Comparative evaluation of shear bond strength at the interface of monolithic zirconia with two distinct core build-up materials: An in vitro study

Abstract

Aim:

The study aimed to evaluate and compare the shear bond strength (SBS) at the interface of monolithic zirconia with zirconomer (Zr) core build-up, a new type of glass ionomer cement to monolithic zirconia with composite resin core build-up material.

Setting and Design:

In vitro a comparative study.

Materials and Methods:

A total of 32 disk-shaped samples of monolithic zirconia and two distinct core build-up materials: Zr (n = 16) and composite resin (n = 16) were used. The two components, monolithic zirconia with Zr core build-up and monolithic zirconia with composite resin core build-up, were bonded using zirconia primer and self-adhesive, dual-cure cement. The samples were subsequently thermocycled, and the SBS was tested at their interfaces. The failure modes were determined using a stereomicroscope. Data were evaluated using the descriptive analysis for mean, standard deviation, confidence interval, and independent t-test for intergroup comparison.

Statistical Analysis Used:

Descriptive analysis, independent t-test, Chi-square test.

Results:

The mean SBS (megapascals) of monolithic zirconia to Zr core build-up (0.74) was statistically significant when compared to monolithic zirconia with composite resin core build-up material (7.25) (P ≤ 0.001). Zirconomer core build-up showed 100% adhesive failure; composite resin core build-up had 43.8% cohesive, 31.2% mixed, and 25.0% adhesive failures.

Conclusion:

When evaluating the two core build-up materials’ bindings to monolithic zirconia, Zr and composite resin core build-up showed statistically significant differences. Although Zr has been demonstrated to be the optimal core build-up material; however, additional investigation is required to determine how it bonds to monolithic zirconia more effectively.

INTRODUCTION

Dental use of zirconia began in the early 1990s. Zirconia’s superior mechanical and biocompatible qualities have allowed it to play a significant role in prosthetic dentistry. Zirconia manufactured using computer-aided design-computer-aided manufacturing (CAD-CAM) technology gives several benefits, including excellent flexural strength, pleasing cosmetic outcomes, minimum tooth preparation, and reduced lab time.[1] Despite having these characteristics, its bonding is quite challenging. Zirconia has a glass free polycrystalline structure, i.e., unlike glass ceramics, it is not susceptible to etching, making it difficult for adhesive procedures.[2] Therefore, applying hydrofluoric acid (5%–9.5%) and a subsequent silane-coupling agent does not help to its bonding.[3]

Over the years, there have been a lot of data collected from studies that suggest various mechanical and chemical methods to enhance the bonding of zirconia. The improvement of the bonding between resin cement and zirconia includes techniques such as airborne-particle abrasion with alumina, silica deposition methods, plasma spraying, selective infiltration etching, and the application of primers such as 10-methacryloyloxydecyl dihydrogen phosphate (MDP), 4-methacryloxyethyl trimellitic anhydride, and thiophosphoric acid methacrylate.[4]

Magne et al.[4] proved in their study that MDP primer forms a better bond for zirconia. The MDP molecule allows copolymerization between the organofunctional part of the organophosphate monomer and the monomers of the resin-luting agents, besides the establishment of a bond between its phosphoric acid groups and the metal oxide in zirconia. Carboxylic acid is the other constituent monomer in this primer, contributing to the bond’s development.[3]

A severely damaged tooth requires an intracoronal support which is cemented to extra coronal prosthesis. The intracoronal support is provided by core build-up or foundation restoration. The core build-up material should have desirable properties such as sufficient compressive strength, flexural strength, and biocompatibility with surrounding tissues and should also have a good bond with the tooth structure, pins, posts, and luting cement. Certain materials have been employed as core build-up materials such as casting core build-up, amalgam, composite resin, glass ionomer cement (GIC), porcelain, compomer, and cention N.[3]

A new material, zirconomer (Zr), has recently been introduced as a restorative material. It is a composition of powder: fluoroaluminosilicate glass, zirconium oxide, pigment, etc., and liquid: polyacrylic acid and tartaric acid.[5] The homogenous incorporation of zirconia particles in the glass component further reinforces the material for lasting durability and high tolerance to occlusal load.[6] It shows better mechanical properties and claims to have a similar shear bond strength (SBS) to that of amalgam. However, Zr has not been challenged clinically, and there is only laboratory-based evidence.

