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30 January 2025: Lab/In Vitro Research  

Effect of Multiple Firing Cycles on Shear Bond Strength Between Zirconia and Lithium Disilicate

Mohammad Zarbah1ABEG, Omir Aldowah2CDE*, Saeed M. Alqahtani3ABF, Maha S. Alabsi4ACF, Elmaha N. Hidah4BDF, Hanan Q. Alfafi4ACF, Nawal A. Alshehri4ABDF

DOI: 10.12659/MSM.946762

Med Sci Monit 2025; 31:e946762

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Abstract

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BACKGROUND: Indirect ceramic restorations often need multiple firings to match the shade of natural teeth or need after-correction and ceramic addition during the clinical trial stage. Many studies have examined how multiple firings affect the mechanical characteristics of zirconia-veneered prostheses. The effect of firing number on adhesion between these core and heat-pressed lithium disilicate veneering ceramics is unclear. This study aimed to evaluate and compare the effects of repeat firings on the planar interface shear bond strength (SBS) using the Schmitz-Schulmeyer test between zirconia core and veneering heat-pressed lithium disilicate.

MATERIAL AND METHODS: Forty standard-sized rectangular zirconia framework specimens were fabricated using the Schmitz-Schulmeyer test, a test to assess bonding quality, and categorized into 4 groups: control, 1 firing cycle, 2 firing cycles, and 3 firing cycles. The additional firing cycles aimed to simulate clinical scenarios. SBS was tested using a universal testing machine.

RESULTS: The study found that mean SBS values were similar across experimental groups, with the control group showing slightly higher values. Analysis using ANCOVA revealed no significant difference in mean SBS between groups, but maximum load significantly influenced SBS. Regression analysis and Pearson correlation confirmed the significant relationship between maximum load and SBS (P<0.0001).

CONCLUSIONS: There was no apparent effect on the SBS from increasing the firing from 1 to 3 cycles, indicating that the study’s hypothesis was not supported. The mode of failure, which was largely adhesive, did not tend to vary as the number of firing cycles rose, despite the fact that some samples in all groups showed mixed failure.

Keywords: Adhesives, Shear Strength

Introduction

All ceramic dental indirect restorations, which are fabricated in the dental laboratory, have been continually improved in the last few decades. Most restorative dentists prefer this kind of material because of their superior esthetics, enhanced strength, and biocompatibility. However, the brittleness of some types of ceramics, such as glass-matrix lithium disilicate, can lead to premature failure and fracture. Thus, due to its high fracture strength, yttria-stabilized tetragonal zirconia (Y-TZP) has been used as the core material to overcome the low fracture strength of glass-matrix ceramic. The combination of these ceramic materials generates restorations, crowns, and fixed denture prostheses that have the advantages of being high esthetic and resistant to occlusal forces [1]. Despite the widespread use of this multi-layered ceramic restoration, chipping and delamination of the veneering ceramic have been reported in the literature [2–6] as the most common mechanical complications, since the bond at the interface between zirconia core and veneering glass ceramic is considered weak due to different thermal and chemical characteristics. One of the causes of ceramic chipping appears to be the growth of internal stresses, because zirconia is a brittle material and less capable of mitigating them. Hence, it is crucial to minimize the stresses of the restorative process as much as possible. This also refers to the stresses that arise from external factors, such as chewing, grinding, and clenching. The ceramic fracture has been asserted to be a cohesive fracture. Therefore, it can be ruled out that there is a lack of bond strength between the veneering and the framework material [7–9]. However, the veneered ceramic in the previous studies was mainly feldspathic ceramic.

Multiple firings of indirect ceramic restorations are required in many cases to mimic the shade of different areas of a natural tooth or after the adjustment and ceramic addition at the clinical try in stage. Several studies have been conducted on the effects of multiple firings on the mechanical properties of zirconia-veneered prostheses [10–12]. However, to the best of our knowledge, the effect of the number of firings on the adhesion between these specific core and heat-pressed lithium disilicate veneering ceramics is not totally clear and needs to be clarified. Shear bond strength (SBS) is essential to examine interfaces between 2 materials, such as ceramic and metal [13]. Therefore, this study aimed to evaluate and compare the effects of repeat firings on the planar interface SBS using the Schmitz-Schulmeyer test between zirconia core and veneering heat-pressed lithium disilicate. The hypothesis of this study was that multiple firing cycles have the potential to adversely affect the SBS of zirconia core to heat-pressed veneering lithium disilicate.

