EFFECT OF ZIRCONIA ADDITION TO LITHIUM DISILICATE … 2020. 11. 24. · for three minutes. The silane bonding agent (ESPE Sil, 3M ESPE, Germany) was then applied to the bonding surface

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EGYPTIANDENTAL JOURNAL

Vol. 61, 4519:4533, October, 2015

* Lecturer, Fixed Prosthodontics Department, Modern Science and Arts University, Faculty of Dentistry, Cairo, Egypt.** Lecturer, Operative Dentistry Department, Modern Science and Arts University, Faculty of Dentistry, Cairo, Egypt.*** Lecturer, Fixed Prosthodontics Department, Faculty of Dentistry, Cairo University, Cairo, Egypt.

EFFECT OF ZIRCONIA ADDITION TO LITHIUM DISILICATE CERAMIC ON TRANSLUCENCY AND BOND STRENGTH USING

DIFFERENT ADHESIVE STRATEGIES

Sherif Fayez Ahmed Bahgat *; Rasha Ramadan Basheer ** and Shereen M El Sayed ***

ABSTRACTObjectives: The aim of this study was to investigate the effect of addition of zirconia to

lithium disilicate ceramics on translucency and bonding ability to resin cements using two adhesive strategies.

Methods: Two groups of ceramic discs (n=10) were constructed from lithium disilicate glass- ceramic (IPS e.max CAD, Ivoclar Vivadent, Schaan, Liechtenstein) and zirconia-reinforced lithium disilicate glass –ceramic (ZLS, Vita Suprinity, Vita Zahnfabrik, Bad Sackingen, Germany) using CAD/ CAM system of 10 mm diameter and 1 mm thickness for translucency measurements (∆E) using a spectrophotometer. For shear bond strength measurements, forty intact maxillary premolars were selected, then sectioned transversely to remove the occlusal surface of the teeth exposing the superficial dentin. Four groups (n=10) of ceramic discs of 5mm in diameter and 2mm thickness were constructed from the aforementioned ceramic materials and were bonded to the exposed dentin with two different bonding strategies: total-etch adhesive resin cement (Bifix QM, Voco, Germany) and self-adhesive one (Rely X Unicem resin cement, 3M Espe), according to manufacturer instructions. Shear bond strength test was carried out using universal testing machine Data were statistically-analyzed using independent t-test to study the effect of different ceramics on translucency parameter. Regarding shear bond strength (MPa), Two Way-ANOVA was used to study the effect of different ceramics and adhesive strategies on mean Shear bond strength (MPa). The significance level was set at P ≤ 0.05.

Results: IPS e.max CAD ceramic discs presented lower mean significant ΔE (20.41±0.41) compared to Vita Suprinity ceramic discs (22.43±0.69) at p≤0.001. Vita Suprinity ceramic discs cemented using total-etch adhesive strategy showed the highest mean Shear bond strength (31.78±1.73MPa) followed by IPS e.max CAD ceramic discs cemented with total-etch adhesive strategy (25.91±1.96MPa) followed by Vita Suprinity ceramic discs cemented with self-adhesive strategy (11.88±0.42MPa) followed by IPS e.max CAD ceramic discs cemented using self-adhesive strategy (9.49±0.48MPa) with a significant difference between each others at p≤0.001.

Conclusions: Under the test conditions, Zirconia-reinforced Lithuim disilicate ceramic demonstrated better translucency and shear bond strength, when compared to Lithuim disilicate one. Meanwhile, total etch dual-cured adhesive resin cement showed better bonding ability between ceramic and dentin, when compared to self-adhesive ones.

KEYWORDS: Lithuim-disilicate, Zirconia-reinforced, total-etch, self-adhesive, translucency, bond strength.

(4520) Sherif Fayez Ahmed Bahgat, et al.E.D.J. Vol. 61, No. 4

INTRODUCTION

The Patients’ demand for natural looking restorations, such as laminates, inlays, onlays and full coverage crowns, that mimic tooth structure has led to development of new all-ceramic systems.1,2 Esthetically pleasing restoration should be an exact replica of shape, size, translucency and surface texture of the natural tooth.3 In spite of the clinical success that was offered by the porcelain fused to metal restorations, unpleasant esthetic light reflection from the opaque metal substructure can compromise the natural appearance and affect the overall esthetic result of the restoration.4