The post and core build-up, as well as the dental prosthesis, are subjected to a variety of stresses during mastication. The type of core build-up material, the type of luting cement, and the bond between the two all determine the durability of fixed prosthesis. Therefore, this study attempted to explore the combinations of Zr and composite resin as core build-up materials in terms of SBS when cemented to MZ. The null hypothesis was formulated as, there is no difference in the SBS at the interface of MZ to Zr core build-up and MZ to composite resin core build-up material.

MATERIALS AND METHODS

Before the start of the study, permission to conduct the study and ethical clearance was obtained from the Institutional Ethics Committee. The sample size was calculated for the study using STATA/ IC 13. Stata Corp LP, College Station, Texas to check the midpoint difference in SBS between the two groups by 1.2 with standard deviations (SDs) of 1.15 at 95% confidence and 80% power. Hence, for this experimental in vitro study, 32 samples (16 samples of MZ to Zr, Group A, and 16 samples of MZ to composite resin, Group B) were fabricated.

Sample preparation

The thirty-two MZ disk-shaped samples (5 mm × 3 mm) were fabricated using the CAD-CAM technique (3M Lava ESPE St Paul, MN). For preparing the samples, a same-dimension composite disc (Ivoclar Vivadent Te Econom Plus Composite Resin—Refills, Schaan Principality of Liechtenstein) was prepared and scanned using EXOCAD software. The zirconia lava block (3M ESPE, St Paul, MN) was utilized to mill the scanned specimen. A 7–8-min 3-axis milling process was followed by a 1200 C sintering process for 8 h. These samples were examined for any irregularities and further polished. They were divided in two groups (n = 16) based on the use of core build-up materials which were Zr and composite resin. Figure 1 shows the groups of the specimens.

Figure 1.

Flowchart of the groups

The disk-shaped core build-up specimens (n = 16 per core build-up material) were prepared in a (7 mm × 7 mm) mold. A polymerized box was filled with addition silicone elastomeric impression material (Aquasil Dentsply/caulk, Kontanz, Germany) in which a similar dimension prototype was impressed to form a mold. The Zr (Conventional GIC, SHOFU, Japan) core build-up specimens were made by simply hand mixing a powder-liquid ratio of 2:1 according to the manufacturer’s instructions on a glass slab with an agate spatula. This mix was packed into the mold and allowed to be set at the room temperature. The composite resin (Ivoclar Vivadent Te Econom Plus Composite Resin—Refills, Schaan Principality of Liechtenstein) core build-up specimens were built up in two 3.5 mm high increment layer within the mold cavity. Each layer was segmentally light-polymerized using a light-emitting diode device (Woodpecker Light Cure Unit LED D) at 800 mW/cm² for the 20s.

Each sample was evaluated for irregularities and the final dimension was confirmed using a micromotor (Kolylong 150 mm, LCD Digital Electronic Carbon Fiber Vernier Caliper Gauge). All the core build-up samples were embedded in an acrylic resin mold to a height such that 1 mm of the core build-up material was exposed, and this was verified using a surveyor. Figure 2 shows the samples of Group A and Group B embedded in acrylic.

Figure 2.

Samples of Group A and Group B embedded in acrylic, respectively

The MZ disks were painted with zirconia primer (ZPrime Plus; Bisco, Illinois, USA) on the front side. The front side of core build-up materials was covered with dual-cure resin cement (Kerr Maxcemelite, Kerr Corp., Orange, CA, USA), and then, the core build-up specimens were pressed onto the MZ specimens. A 5 kg weight was placed over all specimens, and excessive cement was wiped with a brush followed by light polymerization on both side for a total of 80s.

