Effect of ‘Try-in’ paste removal method on bond strength to lithium disilicate ceramic

Effect of ‘Try-in’ paste removal method on bond strength to lithium disilicate ceramic

Journal of Dentistry, 2011-12-01, Volume 39, Issue 12, Pages 863-870, Copyright © 2011 Elsevier Ltd

Abstract

Objectives

The aim of this study was to evaluate four methods of Try-in paste removal from a lithium disilicate ceramic surface before cementation and assess the influence on bond strength to a resin luting cement.

Methods

Slices of dentine (1.6 mm thick) were sectioned from the crown of human third molars and truncated cone shape cavities were made with diamond burs. IPS e.max Press ceramic blocks were obtained for each cavity, etched with hydrofluoric acid and fixed with temporary paste, except for Group G1, which was cemented according to the manufacturer’s instructions. Try-in paste was removed from the experimental groups using different techniques: G2, ultrasonic bath in distilled water (5 min); G3, air/water spray (1 min); G4, etched with phosphoric acid (2 min) + air/water spray (30 s) and G5, etched with phosphoric acid before and after hydrofluoric acid application + air/water spray (30 s). The ceramic restorations were cemented with the same procedures as G1. Push-out bond strength testing (MPa) was performed in a universal testing machine with a crosshead speed of 0.5 mm/min.

Results

One way ANOVA and Tukey’s test ( P < 0.05) were applied: G4(19.21)a, G2(17.09)ab, G1(16.82)ab, G3(16.18)ab and G5(15.9)b. The results showed that all of the experimental groups showed remaining particles of Try-in paste. The bond strength in G4 was statistically higher than in G5. No significant difference was shown in the other groups. There was a predominance of adhesive failures between resin cements and ceramics in G5.

Conclusion

The techniques used were not totally effective at removing the Try-in paste; however, the presence of remaining material did not influence the bond strength of restorations. The methods of temporary paste removal may affect the bond strength of ceramic restorations.

Introduction

Dental offices have experienced a growth in the number of patients seeking an enhanced appearance of their smile, which has encouraged the development and improvement of materials, allowing the professional to select the most suitable option for each case. As a result, adhesive techniques have become necessary in practically all restorative procedures. Restorative materials allied with the adhesive technique have made it possible to implement a conservative philosophy regarding preparations, particularly in anterior regions, where cosmetics are of primary importance. Therefore, the indication of indirect restorations that require planned and controlled wear may be much more effective in terms of cosmetics and durability.

Ceramics have characteristics capable of mimicking the natural structures present in a tooth and can be indicated in a majority of restorative treatments. In addition to being chemically stable, ceramics present excellent optical properties, thus guaranteeing their outstanding place amongst restorative materials. Historically, the low strength and high brittleness of ceramics made them dependent on metal reinforcement. Over the years, these metal copings have been replaced by metal-free reinforced ceramics. Therefore, the indication of laminated ceramics reinforced with lithium disilicate may be indicated for cosmetic restorative procedures.

Adhesive cementation systems have been considered the best option for luting ceramic restorations and the application of low viscosity resins on prepared teeth has demonstrated good biomechanical behaviour, particularly with ceramic restorations. Nevertheless, with the advent of more conservative preparations, the cement shade has a direct influence on the end result of a restoration. In many cases, the laminated ceramic is less than 1 mm thick and the aesthetics of the end result depend on the shade of the substrate, shape and anatomy of the teeth, restoration thickness, and finally, on the cement.

In order to obtain a satisfactory aesthetic result, the clinician may use Try-in pastes to simulate the final colour of the restoration. According to the manufacturer, these pastes contain water soluble glycerin, reinforced by mineral elements and colouring agents, and when applied before definitive cementation, allow the professional to evaluate the final shade of a restoration, considering the colour of the substrate, cement colour, and the translucency of the ceramic, providing an adequate cosmetic approximation of the restoration. However, the application of these agents may lead to internal contamination of the ceramic and interfere in the bond strength of the restoration. Effective bonding depends on the cleanliness of the bonding surfaces. Therefore, removal of these contaminants before ceramic cementation is a critical procedure for the long term clinical success of a restoration. Various methods, such as the use of an ultrasonic bath or acid agents have been used to obtain decontaminated surfaces, allowing better bonding ; however, there are no conclusive studies regarding the efficiency of these methods of decontamination.

Thus, the aim of this current study was to evaluate surface cleaning techniques after the use of a Try-in paste, and to verify their influence on bond strength to lithium disilicate-based ceramic restorations. The null hypotheses in the present study suggest that: (i) there would be no difference amongst the techniques for removing the temporary Try-in cementation paste from the surface of the lithium disilicate-based ceramic surface. (ii) The use of the temporary Try-in cementation paste would not interfere in the bond strength between the ceramic and dentine.

Materials and methods

The study was approved by the Research Ethics Committee of the Dental School of the University of Uberaba, Brazil (Protocol CAAE-0009.0.227.000-09). Extracted sound human third molars were immersed in distilled water at 4 °C and used within 4 months after extraction.

Dentine preparation

Slices of dentine were sectioned from the crowns of human third molars using a water-cooled diamond saw (Isomet 1000, Buehler Ltd., Lake Bluff, IL, USA). The slices were embedded in epoxy resin (Buehler) and wet-ground with 600-grit SiC abrasive papers to obtain a flat surface only in dentine. The slices were standardized to a thickness of 1.6 mm (±0.1 mm) using a digital calliper (Mitutoyo Corporation, Kawasaki, Japan) with an accuracy of 0.01 mm. Cavities were prepared with #3131 diamond burs (KG Sorensen, Barueri, SP, Brazil) at high-speed, under air/water cooling and using a custom-made preparation device ( Fig. 1 ) to standardize the cavity dimensions: widest diameter of 2.3 mm, smallest diameter 1.9 mm and height 1.6 mm. The cavity, before cementation, is presented in Fig. 2 .