Color is a psychophysical sensation resulting from the human visual system in response to the light reflected from objects.5 New all-ceramic systems have been developed to satisfy the patients’ need,1,2 with a deeper translucency similar to natural tooth,4 that is considered one of the main factors in controlling esthetics.6 Recently, many types of all-ceramic systems are available, among which zirconia-based ceramics, which are considered to be very attractive for dentists,7 as they fulfill the biomechanical requirements; as dimensional and chemical stability, high fracture toughness, and mechanical strength.2,8 Unfortunately, the increase in the crystalline content of all-ceramic materials leads to increase in strength but it also increases its opacity as well,5,9 which could be attributed to the difference in homogeneity of the crystals and the refractive indices.10 Zirconia cores are of poor translucency and extremely white in esthetic appearance when compared to other all-ceramic systems.11-12 This has led to the development of shaded zirconia cores, to improve the optical characteristics of these restorations,8,3 through offering better transmission of light and so the reproduction of color and translucency in relation to the natural teeth was highly improved.13

More specifically, tooth color results from the light reflected from the tooth surface combined

with that redirected from dentin, which results from internal reflection and refraction.14-15 Furthermore, color has three dimensions: chroma, hue, and value.11

However, translucency is considered an important property that should be considered when replication of tooth with an esthetic restoration is aimed, which means extent to which light is diffused rather than absorbed or reflected.16 Consequently, the incisal third and the proximal surfaces of a tooth have high translucent enamel,17 While, the middle third of a tooth has greater amount of yellowish dentin, that affects markedly the color of the enamel.18

Lithium disilicate glass ceramic (IPS e.max CAD) was introduced in 2007 by Ivoclar Vivadent (Schaan, Liechtenstein) company, using CAD/CAM technology and was commonly used as all-ceramic restoration, because of the low refractive index of the lithium disilicate needle-shaped crystals, which are considered to be very translucent,19-21 thus offering outstanding esthetics properties, high strength, and can be adhesively bonded.19

In 2013, zirconia-reinforced lithium disilicates (ZLSs) (VITA SUPRINITY) were introduced by VITA Zahnfabrik (Bad Sackingen, Germany). ZLS materials comprise a lithium disilicate glass ceramic that is strengthened with approximately 10% zirconia crystals by weight.22 Although these materials are recently introduced to the market, initial in vitro testing shows they have excellent optical and physical properties similar to lithium disilicates as it features a special fine-grained and homogeneous structure.22,23

Conventional cements and resin cements can be used efficiently to bond the tooth structure to all-ceramic restorations.24 Yet resin cements are more preferred as it provides ultimate aesthetics with low solubility in oral environment, high bond strength, superior mechanical properties,25-27 a better marginal seal, together with high retention.28 In addition, resin cements are available by several manufacturers and in different polymerization types and shades.27,29,30

EFFECT OF ZIRCONIA ADDITION TO LITHIUM DISILICATE CERAMIC (4521)

Owing to the fact that the total-etch technique is considered to be technique sensitive, new luting materials start using self-etching or self-adhesive components. Self-adhesive materials depends on modifying the tooth structures with their acidic components, and also less technique sensitive.31

Hence, it was our interest to investigate the effect of addition of zirconia to lithium disilicate ceramics on translucency and bonding ability to resin cements using two adhesive strategies. The null hypothesis of this study was that addition of zirconia to lithium disilicate would not affect the translucency and the bond strength of the restoration to total-etch and self-adhesive resin cements.

MATERIALS AND METHODS

Discs specimens for translucency testing:

Two groups of ceramic discs were constructed from lithium disilicate glass- ceramic (IPS e.max CAD, Ivoclar Vivadent, Schaan, Liechtenstein) and zirconia-reinforced lithium disilicate glass –ceramic (ZLS, Vita Suprinity, Vita Zahnfabrik, Bad Sackingen, Germany) using CAD/ CAM system (Cerec inLab, Sirona, Germany). Each group consisted of ten discs of 10 mm diameter and 1 mm thickness. In order to standardize the disc dimensions, a machine-made metal mold with a cavity of 10 mm diameter and 1 mm thickness was used.

The upper surface of the metal mold including the cavity with the standardized dimensions was sprayed with scanspray (Cerec Optispray, Sirona) for scanning in the inLab scanner (inEos, Sirona, Germany) (Fig.1).The design of the disc shaped specimens was chosen using the Cerec InLab software. IPS e.max CAD and Vita Suprinity blocks were milled using the CAD/CAM milling machine (Cerec inLab MC XL milling machine, Sirona, Germany) to obtain ten discs for each ceramic material. A2 shade was selected for both ceramic

materials. The ceramic materials used in this study were described in table (1).