The prepared specimens were kept in artificial saliva (Wet Mouth ICPA Health Products Ltd.) at 37°C for 24 h in an incubator (LG Model:-051SA Mahavir, India). The specimens were mounted in the jig of a universal testing machine (Model: UNITEST 10, ACME., Maharashtra, India) [Figure 3]. The adhesive interface was then loaded with 0.5 mm/min force at a constant crosshead speed until failure occurred. The results obtained for load depending on the moment of fracture for each specimen, and the maximum load was recorded at the fracture. The maximum load was calculated by dividing the load (N) by the bonding area (mm²).

Figure 3.

Prepared sample placed in the universal testing machine for shear bond strength

All the specimens were observed for the three types of failure adhesive, cohesive, and mixed and were analyzed using a stereomicroscope (Wuzhou New Found Instrument Co., Ltd., China, Model 3400E) at ×10. Then images were viewed using an image analysis system (Chroma System Pvt. Ltd., India) (MVIG 2005). The failure mode was defined as an adhesive when more than 75% of the core build-up surface was visible. The cohesive failure mode was defined when more than 75% of the core build-up surface was covered with resin or the fracture was inside the core build-up material. All other cases were classified as having mixed failure modes.

The statistical analysis was performed using the SPSS software (Version 20.0; IBM Corp., Illinois, USA). The quantitative data were subjected to descriptive analysis for mean ± SD. The qualitative data were subjected to an independent t- test and a Chi-square test P < 0.05 was considered statistically significant.

RESULTS

Graph 1 shows the maximum load of Groups A and B. As shown in Graph 2, the SBS of Group B, i.e., MZ to composite resin core build-up is higher than Group A, i.e., MZ to Zr core build-up material.

Graph 1.

Maximum load (N) between Group A and Group B

Graph 2.

SBS between Group A and Group B. SBS: Shear bond strength

As displayed in Table 1, the results of the independent t-test on comparing the two groups. A comparison of the maximum load (N) between the two groups showed the maximum load (N) is higher in Group B with a t = −5.63 and is statistically significant (P < 0.001). As well as the SBS megapascals (MPa) between the two groups showed that it is higher in Group B with a t = −5.732 and is statistically significant (P < 0.001).

Table 1.

Independent t-test comparing the two groups A and B

	Mean±SD		t	P
	Group A (n=16)	Group B (n=16)
Maximum load (n)	14.55±5.96	143.36±91.32	−5.63	<0.001*
SBS (MPa)	0.74±0.3	7.25±4.53	−5.732	<0.001*

*Significant (P<0.05). SBS: Shear bond strength, SD: Standard deviation

According to Table 2, the failure mode for Group A was 100% adhesive and Group B was 25% adhesive, 43.8% cohesive, and 31.2% mixed. Figures 4 and 5 depict the failure patterns in Groups A and B, respectively. The test used to compare the failure modes was the Chi-square test [Table 3].

Table 2.

Rate of failure and failure modes

Groups	Failure modes (%)
	Adhesive	Cohesive	Mixed
Group A	100	-	-
Group B	25.0	43.5	31.2

Figure 4.

Adhesive failure pattern of Group A at ×10 magnification by a stereomicroscope

Figure 5.

(a) Cohesive failure (b) adhesive failure (c) mixed failure pattern of Group B at ×10 magnification by a stereomicroscope

Table 3.

The results of the Chi-square test on comparing failure modes

Chi-square test	Value	df	P (<0.05)
Pearson’s Chi-square	19.200	2	<0.001

df: Degree of freedom

DISCUSSION

This study tried to investigate if the SBS of MZ to two different core build-up materials: Zr and composite resin. The findings proved that MZ to Zr core build-up had a significantly lower SBS compared to MZ to composite resin core build-up material.