Detail of cavity preparation.
Fig. 1
Detail of cavity preparation.
Top-view of the cavity before cementation.
Fig. 2
Top-view of the cavity before cementation.

Ceramic block production and cementation process

After cavity preparation, an impression was taken using a polyvinyl siloxane material (Virtual – Ivoclar-Vivadent, Liechtenstein) and reproduced in type V dental stone (Exadur, Polidental, Cotia, SP, Brazil) to obtain a wax pattern. The e.max Press ceramic blocks (Ivoclar-Vivadent, Schaan, Liechtenstein) were fabricated using the heat-pressed ceramic procedure and following the manufacturer’s instructions.

The ceramic restorations were etched with hydrofluoric acid (IPS Ceramic Etching Gel – Ivoclar-Vivadent, Liechtenstein) for 20 s, rinsed with distilled water in an ultrasonic bath for 5 min and air-dried. The specimens were divided into five groups ( n = 10) and treated as shown in Table 1 . After removing the Try-in material, a silane agent was applied for 15 s and air-dried for cementation. In all of the groups, the dentine was etched with 37% phosphoric acid (Total Etch – Ivoclar Vivadent) for 60 s, rinsed for 15 s and dried using a cotton pellet. A one-bottle adhesive system (Excite – Ivoclar-Vivadent) was applied according to the manufacturer’s instructions and photoactivated for 10 s with 1200 mW/cm 2 using the LED light source, Radii-cal (SDI Ltd. – Bayswater, Victoria, Australia). Dual curing resin cement Variolink II (Ivoclar-Vivadent) was mixed and inserted into the cavity and the ceramic blocks were placed under static load (0.454 kgF for 1 min). The excess cement was removed with a disposable applicator (Microbrush International – Grafton, Wisconsin, USA). The Radii-cal (SDI Ltd., 1200 mW/cm 2 ), was used to photoactivate the specimen surfaces for two periods of 40 s, one from top and other from the bottom surface. The specimens were stored in 100% relative humidity at 37 °C, for 24 h.

Table 1
Division of the groups and respective treatment.
Groups Try-in Try-in removal procedure
G1 No No
G2 Yes 5 min ultrasonic bath
G3 Yes Air/water spray for 1 min
G4 Yes Application of phosphoric acid 37% for 1 min + air/water spray for 1 min
G5 Yes Application of phosphoric acid 37% for 1 min + air/water spray for 1 min + application of hydrofluoric acid 10% for 1 min + application of phosphoric acid 37% for 1 min + air/water spray for 1

Push out bond strength test and Scanning Electron Microscope (SEM) Analysis

The push out test was performed in a universal testing machine (EMIC DL 3000, Emic, São José dos Pinhais, PR, Brazil). A metal cylinder with a central orifice adapted to the machine was used as the base for the specimens ( Fig. 3 ). Each specimen was placed on the cylinder with the larger base of its cavity against the orifice on the metal cylinder surface. The small base was then loaded with a 1.9 mm diameter cylindrical plunger (touching only the ceramic base) at a cross-head speed of 0.5 mm/min until failure of the bond on the lateral walls between the ceramic and cavity. The load was recorded by the testing machine and used to calculate the push-out force (PF) in N as follows: PF = ( Lg / A 1), where L was the fracture load (kgF), g the acceleration of gravity (9.80665 m/s 2 ) and A 1 the lateral area of the ceramic cone (mm 2 ). The push-out bond strength data were submitted to one-way ANOVA and Tukey’s test ( P > 0.05).

Push-out bond strength test.
Fig. 3
Push-out bond strength test.

After testing, the fractured specimens were coated with gold and examined by SEM (JSM 5600LV, JEOL Inc., Peabody, MA, USA) in order to classify the failure modes. For the interface analyses, the SEM examinations were performed as described before the bond strength tests. After storage, the specimens were longitudinally sectioned through the middle portion of the ceramic cone base and the specimens were embedded cross-sectionally in epoxy resin (Buheler). After 24 h (to allow for resin polymerization), the specimens were wet-polished with 600-, 1200- and 2000-grit SiC paper, and then polished with 1 μm, 0.3 μm and 0.05 μm Al 2 O 3 suspensions. The cross-sectional profiles were examined by SEM, focusing on integrity, homogeneity and continuity along the bond interface.

In order to observe the topography of the surfaces and evaluate the presence of Try-in paste contamination, etched and non-cemented specimens ( n = 3) for each cleaning procedure and control were coated with gold and examined by SEM.

Results

SEM images of surface topography and ceramic/cement bond interface are shown in Figs. 4–6 . For G1, dissolution of the vitreous phase was observed, with complete exposure of the lithium disilicate crystals ( Fig. 4 A), providing an adequate substrate for adhesion and resulting in a high quality cement interface ( Fig. 4 B). In the other groups (G2–G5), the presence of contamination and the cross-sections of the ceramic-cement interfaces can be seen in Figs. 5 and 6 .