IPS e.max CAD discs were fully crystallized in a furnace (Programat P500, Ivoclar Vivadent, Schaan, Lieichtenstein) for thirty minutes at 850˚C according to manufacturer instructions. Vita Suprinity disc specimens were fully crystallized in the Programat furnace at 840˚C according to manufacturer instructions. Specimens grouping were showed in table (2).

Translucency testing:

For translucency measurements, a spectrophotometer (UV-3101PC, SHIMADZU, Japan) with a 10 mm opening was used. The CIELAB values (L*, a*, b*) of all-ceramic discs were measured against white background and then black background. The translucency parameter (TP) measured as ∆E was calculated using the following equation:

TP (∆E) = [ (L*B-L*

W)+(a*B-a*W)+(b*

B-b*W)]1/2

(∆E= Total color difference, L= Light, a= red-green, b= yellow-blue, B= Black background and W= White background.)

Specimens preparation for shear bond strength testing:

Disc specimen preparation:

Two groups of Ceramic discs were constructed from IPS e.max CAD and Vita Suprinity ceramic materials. The dimensions of the discs were standardized to be 5 mm diameter and 2 mm thickness. Twenty discs were constructed from each ceramic material according to manufacturer instructions using the same procedure previously described with the aid of another machine-made metal mold containing a cavity of 5 mm diameter and 2 mm thickness (Fig. 2). Specimens grouping were illustrated in table (2).


Dentin specimen preparation:

Forty intact maxillary premolars were selected. The teeth were stored in 0.1% thymol at 4˚C, and then sectioned transversely to remove the occlusal surface of the teeth, exposing the superficial dentin using a wet model trimmer (Model Trimmer, Aurora Labs, Aurora, CO, USA). The teeth were vertically mounted in acrylic resin blocks such that the coronal portion remained free of the acrylic and the roots were covered to a height of 2 mm below the CEJ (Fig. 3).

Cementation of ceramic discs to tooth structure

To demarcate the bonding area in all specimens, a piece of polyethylene tape with a circular hole of 5 mm in diameter was positioned in the same place on dentin surface in all the specimens.

i) Total etch bonding strategy:

Conditioning of the restoration: The bonding surface of all the all-ceramic specimens (both IPS e.max CAD and Vita Suprinity discs) was cleaned using alcohol. The bonding surface of the all-ceramic discs was etched using 5% hydrofluoric acid (Vita Ceramics Etch, Vita Zahnfabrik, Bad Sackingen, Germany) for 20 seconds. The hydrofluoric acid was rinsed off with forceful water spray and the ceramic discs were cleaned in an ultrasonic bath for three minutes. The silane bonding agent (ESPE Sil, 3M ESPE, Germany) was then applied to the bonding surface of the ceramic discs. The silane was allowed to react for one minute before dispersion to obtain a very thin silane coat. Conditioning of the dentin was performed using 37% phosphoric acid (Ivoclar Vivadent, Schaan, Liechtenstein), followed by bonding agent (Solobond M, Voco, Germany) application and light curing, according to manufacturer’s instructions.

Cementation of ceramic discs to dentin: The dual cure adhesive resin cement (Bifix QM, Voco, Germany) was auto mixed and applied to the bonding surface of the ceramic discs. The discs were

Fig. (1): Metal mold used for construction of all-ceramic discs for translucency testing. The mold is sprayed with optispray for scanning in CAD/CAM machine.

Fig. (2): Metal mold used for construction of all-ceramic discs for shear bond strength testing.

Fig. (3): Maxillary premolars after transverse sectioning to expose dentin for bonding.


seated on the dentin bonding area in each tooth and the cement was allowed to set under a standardized load of 1 kg. The excess cement was removed. Light curing was applied from each side for 20 seconds using light curing unit (Mini LED, 1250 mW/cm2, Satelec, Acteon).

ii) Self adhesive bonding strategy:

Both IPS e.max CAD and Vita Suprinity discs were treated as previously described. Self-adhesive Rely X Unicem resin cement (3M Espe) was used. Rely X Unicem capsule was mixed in a high frequency mixing unit for 10 seconds, then

TABLE (1) Materials used in this study:

