ORIGINAL RESEARCH

https://doi.org/10.5005/jp-journals-10019-1406
International Journal of Prosthodontics and Restorative Dentistry
Volume 13 | Issue 2 | Year 2023

An In Vitro Study to Comparatively Evaluate the Shear Bond Strength of Two Layering Materials on Zirconia Cores

Abhishek Nagpal¹ https://orcid.org/0000-0002-6801-0171, Akshay Bhargava², Satyavir Singh³, Priyanka Thukral⁴, Puja Malhotra⁵ https://orcid.org/0000-0002-7003-7437, Mansi Singh⁶

^1-6Department of Prosthodontics, Santosh Dental College, Santosh University (Deemed to be University), Ghaziabad, Uttar Pradesh, India

Corresponding Author: Puja Malhotra, Department of Prosthodontics, Santosh Dental College, Santosh University (Deemed to be University), Ghaziabad, Uttar Pradesh, India, Phone: +91 9810857171, e-mail: pujchow@yahoo.com

Received on: 11 April 2023; Accepted on: 15 May 2023; Published on: 28 June 2023

ABSTRACT

Purpose: The aim of the present study was to evaluate and compare the shear bond strength of feldspathic ceramic (VITA VM 9), and IPS e.max Ceram veneered on zirconia cores.

Materials and methods: A total of 50 zirconia cylindrical discs measuring 10 mm in diameter and 3 mm in thickness were milled. 3M zirconia blanks were used to fabricate the zirconia cores. Half of the samples (n = 25) were layered with VITA VM 9 (group I), and the remaining samples were layered with IPS e.max Ceram (group II). The layered specimens were compressively loaded under a universal testing machine till fracture occurred. An independent samples t-test was done to compare the mean shear bond strength of two independent groups.

Results: Specimens in group II, layered with IPS e.max Ceram, showed higher bond strength (1466.76 N) compared to group I, layered with VITA VM 9 (1274.12 N). The difference was significant (p = 0.047).

Conclusion: Specimens layered with IPS e.max Ceram showed better bond strength than specimens layered with VITA VM 9. Hence, in patients with heavy occlusal loads, parafunction habits, multiunit restorations, or in cases of posterior rehabilitation, layering with IPS e.max Ceram can be considered.

How to cite this article: Nagpal A, Bhargava A, Singh S, et al. An In Vitro Study to Comparatively Evaluate the Shear Bond Strength of Two Layering Materials on Zirconia Cores. Int J Prosthodont Restor Dent 2023;13(2):65-69.

Source of support: The authors would like to acknowledge Santosh (Deemed to be University) for financial support for the research as part of seed money project from the University.

Conflict of interest: None

Keywords: IPS e.max Ceram, Shear bond strength, Tooth colored restorations, Vita VM 9, Zirconia

INTRODUCTION

In the second half of the 20th century, dentistry saw challenges so as to meet escalated esthetic needs.¹ Esthetic veneering ceramics are widely used in dentistry. Dental ceramics are known to be brittle. External loading creates tensile stresses, which cause flaws and other defects, eventually leading to the propagation of cracks.² Advancements in the field have led to the invention of various dental ceramic materials viz, IPS Empress 2, zirconia systems, Procera AllCeram, and Denzir.³

As stated in the literature, the fracture strength of a ceramic crown depends on the mechanical properties of the veneer ceramics, the shape, and thickness of the core vis-à-vis the veneer ceramic.⁴^,⁵ Recently, with the advent of computer-aided design and computer-aided manufacturing technique, new ceramics, namely yttrium oxide partially-stabilized zirconia (Y-TZP), has become available to dentistry.⁶^,⁷ These newer ceramics are characterized by superior strength and superior fracture resistance compared to traditional ceramics.⁸^,⁹ Y-TZP has advantageous mechanical properties combined with excellent esthetics; it has started replacing porcelain-fused-to-metal as a bridge material, especially in the premolar and molar regions. However, to further improve the esthetic appearance, the milled frameworks or copings are layered with veneering ceramics compatible with Y-TZP.¹⁰