(A) SEM images of lithium disilicate crystals exposed after the hydrofluoric acid etching (G1). (B) Resin cement (RC) and ceramic (Cm) interface. The pointers show a continuous interface without gaps or unfilled voids.
Fig. 4
(A) SEM images of lithium disilicate crystals exposed after the hydrofluoric acid etching (G1). (B) Resin cement (RC) and ceramic (Cm) interface. The pointers show a continuous interface without gaps or unfilled voids.
SEM images of G2 (A) show a surface topography with the presence of Try-in adhered to lithium disilicate crystals (white arrows); the ellipse shows an area where the debris prevented crystal observation. The bond interface between the resin cement (Rc) and ceramic (Cm) can be seen in image B, no gaps or unfilled voids were observed in this group. For G3, few crystals are observed in C (white arrows), as the largest amount of contamination with porous aspect is identified with (*). The SEM interface analysis of G3 shows the presence of cracks (pointers) between the resin cement (Rc) and ceramic (Cm) (D).
Fig. 5
SEM images of G2 (A) show a surface topography with the presence of Try-in adhered to lithium disilicate crystals (white arrows); the ellipse shows an area where the debris prevented crystal observation. The bond interface between the resin cement (Rc) and ceramic (Cm) can be seen in image B, no gaps or unfilled voids were observed in this group. For G3, few crystals are observed in C (white arrows), as the largest amount of contamination with porous aspect is identified with (*). The SEM interface analysis of G3 shows the presence of cracks (pointers) between the resin cement (Rc) and ceramic (Cm) (D).
Image A shows a large quantity of contamination demonstrated by the ellipses. In Image B, small failures are pointed out in the resin cement and ceramic interface. For G5, a large quantity of contamination with a porous aspect was observed in image C (*). The crystals indicated with white arrows present a large number of porosities. Another aspect observed only in image C is the presence of empty spaces marked by # and delimited by a dotted line. The separation that occurred at the interface between the resin cement and ceramic can be seen in image D.
Fig. 6
Image A shows a large quantity of contamination demonstrated by the ellipses. In Image B, small failures are pointed out in the resin cement and ceramic interface. For G5, a large quantity of contamination with a porous aspect was observed in image C (*). The crystals indicated with white arrows present a large number of porosities. Another aspect observed only in image C is the presence of empty spaces marked by # and delimited by a dotted line. The separation that occurred at the interface between the resin cement and ceramic can be seen in image D.

After normality ( P = 0.716) and equal variance testing ( P = 0.901), the Push-out bond strength values were submitted to one-way ANOVA ( Table 2 ). The results of the Push-out Bond strength test and the distribution of the failure mode are shown in Table 3 . The G4 means were statistically higher than G5. The other groups did not show any statistical differences. The adhesive failures between cement and dentine (ACD) were predominant in G1 and G2. Mixed failures between cement and dentine (MIX) were predominant in G3 and G4. G5 was the only one group that presented adhesive failures between the ceramic and resin cement (ACC). Details on the failure modes are shown in Fig. 5 . Adhesive failures can be seen in Fig. 7 A (dentine side) and A′ (ceramic side). Fig. 7 B presents a mixed failure. ACC is shown in Fig. 7 C (dentine side) and Fig. 7 C′ (ceramic side). Details of this failure are shown in Fig. 7 D and E.

Table 2
One-way ANOVA.
Source of variations DF SS MS F P
Between groups 4 67.869 16.967 2.581 0.050
Residual 45 295.849 6.574
Total 49 363.718
Table 3
Mean of the push-out bond strength and distribution of the failure mode.
Groups Push-out bond strength Distribution of failure mode
ACD MIX ACC
G1 16.82 (2.98) ab 6 4
G2 17.09 (2.27) ab 9 1
G3 16.18 (2.32) ab 5 5
G4 19.21 (2.20) a 5 5
G5 15.90 (2.92) b 2 3 5
Different small letters in column represent statistic difference to the Tukey’s Test ( P < 0.05).
ACD, adhesive failure between ceramic and dentine; MIX, failure mixed; ACC, adhesive failure between ceramic and resin cement.
In images A (dentine side) and A′ (ceramic side), the adhesive failure between resin cement and dentine (ACD) can be seen. In images B and B′, a mixed failure (FM) with a portion of adhesive fracture between dentine and resin cement (Dn) and part of cohesive failure in resin cement (Rc) is shown. On the ceramic side (B′), the opposite part of cohesive failure (Rc) and adhesive portion of failure (*) can be seen. In images C (dentine side) and C′ (ceramic side), the adhesive failure mode between ceramic (Cm) and resin cement (Rc) is shown, a small part of dentine can still be seen (Dn). The details of this failure mode are observed at higher magnification in images D and E.
Fig. 7
In images A (dentine side) and A′ (ceramic side), the adhesive failure between resin cement and dentine (ACD) can be seen. In images B and B′, a mixed failure (FM) with a portion of adhesive fracture between dentine and resin cement (Dn) and part of cohesive failure in resin cement (Rc) is shown. On the ceramic side (B′), the opposite part of cohesive failure (Rc) and adhesive portion of failure (*) can be seen. In images C (dentine side) and C′ (ceramic side), the adhesive failure mode between ceramic (Cm) and resin cement (Rc) is shown, a small part of dentine can still be seen (Dn). The details of this failure mode are observed at higher magnification in images D and E.

Discussion

Bonding ceramic to dental structures by using resin adhesive systems increases the fracture strength of the tooth and restoration, whilst also minimizing crack formation, a determining factor in treatment success or failure. However, contaminants on the internal surface of the bonding surfaces may diminish the bond strength and must be removed before definitive cementation. In the present study, all of the methods for removing the temporary cementation agent were shown to not be efficient, as the presence of the material were left behind. Nevertheless, the presence of remnants of the material did not interfere with the bond strength results of all of the groups, as the results from Group G4 were statistically superior to those of G5. Therefore, the null hypotheses of the study were partially rejected.