Material Composition Manufacturer

IPS e.max CAD57–80% SiO2, 11-19% Li2O, K2O, MgO, Al2O3, P2O5 and other oxides

Ivoclar Vivadent, Schaan,Liechtenstein

Vita Suprinity

8-12 wt% ZrO2 (zirconia)56-64 wt% SiO2 (silicon dioxide)15-21 wt% Li2O (lithium oxide)<10 wt% Pigments>10 wt% Various

Vita Zahnfabrik, Bad Sackingen, Germany

Bifix QM Resin Cement 10-25% HEDMA and 10-25% BIS-GMA Voco, Germany

Rely X UnicemResin Cement

Powder:- Alkaline (basic) fillers- Silanated fillers- Initiator components- PigmentsLiquid:- Methacrylate monomers containing phosphoric acid groups- Methacrylate monomers- Initiator components- Stabilizers

3M ESPE, Germany

TABLE (2) Specimens Grouping:

A) Specimens for Translucency Testing

Type of ceramic materialGroup I

IPS e.max CADGroup II

VITA SUPRINITYNumber of samples 10 10

Total number of samples 20

B) Specimens for Shear Bond Strength Testing

Type of ceramic materialGroup III

IPS e.max CADGroup IV

VITA SUPRINITY

Type of cementSubgroup A

(Total-etch adhesive)Subgroup B

(self-adhesive)Subgroup A

(Total-etch adhesive)Subgroup B

(self-adhesive)

Number of samples 10 10 10 10

Total number of samples 40


the cement was applied to the disc. The discs were seated and cemented on the dentin bonding area as previously described.

Shear bond strength testing:

All specimens were subjected to 2000 thermal cycles between 5°C and 55°C (Willytec thermocycler, Germany) with a dwell time of 30 seconds in each bath. Bond strength was measured as shear bond strengths (SBSs). In order to carry out the shear bond strength test, the test specimens were stored in distilled water (37°C) and then immediately placed in the holder of a universal testing machine (Lloyd Instruments LRX Material Testing Machine, Lloyd Instruments, Fareham, UK) measuring shear bond strength . A cross-head speed of 1.0 mm/min with a parallel knife-edge blade touching the interface of the all-ceramic discs and dentin bonding area was used, and the debonding fracture load was registered. Fig. (4). Debonding loads were calculated as shear stress (MPa), i.e. by dividing the failure load (N) by the bonding area (mm2):

τ=F/A

(τ is the shear stress in megapascals (MPa), F is the failure load in newtons (N) and A is the surface area in squaremillimeters (mm2).)

Failure mode

The bond surfaces of the failed specimens were examined using a stereomicroscope32 (Olympus, SZ-PT: Japan) at 100X magnification. The failure modes were classified as:

a) Cement/Dentin bonding failure (Dentin adhe-sive).

b) Substrate cohesive failure (Dentin Cohesive).

Statistical Analysis:

Statistical analysis was performed with IBM® SPSS® (SPSS Inc., IBM Corporation, NY, USA) Statistics Version 22 for Windows. Data were presented as mean and standard deviation (SD) values. Data explored for normality using D’Agostino-Pearson test for Normal distribution for both translucency parameter (∆E) and shear bond strength. (∆E) showed normal distribution, so Independent t-test was used to study the effect of different ceramics on translucency parameter.

Regarding shear bond strength (MPa), it showed normal distribution, and Two Way-ANOVA was used to study the effect of different ceramics and adhesive strategies on mean Shear bond strength (MPa). Tukey’s post-hoc test was used for pair-wise comparison between the means when ANOVA test is significant. One Way ANOVA was used to compare between the interactions between variables. The significance level was set at P ≤ 0.05.

RESULTS

Translucency results

Means and standard deviations (SD) of ΔE for different Ceramics were presented in table (3) and figure (5). IPS e.max CAD ceramic discs presented statistically significant lower mean ΔE (20.41±0.41) compared to Vita Suprinity ceramic discs (22.43±0.69) at p≤0.001.

Fig.(4): Shear bond strength testing.


TABLE (3) Means and standard deviations (SD) of ΔE for different ceramics.

Group

p-valueVita Suprinity IPS e.max CAD

Mean SD Mean SD

ΔE 22.43 0.69 20.41 0.41 ≤0.001*

*=Significant

Shear bond strength results:

1. Two Way-ANOVA used to study the effect of different ceramics and adhesive strategies on mean shear bond strength (MPa):

Two Way ANOVA showed that different ceramics and adhesive strategies had a significant effect on mean shear bond strength (MPa) at P≤0.001. The interaction between different ceramics and adhesive strategies presented a significant difference on mean shear bond strength (MPa) at p=0.011 (table 4).

