Shear Bond Strength Between Lithium Disilicate and Zirconia Core After Addition of Nanoparticles: A Comparative Study

Introduction

Since the 1980s, ceramic veneer has been the most dependable, long-lasting, and recommended restorative material for anterior teeth, particularly with the ongoing advancement of adhesive procedures [1, 2]. Historically, tetragonal zirconia crystals have been the primary raw material used to make dental zirconia, with a small amount of yttria stabilizer (3Y-TZP). This kind has a low translucency yet is incredibly powerful. To do this, partially stabilized zirconia with a higher yttria concentration—4 mol% (4Y-PSZ) or 5 mol% (5Y-PSZ) was created. Zirconia's stress-induced toughening is lessened by the c-phase, which lessens its toughness and strength [3, 4]. Since its introduction as a restorative core material, zirconia has significantly increased the use of ceramic restorations in dentistry. The most often used core material is zirconia because of its remarkable mechanical properties, great strength, and eye-catching color. Since zirconia lacks the appearance of natural teeth, porcelain lithium disilicate is required to give it a more appealing appearance [5, 6]. Furthermore, the most popular method now in use for creating monolithic zirconia dental prosthesis is apparently computer-aided design and manufacturing (CAD/CAM), which is quick, simple, and time-efficient [7, 8].

Clinical studies on zirconia-based all-ceramic restorations have produced high survival rates and favorable results [9-16]. Nevertheless, the most frequent issues with these restorations are chipping, cracking, and delamination of the porcelain lithium disilicate [17-19]. It must be possible to transfer functional stresses from the porcelain lithium disilicate to the overall structure with a strong enough bond between the fundamental system and the porcelain lithium disilicate [20]. In aesthetic dentistry, porcelain is used to make cosmetic restorations that provide the desired optical transparency. Nonetheless, porcelain needs to fulfill more criteria than just aesthetic ones; it must also be durable and robust enough to support the functions of mastication [18, 19, 21]. The utilisation of lithium disilicate (LD) glass ceramics in dentistry is on the rise, and its unique mechanical and aesthetic properties make its synthesis and manufacture extremely important characteristics, as well as the difficulty of making it [22, 23]. Both the right tooth color and materials with mechanical qualities are crucial because of the forces operating on dental restorations [24].

Nanotechnology has been applied to dentistry in a number of ways, particularly for material improvement [25, 26]. The small size and high specific surface area of nanoparticles (0.1–100 nm) contribute to their special qualities as excellent mechanical, chemical, optical, and magnetic characteristics in contrast to their bulk counterparts [27, 28], including silica which has traditionally been used in dentistry due to its color, shine, and durability. Research is now being done on a silica-based nanomaterial to enhance these intrinsic qualities [29, 30]. As a novel material for dental lithium disilicates, titanium nanoparticles (Ti-NPs), a substance with appropriate mechanical qualities, are combined with lithium disilicate synthesis to create dental ceramics that have greater strength [24, 31, 32]. Titanium dioxide possesses antimicrobial properties, is chemically inert, non-toxic, and resistant to corrosion [33], and it has outstanding photocatalytic properties and is an incredibly stable particle. The output oxidative stress is caused by reactive oxygen species [34-36].

They have demonstrated that even at low concentrations of Titanium dioxide nanoparticles, new physicochemical, electrical, and optical characteristics can be induced, leading to an enhanced new class of nanocomposite materials with high hardness. We have not yet examined how nanoparticles affect the strength of the binding to the underlying zirconia core. Thus, the purpose of the current study was to assess how lithium disilicate treated with silica and titanium nanoparticles would affect the strength of the bond at the lithium disilicate-zirconia contact, so the null hypothesis is that the addition of Silica dioxide and Titanium dioxide nanoparticles to lithium disilicate does not affect the shear bond strength of lithium disilicate veneer to the zirconia core.