Fig. 4 presents the recommended condition for the bonding surfaces performed in G1 as recommended by the restorative material manufacturer, and without the application of the temporary cementing agent, Try-in. The application of hydrofluoric acid for 20 s removed the vitreous matrix, exposing the lithium disilicate crystals and leaving the surface free of impurities, which favoured the adhesive procedure and has been demonstrated in previous studies. The time of 20 s for etching ceramics reinforced with lithium disilicate is recommended by the manufacturer, although removal of the vitreous matrix did not affect the integrity of the crystal reinforcement, as seen in Fig. 4 A. This condition allowed the formation of an interface without interrupting the continuity between the resin cement and ceramic, thus contributing to no gap formation between the materials ( Fig. 4 B).

Ultrasonic cleaning for 5 min (G2) ( Fig. 5 A) partially removed the temporary cementation paste; nevertheless, residues of the material remained adhered to the lithium disilicate crystals. The vibration promoted by the ultrasonic bath appears to have fragmented the Try-in residues, particularly when compared with the other experimental groups, which presented plates of the material. The bath time of 5 min was based on cleaning ceramic test specimens in a previous study. However, this duration seems too short for the complete removal of Try-in, although this material is composed of water soluble glycerin, as informed by the manufacturer. The presence of spaces could be perceived between the crystals, thus allowing the interaction between the resin cement and ceramic whilst not affecting the bond strength. This resulted in a bond interface that was continuous and without the presence of gaps, similar to that observed in Group G1. The cement/ceramic bond interface of G2 can be seen in Fig. 5 B.

The samples submitted to cleaning with a jet of air/water (G3) presented a larger amount of contamination, resulting in the formation of plates with a porous aspect and the exposure of few lithium disilicate crystals ( Fig. 5 C). This result indicates that the mechanical action of the jet of air/water was less efficient for removing Try-in than the ultrasonic bath. Although the difference in surface topography was perceptible, this difference was not reflected in the bond strength values, although the analysis of the interface of this group demonstrated the presence of gaps between the cementing agent and ceramic ( Fig. 5 D). This leads one to believe that, even with less interaction between the cement and ceramic, the bond between the dentine and resin cement was still superior, which was confirmed by the high rate of adhesive fractures between the resin cement and dentine found in this group.

Although the manufacturer recommends these methods for removing the temporary cementation paste, no bath time or any form of application of water or any other solvent was indicated in their recommendations, despite the manufacturer itself emphasizing that the presence of residues could affect the bond of the material.

Some studies have shown that the use of phosphoric acid could be efficient for cleaning and decontaminating ceramic surfaces before cementation. In the present study, the use of phosphoric acid provided distinct results, both with regard to the surface topography and the bond strength. Fig. 6 A shows a large quantity of contamination demonstrated by the ellipses. The partially exposed crystals presented small porosities (white arrows). The porosities may have resulted from the exposure to acid, since this was not observed in G1–G3. This finding corroborates a previous study that showed irregular etching patterns when a leucite-based ceramic was submitted to different time intervals of hydrofluoric acid application. This may also explain the small failures indicated in Fig. 6 B. The bond strength in this group did not differ statistically from G1 to G3; however, it was statistically higher than that of G5, which also did not differ from that of the others. The number of mixed failures in G4 was similar to those in G3, which also presented failures at the interface; however, the bond strength to dentine still appears to be the weakest link when bonding to ceramic, equaling the bond strength values.

For G5, a large quantity of contamination with a porous aspect was observed in Fig. 6 C. The crystals pointed out by the white arrows present a large number of porosities, which possibly weakened the lithium disilicate crystals. Another aspect observed only in Fig. 6 C was the presence of empty spaces. In a previous study, Naves et al. also related the appearance of these spaces in vitreous ceramics reinforced with leucite, when submitted to prolonged etching times or successive applications of acid, as performed for cleaning in this group. In that previous study, a reduction in bond strength values and an increase in cohesive fractures in ceramic and mixed fractures were also observed. Fig. 6 D shows the separation that occurred at the interface between the resin cement and ceramic, probably caused by the action of successive applications of acids on the crystals and vitreous matrix, causing it to weaken. This is reflected in the failure pattern found in the bond strength samples of G5 ( Fig. 7 C and C′), in which adhesive failure between the ceramic and resin cement predominated, as shown in Table 3 . The details of this failure mode are observed at higher magnification in Fig. 7 D and E.

Another hypothesis that could explain the adhesive behaviour of the restorations in G5 would be the repeated application of hydrofluoric acid, before and after the use of the tested paste. This procedure increased the etching time recommended by the manufacturer (overetching) and could have caused the accumulation of crystalline residues deposited on the ceramic surfaces and greater depth of dissolution promoted by the increase in etching time. According to previous studies, extended contact of hydrofluoric acid on the silica-based ceramics could produce insoluble silicon fluoride salts, which may remain on the surface as a byproduct affecting the bond strength and weaken the ceramic or create unfilled voids underneath the resin cement layer.

The first hypothesis was partially rejected, since the Try-in removal methods showed different surface conditions resulting in the presence of Try-in paste contamination (G2 and G4) or unconventional vitreous matrix dissolution (G3, G4 and G5) when compared with Group G1. The second hypothesis was partially rejected, since the bond strength for G4 was higher than for G5, and there were no significant differences for the other groups.