2. Effect of different ceramics and adhesive strate-gies on mean shear bond strength (MPa) within each variable:

Mean and standard deviation (SD) of shear bond strength (MPa) for different ceramics and adhesive strategies were presented in table (5) and figure (6 and 7).

Effect of different ceramics

For total-etch adhesive strategy, IPS e.max CAD ceramic had statistically significant lower mean

Fig (5): Bar chart showing mean (∆E) for different ceramics.

TABLE (4) Two Way ANOVA used to study the effect of different ceramics and adhesive strategies on mean shear bond strength (MPa)

SourceType III Sum of

Squaresdf Mean Square F Sig.

Corrected Model 1749.987a 3 583.329 322.949 ≤0.001*

Intercept 7812.234 1 7812.234 4325.096 ≤0.001*

Ceramics 85.291 1 85.291 47.220 ≤0.001*

Adhesive 1649.620 1 1649.620 913.281 ≤0.001*

Ceramics * Adhesive

15.076 1 15.076 8.347 0.011*

Error 28.900 16 1.806

Total 9591.121 20

Corrected Total 1778.887 19

(df= degrees of freedom, Sig. = Significant (Probability level), *= Significant at p≤0.001 and NS= Non-Significant)


shear bond strength (25.91±1.96MPa) compared to Vita Suprinity ceramic (31.78±1.73MPa) at p=0.001. Regarding self-adhesive strategy, IPS e.max CAD showed statistical significant lower mean shear bond strength (9.49±0.48 MPa) compared to Vita Suprinity (11.88±0.42 MPa) p≤0.001. (table 5, fig 6)

Effect of different adhesive strategies:

As for IPS e.max CAD ceramic, total-etch adhesive strategy showed statistically significant higher mean shear bond strength (25.91±1.96 MPa) compared to self-adhesive strategy (9.49±0.48 MPa) at p≤0.001. Regarding Vita Suprinity ceramic, the same results were obtained where total-etch adhesive strategy presented statistically significant

higher mean shear bond strength (31.78±1.73MPa) compared to self-adhesive strategy (11.88±0.42MPa) p≤0.001. (table5, fig 7)

3. Effect of different groups on mean Shear bond strength (MPa):

Means and standard deviations (SD) of shear bond strength (MPa) for different groups were presented in table (6) and figure (8).

Vita Suprinity ceramic discs cemented using total-etch adhesive strategy showed the highest mean Shear bond strength (31.78±1.73MPa) followed by IPS e.max CAD ceramic discs cemented with total-etch adhesive strategy (25.91±1.96MPa) followed by Vita Suprinity ceramic discs cemented with self-adhesive strategy (11.88±0.42MPa) followed by

TABLE (5): Means and standard deviations (SD) of mean shear bond strength (MPa) for different ceramics and adhesive strategies.

Ceramic materialsp-valueIPS e.max CAD Vita Suprinity

Mean SD Mean SD

Shear bond strength (MPa)

Total etch 25.91 1.96 31.78 1.73 0.001*Self-adhesive 9.49 0.48 11.88 0.42 ≤0.001*

p-value ≤0.001* ≤0.001*

*=Significant

Fig (6): Bar chart showing the mean Shear bond strength (MPa) for different ceramics.

Fig (7): Bar chart showing the mean Shear bond strength (MPa) for different adhesive strategies.


IPS e.max CAD ceramic discs cemented using self-adhesive strategy (9.49±0.48MPa) with a significant difference between each other’s at p≤0.001.

TABLE (6): Means and standard deviations (SD) of mean shear bond strength (MPa) for different groups.

Shear bond strength (MPa) Rank p-valueMean SD

Inte

ract

ion

IPS e.max CAD +Self-adhesive

9.49 0.48 d

≤0.001*

Vita Suprinity +Self-adhesive

11.88 0.42 c

IPS e.max CAD +Total-etch

25.91 1.96 b

Vita Suprinity +Total-etch

31.78 1.73 a

Means with the same letter within each column are not significantly different at p=0.05.

