Comparative evaluation of shear bond strengths of veneering porcelain to base metal alloy and zirconia substructures before and after aging - An in vitro study.

OBJECTIVE: The aim of this study was to evaluate and compare the shear bond strength of veneering porcelain to base metal alloy and zirconia substructures before and after aging. Scanning electron microscopy (SEM) was used to determine the failure pattern.
MATERIALS AND METHODS: Twenty rectangular blocks (9 mm length × 4 mm height × 4 mm width) of base metal alloy (Bellabond plus, Bego, Germany) and zirconia (Will ceramZ zirconia K block) were fabricated for shear bond strength test. Surface of the base metal alloy block (4 mm × 4 mm area) was veneered with corresponding veneering porcelain (Ivoclar, IPS classic, vivadent). Similarly, surface of the zirconia rectangular block (4 mm × 4 mm) was veneered with corresponding veneering ceramic (Cercon ceram kiss, Degudent). Out of forty rectangular porcelain veneered core specimen, ten porcelain veneered base metal alloy specimen and ten porcelain veneered zirconia specimen were immersed in water at 37°C for one month to simulate the oral environment.
RESULTS: On comparison, the highest shear bond strength value was obtained in porcelain veneered base metal alloy before aging group followed by porcelain veneered base metal alloy after aging group, Porcelain veneered zirconia before aging group, porcelain veneered zirconia after aging group. SEM analysis revealed predominantly cohesive failure of veneering ceramic in all groups.
CONCLUSION: Porcelain veneered base metal alloy samples showed highest shear bond strength than porcelain veneered zirconia samples. Study concluded that aging had an influence on shear bond strength. Shear bond strength was found to be decreasing after aging. SEM analysis revealed cohesive failure of veneering ceramic in all groups suggestive of higher bond strength of the interface than cohesive strength of ceramic. Hence, it was concluded that veneering ceramic was the weakest link.

INTRODUCTION

Core veneered restorations are the cornerstone for prosthetic dentistry, and combination of a strong core and an esthetic veneer ceramic has proven successful for many decades. Porcelain-fused-to metal restorations have been in use for more than five decades due to their improved mechanical properties.[1] However, metal ceramic restorations show the problem of metal discoloration at the margins, allergic reactions, and sensitivity to various metals. The metal substructure is opaque and does not duplicate the inherent translucency of natural teeth. Hence, the need for a restoration that mimics the natural tooth in esthetics and strength led to the development of yttrium tetragonal zirconia polycrystal (Y-TZP)-based materials.[234]

Superior mechanical properties, unique chemical stability, and esthetics combined with computer-aided design/computer-aided manufacturing (CAD/CAM) technology have made zirconia the core material of choice in recent years. Y-TZP also has a unique property of being resistant to crack propagation.[5]

Zirconia is the only ceramic material which meets the flexural strength requirements for fixed partial dentures (FPDs) of four or more units, as recommended by the International Organization for Standardization (ISO).[678] Yet, even while being strong, due to limited translucency, zirconia has been veneered with esthetic porcelain for better clinical acceptance. Veneering porcelain is a highly esthetic, fine-structure feldspar ceramic that is perfectly adapted to the coefficient of thermal expansion (CTE) value of zirconia frameworks and base metal frameworks as well.

Although zirconia core exhibited high stability as a framework material, chipping of veneering porcelain was found out to be a common cause of failure.[6] The common reasons cited as causes include lack of proper veneering ceramic support, ceramic layer thickness, and improper framework design.[9]

The success of the metal ceramic or ceramic veneered to zirconia core restorations depends primarily on the strong bond between the veneering ceramic and the substructure.

Shear bond strength test is one of the most reliable methods to evaluate the bond strength because it concentrates the applied tension on the interface between two materials.[10]

Generally, ceramics are brittle and have low tensile strength, and are prone to degradation in a moist environment. Hence, the presence of saliva in the oral cavity can have an impact on the ceramic restorations contributing to fatigue failure which occurs in dental prosthesis based on ceramics.[11121314]

In view of the above considerations, the aim of the present in vitro study was to evaluate and compare the shear bond strength of veneering porcelain to base metal alloy and zirconia substructures before and after aging. Scanning electron microscope (SEM) was used to analyze the mode of failure.