The low influence of the treatments on bond strength may have occurred as a result of the test having been performed 24 h after cementation. It is necessary to conduct a study that takes into consideration the degradation of the interface by ageing to provide more precise data about the longevity of indirect restorations cemented after use of the temporary cementation paste, Try-in.

Conclusion

The techniques used in the current study were not totally effective at removing the Try-in paste; however, the presence of remaining material did not influence the bond strength of the restorations. It was observed that the methods of the temporary paste removal may affect the bond strength of ceramic restorations.

Acknowledgments

This study was supported in part by PAPE – Programa de Apoio à Pesquisa – University of Uberaba . We thank Ivoclar-Vivadent, Liechtenstein, for the financial support for this study. We also thank Marcos Massao Shimano for help us push-out test.

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Effect of ‘Try-in’ paste removal method on bond strength to lithium disilicate ceramic Renato Abdalla Prata , Vinicius Pires de Oliveira , Fernando Carlos Hueb de Menezes , Gilberto Antônio Borges , Osvaldo Scopin de Andrade and Luciano de Souza Gonçalves Journal of Dentistry, 2011-12-01, Volume 39, Issue 12, Pages 863-870, Copyright © 2011 Elsevier Ltd Abstract Objectives The aim of this study was to evaluate four methods of Try-in paste removal from a lithium disilicate ceramic surface before cementation and assess the influence on bond strength to a resin luting cement. Methods Slices of dentine (1.6 mm thick) were sectioned from the crown of human third molars and truncated cone shape cavities were made with diamond burs. IPS e.max Press ceramic blocks were obtained for each cavity, etched with hydrofluoric acid and fixed with temporary paste, except for Group G1, which was cemented according to the manufacturer's instructions. Try-in paste was removed from the experimental groups using different techniques: G2, ultrasonic bath in distilled water (5 min); G3, air/water spray (1 min); G4, etched with phosphoric acid (2 min) + air/water spray (30 s) and G5, etched with phosphoric acid before and after hydrofluoric acid application + air/water spray (30 s). The ceramic restorations were cemented with the same procedures as G1. Push-out bond strength testing (MPa) was performed in a universal testing machine with a crosshead speed of 0.5 mm/min. Results One way ANOVA and Tukey's test ( P < 0.05) were applied: G4(19.21)a, G2(17.09)ab, G1(16.82)ab, G3(16.18)ab and G5(15.9)b. The results showed that all of the experimental groups showed remaining particles of Try-in paste. The bond strength in G4 was statistically higher than in G5. No significant difference was shown in the other groups. There was a predominance of adhesive failures between resin cements and ceramics in G5. Conclusion The techniques used were not totally effective at removing the Try-in paste; however, the presence of remaining material did not influence the bond strength of restorations. The methods of temporary paste removal may affect the bond strength of ceramic restorations. 1 Introduction Dental offices have experienced a growth in the number of patients seeking an enhanced appearance of their smile, which has encouraged the development and improvement of materials, allowing the professional to select the most suitable option for each case. As a result, adhesive techniques have become necessary in practically all restorative procedures. Restorative materials allied with the adhesive technique have made it possible to implement a conservative philosophy regarding preparations, particularly in anterior regions, where cosmetics are of primary importance. Therefore, the indication of indirect restorations that require planned and controlled wear may be much more effective in terms of cosmetics and durability. Ceramics have characteristics capable of mimicking the natural structures present in a tooth and can be indicated in a majority of restorative treatments. In addition to being chemically stable, ceramics present excellent optical properties, thus guaranteeing their outstanding place amongst restorative materials. Historically, the low strength and high brittleness of ceramics made them dependent on metal reinforcement. Over the years, these metal copings have been replaced by metal-free reinforced ceramics. Therefore, the indication of laminated ceramics reinforced with lithium disilicate may be indicated for cosmetic restorative procedures. Adhesive cementation systems have been considered the best option for luting ceramic restorations and the application of low viscosity resins on prepared teeth has demonstrated good biomechanical behaviour, particularly with ceramic restorations. Nevertheless, with the advent of more conservative preparations, the cement shade has a direct influence on the end result of a restoration. In many cases, the laminated ceramic is less than 1 mm thick and the aesthetics of the end result depend on the shade of the substrate, shape and anatomy of the teeth, restoration thickness, and finally, on the cement. In order to obtain a satisfactory aesthetic result, the clinician may use Try-in pastes to simulate the final colour of the restoration. According to the manufacturer, these pastes contain water soluble glycerin, reinforced by mineral elements and colouring agents, and when applied before definitive cementation, allow the professional to evaluate the final shade of a restoration, considering the colour of the substrate, cement colour, and the translucency of the ceramic, providing an adequate cosmetic approximation of the restoration. However, the application of these agents may lead to internal contamination of the ceramic and interfere in the bond strength of the restoration. Effective bonding depends on the cleanliness of the bonding surfaces. Therefore, removal of these contaminants before ceramic cementation is a critical procedure for the long term clinical success of a restoration. Various methods, such as the use of an ultrasonic bath or acid agents have been used to obtain decontaminated surfaces, allowing better bonding ; however, there are no conclusive studies regarding the efficiency of these methods of decontamination. Thus, the aim of this current study was to evaluate surface cleaning techniques after the use of a Try-in paste, and to verify their influence on bond strength to lithium disilicate-based ceramic restorations. The null hypotheses in the present study suggest that: (i) there would be no difference amongst the techniques for removing the temporary Try-in cementation paste from the surface of the lithium disilicate-based ceramic surface. (ii) The use of the temporary Try-in cementation paste would not interfere in the bond strength between the ceramic and dentine. 