Journal of Oral Research and Review

ORIGINAL ARTICLE
Year
: 2021  |  Volume : 13  |  Issue : 2  |  Page : 86--93

Effect of thermocycling on shear bond strength of cemented zirconia-reinforced lithium silicate (Celtra Duo)


Kamilia Faisal Abdulkader1, Lamia Sayed Kheiralla2, Gihan Abd Elhady Elnaggar2,  
1 Department of Fixed Prosthodontic, Cairo University, Giza, Egypt
2 Department Fixed Prosthodontics, Faculty of Dentistry, Cairo University, Giza, Egypt

Correspondence Address:
Kamilia Faisal Abdulkader
Department of Fixed Prosthodontic, Cairo University, Giza
Egypt

Abstract

Purpose: The aim of the present in vitro study is to evaluate the effect of thermocycling on the shear bond strength (SBS) of cemented zirconia-reinforced lithium silicate (ZLS) discs (Celtra Duo). Materials and Methods: Thirty-six Celtra Duo ceramic discs cemented (n = 36) to composites discs background (Master dent) by adhesive resin cement (Biscem). The baseline group (n = 12) did not subject to thermocycling, the second group (n = 12) was subject to 2500 thermal cycle, and the third group was subject to 5000 thermal cycles, then SBS and the failure mode were evaluated. Results: There was a statistically significant difference between the groups (P = 0.001) where the value of SBS (Mpa) was significantly decreased in both 2500 thermocycling group (P = 0.021) and 5000 thermocycling group (P = 0.001) when compared with their corresponding level measured at baseline group. In addition, there was a significant decrease in bond strength of 5000 thermocycling group when compared with its corresponding value in the 2500 thermocycling group (P = 0.001). Conclusion: Based on the results of the present study and within the limitations of this in vitro study, it can be concluded that the SBS of cemented ZLS restorations will be decreased when samples are subjected to thermocycling to simulate the clinical situation.



How to cite this article:
Abdulkader KF, Kheiralla LS, Elhady Elnaggar GA. Effect of thermocycling on shear bond strength of cemented zirconia-reinforced lithium silicate (Celtra Duo).J Oral Res Rev 2021;13:86-93


How to cite this URL:
Abdulkader KF, Kheiralla LS, Elhady Elnaggar GA. Effect of thermocycling on shear bond strength of cemented zirconia-reinforced lithium silicate (Celtra Duo). J Oral Res Rev [serial online] 2021 [cited 2021 Dec 5 ];13:86-93
Available from: https://www.jorr.org/text.asp?2021/13/2/86/319006


Full Text



 Introduction



Ceramics in dentistry are denoted to nonmetallic, inorganic structures mainly containing compounds of oxygen with one or more metallic or semi-metallic elements such as calcium, lithium, aluminum, magnesium, potassium, silicon, phosphorus, sodium, zirconium, and titanium. Ceramics are used to gain optimal esthetic results in clinical practice.

In addition to lithium oxide and silicon dioxide, Celtra contains about 10% zirconium dioxide (ZrO2) in highly dispersed form in the glass phase of the ceramic, which prevents crystallization of the zirconium oxide. This unique dispersion provides it with high translucency and opalescence and avoids the opaquer look of zirconium oxide ceramics.

The fully crystalized Celtra Duo zirconia-reinforced lithium silicate (ZLS) block contains high glass content and is the only material that can be processed in two different ways, providing the clinician complete control for each individual restorative case.

Its unique capacity allows it to be processed in two ways – either milled and polished or milled and fired. This dual pathway component renders complete clinician control in determining the appropriate solution to apply for each individual restorative.

A durable bonding between restorations and teeth is mandatory, especially in some clinical situations such as short clinical crowns or over tapered teeth.[1]

Moreover, thermocycling is an approved method used to simulate clinical performance and long-term durability of tooth/restoration complex.