*=Significant

Failure mode results:

With regard to the type of failure, adhesive failure was reported between cement and dentin in all self-adhesive groups for both ceramic materials. (Figure 9) As for total-etch groups, the failure mode was cohesive within dentin substrate for both ceramic materials. (Figure 10)

DISCUSSION

Ceramic was one of the primary materials used as a definitive anterior esthetic restorative material, due to its color and optical properties; simulating natural teeth, good wear resistance, and color stability.33-36 Lately, the manufacturers claim that the newly introduced all-ceramic systems in dentistry have translucent properties comparable to feldspathic porcelains along with improved mechanical resistance.37 Accordingly, for a correct selection, longevity and esthetics have to be considered from the main parameters.38

Fig (8) Bar chart showing the mean Shear bond strength (MPa) for different groups.

Fig (9): Dentine adhesive failure of self-adhesive cement groups.

Fig (10): Dentine cohesive failure of total-etch cement groups.


Lithium disilicate ceramic material which has superior esthetics and durability is considered one of the important ceramic materials available nowadays. The translucency and light diffusion property of IPS e.max ceramics were reached to replicate natural tooth structure for esthetic undetectable restoration.39 New additions to the category of glass ceramics as being classified in an updated classification in 2015 are the zirconia-reinforced lithium disilicate 40. Initial in vitro testing of the ZLS showed a positive combination of the material characteristics of zirconia and glass-ceramics, however these materials are new to the market.40 Therefore, the study reported here aimed to assess the effect of zirconia addition to lithium disilicate ceramic on translucency and bond strength using total-etch and self-adhesive systems.

The null hypothesis of this study that addition of zirconia to lithium disilicate would not affect the translucency and the bond strength of the restoration to either total-etch and self-adhesive resin cements must be rejected.

Regarding the translucency parameter (TP), it was found that zirconia-reinforced lithium disilicate glass-ceramic discs showed statistically significant higher mean translucency (22.43±0.69) compared to lithium disilicate glass-ceramic discs (20.41±0.41) at p≤0.001. The results of the current study was consistent with many other studies,11,41-49

that demonstrated that different porcelains showed different translucencies.

The significantly higher translucency of zirconia-reinforced lithium disilicate glass-ceramic may be due to the addition of zirconia and the ensuing nucleation process, resulting in more homogenous crystalline structure and finer crystal size (0.5 µm) compared to the needle-shaped coarser crystalline structure (1.5 µm) of lithium disilicate glass- ceramic.50

This was found in accordance with Heffernan et al,11,43 who stated that the amount of light absorbed, reflected and transmitted is dependent on several

factors including the particles’ size compared to the incident light’s wavelength (0.4-0.7µm), as porcelain translucency increases when the size of particles decreases in its composition, irregularities in the distribution of the phases, and optical anisotropy of the grains. However, the translucency results of this study was not consistent with Giordano,38 and Denry,51 who claimed that decrease in glass content in ceramics results in greater opacity, despite of the fact that the IPS e.max CAD has higher glass content (57-80%) than the Vita Suprinity (56-64%) before crystallization, so assumingly after crystallization, the former will still have higher glass content than the latter, however, this needs further investigations.

Proper adhesive cementation is important between indirect ceramic restorations and dental tissues especially for minimally retentive preparations.32,52 Adhesive luting agents increased the fracture resistance of all ceramic materials through its penetration into the irregularities of the restoration fitting surface inhibiting crack propagation. The stresses at the interface of the tooth-cement-restoration are complex as they may be primarily tensile or shear and are created by forces working either perpendicular or parallel to the tooth surface.53 Therefore a dental adhesive must be able to provide retention for a variety of materials;40 to tooth structure through the formation of hybrid layer between the resinous material and dental tissues from one side and micro-retention and/or chemical bonding with ceramic material from the other side.52,54

For the shear bond strength of the two tested materials (Vita Suprinity and IPS e.max CAD) using different adhesive strategies (total-etch and self-adhesive techniques), specimens were subjected first to thermocycling where they were subjected to 2000 thermal cycles between 5°C and 55°C (Willytec thermocycler, Germany) with a dwell time of 30 seconds in each bath, in order to simulate the physical process in the oral environment that more frequently influences the integrity of the bonding union between restorative materials and luting agents.55