There are various materials that are used as veneering material over zirconia frameworks. However, chipping is a common complaint of patients having veneered zirconia crowns, the fractures being adhesive in nature; it is believed that the shear bond strength of layering material on zirconia frameworks is weak.²^,¹⁰ This is especially true for Emax veneering materials, which have been commonly used for veneering zirconia crowns. This makes veneered zirconia restorations unsuitable for use in high-stress bearing areas of the mouth (posterior areas/bruxers), long span posterior bridges. Although monolithic zirconia restorations have commonly been used in these areas, these restorations are known to be harsh on the opposing dentition.¹¹

During the function, the functional forces may vary from 200 to 1000 N.¹² This type of force can create substantial stresses on the tiny tooth surfaces. This type of force is damaging and produces a strain that may eventually damage the contacting occluding surface of the restoration. Dental ceramics are brittle in nature; they are not capable of absorbing very high occlusal or chewing forces.¹² Conventional dental ceramics were known to chip and fracture under heavy occlusal loads; this led to the development of better high-strength ceramics that were used with a metal core and were able to withstand occlusal forces as esthetics became important metal was soon replaced with zirconia.¹³ The optical properties of a dental material are also very important for the selection of dental material for restorative purposes. At times to make a material more aesthetic, strength has to be compromised by either making the veneer layer of porcelain thicker or by reducing the highly opaque core layer of alumina and zirconia.¹⁰

The bonding between two materials (zirconia and ceramic) is a delicate and much-studied aspect of ceramic restoration. The bond strength is potentially influenced by the type of zirconia framework and the layering technique.¹³^,¹⁴

Therefore, the aim of the present study was to assess the bond strength of IPS e.max Ceram (a fluorapatite veneering ceramic) layered onto standard zirconia copings, which was compared with that of standard zirconia copings layered with feldspathic (lithium disilicate) porcelain. This null hypothesis considered for this study stated that the test materials exhibit similar bond strength under standardized test conditions.

MATERIALS AND METHODS

The in vitro study was conducted at Santosh Dental College and Hospital, Ghaziabad, India after the study received a clearance from the ethical committee of Santosh deemed to be University on 20/07/2021 with ethical approval no. SU/2021/1852(3).

Sample Preparation

A total of 50 zirconia discs measuring 10 mm in diameter and 3 mm in thickness were milled using 3M zirconia blanks with the help of a five-axis milling machine (Fig. 1). In order to obtain a uniform thickness of 2 mm over the zirconia discs the samples were layered using a silicon jig (Fig. 2). In group I, zirconia discs (n = 25) were layered with VITAVM 9 (VITA Zahnfabric, Gmbh,Postfach, Bad Säckingen) (Fig. 3) and group II specimens (n = 25) were layered with IPS e.max Ceram (Ivoclar Vivadent Inc., New York, United States of America) layering ceramic (Fig. 4) using the conventional powder slurry method. Zirliner, 0.1 mm thick, was applied to the specimen and fired in a furnace at 960°C according to the manufacturer’s instructions. The final dimension of the sample was 10 × 5 mm.¹²^,¹⁵^,¹⁶ The samples in each group were further glazed, finished, and polished.

Fig. 1: Study specimen design

Fig. 2: Silicone jig for uniform layering of zirconia specimen

Fig. 3: Study specimens layered with VITA VM 9

Fig. 4: Study specimens layered with IPS e.max Ceram

Fabrication of Metal Jig and Chisel

In order to hold and test the specimens for shear bond strength under the universal testing machine, a metal jig with a trough in the center and a metal chisel with a curved end were designed and fabricated (Figs 5 and 6).

Fig. 5: Metal jig and chisel design

Fig. 6: Metal jig and chisel for testing of zirconia specimens under the universal testing machine

Testing of Specimens

Each zirconia specimen was mounted on the trough of the metal jig and positioned on the table of the computer attached to the universal testing machine (Instron, model 3345, Buckinghamshire, United Kingdom) (Fig. 7). Each assembly was statically loaded under compression until fracture occurred. The machine was operated at a crosshead speed of 0.5 mm/minute, using a metal chisel placed centrally at the junction of ceramic and zirconia disks. The fracture of the specimens was noticed when a crack was heard, and a sudden drop in the load-deflection curve occurred (Fig. 8). The software of the machine recorded the force at which the fracture occurred in Newton.