Most studies that apply these methods reveal significant differences between early and late bond strength values.[2]

Therefore, the aim of the present in vitro study was to evaluate the effect of thermocycling on the shear bond strength (SBS) of cemented ZLS discs (Celtra Duo).

The null hypotheses

SBS of cemented ZLS discs (Celtra Duo) will not be affected by different thermal cycles.

 Materials and Methods



The name, composition, and manufacturer of different materials used in the present study are illustrated in [Table 1]. A ZLS block of size 14 was carefully chosen to be used for disc sample construction. ZLS discs were cut by using low-speed micro-saw Isomet (Isomet 4000, Buehler, U.S.A) with water coolant to gain an even size of 10 mm diameter and 2.2 thickness discs.[3]{Table 1}

The discs thickness was checked using digital caliber Digimatic (Mitutoyo corporation) at different points. Thirty-six specimens were milled (n = 36). All discs were polished by using a polishing kit according to the manufacturer's instructions, resulting in 2.00 mm thick discs. The final thickness was measured using a digital caliper Digimatic at different points, with an accuracy of ± 0.01 mm [Figure 1]. All discs samples were ultrasonically washed by distilled water for 10 min (EasyClean Ultrasonic Cleaner, Renfert Gmbh and Co, Hilzingen, Germany) and dried with compressed air. The ceramic disc samples were glazed according to the manufacturer's instruction using furnace (Ivoclar Vivadent) P310 following their prescribed chart as manufacturer's instructions. The outer untreated surface was marked, to be easily identified from the treated surface.{Figure 1}

All samples were numbered from 1 to 36 then were divided by website (www.Random.org) into 3 equal groups according to the method of surface treatment used.

Construction of composite background

A composite background for the ceramic discs was formed by constructing a Teflon mold with an inner hole to produce composite discs with dimensions 5 mm diameter and 2 mm thickness. Micro-hybrid composite (Master Fill Biodinamica USA) was applied and cured inside the mold between two glass slabs. It was cured for 60 s using LED light cure unit with a light intensity of 1500 mW/cm2 (TY309 LED Root Toye). After curing, all composite disc samples were finished and polished to be ready for cementation to the ceramic discs.

Surface treatment of the samples

Acid etching of the ceramic discs

All ceramic discs surfaces were etched with a 10% hydrofluoric acid gel (Bisco-etch Bisco Inc., Schaumburg, IL, USA) for 20 s. A stopwatch was used to standardize the etching time for all samples. Then, etched surfaces were washed under a copious amount of water for 60 s then air-dried for 60 s.

Examination of the ceramic surfaces

Examination of etched surfaces was done by USB Digital microscope (U500X Capture Digital Microscope, Guangdong, China). Specimens were photographed using a USB Digital microscope. Subsequently, a three-dimensional image of the surface profile of the specimens was created. WSxM software was used to calculate an average of heights (Ra) expressed in micrometer which can be assumed as a reliable index of surface roughness.

Cementation of the disc samples to the composite background

A specially designed cementing device was constructed to ensure standardization of the load applied and its direction during cementation.

It is constructed of an elevated cylinder with a hall of 10 mm width and 2 mm depth to receive the discs assembly, loaded rod to transfer the load to the samples, and load holding plate that carried a load of 500 g.[4]

Silanization of etched ceramic surfaces was done by applying the silane coupling agent for 1 min and air-dried, then another layer was applied according to the manufacturer's instructions. Each ceramic specimen was fixed in place in the central hole of the metal base to be cemented by using adhesive resin cement to the composite disc substrates.

Biscocem dual-cure luting pastes were auto mixed and dispensed to the center of the bonding surface of the ceramic discs. Then, each composite disc was centralized on the ceramic disc. Constant load of 500 g was applied on the upper surface of the assembly using the specially constructed cementing device, initial curing for 2 s was applied, then excess cement was removed using stainless steel carver followed by final curing for 60 s from four sides of the disc circumferentially parallel to the cement interface, and each quadrant received 60 s curing time to ensure optimal polymerization of the resin cement [Figure 2].{Figure 2}

The weight was removed after 4 min, then the assembly was removed from its place in the holding device. All specimens were stored in distilled water at room temperature for 24 h.