MATERIALS AND METHODS

Materials used in the study

Bego, bremen, germany

Ivoclar vivadent, Bendererstrasse, Leichtenstein

Degudent, Hanau, Germany

Al dente, Horgenzell, Germany

GC corporation, Tokyo, Japan.

Methodology

A. Preparation of base metal alloy core–porcelain veneer samples [Figure 1]

Figure 1

Base metal alloy core–porcelain veneer samples

Twenty wax blocks of size 9 mm length × 4 mm height × 4 mm width were fabricated using Inlay wax (GC Corporation, Japan). The prepared wax patterns with sprue were invested in phosphate-bonded investment material (Bellasum; Bego, Germany). After setting, the investment mold was taken to the furnace and kept at room temperature. Then it was heated continuously till 950°C at the rate of 8°C/min and held for 30 min at 950°C in a centrifugal induction casting machine. Nickel–chromium alloy (Bellabond plus, Bego) was heated till the alloy ingot turned into molten state and the casting procedure was completed. The investment was left to cool to room temperature. Divestment was done and casting was retrieved. Sprues were cut with carborundum disk. A total of 20 rectangular base metal alloy samples were obtained.

Surface of the rectangular base metal alloy block (4 mm × 4 mm area) which had to be veneered with porcelain was sandblasted with 50 µ Al3O2 particles and steam cleaned. One layer of opaque porcelain was applied to the base metal alloy surface and fired in a dental porcelain furnace (Vita vacumat 100, Bad Säckingen, Germany) following the manufacturer's recommendation. Dentine porcelain was applied over the same area and fired. The excess porcelain was removed by using diamond burs with low-speed handpiece, so that the final dimension of the veneering ceramic was 3 mm length × 4 mm height × 4 mm width. The samples were finished and polished.

In this manner, 20 porcelain veneered base metal alloy samples were prepared and divided into two groups (group I and group II). Each group contained 10 samples. Group I and group II test samples were used to determine the shear bond strength before aging and after aging, respectively.

B. Preparation of zirconia core–porcelain veneer samples [Figure 2]

Figure 2

Zirconia core–porcelain veneer samples

The required dimension for the zirconia substructure in the present study was 9 mm length × 4 mm height × 4 mm width.

CAM system was used for preparation of zirconia samples.

CAD/CAM wax (Al dente, Horgenzell, Germany) was used to make rectangular block having dimension 9 mm × 4 mm × 4 mm. Rectangular block of CAD/CAM wax was mounted on a frame with the help of Cercon wax sticks which were connected to the milling machine. CAD/CAM brain (Milling Machine; Degudent, Germany) was used to mill zirconia block. Milling of the block was done with an enlargement factor of approximately 26% relative to the final dimension. This was compensated for the shrinkage that occurred during full sintering. On completion of milling, the zirconia core was finished and prepared for sintering. The zirconia block (green state) was sintered in a sintering furnace (Cercon heat; Degudent, Germany) according to manufacture; s recommendation. The surface of the rectangular zirconia block which had to be veneered was sandblasted with 50 µ Al3O2 particles and steam cleaned. After that, the liner (Cercon ceram Kiss liner; Degudent, Germany) was applied and fired. Veneering with ceramic (Cercon ceram Kiss; Degudent, Germany) was done using the layering technique as recommended by the manufacturer. Final dimension of the veneering ceramic was 3 mm length × 4 mm height × 4 mm width.

In this manner, 20 porcelain veneered zirconia samples were prepared and divided into two groups (group III and group IV) of 10 samples each. Group III and group IV test samples were used to determine the shear bond strength before aging and after aging, respectively.

Aging of the samples

Group II and group IV samples were immersed in distilled water separately in a stainless steel tray with lid and kept in an incubator at 37°C for 1 month to simulate the oral environment prior to testing.

Mounting the samples for shear bond strength test

Each test sample was embedded in the self-cure clear acrylic (DPI-RR) which was confined within a galvanized iron (GI) pipe mold of dimension 5 mm width and 20 mm diameter [Figure 3a and b].

