fisher's exact test was applied to compare the distribution of fracture types between different materials. The analysis was performed using the SAS statistical package on the Windows platform. All tests were performed with a significance level of α = 0.05.

Results

For all tested protocols, the distribution did not deviate from the normal distribution (Table 1, p> 0.05).

Table 1

Data normality testing for shear bond strength

		Shear bond strength
Material	N	Mean val.	Stand dev.	W**	p*
Control group	5	2.51	3.11	0.83	0.14
Acid	7	11.45	5.15	0.83	0.13
Silanization	7	12.92	4.23	0.89	0.29
Acid + silanization	7	8.92	4.09	0.95	0.74
Sandblasting + silaniz.	7	11.93	6.18	0.98	0.97
Nd:YAG + silanization	8	15.91	7.28	0.92	0.44
Er:YAG + silanization	6	6.78	2.64	0.87	0.23

* p-value for Shapiro-Wilk test

** W - statistics value for Shapiro-Wilk test

Surface roughness analysis has shown that, for both sets of roughness data (Ra - arithmetic mean deviation of the profile and Rz - mean height of irregularities), the data distribution did not deviate significantly from the normal distribution (p> 0.05).

Table 2 Shows Ra from a different surface.

Table 2

Surface roughness measurements

Material • Materijal	Roughness (Ra) • Hrapavost (Ra)
Control group • Kontrolna skupina	127.8
Acid • Kiselina	90.2
Silanization • Silanizacija	75.2
Acid + silanization • Kiselina + silanizacija	106.8
Sandblasting + silanization • Pjeskarenje + silanizacija	282.1
Nd: Yag + silanization • Nd:Yag + silanizacija	100.7
Er: Yag + silanization • Er:Yag + silanizacija	1194.5

Table 3 shows the mean value, standard deviation, median, first quartile, third quartile, minimum and maximum values ​​of shear bond strength between dental lithium disilicate ceramics reinforced with zirconium dioxide and self-adhering composite cement.

Table 3

Comparison of shear bond strength for different pretreatment protocols

Pretreatment protocol	Shear bond strength
	Mean value	Stand.dev.	Median	Q1	Q3	Minimum	Maximum
Control group	2,51	3,11	0,75	0,29	4,24	0,10	7,19	abc
Acid	11,45	5,15	10,26	9,13	13,53	5,46	21,81
Silanization	12,92	4,23	14,02	8,47	17,33	7,72	17,89	a
Acid + silanization	8,92	4,09	7,96	4,93	13,18	3,92	14,91
Sandblasting + silan.	11,93	6,18	11,13	6,40	16,21	3,67	22,23	b
Nd:YAG + silanization	15,91	7,28	17,82	8,72	21,32	6,17	25,41	cd
Er:YAG + silanization	6,78	2,64	5,70	5,14	9,27	4,08	10,82	d

a,b,c,d 'post-hoc' test, the same letter denotes materials that differ from each other

Table 4 shows fracture's type share after tested protocols for preparation of lithium disilicate ceramics reinforced with zirconium dioxide.

Table 4

Fracture's type share after tested protocols for preparation of lithium disilicate ceramics reinforced with zirconium dioxide.

Material	Fracture typeTotal Adhesive fract. Cohesive fract. Mixed fracture
	N	N	%	N	%	N	%
Control group	5	5	100.0	0	0.0	0	0.0
Acid	7	0	0.0	1	14.3	6	85.7
Silanization	7	0	0.0	0	0.0	7	100.0
Acid + silanization	7	0	0.0	0	0.0	7	100.0
Sandblasting + silaniz.	7	5	71.4	1	14.3	1	14.3
Nd:YAG + silanization	8	4	50.0	1	12.5	3	37.5
Er:YAG + silanization	6	2	33.3	0	0.0	4	66.7

Figure 1 shows graphically the mean values ​​of shear bond strength with standard deviation after different surface preparation protocols of lithium disilicate ceramics.

Figure 1

Shear bond strength of lithium disilicate ceramic to a self-adhesive cement after its different surface pretreatment

The results showed that there was a significant difference in the shear bond strength between self-adhering cement and dental lithium disilicate ceramics reinforced with zirconium dioxide after different preparation protocols (p<0.05). There was significantly higher shear bond strength after the treatment of lithium disilicate ceramics reinforced with zirconium dioxide by silanization, sandblasting + silanization, Nd: YAG + silanization compared to the control group (in average 12.9 MPA, 11.9 MPa, and 15.9 MPa compared to 2.5 MPa for the control group). The pretreatment of lithium disilicate ceramics with Nd: YAG + silanization resulted in significantly higher shear bond strength compared to the treatment with Er: YAG + silanization (on average 15.9 MPa compared to 6.8 MPa) (p <0.05).