Thermocycling procedure

Ceramic/composite assembly samples were divided into three groups: first group is composed of 12 assembly disc specimens which were not subjected to thermocycling (baseline group).

The second group is composed of 12 assembly disc samples which were subjected to thermocycling (Robota automated thermal cycle; BILGE, Turkey) for 2500 cycles, at 5°C to 55°C, with dwell times of 25 s in each water bath with a lag time 10 s, as recommended by[5] Al-Thagafi et al. 2016. A 2500 thermal cycle is equivalent to 6 months intraorally, as stated by Bayne, 2011 [Figure 3].[6]{Figure 3}

The third group is composed of 12 assembly disc samples which were subjected to thermocycling for 5000 cycles, at 5°C to 55°C, with dwell times of 25 s in each water bath with a lag time of 10 s. A 5000 thermal cycle is equivalent to 1 year intraorally, as stated by Bayne, 2011.[6]

Shear Bond Strength test

Testing procedure

A universal testing machine (Model 3345; Instron Industrial Products, Norwood, USA) was used to evaluate the SBS between ceramic and composite discs. A circular interface shear test was designed to evaluate the bond strength [Figure 4]. All samples were individually and vertically mounted on a computer-controlled material testing machine with a load cell of 5 kN and data were recorded using computer software (Bluehill Lite; Instron Instruments). Failure was manifested by an audible sound and evidence of debonding between the cemented discs.{Figure 4}

SBS test was done after 24 h of water storage to ensure complete polymerization of the resin cement for the first group (baseline group), the second group was subjected to SBS test after 2500 thermal cycles, and the third group was subjected to SBS test after 5000 thermal cycles.

Shear bond strength calculation

The load at failure was divided by bonding area to express the bond strength in MPa τ = P/πr2

where τ = shear bond strength (MPa), P = load at failure (N), π = 3.14, and r = radius of disc (mm). These tests were performed using software (Bluehill Lite Software from Instron).

Statistical analysis

The results were expressed as mean ± standard deviation or number (%). A comparison between categorical data (number [%]) was performed using Chi-square test. Test of normality, Kolmogorov–Smirnov test, was used to measure the distribution of SBS data. Accordingly, data were not normally distributed, so a comparison between the three groups was performed using Kruskal–Wallis ANOVA test followed by Mann–Whitney test if significant results were recorded. Chi-square test was used to detect significance between failure mode patterns. The significance level was set at P ≤ 0.05 and 95% Confidence interval. Statistical Package for Social Sciences (SPSS) computer program (version 19 windows) was used for data analysis. P ≤ 0.05 was considered significant.

 Results



Results of shear bond strength testing

The mean value of SBS (Mpa) in the baseline group, in the 2500 thermocycled group, and the one after 5000 thermocycled group were 13.34 ± 1.94, 10.68 ± 2.61, and 6.21 ± 2.50 MPa, respectively. There was a statistically significant difference between the groups (P = 0.001) where the value of SBS (Mpa) was significantly decreased in both 2500 thermocycled group (P = 0.021) and 5000 thermocycled group (P = 0.001) when compared with their corresponding level measured at baseline group. Furthermore, there was a significant decrease in bond strength of the 5000 thermocycled group when compared with its corresponding value in the 2500 thermocycled group (P = 0.001) [Table 2] and [Figure 5].{Table 2}{Figure 5}