Figure 3

(a) Porcelain veneered base metal alloy sample embedded in the mold. (b) Porcelain veneered zirconia core embedded in the mold

Test for shear bond strength

A total of 40 test samples (groups I–IV) were tested for shear bond strength in Universal Testing Machine (model LR 100K Lloyd instrument) [Figure 4]. The sample was aligned such that the bevelled blade of the machine was in line with the core veneer interface. Shear bond force (Newton) was exerted to the bonding interface at a cross-head speed of 0.5 mm/min until fracture occurred. Shear bond strength was calculated as: Shear bond strength (MPa) = shear bond force (N)/surface area (mm2).

Figure 4

Sample in the Universal Testing Machine for shear bond strength test

To determine the mode of failure, the fractured samples were examined under SEM (JSM 6390LA; Jeol, Massachusetts, USA) under 30× and 250× magnification.

RESULTS

Basic values of shear bond strength of all test samples in the four groups were tabulated. The mean shear bond strength for each group was calculated and tabulated. The results were subjected to statistical analysis. Tested samples were subjected to qualitative analysis using SEM.

Tables 1–4 shows the data of the results obtained in this study for the shear bond strengths of samples in groups I–IV, respectively. Table 5 shows a comparison of the mean shear bond strengths obtained from basic values of the four groups.

Table 1

Values of shear bond strength of veneering porcelain to base metal alloy substructure before aging (group I)

Table 4

Values of shear bond strength of veneering porcelain to zirconia substructure after aging (group IV)

Table 5

Mean shear bond strength obtained from basic values of the four groups

Statistical analysis

The data were analyzed using the software SPSS 10.0. Mean and standard deviations were estimated for the samples of each study group.

Descriptive statistics was used to find the mean and standard deviation of the variables. Independent Student's t-test was used to compare the bond strengths between groups. P < 0.05 was considered as the level of significance [Tables 6–9].

Table 6

Comparison between mean values obtained from group I and group II

Table 9

Comparison between mean values obtained from group II and group IV

On comparison of groups, the highest shear bond strength value was obtained in porcelain veneered base metal alloy before aging group, followed by porcelain veneered base metal alloy after aging group, porcelain veneered zirconia before aging group, and porcelain veneered zirconia after aging group (group I > group II > group III > group IV).

To evaluate the mode of failure, the interfaces of the fractured core surface and fractured veneer surface were examined under SEM under 30× and 250× magnification [Figures 5–8].

Figure 5

(a and b): Tested porcelain veneered base metal alloy samples before aging under 30× (a) and 250× (b) magnification. (c and d) Fractured veneer surface under 30× (c) and 250× (d) magnification. Arrow indicates the direction of load. The loaded side demonstrates cohesive failure within the veneering porcelain. Many pores are visible within the veneering ceramic where the fracture originated. Fractured veneer surface does not show metal oxide layer suggesting cohesive failure of veneering ceramic

Figure 8

(a and b): Tested porcelain veneered zirconia samples after aging under 30× (a) and 250× (b) magnification. (c and d) Fractured veneer surface under 30× (c) and 250× (d) magnification. Arrow represents the loaded side and shows veneering ceramic attached on zirconia substructure suggesting cohesive failure of veneering ceramic. High magnification of fractured veneer surface show a big pore inside the veneering ceramic

(a and b): Tested porcelain veneered base metal alloy samples after aging under 30× (a) and 250× (b) magnification. (c and d) Fractured veneer surface under 30× (c) and 250× (d) magnification. Arrow indicates the direction of load. The loaded side demonstrates both adhesive and cohesive failure, but predominantly cohesive failure within the veneering porcelain. High magnification of fractured veneer surface shows traces of metal oxide. Many pores are visible within the veneering ceramic

(a and b): Tested porcelain veneered zirconia samples before aging under 30× (a) and 250× (b) magnification. (c and d) Fractured veneer surface under 30× (c) and 250× (d) magnification. Arrow indicates the direction of load. The loaded side demonstrates presence of veneering ceramic on the zirconia substructure suggesting cohesive failure of the veneering porcelain. High magnification of fractured veneer surface and tested zirconia surface shows numerous pores in the veneering ceramic

SEM analysis revealed the presence of veneering porcelain on the fractured surface of base metal alloy and zirconia samples in all groups suggesting a predominantly cohesive failure of veneering ceramic. Since the bond strength of the interface was higher than the cohesive strength of ceramic, it was concluded that the veneering ceramic was the weakest link.