2 Materials and methods The study was approved by the Research Ethics Committee of the Dental School of the University of Uberaba, Brazil (Protocol CAAE-0009.0.227.000-09). Extracted sound human third molars were immersed in distilled water at 4 °C and used within 4 months after extraction. 2.1 Dentine preparation Slices of dentine were sectioned from the crowns of human third molars using a water-cooled diamond saw (Isomet 1000, Buehler Ltd., Lake Bluff, IL, USA). The slices were embedded in epoxy resin (Buehler) and wet-ground with 600-grit SiC abrasive papers to obtain a flat surface only in dentine. The slices were standardized to a thickness of 1.6 mm (±0.1 mm) using a digital calliper (Mitutoyo Corporation, Kawasaki, Japan) with an accuracy of 0.01 mm. Cavities were prepared with #3131 diamond burs (KG Sorensen, Barueri, SP, Brazil) at high-speed, under air/water cooling and using a custom-made preparation device ( Fig. 1 ) to standardize the cavity dimensions: widest diameter of 2.3 mm, smallest diameter 1.9 mm and height 1.6 mm. The cavity, before cementation, is presented in Fig. 2 . Fig. 1 Detail of cavity preparation. Fig. 2 Top-view of the cavity before cementation. 2.2 Ceramic block production and cementation process After cavity preparation, an impression was taken using a polyvinyl siloxane material (Virtual – Ivoclar-Vivadent, Liechtenstein) and reproduced in type V dental stone (Exadur, Polidental, Cotia, SP, Brazil) to obtain a wax pattern. The e.max Press ceramic blocks (Ivoclar-Vivadent, Schaan, Liechtenstein) were fabricated using the heat-pressed ceramic procedure and following the manufacturer's instructions. The ceramic restorations were etched with hydrofluoric acid (IPS Ceramic Etching Gel – Ivoclar-Vivadent, Liechtenstein) for 20 s, rinsed with distilled water in an ultrasonic bath for 5 min and air-dried. The specimens were divided into five groups ( n = 10) and treated as shown in Table 1 . After removing the Try-in material, a silane agent was applied for 15 s and air-dried for cementation. In all of the groups, the dentine was etched with 37% phosphoric acid (Total Etch – Ivoclar Vivadent) for 60 s, rinsed for 15 s and dried using a cotton pellet. A one-bottle adhesive system (Excite – Ivoclar-Vivadent) was applied according to the manufacturer's instructions and photoactivated for 10 s with 1200 mW/cm 2 using the LED light source, Radii-cal (SDI Ltd. – Bayswater, Victoria, Australia). Dual curing resin cement Variolink II (Ivoclar-Vivadent) was mixed and inserted into the cavity and the ceramic blocks were placed under static load (0.454 kgF for 1 min). The excess cement was removed with a disposable applicator (Microbrush International – Grafton, Wisconsin, USA). The Radii-cal (SDI Ltd., 1200 mW/cm 2 ), was used to photoactivate the specimen surfaces for two periods of 40 s, one from top and other from the bottom surface. The specimens were stored in 100% relative humidity at 37 °C, for 24 h. Table 1 Division of the groups and respective treatment. Groups Try-in Try-in removal procedure G1 No No G2 Yes 5 min ultrasonic bath G3 Yes Air/water spray for 1 min G4 Yes Application of phosphoric acid 37% for 1 min + air/water spray for 1 min G5 Yes Application of phosphoric acid 37% for 1 min + air/water spray for 1 min + application of hydrofluoric acid 10% for 1 min + application of phosphoric acid 37% for 1 min + air/water spray for 1 2.3 Push out bond strength test and Scanning Electron Microscope (SEM) Analysis The push out test was performed in a universal testing machine (EMIC DL 3000, Emic, São José dos Pinhais, PR, Brazil). A metal cylinder with a central orifice adapted to the machine was used as the base for the specimens ( Fig. 3 ). Each specimen was placed on the cylinder with the larger base of its cavity against the orifice on the metal cylinder surface. The small base was then loaded with a 1.9 mm diameter cylindrical plunger (touching only the ceramic base) at a cross-head speed of 0.5 mm/min until failure of the bond on the lateral walls between the ceramic and cavity. The load was recorded by the testing machine and used to calculate the push-out force (PF) in N as follows: PF = ( Lg / A 1), where L was the fracture load (kgF), g the acceleration of gravity (9.80665 m/s 2 ) and A 1 the lateral area of the ceramic cone (mm 2 ). The push-out bond strength data were submitted to one-way ANOVA and Tukey's test ( P > 0.05). Fig. 3 Push-out bond strength test. After testing, the fractured specimens were coated with gold and examined by SEM (JSM 5600LV, JEOL Inc., Peabody, MA, USA) in order to classify the failure modes. For the interface analyses, the SEM examinations were performed as described before the bond strength tests. After storage, the specimens were longitudinally sectioned through the middle portion of the ceramic cone base and the specimens were embedded cross-sectionally in epoxy resin (Buheler). After 24 h (to allow for resin polymerization), the specimens were wet-polished with 600-, 1200- and 2000-grit SiC paper, and then polished with 1 μm, 0.3 μm and 0.05 μm Al 2 O 3 suspensions. The cross-sectional profiles were examined by SEM, focusing on integrity, homogeneity and continuity along the bond interface. In order to observe the topography of the surfaces and evaluate the presence of Try-in paste contamination, etched and non-cemented specimens ( n = 3) for each cleaning procedure and control were coated with gold and examined by SEM. 3 Results SEM images of surface topography and ceramic/cement bond interface are shown in Figs. 4–6 . For G1, dissolution of the vitreous phase was observed, with complete exposure of the lithium disilicate crystals ( Fig. 4 