The fracture type distribution in tested pretreatment protocols is shown in Table 3. Adhesive fracture dominates in the control group (Figure 2) and in the sandblasting + silanization group (Figure 3), while in the acid (Figure 4), silanization (Figure 5), acid + silanization (Figure 6), and Er: YAG + silanization group (Figure 7) mixed fracture dominates. In group Nd: YAG, the adhesive fracture also dominates (Figure 8), but with a share of 50%, while the share of mixed fracture is 37.5%.

Figure 2

Adhesive fracture in the control group

Figure 3

Adhesive fracture in the sandblasting + silanization group

Figure 4

Mixed fracture in the acid

Figure 5

Mixed fracture in silanization

Figure 6

Mixed fracture in acid + silanization

Figure 7

Mixed fracture in Er: YAG + silanization group

Figure 8

Adhesive fracture in Nd: YAG

Topographic architecture after surface treatment with different processing methods at magnifications of 300 and 1000 x was reviewed by SEM analysis.

Figure 9 SEM analysis after surface treatment with different processing methods

Figure 9a

SEM analysis of the control group

Figure 9b

SEM analysis of the acid

Figure 9c

SEM analysis of the silanization .

Figure 9d

SEM analysis of the acid + silanization

Figure 9e

SEM analysis of the sandblasting

Figure 9f

SEM analysis of the the Er:YAG laser

Figure 9g

SEM analysis of the Nd:YAG laser

Discussion

Despite continuous development and technological innovations, there is no material that is perfect on the market; therefore it is important to mention the shortcomings and complications in the therapy with these materials that occur due to fracture of the restoration, cement loosening, hypersensitivity, or abutment tooth caries. To ensure the durability and favorable biomechanics of such a ceramic appliance, its surface must be treated with chemical agents prior to cementing to the abutment tooth of the fixed prosthetic appliance. Technology development raises the question of the use of alternative systems that can affect the treatment and the roughness of the surface of ceramic material by acting on the shear bond strength of ceramics with dental cement. The use of dental laser is one of such alternative systems. Dental lasers are increasingly used in dental medicine, which places this area of ​​research in the scientist’s focus.

Although dental laser has been used for industrial purposes for a long period of time, its surface preparation is more recent. Laser preparation can change the surface microstructure of many materials and can be easily controlled (, ).

Previous research has shown that certain types of lasers, adjusted to certain parameters, can affect the surface roughness and characteristics of the material, thus directly affecting the shear bond strength system of ceramics. The energy released by laser can have beneficial effects on surface roughness by creating microcracks, thus forming an additional retention surface and improving the shear bond strength. However, laser-generated energy can also reduce the quality of the bond by dissolving the ceramic surface. In accordance with the newly created, smooth, dental surface of ceramic material, the laser reduces the shear bond strength. Ural and Kalyoncuoglu proved this in their research in 2012 by treating the surface of zirconium oxide ceramics with a CO2 laser. They observed that increasing the laser output power led to a decrease in the shear bond strength potential due to molten area formation on the ceramic surface ().

A similar result was obtained by Hoosmand in 2015 using a Nd: Yag laser (shear bond strength potential weakening by creating molten areas on the ceramic surface), while Akin et al. showed that the Er: Yag laser had a beneficial effect on the shear bond strength creating microcracks and additional retention surfaces (). It can be concluded that lasers and their influence on the shear bond strength when treating the dental ceramic's surface are unpredictable. The multitude of variables in the dental ceramics system, a large number of laser parametric combinations and the laser's surface length treatment provide research breadth, numerous opportunities for new observations, and innovativeness of the obtained results of individual research.

By standardizing the parameters for individual lasers, certain types of dental ceramics and variables responsible for shear bond strength, there will be a better understanding of bonding and processes that contribute to, or lead to bond failure, which is especially important to clinical work, thus ensuring quality and longevity of prosthetic therapy.