Materials and Methods

Samples preparation for shear bonding strength test

Cylindrical shape zirconia cores were made by a CAD/CAM system with (9 mm × 4 mm × 4 mm) dimensions. After zirconia core construction, the surface of the lithium disilicate was roughened by a (50 µm Al₂O₃) sandblast procedure and then cleaned with ultra-sonic device [37]. Addition of silica nano-particles to lithium disilicate at a ratio of (5% by weight) [38]. The incorporation of titanium nano-particles in lithium disilicate at a ratio of 5% by weight by using a delicate electrical balance (with 0.000 digits). These concentrations of silica and titanium nano-particles were selected according to the references (Table 1) [35, 38].

Table 1. Materials used in this study

Study grouping

At first, we determined the number of samples in each group by using G*Power (3.1.9.2) software according to previous studies [39], so the total number of samples was 40, divided into four groups (each group containing 10 samples) according to the type of nano-particles added to the lithium disilicate material as follows:

• Group 1: Lithium disilicate without any type of nanoparticle addition, over a zirconia core.
• Group 2:  Lithium disilicate with (5% wt/wt) Titanium nanoparticles addition over zirconia core.
• Group 3: Lithium disilicate with (5% wt/wt) Silica nanoparticles addition over zirconia core.
• Group 4: Lithium disilicate with mixed addition of (2.5% wt/wt) of Titanium nanoparticles and (2.5% wt/wt) of Silica nanoparticles over zirconia core.

All samples were collected and measured for shear bond strength by the universal testing machine with a speed of cross head (0.5 mm/min), the blade of the testing machine should be set as close as possible to the interface of porcelain‑zirconia (Figure 1). The force at which zirconia-emax bond failure was calculated by dividing the failure force (Newton) by the bonded surface area (mm²).

Nano-particles mixed with lithium disilicate lithium disilicate powder in a correct ratio by loading these two materials for each sample into empty and clean capsule of amalgam and then mixing by using amalgamator for (2 minutes), and then this powder of lithium disilicate and nano-particles mixed with lithium disilicate molding liquid to form a past, by using moistened brush we would apply each layer of lithium disilicate paste on zirconia core as lithium disilicate with (3mm × 4mm× 4mm) dimensions until it reached the selected height, which was measured by digital caliper [40, 41]. In order to dry the samples, the furnace's open entry was heated gradually. This process is used to remove extra water. Steam can emerge as a result. After drying off the compact. The makers' instructions were followed when firing each sample in a porcelain furnace.

After completing all samples, the zirconia core of each sample should be inserted vertically in acrylic resin, and put in a plastic PVC tube until the zirconia-emax interface, which should be placed at the same plane as the shear test blade for the universal testing machine.

Statistical analysis

With SPSS ver. 11.5.0 (SPSS Inc., Chicago, IL, USA), the data were analyzed. After the Shapiro-Wilk test was used to analyze the data, it was discovered that symmetrically distributed. Consequently, the statistical analysis One-way analysis of variance, was used to analyze the data. Duncan's test for multiple comparisons comes next. The (P < 0.05) was designated as the statistical significance level.

Results and Discussion

The means of shear bond strength of control group and nanoparticles- modified lithium disilicate lithium disilicate (Table 2). The presence of (5% wt/wt) titanium oxide nanoparticles in lithium disilicate (group 2) significantly increased the (SBS) to zirconia core (P < 0.05). The presence of (5% wt/wt) silica oxide nanoparticles in lithium disilicate lithium disilicate (group3) significantly increased the (SBS) to zirconia core (P < 0.05) in comparison to control group (group1),  but the addition of two nano-materials at one time is significantly increased and have the highest (SBS) to zirconia core (P < 0.05). All data collected were normally distributed, and it was parametric data.

Table 2. The means of Shear bond strengths (MPa) of nanoparticles‑containing lithium disilicate to zirconia core.

One-way ANOVA (The analysis of variance) was done to show if there is a significant difference between the groups (Table 3). In results of One-Way Anova test observed significance differences between all groups at (p > 0.05), but in Table 4 which represent Duncan’s multiple range test results showed that group2 (which is Titanium dioxide modified LDGC) and group3 (which is Silica dioxide modified LDGC) these two groups had no statistical difference between them, but these two groups had statistical difference from group1 (Control) and group4 ( which is combine Titanium dioxide+SiSO2 modified LDGC).