In baseline group, mixed failure was present in 10 samples (83.3%), and cohesive mode of failure was present in 2 samples (16.5%). In 2500 thermocycled group, mixed, cohesive, and adhesive modes of failure were present in 6 (50.0%), 2 (16.7%), and 4 (33.3%) of samples, respectively, while in the 5000 thermocycled group, they were present in 2 (16.7%), 7 (58.3%), and 3 (25.0%) of samples, respectively. There was no statistically significant difference between baseline and 2500 thermocycled groups (P = 0.082). On contrary, the percent of mixed failure in 5000 thermocycled one (2 [16.7%]) was significantly lower than in baseline group (10 [83.3%]) (P = 0.004). Again, there was no statistically significant difference between the percent of mixed failure in 2500 thermocycled groups (6 [50%]) and its corresponding value in 5000 thermocycled groups (2 [16.7%]) (P = 0.085) [Table 3] and [Figure 6].{Table 3}{Figure 6}

 Discussion



The retentive strength and the clinical durability of a ceramic restoration are determined by the quality and nature, of the ceramic resin interface bonding with changing intraoral conditions in part.[7] Morphology of the ceramic surface plays a role in the micromechanical retention to develop a durable bond strength of the resin cement to the ceramic surface in other parts.[8]

Consequently, the current in vitro study was made to estimate the SBS and longevity of cemented ZLS (Celtra Duo) ceramics subjected to thermocycling to simulate the intraoral condition.

In the current study, the null hypothesis was rejected, as there was a statistically significant difference in SBS when cemented ZLS glass-ceramic (Celtra DuO) discs were subjected to different thermocycling cycles.

The ZLS (Celtra Duo) was chosen to be tested as it is a new material that was claimed to combine the characteristics of zirconia (ZrO2) and glass ceramic. The zirconia particles are incorporated to reinforce the ceramic structure by crack disruption. It has been supposed that the structure which is found after crystallization, shows improved mechanical characteristics and accomplishes the highest esthetic requirements at the same time.[9]

Thermal cycling is one of the main broadly utilized procedures to mimic the physiological aging experienced by biomaterials in clinical implementation; accordingly, it is regularly used in experimental studies to assess materials' performance.[10]

Thermal cycling was done for the second and third groups with different thermal cycles, as the second group was subjected to 2500 thermocycles which is equivalent to 6 months intraorally, while the third group was subjected to 5000 thermocycles which is equivalent to 1 year intraorally to detect the effect of variable intraoral conditions in the mouth on the SBS.

Hydrofluoric acid etching is a modifier and etching promoter for silica-containing ceramics to modify the vitreous phase, exposing crystals and resulting in microporosities on the ceramic structure. This attends to amplify surface area and improve bonding value which delivers better contact between the restoration material and the resin cement.[11]

Formation of Hexafluorosilicates as a result of a reaction between the Hydrofluoric acid and the silica containing glass matrix. As a consequence of the elimination of the glass matrix, the crystalline structure is uncovered in the surface of the ceramic therefore the ceramic surface becoming rough, which is required for micromechanical retention. Its rough surface also provides more surface energy prior to combining the silane solution.[12] To ensure the success of acid etching surface treatment, the etchant concentration was adjusted to be 5% for 20 s as recommended by the manufacturer.[13]

Bis-Silane Porcelain Primer which is a two-part silane coupling agent helps to enhance bonding between porcelain restorations and resin cements. It offers additional shelf-life stability to ensure long-lasting effective bonding to porcelain, it also protects porcelain restorations from contamination, increases mechanical and chemical bonding of resin to the porcelain, and yields greater resistance to water leakage at the bonding interface.[14]

To standardize cementation load, a special loading device with a constant load of 500 g was used during cementation to ensure standardization of pressure adjusted on the samples throughout cementation. Standard load application aids the cement film thickness to be free from any structural flaws.

The resin cement used in this study was Biscem (Bisco, Schaumburg, IL, USA), which is a dual-cured auto-mix self-adhesive resin cement. This cement was carefully chosen since it has different cementation techniques and is largely used in clinical practice for cementation of indirect restorations because it has low solubility and high mechanical properties. It contains base: Bis-GMA >10%, uncured dimethacrylate monomer <20%, glass filler <50%, catalyst: phosphate acidic monomer <10%, and glass filler <50%, as reported by the manufacturer.