A), providing an adequate substrate for adhesion and resulting in a high quality cement interface ( Fig. 4 B). In the other groups (G2–G5), the presence of contamination and the cross-sections of the ceramic-cement interfaces can be seen in Figs. 5 and 6 . Fig. 4 (A) SEM images of lithium disilicate crystals exposed after the hydrofluoric acid etching (G1). (B) Resin cement (RC) and ceramic (Cm) interface. The pointers show a continuous interface without gaps or unfilled voids. Fig. 5 SEM images of G2 (A) show a surface topography with the presence of Try-in adhered to lithium disilicate crystals (white arrows); the ellipse shows an area where the debris prevented crystal observation. The bond interface between the resin cement (Rc) and ceramic (Cm) can be seen in image B, no gaps or unfilled voids were observed in this group. For G3, few crystals are observed in C (white arrows), as the largest amount of contamination with porous aspect is identified with (*). The SEM interface analysis of G3 shows the presence of cracks (pointers) between the resin cement (Rc) and ceramic (Cm) (D). Fig. 6 Image A shows a large quantity of contamination demonstrated by the ellipses. In Image B, small failures are pointed out in the resin cement and ceramic interface. For G5, a large quantity of contamination with a porous aspect was observed in image C (*). The crystals indicated with white arrows present a large number of porosities. Another aspect observed only in image C is the presence of empty spaces marked by # and delimited by a dotted line. The separation that occurred at the interface between the resin cement and ceramic can be seen in image D. After normality ( P = 0.716) and equal variance testing ( P = 0.901), the Push-out bond strength values were submitted to one-way ANOVA ( Table 2 ). The results of the Push-out Bond strength test and the distribution of the failure mode are shown in Table 3 . The G4 means were statistically higher than G5. The other groups did not show any statistical differences. The adhesive failures between cement and dentine (ACD) were predominant in G1 and G2. Mixed failures between cement and dentine (MIX) were predominant in G3 and G4. G5 was the only one group that presented adhesive failures between the ceramic and resin cement (ACC). Details on the failure modes are shown in Fig. 5 . Adhesive failures can be seen in Fig. 7 A (dentine side) and A′ (ceramic side). Fig. 7 B presents a mixed failure. ACC is shown in Fig. 7 C (dentine side) and Fig. 7 C′ (ceramic side). Details of this failure are shown in Fig. 7 D and E. Table 2 One-way ANOVA. Source of variations DF SS MS F P Between groups 4 67.869 16.967 2.581 0.050 Residual 45 295.849 6.574 Total 49 363.718 Table 3 Mean of the push-out bond strength and distribution of the failure mode. Groups Push-out bond strength Distribution of failure mode ACD MIX ACC G1 16.82 (2.98) ab 6 4 – G2 17.09 (2.27) ab 9 1 – G3 16.18 (2.32) ab 5 5 – G4 19.21 (2.20) a 5 5 – G5 15.90 (2.92) b 2 3 5 Different small letters in column represent statistic difference to the Tukey's Test ( P < 0.05). ACD, adhesive failure between ceramic and dentine; MIX, failure mixed; ACC, adhesive failure between ceramic and resin cement. Fig. 7 In images A (dentine side) and A′ (ceramic side), the adhesive failure between resin cement and dentine (ACD) can be seen. In images B and B′, a mixed failure (FM) with a portion of adhesive fracture between dentine and resin cement (Dn) and part of cohesive failure in resin cement (Rc) is shown. On the ceramic side (B′), the opposite part of cohesive failure (Rc) and adhesive portion of failure (*) can be seen. In images C (dentine side) and C′ (ceramic side), the adhesive failure mode between ceramic (Cm) and resin cement (Rc) is shown, a small part of dentine can still be seen (Dn). The details of this failure mode are observed at higher magnification in images D and E. 4 Discussion Bonding ceramic to dental structures by using resin adhesive systems increases the fracture strength of the tooth and restoration, whilst also minimizing crack formation, a determining factor in treatment success or failure. However, contaminants on the internal surface of the bonding surfaces may diminish the bond strength and must be removed before definitive cementation. In the present study, all of the methods for removing the temporary cementation agent were shown to not be efficient, as the presence of the material were left behind. Nevertheless, the presence of remnants of the material did not interfere with the bond strength results of all of the groups, as the results from Group G4 were statistically superior to those of G5. Therefore, the null hypotheses of the study were partially rejected. Fig. 4 presents the recommended condition for the bonding surfaces performed in G1 as recommended by the restorative material manufacturer, and without the application of the temporary cementing agent, Try-in. The application of hydrofluoric acid for 20 s removed the vitreous matrix, exposing the lithium disilicate crystals and leaving the surface free of impurities, which favoured the adhesive procedure and has been demonstrated in previous studies. The time of 20 s for etching ceramics reinforced with lithium disilicate is recommended by the manufacturer, although removal of the vitreous matrix did not affect the integrity of the crystal reinforcement, as seen in Fig. 4 A. This condition allowed the formation of an interface without interrupting the continuity between the resin cement and ceramic, thus contributing to no gap formation between the materials ( Fig. 4 B). Ultrasonic cleaning for 5 min (G2) ( Fig. 5 A) partially removed the temporary cementation paste; nevertheless, residues of the material remained adhered to the lithium disilicate crystals. The vibration promoted by the ultrasonic bath appears to have fragmented the Try-in residues, particularly when compared with the other experimental groups, which presented plates of the material. The bath time of 5 min was based on cleaning ceramic test specimens in a previous study. However, this duration seems too short for the complete removal of Try-in, although this material is composed of water soluble glycerin, as informed by the manufacturer. The presence of spaces could be perceived between