Material and Methods

PREPARATION OF THE SAMPLES:

A zirconia framework of specified dimensions was created with CAD/CAM technology. Forty standard-size rectangular zirconia framework specimens with dimensions of 10 mm in length, 5 mm in width, and 5 mm in thickness were milled from pre-sintered zirconia blocks (Ceramill ZI, Amann Girrbach AG) following the CAD design, to assemble the samples based on the Schmitz-Schulmeyer model. The zirconia framework specimens were cleaned using an ultrasonic cleaner for 10 min. A 5×5×5 mm wax pattern for the veneering lithium disilicate block was produced on the lowest section of the zirconia sample framework using inlay wax. The wax pattern was placed in a phosphate-bonded investment substance called IPS Pressvest premium. The investment was subjected to dewaxing in the furnace after being set, followed by preheating the investment material to a temperature of 403°C. The sintering unit, Programat EP 3000 by Ivoclar, was preheated to 730°C prior to placing the investment and the hot press ceramic ingot (IPS e.max Press). Once the temperature reached 900°C, the ingots were automatically forced into the mold cavity against the core connection surface. The sprue channels were cut, and the measurements of the veneering materials were verified. The veneered samples were fired once at the pressing temperature (Figure 1). The 40 samples were randomly divided into 4 groups based on the amount of repeated firing cycles: (A) control group, (B) 1 firing cycle group, (C) 2 firing cycles group, and (D) 3 firing cycles. Each group contained 10 specimens. For the repeated firing cycles groups (B, C, and D), the additional firing procedures were done 1, 2, and 3 times, respectively, after application of IPS e.max Ceram (Ivoclar Vivadent) on the veneering lithium disilicate to mimic the clinical scenarios. SBS was tested using a universal testing machine.

TESTING AND ANALYZING THE SAMPLES:

Specimens were placed and immobilized in a custom-made acrylic resin holder. The apparatus of the universal testing machine was positioned perpendicular to the core-veneer bond surface, and the force was applied until the delamination happened.

The fractured surfaces at the interface between the 2 materials were analyzed using a digital microscope and scanning electron microscope to determine the mode of failure.

During the preparation of samples for the analysis of fractured surfaces at the interface of the 2 materials, using digital microscopy and scanning electron microscopy, the fractured samples were subjected to visual inspection to discern critical features of the interface and ascertain the principal fracture pathways. The samples were subsequently sorted and sectioned into manageable sizes, using precision cutting instruments. Subsequent to sectioning, the samples were carefully polished to eliminate loose particles and pollutants, using ultrasonic cleaning in a solvent.

For scanning electron microscopy analysis, samples were meticulously affixed to a specimen holder or stub with conductive adhesives to mitigate charging by grounding the sample. A slender conductive layer was deposited, using a sputter coater or an evaporative coater. The polishing procedure included using increasingly finer abrasives to provide a clean surface that uncovered subsurface characteristics, without generating artefacts that could confuse the research.

After surface coating, photographs were taken at different magnifications, and then the failure mode of specimens was classified as core-veneer interface adhesive failure, cohesive in veneer failure, or mixed failure.

STATISTICAL ANALYSIS:

The data were taken from the computerized machine and transferred to SPSS (version 24.0). Descriptive statistics were computed for maximum load (ML) and SBS. Analysis of covariance was applied for SBS with ML as a cofactor. Kolmogorov-Smirnov non-parametric statistics verified the normality assumption, and regression analysis was used to observe the dependence of SBS on ML. The P value of less than 5% was considered as significant cut-point.

Results

Table 1 shows the descriptive statistics of SBS and ML for each group. The mean values of SBS and ML of all the three experimental groups showed quite closed. However, the mean SBS and ML of the control group was slightly higher.

The dependent variable in this study was SBS, while ML should be considered as cofactor. Analysis of Covariance (ANCOVA) was applied to see the significant difference between mean SBS with ML as cofactor. To check the normality assumption of SBS observations, the Kolmogorov-Smirnov non-parametric statistic was applied, and it showed that the SBS observations could be considered as normally distributed. The check the assumption of significant relationship of ML on SBS, regression analysis was used. The analysis showed that the regression coefficients were significant and exactly the same in each group (regression coefficient=0.038, P<0.0001), implying that the slope of the line was significant and parallel to each other. Therefore, this assumption of ANCOVA was also fulfilled. It was also verified by Pearson correlation, which showed the correlation value as exactly 1.0.

Table 2 shows the ANCOVA of SBS with ML as cofactor. There was no significant difference between the mean values of SBS (P>0.05), while the cofactor ML was significantly related to the SBS (P<0.0001), as shown in Table 2. It implies that almost all the variability of SBS between the groups was due to the ML, and the groups showed no effect in the change of SBS.

Regarding the mode of failure, digital microscopy and scanning electron microscopy showed that 77.5% of specimens had adhesive failure. The other samples (22.5%) had mixed failure: adhesive and cohesive (Figure 2).

