Evaluation of shear bond strength between zirconia core and ceramic veneers fabricated by pressing and layering techniques: In vitro study.

STATEMENT OF PROBLEM: Although ceramic veneered on to zirconia core have been in use for quite some time, information regarding the comparative evaluation of the Shear bond strength of Pressable & Layered ceramic veneered on to zirconia core is limited. PURPOSE OF STUDY: To evaluate the shear bond strength of zirconia core and ceramic veneer fabricated by two different techniques, Layering (Noritake CZR) and Pressing (Noritake, CZR Press). MATERIALS AND
METHOD: 20 samples of zirconia blocks were fabricated and the samples were divided into group A & B. Group A - Ceramic Veneered over zirconia core by pressing using Noritake CZR Press. Group B - Ceramic Veneered over zirconia core by layering using Noritake CZR. The veneered specimens were mounted on to the center of a PVC tube using self-cure acrylic resin leaving 3 mm of the veneered surface exposed as cantilever. Using a Universal testing machine the blocks were loaded up to failure. RESULT: The results were tabulated by using independent samples t-test. The mean shear bond strength for Pressed specimens was 12.458 ± 1.63(S.D) MPa and for layered specimens was 8.458 ± 0.845(S.D) MPa.
CONCLUSION: Pressed specimens performed significantly better than the layered specimen with a P value 0.001. Clinicians and dental laboratory technicians should consider the use of pressed ceramics as an alternative to traditional layering procedures to reduce the chances of chipping or de-lamination of ceramics.

The increasing esthetic demands of patients and clinicians have led to a search of an ultimate esthetic restorative material, which has resulted in the development of tooth-colored restorations rather than porcelain-fused-to-metal and full metal restorations. Introduction of newer all-ceramic materials is increasing at an exponential rate as it offers the potential for excellent esthetics, biocompatibility, and long-term stability.[1] Zirconia is currently of great interest among the all-ceramic materials. Zirconium (Zr) is found in the minerals as baddeleyite and zircon (ZrSio4) and in nature it exists in conjunction with silicate oxides or as zirconia oxide (ZrO2). Zirconia oxide stabilized with an addition of yttrium oxide results in a high strength (900–950 MPa), high elastic modulus (200 GPa) and better fracture toughness. The toughening mechanism is related to a martensitic-like transformation of tetragonal metastable grains into a monoclinic state occurring at the crack tip.[2] Thus, rendering it a most preferred material for core, posterior crowns, and fixed partial dentures.[3]

Earlier, the most common technique to fabricate all-ceramic restorations was brush application of veneering porcelain onto sintered zirconia cores. However, heat pressing technique is now available for veneering ceramic on to a zirconia core. The technique involves investing of wax pattern in a refractory material, which is preheated and transferred to a specially designed press furnace. A preheated ceramic ingot is then pressed into the mold via an alumina plunger under pressure from the pneumatic press furnace.[4] This procedure is very technique sensitive, and the long-term success of these veneered restorations may be at a risk of chipping of restoration and high failure rate. Irena Sailer reported chipping of the veneering ceramic is 13.0% in a 3 years follow-up study and 15.2% in a 5 years follow-up study.[56]

The possible reasons for chipping may be insufficient bond strength and excessive tensile stress due to a coefficient of thermal expansion mismatch, excessive load due to premature contacts, insufficient substrate support, tensile stress established during cooling after firing, especially when a considerable thermal gradient develops through the layered system upon rapid cooling.[7] The objective of this study was to evaluate the shear bond strength of ceramic material veneered to zirconia core by two different techniques, layering, and pressing. The null hypothesis was that the shear bond strength of zirconia core to pressed ceramics would be the same as that of layered ceramic.

Material and Methods

A metal block of dimension 3 mm × 3 mm × 3 mm was fabricated using Auto-Cad Wire Cut Iridium Technology (Electronic, Electra, Mektron, Maxicut 734) for standardizing the dimensions of the zirconia blocks. The metal block was scanned using a software for CAD/CAM machine (INTELLI denta, Dental Smart Products, Italy). The three-dimensional (3D) design was fabricated with software (MiniMagics, version 2.0). Using the 3D design 20 zirconia blocks (Noritake Cerabien) were fabricated in the same dimension as that of the metal block by CAD/CAM technology.

The 20 zirconia blocks obtained were divided into 2 groups of 10 each

Group A: Ceramic veneered onto zirconia core by pressing

Group B: Ceramic veneered onto zirconia core by layering.

The dimension of the zirconia blocks was checked using a separable metal mold of 3 mm × 3 mm × 3 mm and the zirconia blocks were cleaned with 70% ethanol [Figure 1].

Figure 1

Dimensions of the zirconia block checked in a removable metal mold

The pressed samples were fabricated by fabricating a 2 mm thickness wax pattern on the clean end of Group A zirconia blocks and the dimension of the wax pattern was checked in the separable metal mold [Figure 2]. These wax patterns were invested in gypsum bonded investment (Noritake press investment) as per the manufacturer's instructions. Burnout was carried out and Noritake CZR Press ingots (Lot no.: OEXZ01, 201412) were pressed using Noritake Plunger in the ceramic furnace (EP600, EMU FRU32, Ivoclar Vivadent), as per the manufacturer's instructions [Figure 3]. The dimension of pressed specimens was checked by placing it in the separable metal mold.