Self-adhesive cements make the bonding technique easier and simple by eliminating the several steps needed for total etch cements. Yet, bond strengths differ among specific cements, but total etch cements generally provide the highest preservation of bond, self-etching systems are midway, and self-adhesive cements can deliver almost similar bond strength to self-etching systems.

Self-adhesive cements provide more retention than resin-modified glass ionomers and are particularly useful with high strength all-ceramic restorations.[15]

Long-term water storage at a constant temperature or thermal cycling is the condition most often used to simulate aging of resin bond.[16] Therefore, the current study evaluates the effect of thermocycling on SBS as it simulates clinical conditions following 2500 thermocycles (equivalent to 6 months intraorally) and 5000 thermocycles (equivalent to 1 year intraorally), compared to the baseline group.[6]

The SBS test has the lowest operational defect and is the simplest to accomplish. In addition, shear force is practiced in the clinical condition; therefore, shear test is the most common method used to evaluate adhesive bonding.[17]

The results of the SBS of cemented Celtra Duo discs with and without thermocycling showed statistically significant difference, where the value of SBS (Mpa) was significantly decreased in both 2500 thermocycled (P = 0.021) and 5000 thermocycled groups (P = 0.001) when compared with their corresponding level measured at baseline group. In addition, it was significantly decreased in the 5000 thermocycled group when compared with its corresponding value in the 2500 thermocycled group (P = 0.001).

This could be attributed to two important factors; it might be due to degradation of the luting cement itself[18],[19] and the hydrolytic effect of water at the luting cement/ceramic interface due to thermal expansion of the bonded specimens[20],[21] (Celtra Duo luting agent and composite resin), which could result in hoop stress during thermocycling.[21] Another factor could be the fact that silanized surfaces were unstable in contact with moisture, as reported by Derand et al.[20]

However, we should not relay on the bond value as indicator for the strength of the bonding data alone, since the failure mode is also important; as it is considered an estimation of clinical performance. Following the shear bond testing procedure,[22] all the samples were observed below a USB digital microscope to recognize the nature of bond failure. The mode of failure was classified as follows: Mode 1: [Figure 7] shows adhesive mode of failure at the ceramic/resin interface, indicating lower bond strength as shown in 33% of the second group (2500 thermocycles) and 25% of the third group (5000 thermocycles), while the first group (baseline) reported 0% adhesive failure mode. Mode 2: [Figure 8] shows a cohesive mode of failure within either the ceramic or the composite, as shown in 2% of the first group (baseline) and of the second group (2500 thermocycles), while the third group (5000 thermocycles) reported 58% cohesive failure mode. Mode 3: [Figure 9] shows mixed mode of failure, involving composite, ceramic, and cement as shown in 83% of the first group (baseline), 50% of the of the second group (2500 thermocycles), and 16% of the third group (5000 thermocycles), which supported the bond strength values of each group. Since the baseline group with higher bond strength mean value showed 83% mixed mode of failure which emphasized the higher SBS values obtained with this group. Moreover, we can notice the decrease in the mixed mode of failure percentage as the bond strength decreases in the second (2500 thermocycles) and third groups (5000 thermocycles).{Figure 7}{Figure 8}{Figure 9}

Almost all literature support the current study which denoted that thermal aging significantly decreases the ceramic resin bond strength.[18],[19],[20],[21],[22]

In 2011, Heikkinen[23] clearly expressed that water storage in combination with thermocycling will decrease the shear bond up to 60%–85%; in addition, Peumans et al., 2016, stated that the bond strength decreases with time.[24]

Further in vitro studies are recommended to demonstrate the clinical longevity of Celtr Duo glass ceramic.

 Conclusion



Based on the results of the present study and within the limitations of this in vitro study, it can be concluded that the SBS of cemented ZLS restorations will be decreased when samples are subjected to thermocycling to simulate the clinical situation.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

Ethical clearance

Ethical from Cairo university ethical committee for an in vitro study.

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