Discussion

Multiple firing procedures are clinically considered to achieve better shade, occlusion, and contour. Anecdotally, the repeated firing cycles are thought to be a major problem in changing the thermal specifications of the veneering lithium disilicate restorations and subsequently the bond strength at the interface with the zirconia core, although there is no specific scientific evidence. Therefore, the hypothesis of this study was that multiple firing cycles have the potential to adversely affect the SBS of zirconia core to veneering lithium disilicate. The shear test is the predominant method for assessing the interlayer bonding property, which was most suitable for this study [14]. The Schmitz-Schulmeyer test is an effective method because it uses fewer experimental variables, and the force applies directly to the junction to create a uniform interlayered stress [15].

The results showed no statistically significant difference in SBS between all groups, although the control group was slightly higher. It is challenging to evaluate the results of this study within the context of the existing literature, due to the limited availability of studies on this particular topic. Even though there are some differences in the type of veneering materials and surface treatment methods used, the findings of our study are in contrast to a study done by Zeighami et al [11], which showed a decrease in the bond strength with increasing number of firing cycles. However, a full comprehensive comparison between the 2 studies is not applicable, as the veneering ceramic and manufacturing technique that were used by Zeighami et al [11] were totally dissimilar to the tested veneering material and manufacturing technique in our study. To the same extent, findings of our study also do not agree with results of other studies [16–18] that revealed the opposite of Zeighami et al [11], as these trials showed that the bond strength increased with an increasing number of firing cycles. Again, caution should be exercised with this relative comparison, as different veneering ceramic materials were tested and none of them matched the veneering ceramic material that we assessed in our study. On the other hand, a study conducted by Rayyan supports our study’s results [19]. Rayyan conducted a trial on the impact of firing cycles on the SBS and failure mode of the interface between veneering feldspathic ceramic and zirconia core. He concluded that repeated firing cycles from 1 to 4 had no impact on SBS. However, comparing the results of the 2 studies should be also cautioned due to the differences in mechanical and chemical properties of veneering materials used in both trials.

One of the most common mechanical complications of zirconia restorations veneered with glass-based ceramic is the separation of veneering ceramic. Failure of the bond at the interface can be explained by the coefficients of thermal expansion difference between the core and veneering ceramic [1], poor wetting of the veneering ceramic on the zirconia core [20], and the type of processing method [1]. Moreover, firing shrinkage of ceramic associated with heating and cooling rates has a potential effect on the failure of interfacial bond between layered all-ceramic prostheses [7]. With the added multiple firing cycles needed in some clinical situations, it is thought that the change of chemical content of veneering ceramic as well as the microcrack formation within the veneering layer resulted from the differences in coefficients of thermal expansion might explain the decrease in the SBS that leads consequently to delamination of the veneering ceramic [19]. Nevertheless, this later explanation is not proved by the results of our study.

Most of the samples in all groups, including the control group, had adhesive failure mode, which was verified by digital microscopy and scanning electron microscopy. However, some samples in each group had mixed failure mode: control group (A): 2 samples; group B: 1 sample; and groups C and D: 3 samples each. Opposite results were found in other studies, as cohesive failure was observed more often [11,19]. This is demonstrated by when the veneering ceramic is able to effectively fill the gaps and imperfections on the zirconia core, it enables the system to function as a single layer of ceramic when subjected to external forces [19]. However, the results of our study do not fulfil this hypothesis, even though it is believed that heat-pressed ceramics have little or an absence of internal defects, resulting in a lower incidence of delamination [21]. Based on our study findings, and compared with the control group, there was no significant difference in the failure mode between all groups, which is supported by findings of other studies [11,18]. The result is not surprising since the fracture toughness of lithium disilicate is 2.75 MPa m−0.5, whereas zirconia ceramics exhibit a fracture toughness greater than 6.0 MPa m−0.5 [13].

The main limitations of our study were that the tests were not performed on anatomical dental prostheses and they were conducted in vitro. Additionally, the effect of aging on the SBS of zirconia core veneered with lithium disilicate was not evaluated. Since this was non-clinical study, it was not possible to replicate all of the dynamic changes that occur in the oral environment, which may have affected the outcomes. Thus, further trials are needed to investigate oral simulation conditions, such as aging and complex dynamic changes.

Conclusions

Within the limitations of this study, the following conclusions could be drawn: (1) An increase in firing cycles from 1 to 3 cycles had no significant effect on SBS; therefore, the hypothesis of the study was not fulfilled. (2) Even though some samples in all groups had mixed failure, the mode of failure, which was mostly adhesive, did not tend to change as the number of firing cycles increased.

References

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