This study aimed to analyze the shear bond strength of self-adhering composite cement (RelyX U200 Automix, 3M, ESPE, Neuss, Germany) to the surface of glass ceramics reinforced with zirconium oxide (Suprinity, Vita Zahnfabrik, Bad Sackingen, Germany) after different surface pretreatments. The aim of the survey was also to examine the effect of laser radiation (Er: YAG, Nd: YAG) on the surface (qualitative micromorphological analysis of the ceramics surface samples using SEM, microchemical X-ray spectroscopy using EDS and X-ray diffraction analysis, XRD), surface roughness (profilometry) and samples shear strength with fracture analysis and to compare with conventional surface preparation protocols (sandblasting, etching with hydrofluoric acid, silanization).

After the process of controlled glass crystallization, the introduction and invention of glass- ceramics, from the first Nycor glass through leucite (IPS Empress) and ceramics with mica crystals (Dicor ceramics), then lithium disilicate (IPS Empress 2 and E-max ceramics) and finally hybrid, lithium disilicate glass-ceramics reinforced with zirconium dioxide have occurred in a relatively short period. Leucite glass-ceramics were used to make individual crowns in the anterior (layering technique) or posterior (staining technique) segment of dentition and onlay, inlay, and overlay. It was quickly noticed that such a narrow indication area, which characterized both first generations of glass-ceramics, was not profitable. Therefore, various attempts were made to strengthen glass-ceramics with the intention of using it for greater constructions. The crystal structure, quantity, arrangement, and crystals size were changed; secondary phases were introduced until the final reinforcement with zirconium oxide. Such a combination resulted in very desirable building material in fixed prosthetic therapy that combines good properties of glass-ceramics (the possibility of achieving excellent esthetics) and zirconium oxide (outstanding mechanical properties). Ensuring a quality connection between the restoration and the abutment tooth can be observed through two aspects: prosthetic appliance surface preparation made of ceramic material and abutment tooth surface preparation. The inner surface of ceramics must be conditioned to ensure optimal micro-mechanical retention by penetrating the composite into the ceramic micro-roughness surface; this procedure increases the cement mechanical retention by increasing the contact surface with the tooth structure through the microporosity formation. Roughness formation and promotion of micro-mechanical retention, different ways of surface treatment in contact such as abrasive treatment, sandblasting, and acid etching have been well explained in the literature (, ). These procedures were tested in in vitro studies. In vitro studies and the results obtained by such studies have their limitations; therefore it is necessary to take them with reserve. As much as in vitro research can simulate conditions in the oral cavity, it is still difficult to obtain identical conditions since the oral cavity is a specific and complex medium from both a mechanical and a corrosive point of view. However, in vitro studies are easier to conduct, and they are also cheaper and faster. Their results can be used as a guide to help interpret with considerable certainty a wide range of developments in the oral cavity. This attitude is widely accepted in the scientific community.

Tian has stated that the most commonly used technique for preparing the surface of a glass-ceramic restoration before cementation is treatment with hydrofluoric acid and silanization (). During this procedure, the surface was partially dissolved and the crystals were partially stripped, leading to a rough ceramic surface formation that provided micromechanical retention with the composite cement. An additional roughness increase enlarges the surface energy and interaction between the binder and silane, which promoted the chemical-mechanical adhesion between the ceramic / silane/cement surface (). While hydrofluoric acid increases the shear bond strength between cement and ceramics, the acid simultaneously reduces the material mechanical resistance depending on the acid concentration and the conditioning time. These factors can also change the shear bond strength between composite cement and glass-ceramics ().

Cement should ensure good replacement retention and quality edge fitting, but its contribution to modern building materials application is certainly in providing better optical properties of prosthetic appliance. The first types of cements were aqueous suspensions, such as zinc phosphate and glass-ionomer cement. By introducing composite cement, properties such as solubility and adhesion have been improved by enabling a minimally invasive abutment tooth preparation form. The esthetic property of composite cement is of great importance in modern esthetic prosthetics; therefore composite cement is becoming more and more common in dental medicine. It consists of three parts: an organic resin matrix consisting of bis-GMA or urethane-dimethacrylates (UDMA), inorganic filler particles, and a binder (bonding intermediate layer). They are characterized by high compressive and tensile strength, the ability to achieve a micromechanical bond with enamel, dentin, dental alloys, and ceramics. Composite cement together with an adhesive bonding system makes up adhesive cementation. The bond achieved by adhesive cementation can be mechanical, micromechanical, and chemical at the molecular level.