Table 3. The one-way ANOVA test of (SBS) between lithium disilicate and zirconia core.

Table 4. Results of Duncan's multiple range test.

Numerous fields of study have incorporated nanotechnology since it offers a range of significant answers to complex scientific and medical problems and difficulties [42-51]. A single gram of powder's surface area could equal at different spherical diameters under test, and the surface Below 100 nm, the area per gram grows dramatically. In some materials, this results in a phase shift that raises the surface energy per gram of substance. There are several possible uses for this extra surface area of causes [52, 53]. The entire binding strength between dentin and biomaterials is strengthened by these nanoparticles. Studies conducted in vitro have shown that these nanoparticles slow the spread of cracks and make dental ceramics more resistant to fracture, avoiding crowns and other porcelain restorations, lithium disilicates, and bridges from cracking [54, 55].

Addition of nanoparticles with a reduced percentage, such as nano titanium and graphene-silver nanoparticle (R-GNs/Ag) nanocomposite, nano silicon dioxide (Silica dioxide), nano titanium dioxide (Titanium dioxide), nano chitosan, nano zirconium dioxide (ZrO₂), showed notable improvement in mechanical characteristics, because it leads to more surface area and energy, as well as more evenly distributed particles. Accordingly, new nanoparticles that exhibit potent antibacterial properties have grown in popularity in the dentistry field [56, 57].

Silicon oxide nanoparticles are a useful material for dental materials because of their many advantageous properties, which include increased strength and toughness, resistance to corrosion and abrasion, and excellent biological properties [54, 58].

In this study, we added different types of nanoparticle materials to lithium disilicate glass ceramic (LDGC) to evaluate shear bond strength with zirconia core. The results of this study found there were significant differences in SBS between LDGC and zirconia core after the addition of different types of nanoparticles to LDGC, so the null hypotheses were rejected. Group 3 (which is modified LDGC with 5% of nano-silica) showed an increase in SBS when compared to control samples. The insertion of silica nanoparticles strengthened the shear bond in the modified samples, as stated by the latest studies conducted by Fattah et al. [54].

Adding nanoparticles improves the consistency and enhances the material's consistency [56, 60], and our results are in line with the results of Amer et al.'s study, who found that the addition of silica nanoparticles to porcelain lithium disilicate causes an increase in shear bond strength with zirconia core [35, 58].

Group 2 (which is modified LDGC with 5% of nano-titanium) showed significant elevation in SBS in comparison with the control group. Fragile LDGC-zirconia contact caused by weak and brittle LDGC can result in bond failure and chipping LDGC lithium disilicate [61, 62]. Consequently, higher bonding may have resulted from LDGC lithium disilicate's enhanced flexural strength due to the addition of nanoparticles, affecting the shear bond strength of LDGC lithium disilicate compared to the control group with regard to the zirconia core. During mixing, LDGC lithium disilicate may create air spaces that titanium oxide nanoparticles may fill. Additionally, the nanoparticles could clog the imperfections at the junction between lithium disilicate and zirconia and enhance the bond strength and surface adhesion [63, 64]. Group 4 (which is modified LDGC with combine 2.5% of Silica oxide and 2.5% of Titanium dioxide) was the highest SBS from control and other groups, because it has combine bond strength of silica and titanium nanoparticles together [65-74].

Conclusion

Shear bond strength between lithium disilicate lithium disilicate and zirconia core can be improved by addition of either (5% wt/wt) of Titanium dioxide nanoparticles or (5% wt/wt) of Silica dioxide nanoparticles to lithium disilicate, but we can increase (SBS) to higher value if using combination of these two types of nanoparticles (Titanium dioxide+ Silica dioxide).

Acknowledgments: The author’s thanks University of Mosul and Aliraqia University for the facilities provided to accomplish this study.

Conflict of interest: None

Financial support: None

Ethics statement: The research was approved and registered at the University of Mosul by the Department of Conservative Dentistry, College of Dentistry (Approval Code CoD06 on 02 Dec 2024).