DISCUSSION

Long-term assessment of conventional FPDs on durability score showed lower failure rates for metal ceramic FPDs.[1516] According to Creugers et al.[15] and Scurria et al.,[16] the failure rates of metal ceramic prosthesis after 10 years were 10% and 8%, respectively. Based on this data, porcelain-fused-to-metal systems still represent the gold standard. The ISO standardized the bond strength of metal ceramic system as >25 MPa.[17] However, bond strength measurement of all ceramic systems cannot be made due to their brittle nature.[1819]

Many factors affect the bonding between metal and porcelain, whereas micromechanical interactions are solely believed to be the bonding mechanism between zirconia core and veneering ceramic.[2021]

There were few short-term clinical studies addressing the clinical performance of zirconium dioxide based restorative systems. Raigrodski et al.,[22] in a study of posterior 3-unit FPD, observed minor veneer chipping in 25% of cases after a mean follow-up of 31.2 months. Sailer et al.[23] reported the success rate of three to five zirconia frameworks for posterior FPD after 5 years of clinical observation to be 97.8% in a clinical study. The survival rate diminished to 73.5% due to other causes.

Exposure to an aqueous environment results in strength degradation of ceramics, which is believed to be caused by a stress corrosion process due to accumulation of pre-existing flaws.

Out of many tests available to check the bond strength, shear bond strength test was found to be the most reliable test as it concentrates the applied tension on the interface between two materials.[24]

In our study, the highest shear bond strength value was obtained in porcelain veneered base metal alloy before aging group (mean value 39.51 MPa), followed by porcelain veneered base metal alloy after aging group (mean value 37.20 MPa), porcelain veneered zirconia before aging group (mean value 28.12 MPa), and porcelain veneered zirconia after aging group (mean value 26.20 MPa).

A study conducted by Choi et al.[25] evaluated the shear bond strength of veneering ceramic to base metal group and found it as 35.87 ± 4.23 MPa. Al-Dohan et al.[19] reported the shear bond strength of porcelain fused to metal as 30.16 ± 5.89 MPa. Drummond et al.[10] reported the shear bond strength of non-precious alloy after 4 months of aging as 25.07 ± 5.23 MPa and after 12 months of aging as 25.01 ± 7.06 MPa.

Choi et al.[25] evaluated the shear bond strength of veneering ceramic to zirconia substructure as 25.43 ± 3.12 MPa. Al-Dohan et al.[19] reported the shear bond strength of porcelain veneered zirconia as 27.90 ± 4.79 MPa. Morena et al.,[11] in their study about dental ceramic fatigue in a simulated oral environment, found the mean dynamic fatigue result for feldspathic porcelain as 44 MPa. In our study, the results obtained are in favor of ISO requirements and in concurrence with the results of previous studies conducted by many authors.

SEM under 30× and 250× magnification indicated that the fracture occurred predominantly in the veneering ceramic. As the veneering ceramic material is weak compared to high-strength core material, the veneering ceramic is prone to fail at low loads. Thus, all tested samples fractured as predominantly cohesive failure within the veneering ceramic. This type of failure mode indicated a sufficient interfacial bond between the core and the veneering material. The cohesive failure of veneering ceramic strongly suggests high residual stresses and minute porosities within the veneer layer. This may be related to the varying thermal diffusivity of core and veneer material. This cooling rate difference may lead to different stress states in the two systems.

CONCLUSION

The highest shear bond strength value was obtained in porcelain veneered base metal alloy before aging group, followed by porcelain veneered base metal alloy after aging group, porcelain veneered zirconia before aging group, and porcelain veneered zirconia after aging group. Thus, it is concluded from this study that aging has an influence on ceramics. Presence of water degrades the strength of ceramic restorations. Since the bond strength of the interface was higher than the cohesive strength of veneering ceramic, it was concluded that the veneering ceramic was the weakest link. Improving the zirconia core–veneer bond strength and the strength of the veneering ceramic may reduce the failure and is paramount to the longevity of the all-ceramic restorations.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

Table 2

Values of shear bond strength of veneering porcelain to base metal alloy substructure after aging (group II)

Table 3

Values of shear bond strength of veneering porcelain to zirconia substructure before aging (group III)

Table 7

Comparison between mean values obtained from group III and group IV

Table 8

Comparison between mean values obtained from group I and group III