The results of this study showed that the best shear bond strength was achieved after surface treatment with Nd: Yag laser in combination with silanization, while the application of Er: Yag laser has achieved lower values ​​of shear bond strength compared to conventional surface preparation methods (sandblasting, etching, silanization), which makes the second null hypothesis (no difference in shear bond strength between laser-treated surfaces and conventional surface treatment methods) rejected, while the second working hypothesis (shear bond strength is higher after laser treatment) is partially accepted. The largest contact area was obtained in samples treated with Nd: Yag laser.

Scanning electron microscopy analysis of fractured surfaces showed the adhesive type of fractures (largest in samples sandblasting + silanization, absent in samples treated with acid, silanization, and acid + silanization), cohesive type of fracture (same within samples treated with acid, sandblasting + silanization, and Nd: laser + silanization, while in other samples this type of fracture did not occur) and mixed type of fracture (highest in samples treated with silanization and acid + silanization, while in the control group there were no fractures of this type).

The quality of the bond, i.e. the shear bond strength of the glass-ceramic and composite bonding agent, is influenced by numerous factors. The imperative is known to everyone - to achieve a quality bond that ensures the durability of prosthetic appliance. Different methods of glass-ceramic surface treatment before cementation have been described in the literature and applied in clinical work, all of them aimed to achieve the best possible bond between the restoration and the abutment tooth. The most common way to test the bond strength is the shear test. This test shows the occurrence of cohesive fracture within a material more often than at the junction of two materials. The result is explained by the strong stress accumulation during testing which can lead to test results misinterpretation. Consequently, it is important to eliminate uneven stress within the adhesive zone. Some authors have used very small test areas of only 1mm2 to create a uniformly transmitted stress to the joint surface (-). In this way, detection of the weakest points of the procedure is enabled. In the machine production process of restoration, the impact of this procedure on the cutting surfaces should be taken into consideration. The milled sample's SEM analysis shows visible crystals on the cutting surface, which improve micromechanical retention and increase the bonding surface with composite cement. These results were confirmed by this study. This interpretation confirms the hypothesis that it may be possible to establish a quality bond with glass-ceramic chains without etching with hydrofluoric acid (). Pollington has claimed that SEM analysis of machine processed lithium-disilicate glass-ceramics surface revealed the presence of microporosity which may be of significant importance in achieving a micromechanical bond between the restoration and the abutment tooth surface. He pointed out that this type of complete ceramic must be subjected to additional surface treatment procedures to provide sufficient micro retention for quality bond achievement (). If a cohesive fracture of the binder dominates, it occurs due to microcracks inside the cement rather than the intermediate joint itself.

Silane use as a bond promoter between the ceramics and the binder is a well-known fact (). The bond with the ceramics is achieved through a condensation reaction between silanol groups (Si-OH) on the ceramic surface and hydrolyzed silane silanol groups, which forms a siloxane bond (Si-O-Si) and produces a water molecule as a by-product (). The presence of a glassy phase in ceramics promotes forming of a better siloxane bond. The silanol groups then react by forming a siloxane network with silicon on the surface (). An important factor in the chemical bond between the two materials is silicon from glass. This finding by Pollington is consistent with an earlier claim that etching is not necessary ().

Mean roughness value and SEM analysis showed an irregular surface with pronounced porosity and undermined sites. This surface is visibly weakened by the action of hydrofluoric acid. This is also confirmed by other authors ().

Glass-ceramics sandblasting leads to extreme destruction of glass and crystals. The crystals obliterate dentinal tubules and lead to impossibility of ensuring a quality connection. Another reason for poor bonding is the contamination and deposition of alumina particles on the ceramic surface. Ustun et al. have claimed that the surface treatment affects surface roughness and stated that sandblasting achieves significantly higher values ​​of bond strength compared to the surface treated with Erbi laser (). The abovementioned facts have been confirmed by this research. The abovementioned authors prefer sandblasting over the application of Er: Yag lasers. Bond quality tests between ceramics and composite cement can be carried out by tests such as aging corrosion and alike (). Water storage leads to gradual water absorption within the composite, which can lead to hydrolytic degradation and consequently to bond weakening between the ceramic and the composite cement; silane bond hydrolysis is likely to occur (, ).

Numerous authors concur that the optimum bond between composite cement and glass-ceramics varies for different ceramic systems. It cannot be expected that a single procedure will be universal for all-ceramic materials. This realization is crucial since new ceramic materials of different compositions and microstructures will appear on the market.