Fig. 7: Testing of zirconia specimens under the universal testing machine

Fig. 8: Fractured zirconia specimen

Statistical Analysis

The data was recorded and fed into a computer using the Statistical Package for the Social Sciences (SPSS) software (IBM Corp. Released 2017. IBM SPSS Statistics for Windows, Version 25.0. Armonk, New York, United States of America: IBM Corp) package. The difference between the strength values of the two groups was calculated at a 95% confidence interval. An independent samples t-test was done to compare the means of two independent groups.

RESULTS

The values of shear bond strength between zirconia cores and veneering ceramic were recorded for all 50 specimens. Group I samples veneered with VITA VM 9 showed a mean shear bond strength of 1274.12 N, while group II samples veneered with IPS e.max Ceram showed a mean shear bond strength of 1466.76 N (Table 1).

**Table 1:** Mean shear bond strength of groups I and II specimens
Group	N	Mean	Std deviation	Std error	95% confidence interval		Minimum	Maximum
Group	N	Mean	Std deviation	Std error	Lower bound	Upper bound	Minimum	Maximum
Group I (zirconia discs layered with VITA VM 9)	25	1274.1188	433. 23701	86.647	1095.287	1452.950	605.07	2940.03
Group II (zirconia discs layered with IPS e.max Ceram)	25	1466.7608	187. 32341	37.464	1389.437	1544.084	1123.84	1838.75

N, sample size; std, standard

A significant difference (p = 0.047) in the mean shear bond strength values for group I (VITA VM 9, 1274 ± 433.23 N) and group II (IPS e.max Ceram (1466.76 ± 187.32 N) was observed. These results suggest that the shear bond strength of group II (IPS e.max Ceram) was significantly more than that observed in group I (VITA VM 9) (Table 2 and Fig. 9). The type of failure observed in both groups was of the adhesive type.

**Table 2:** Independent t-test to compare the shear bond strength of group i with ii specimens
Group	N	Mean	Standard deviation	Standard error mean	p-value	Mean difference	95% confidence interval
Group	N	Mean	Standard deviation	Standard error mean	p-value	Mean difference	Lower	Upper
Group I (zirconia discs layered with VITA VM 9)	25	1274.12	433.237	86.647	0.047*	−192.642	−382.446	−2.837
Group II (zirconia discs layered with IPS e.max Ceram)	25	1466.76	187.323	37.465	0.047*	−192.642	−382.446	−2.837

Level of significance at p < 0.05; *indicates statistically significant

Fig. 9: Comparative analysis of SBS of IPS e.max Ceram layered zirconia cores vs Vita VM 9 layered cores

DISCUSSION

Zirconia has been used as a biomaterial in the field of medicine since the early 1980s. Zirconia also started being used in a variety of dental procedures. It is now used for single crowns, endodontic posts, fixed partial dentures, and even dental implants. Zirconia is a very strong opaque material often referred to as “ceramic steel.” It is characterized by outstanding biocompatibility, super esthetics, and super biomechanical properties making it a very versatile material. However, its weak and less reliable bonding with porcelain when compared with metal-ceramic systems and unpredictable bonding with resin-based adhesives are some drawbacks that limit its dental applications. Delamination of veneering ceramic is a common complaint associated with layered zirconia restorations.¹⁶

The Emax crown is known for its esthetics, strength, and durability. Consequently, it exhibits less chipping. In view of the same, the manufacturer has come out with IPS e.max Ceram, which is a nano fluorapatite layering ceramic material that can be used to layer zirconia frameworks. Fluorapatite glass ceramic is made by a sintering process and has higher bending strength, elastic modulus, and hardness.¹⁷

Bilayered zirconia restorations often exhibit failures like chipping, delamination, and fracture.¹⁷ This questions their use in the posterior regions of the mouth, which have to bear the forces of occlusion and chewing. Literature has documented that occlusal loads are concentrated in the second premolar area irrespective of the type of food.¹⁰^,¹³ Therefore, during mastication, the maximum amount of load is experienced in the second premolar region of the oral cavity, and in order to restore any missing component in this region, the restoration should be both durable and aesthetic.