The bonding mechanism of MZ is quite challenging due to its inert nature. A vitreous phase (<1%) in MZ makes it resistant to acid etching, determining a poor bond strength to its counterpart core build-up material. Various investigations over the years suggest different mechanical and chemical methods to enhance the bonding of MZ. The MDP primer has been suggested to provide favorable bond of all methods. The MDP primer is an organophosphate monomer that exhibits a terminal functional group containing phosphoric acid, which combines with zirconia to create P-O-Zr bonds. The presence of a vinyl terminal group at the opposite end of the molecule permits copolymerization with the resin. When self-adhesive composite cement is used, this chemical attachment increases.[3]

Seabra et al.[7] the study proved that two coats and light polymerization of zirconia primer application effectively promote adhesion between composite resin and zirconia. Torabi Ardakani et al.[8] reported similar results where zirconia primer strengthened the bond between zirconia posts and the root canal dentin cemented with either self-etch or self-adhesive resin cement.

The prosthesis and the core build-up are linked via the cement. Dual cure resin cement has improved mechanical qualities, including flexural strength and hardness, in addition to the advantage of favoring better polymerization to MZ.[9] This has been proven by Magne et al.[4] with different luting types of cement and zirconia primers; zirconia exhibits improved bond strength. Therefore, the current study used a self-adhesive, dual-cure resin cement.

Ideally, a core build-up material should have good mechanical properties to transfer forces like a tooth. Materials such as casting core build-up, amalgam, composite resin, GIC, porcelain, compomer, and cention N have been used for a long time. With newer materials emerging, Zr is introduced to address all the tissues that plague conventional GIC. Composite resin has been shown to have good bond strength to MZ.

Confirming that, in the current study, MZ to composite resin had a higher SBS value at the interface than MZ to the Zr. This higher bond strength is due to the chemical bond between the composite resin and MZ at the interface because of the primer and resin cement. Prabakaran et al.[10] concluded that nanocomposite possessed better mechanical properties as a core build-up material than Zr. A similar study by Abraham et al.[11] compared the SBS of GIC, Zr, and Luxacore build-up core build-up materials to zirconia. The results of the study showed SBS values of 9.51 MPa, 13.94 MPa, and 17.48 MPa for GIC, Zr, and Luxacore build-up to zirconia, respectively. Hence, the study suggested the use of both chemical and mechanical methods to increase the SBS between MZ and Zr. Tavakolizadeh et al.[12] examined the SBS of zirconia to various core build-up materials, including nonprecious gold alloy, zirconia ceramic, natural dentin, and composite resin. Composite resin was the most valuable core build-up material when compared to the others (11.58 to 1.74 MPa).

The failure of the bond between MZ to core build-up materials can be differentiated by where the failure occurs, i.e., adhesive, cohesive, or mixed. For the current study, stereomicroscope analysis revealed a 100% adhesive failure between MZ and Zr; a 25% adhesive, a 43.8% cohesive, and a 31.2% mixed failure mode between MZ and composite resin. In a similar study, Giti and Zarkari[3] discovered 100% cohesive failure for the composite resin to zirconia disc. These failure patterns express the bond strength between MZ and core build-up materials. One hundred percent adhesive failure between MZ and Zr means a lower bond strength, despite the use of MDP primer and self-adhesive dual-cure cement. Therefore, an investigation needs to be done to enhance the bond strength between MZ and Zr.

In viewing the limitations of the study, using crowns instead of geometry specimens could have given better results. As SBS is a technique-sensitive test, the specimens could have been kept in a mold or sealed while being placed in the thermocycler. Zr has proven to be good material in other aspects. A study by Paul et al.[13] concluded that Zr is more efficient in initial and fluoride re-release and cariostatic performance in real environmental circumstances. Along with that, Zr has a higher compressive strength than amalgam. Further, extensive in vitro and in vivo studies are required to examine the performance of a new member of the GIC family, Zr

CONCLUSION

According to the results of this study and keeping in mind the limitations, MZ with Zr has a significantly lower SBS value when compared to MZ with composite resin. Despite the wide range of core build-up materials available, composite resin continues to be the wisest option. Although marketed as a strong, durable, and fluoride-rich material ideal for bulk-filling the structural core build-up, Zr has yet to fill the research gaps as the material of choice for core build-up.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.