All the above-named procedures require micromechanical locking on the joint surface and a chemical bond between the joint surfaces; therefore it is necessary to intervene in some way in the surface structure of the material and/or teeth. Research on non-aggressive procedures is becoming more pronounced by modifying the surface texture and material chemical properties on the surface, which makes the surface more activated, i.e. a functional surface is created (). The acid dissolves the ceramic surface by dissolving the glass phase. It leads to the irregular formation on the surface and increases the contact surface (). Adhesion between ceramics and composite cement is the result of physicochemical interaction in the interface between composites (adhesives) and ceramics (substrates). The surface treatment and its topography will contribute to physical interactions of adhesion. Modifications in topography surface achieved by sandblasting will result in changes in substrate moisture, which correlates with surface energy and adhesive potential (). Rough surface increases mechanical retention by enabling adhesive interlocking (locking) in surface irregularities (). Unfortunately, several studies have demonstrated the possibility of ceramic surface weakening after etching, thus leading to faster fracture of the restoration (). Although the application of dental laser for surface preparation before cementation is not exempt from difficulties, it nevertheless promises. Some tests of CW CO2 lasers impact with 10.6 µm on lithium disilicate () and CAD-CAM ceramic () confirm the presence of micro-cracks and surface dissolution, as a result of the laser thermal effect irradiation at a power higher than 10 W CW (3184.7 W / cm2). However, ceramics structure observations irradiated with a 10 W (14,185 W / cm2) pulsed Nd: YAP laser with 1340 nm show the presence of channels, micro-cracks, and dissolved crystals (). High quantum radiation energy directed over a precisely defined area, over a short period, is the probable cause of an enormous energy accumulation. Micro-cracks on ceramics after CO2 and Nd: YAP laser irradiation can be correlated with high thermal values ​​leading to extreme physical stress and additional ceramic surface hardening (, ). Er: YAG lasers can be used to treat the surface of alumina ceramics, but their result is much weaker than the one achieved by etching. The most probable explanation for this is that energy generated by the Er: YAG laser cannot be absorbed as well as in this type of ceramic, and it does not create a sufficient micro-mechanical retention (). According to this study, some authors recommended the use of very high energy (500 mJ) to achieve a satisfactory retention (). The most recent types of ultra-short pulsed lasers may achieve better results ().

Despite the results of numerous studies, the application of lasers is still an alternative method in surface preparation in order to achieve a better connection between two surfaces in contact. Lasers modify the material surface in a relatively light and simple way. In ZrO2 ceramics, the laser does not form the desired roughness because these irregularities are very shallow and do not provide micromechanical retention, meaning there is no increase in bond strength. Compared to tribochemical processing, the laser is less efficient. During laser surface treatment, created clusters can stick to the melted ceramic surface, leading to quality bond impairment ().

Er: Yag is the most commonly used laser in clinical practice. The wavelength is approximately 2940 nm. These types of lasers make the surface irregular, which increases the micromechanical retention of the ceramic material. Lasers with longer wavelengths can damage the surface by creating cracks, thus weakening the bond (). The findings of previously mentioned studies are contradictory and are not in line with findings of the present survey. Numerous factors such as ceramic moisture, surface roughness, binder, and chemical composition can affect the composite cement quality and stability and its bond to the ceramic surface (). Gomes et al. thermocycled zirconium oxide ceramics samples cemented with composite cement and concluded that shear bond strength is affected by surface treatment, aging, and cement type (). On the contrary, Subasi () believes that cement type has the greatest impact on shear bond strength, while Oyagüe favors pretreatment of the joint surface (). The SEM analysis shows that the once treated surface always remains rough, and has uniform round micro retentions and shallow holes, but without microcracks. Silane contains silicon-bonded to reactive organic radicals that chemically bind to composite molecules forming siloxanes with silicon-coated surfaces. This improves the ceramic humidity (creating better contact and composite infiltration into the ceramic irregularity), protecting it from moisture and creating an acidic environment that can support the bonding mechanisms (, ).

Zirconia ceramics does not contain water, which can affect absorption of laser energy. Therefore, some studies did not find a significant increase of the micromechanical bond between cement and ceramics (). However, Spohr et al. () and Usumezet et al () concluded that Nd:YAG laser irradiation increased both, the surface roughness and shear bond strength. Our results also revealed an increase in surface roughness and bond strength due to Nd:YAG laser irradiation.

Conclusions

Under the limitations of this study, the Nd:YAG irradiation with silanization could be used as pretreatment for providing greater shear bond strength of self-adhesive resin cement to zirconium reinforced lithium disilicate. The pretreatment with ER.YAG irradiation did not increase the bond strength compared to the conventional pretreatment protocol.