Aboushelib et al.,¹⁸^,¹⁹ concluded from a study that zirconia type, white or colored, and its surface finish on the bond strength to two veneer ceramics affect the bond strength of the veneering ceramic. They observed both cohesive and interfacial surface failures.

Therefore, the present study was undertaken to evaluate the shear bond strength of a fluorapatite veneering ceramic (IPS e.max Ceram) layered onto standard zirconia copings and compared it with the shear bond strength of standard zirconia copings layered with feldspathic (VITA VM 9) porcelain. The null hypothesis for the study, which stated that the test materials exhibit similar bond strength under standardized test conditions, was rejected.

For ease of fabrication and standardization, zirconia discs were fabricated and layered with the test materials, and the shear bond strength was evaluated with the help of a universal testing machine. It was seen that layering with IPS e.max Ceram exhibited a significantly higher mean bond strength of 1466.95 N as compared to the VITA VM 9, which demonstrated a mean bond strength of 1274.12 N. This could be attributed to one of the differences in the chemical structure of feldspathic and fluorapatite ceramics, which made fluorapatite (IPS e.max Ceram) ceramic stronger with a bending strength of 160 Mpa, elastic modulus 87 Gpa, and hardness 637 HV whereas that of VM9 was 96 Mpa, elastic modulus 75 Gpa and 539 HV.¹⁸ VM9 is classified as a leucite-based synthetic glass matrix ceramic. On the contrary, IPS e.max Ceram is fluorapatite classified as leucite-based synthetic glass matrix ceramic; the glass phases are combined with apatite crystals in addition to leucite to match the coefficient of thermal expansion (CTE) of their respective frameworks.¹⁹ The CTE difference principally influences the interfacial stresses that cause crazing and ceramic delamination. A compatible CTE of veneering materials results in appropriate bond strength. VITA VM 9 CTE is 9.0–9.2 × 10^−6, while for IPS, Ceram is 9.2–9.75 × 10^−6, and that of zirconia was 10.1 × 10. The mismatch in CTE may increase the development of tensile stresses in veneering ceramic, so it is advisable to have the difference as small as possible.¹⁵

Juntavee and Dangsuwan,²⁰ in their study, concluded that the CTE of veneering ceramic is recommended to be below the CTE of zirconia. In the present study, both the test materials exhibited significantly higher bond strength, that is, 1466.76 N (IPS e.max Ceram) and 1274.12 N (VITA VM 9). The mean shear bond strength for groups I and II is higher than the maximum occlusal force of 200–1000 N that has been reported to be exerted in natural dentition.¹¹^-¹² Therefore, it may be suggested that zirconia cores layered with IPS e.max Ceram can be used as a cost-effective, esthetic, and durable option for our patients in order to replace multiple missing teeth and in areas of heavy occlusal loads.²¹^,²²

In the present study, the type of failure observed in the specimens was adhesive type. Since the objective was a comparative evaluation of the shear bond strength of the two materials under simulated occlusal loading, the values obtained were well within clinical requirements. The present study has limitations in that it is an in vitro study and not simulating oral clinical conditions. So, further in vivo studies needed, which can assess different aspects porcelain and zirconia bonding.

CONCLUSION

In conclusion, it may be suggested that in patients exhibiting heavy occlusal forces, the restorations layered with IPS e.max Ceram may perform better than the restorations layered with VITA VM 9. Hence, in patients with heavy occlusal loads, such as in cases of bruxers, for multiunit restorations or for restorations in the posterior segment of the jaws layering with IPS e.max Ceram can be considered.

ORCID

Abhishek Nagpal https://orcid.org/0000-0002-6801-0171

Puja Malhotra https://orcid.org/0000-0002-7003-7437

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