the crystals, thus allowing the interaction between the resin cement and ceramic whilst not affecting the bond strength. This resulted in a bond interface that was continuous and without the presence of gaps, similar to that observed in Group G1. The cement/ceramic bond interface of G2 can be seen in Fig. 5 B. The samples submitted to cleaning with a jet of air/water (G3) presented a larger amount of contamination, resulting in the formation of plates with a porous aspect and the exposure of few lithium disilicate crystals ( Fig. 5 C). This result indicates that the mechanical action of the jet of air/water was less efficient for removing Try-in than the ultrasonic bath. Although the difference in surface topography was perceptible, this difference was not reflected in the bond strength values, although the analysis of the interface of this group demonstrated the presence of gaps between the cementing agent and ceramic ( Fig. 5 D). This leads one to believe that, even with less interaction between the cement and ceramic, the bond between the dentine and resin cement was still superior, which was confirmed by the high rate of adhesive fractures between the resin cement and dentine found in this group. Although the manufacturer recommends these methods for removing the temporary cementation paste, no bath time or any form of application of water or any other solvent was indicated in their recommendations, despite the manufacturer itself emphasizing that the presence of residues could affect the bond of the material. Some studies have shown that the use of phosphoric acid could be efficient for cleaning and decontaminating ceramic surfaces before cementation. In the present study, the use of phosphoric acid provided distinct results, both with regard to the surface topography and the bond strength. Fig. 6 A shows a large quantity of contamination demonstrated by the ellipses. The partially exposed crystals presented small porosities (white arrows). The porosities may have resulted from the exposure to acid, since this was not observed in G1–G3. This finding corroborates a previous study that showed irregular etching patterns when a leucite-based ceramic was submitted to different time intervals of hydrofluoric acid application. This may also explain the small failures indicated in Fig. 6 B. The bond strength in this group did not differ statistically from G1 to G3; however, it was statistically higher than that of G5, which also did not differ from that of the others. The number of mixed failures in G4 was similar to those in G3, which also presented failures at the interface; however, the bond strength to dentine still appears to be the weakest link when bonding to ceramic, equaling the bond strength values. For G5, a large quantity of contamination with a porous aspect was observed in Fig. 6 C. The crystals pointed out by the white arrows present a large number of porosities, which possibly weakened the lithium disilicate crystals. Another aspect observed only in Fig. 6 C was the presence of empty spaces. In a previous study, Naves et al. also related the appearance of these spaces in vitreous ceramics reinforced with leucite, when submitted to prolonged etching times or successive applications of acid, as performed for cleaning in this group. In that previous study, a reduction in bond strength values and an increase in cohesive fractures in ceramic and mixed fractures were also observed. Fig. 6 D shows the separation that occurred at the interface between the resin cement and ceramic, probably caused by the action of successive applications of acids on the crystals and vitreous matrix, causing it to weaken. This is reflected in the failure pattern found in the bond strength samples of G5 ( Fig. 7 C and C′), in which adhesive failure between the ceramic and resin cement predominated, as shown in Table 3 . The details of this failure mode are observed at higher magnification in Fig. 7 D and E. Another hypothesis that could explain the adhesive behaviour of the restorations in G5 would be the repeated application of hydrofluoric acid, before and after the use of the tested paste. This procedure increased the etching time recommended by the manufacturer (overetching) and could have caused the accumulation of crystalline residues deposited on the ceramic surfaces and greater depth of dissolution promoted by the increase in etching time. According to previous studies, extended contact of hydrofluoric acid on the silica-based ceramics could produce insoluble silicon fluoride salts, which may remain on the surface as a byproduct affecting the bond strength and weaken the ceramic or create unfilled voids underneath the resin cement layer. The first hypothesis was partially rejected, since the Try-in removal methods showed different surface conditions resulting in the presence of Try-in paste contamination (G2 and G4) or unconventional vitreous matrix dissolution (G3, G4 and G5) when compared with Group G1. The second hypothesis was partially rejected, since the bond strength for G4 was higher than for G5, and there were no significant differences for the other groups. The low influence of the treatments on bond strength may have occurred as a result of the test having been performed 24 h after cementation. It is necessary to conduct a study that takes into consideration the degradation of the interface by ageing to provide more precise data about the longevity of indirect restorations cemented after use of the temporary cementation paste, Try-in. 5 Conclusion The techniques used in the current study were not totally effective at removing the Try-in paste; however, the presence of remaining material did not influence the bond strength of the restorations. It was observed that the methods of the temporary paste removal may affect the bond strength of ceramic restorations. Acknowledgments This study was supported in part by PAPE – Programa de Apoio à Pesquisa – University of Uberaba . We thank Ivoclar-Vivadent, Liechtenstein, for the financial support for this study. We also thank Marcos Massao Shimano for help us push-out test. References 1. Conrad H.J., Seong W.J., Pesun I.J.: Current ceramic materials and systems with clinical recommendations: a systematic review. 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