Regarding the effect of ceramic material used, it was found that the shear bond strengths (SBSs) values of Vita Suprinity are significantly higher than the IPS e.max CAD ceramic, using both adhesive strategies. This could be also explained by the homogeneous, fine crystalline structure with an average crystal size of 0.5 μm of the Vita Suprinity, compared to a structure with needle-shaped crystals and an average crystal size of 1.5 μm of the IPS e.max CAD. The smaller particle size of the former compared to the latter for the same given surface area might have led to increased amount of pores, increasing the surface area available for bonding.50 These results were in agreement with Della-Bona,56 who concluded that the bond strength of resin-bonded ceramics is affected by their microstructure which influenced composite ceramic adhesion zone. However, this was not in agreement with many authors who claimed that ZrO2 addition is accompanied by a reduction in the glassy matrix and Si content,57, 58 resulting in more acid-resistant ceramics.59

The choice of the resin cementation system for a given clinical application is critical. Conventionally, adhesive luting protocols include etching of the tooth structure followed by rinsing and application of an adhesive system to tooth substrate. Lately, self-adhesive cements were introduced to simplify the cementing technique as they minimize the clinical steps.32

Regarding the effect of different adhesive strategies, it was found that the total-etch adhesive strategy with both tested ceramic materials (Vita Suprinity and IPS e.max CAD) showed statistical significant higher mean shear bond strength compared to self-adhesive one. The low bond strength values and high adhesive failure between cement and dentin in all self-adhesive groups for both ceramic materials were obvious. This was found in agreement with Holderegger et al,60 who stated that self-adhesive cements have low bond strength compared to total-etch cements,60,61 which may be due to the minimum demineralization effect with

self-adhesive cements on the dentin substrate, with no decalcification recongnized.62,63 This was found also in accordance to Behr et al,64 who demonstrated that there was minimal interaction with the substrate, especially when dentin is observed, which may be attributed to its great quantities of the glass particles and also to the high viscosity of cement that hinders the wetting and infiltrating of the dentin surface by the luting agent leading to this weak interaction. On the contrary, Hikita et al, 65 concluded that self-adhesive systems were superior in dentin only in one case where enamel etching was done before cementation.

For the total-etch group the failure was exclusively cohesive within the tooth structure, thus the application of acidic conditioning on the dentin substrate followed by the application of an adhesive promoted the formation of hybrid layer enabling better bonding to dentin substrate. However, in the self-adhesive group the failure was adhesive at the cement-tooth interface. This may be explained by the superficial interaction of the cement due to chelation of calcium ions by acid groups, which does not promote formation of a hybrid layer or resin tags in the dentinal tubules. This was in agreement with many studies.66-69

Regarding the bonding ability of different adhesive strategies and ceramics used, there was no adhesive failure at the ceramic-cement interface for all groups. These findings prove the excellent micromechanical retention between the ceramic surface for either IPS e.max CAD or Vita Suprinity and cement after hydrofluoric acid etching as well as chemical bonding after silane application. This finding is consistent with many previous studies for lithium disilicate ceramics.70-72 Moreover, Rigolin et al, 54 proved that self-adhesive cement did not significantly differ regarding bond strength to lithium disilicate glass-ceramic from conventional dual resin cement. On the contrary, Hooshmand et al,73 demonstrated that total-etch resin cements had better bonding ability to lithium disilicate glass-ceramic compared to self-adhesive resin ones.


It is of value to mention that the specimen prepa-ration methodology of this study was more clinical-ly relevant, this means that the authors were trying to simulate the clinical conditions through cementa-tion of the ceramic discs to dentin. Thus, there were two major interphases: tooth/cement and ceramic/cement interphases. However, the tooth/cement seems to be the weakest link within this complex. Some studies52, 74 preferred to test each interphase alone to limit the variables and reach the best bond-ing strategy for each interphase. Others 32, 54 sug-gested the examination of the complex as a whole for creating actual clinical environment. This was a limitation in this study and further investigation regarding the cement/ceramic bond is needed.

Finally, more future clinical studies should be carried out as such type of laboratory studies does not eliminate the need for clinical ones.

CONCLUSIONS

Under the test conditions, Zirconia-reinforced Lithuim disilicate ceramic demonstrated better translucency and shear bond strength, when compared to Lithuim disilicate ceramic. Meanwhile, total etch dual-cured adhesive resin cement showed better bonding ability between ceramic and dentin, when compared to self-adhesive ones.

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EFFECT OF ZIRCONIA ADDITION TO LITHIUM DISILICATE … 2020. 11. 24. · for three minutes. The silane bonding agent (ESPE Sil, 3M ESPE, Germany) was then applied to the bonding surface