Figure 2

Wax pattern fabrication and spruing done for Group A zirconia blocks

Figure 3

Group A specimens fabricated by pressing veneering ceramics

For the fabrication of layered samples, the separable metal mold was isolated by ceramic separating solution (Ceramic Separating Stick, Noritake Cerabien) to avoid adhesion of ceramic powder onto the mold during layering.

The ceramic powder (Noritake CZR, Lot No.: OEXZ01, 201412) was mixed with the respective liquid as per manufacturer's instruction and layered on Group B zirconia blocks. Excess liquid was removed using a tissue. The Group B specimens were glazed in the ceramic furnace (VOP, Ceramic Master) as per the manufacturer's instructions. The dimensions of layered specimens were checked by placing it in the separable metal mold [Figure 4].

Figure 4

Group B specimens fabricated by layering veneering ceramics

The veneered Group A and B specimens were mounted onto the center of a polyvinyl chloride tube of 1 cm height using self-cure acrylic resin leaving 3 mm of the veneered surface exposed as cantilever. Flex Kwik (Pidilite Industries Ltd., Batch No, H72214) was used for adhesion of these specimens to the self-cure acrylic resin. The specimens were stressed with notched shear bond test using a universal loading apparatus (Unitek 94100, FIE) The blocks were loaded up to failure with a chisel-shaped piston at the interface parallel to the block at a crosshead speed of 5 mm/min [Figures 5 and 6].

Figure 5

Specimen stressed in a universal loading apparatus

Figure 6

Specimens loaded up to failure

Shear bond strength (Mpa) was calculated dividing the load value (n) at which the failure occurred by the bonding area of the zirconia-veneer interface (mm2). Shear stress (MPa) = load (n) ÷ area (mm2)

Statistical analysis

A comparative evaluation of the shear bond strength of ceramic material veneered onto zirconia core by two different techniques, layering and pressing was done using independent samples t-test.

Results

Pressed ceramics performed significantly better when comparing the shear bond strength of ceramics veneered onto zirconia core by pressing and layering technique. The null hypothesis was rejected. The mean shear bond strength of specimens fabricated by pressing was 12.536 ± 1.633 MPa and by layering was 8.458 ± 0.845 MPa. The t value was 4.96 and P = 0.001 which was statistically significant [Table 1].

Table 1

The shear bond strength of zirconia core veneered by pressing and layering

Discussion

Ceramics veneered onto metal alloys or zirconia core have been available for quite some time but the bonding mechanism between zirconia core and ceramic veneer have not been clearly demonstrated.[8] According to various clinical studies, most of the failure in zirconia veneered ceramic crowns occur at the interface. The reasons for failures may be due to material composition, properties, firing temperatures, cooling rates, operator's skill, porosities, and fabrication process which may affect the quality and strength of bond between the core and the veneered material.[9]

According to Kim et al.[9] and Aboushelib et al.[10] one or more surface treatments like airborne particle abrasion or application of a liner will typically enhance the bond strength by increasing the surface area of contact between zirconia core and veneered ceramic. Smith et al.[11] and Al-Dohan et al.[12] reported that scanning electron microscopy (SEM) observations of the sectioned areas reveals compositional differences across the interface between the core and the veneer due to a chemical alteration of the glass ceramics adjacent to the core, possibly altering the properties such as strength and coefficient of thermal expansion at the interface. Ban and Anusavice[13] suggested that adhesion of porcelain to zirconia core depends on chemical bonding and reaction taking place at the grain boundaries. Taskonak et al.[14] reported that the presence of global residual compressive stresses and porosities at the interface can also lead to poor bonding mechanism.

This study was done to evaluate the significant difference in the shear bond strength of ceramic material veneered to zirconia core by two different techniques, layering and pressing. Noritake CZR Press (pressable ceramics) and Noritake CZR (layered veneer ceramics), Cerabien, Japan was used. The result obtained in this study showed that pressable ceramic perform significantly better than the layered specimen (P = 0.001).

The study result was in accordance to the result reported by Aboushelib et al.[10] and Ansong et al.[15] Wherein, the mean shear bond strength of pressed samples was 12.54 ± 1.63 MPa (standard deviation) and of layered samples was 8.458 ± 0.845 MPa. Aboushelib et al.[10] investigated the micro tensile bond strength between one type of zirconia substrate and various commercially available ceramic material for veneering by both pressing and layering techniques, the mean shear bond strength ranged from 25 to 48.8 MPa for pressable ceramic and 17.2–41.1 MPa for manual layered ceramic. Ishibe et al.[16] in his study reported that the mean shear bond strength for pressed zirconia ceramic specimen ranged from 21.34 (24.30) MPa to 40.41 (10.28) MPa, (P < 0.05) and mean shear bond strength for layered zirconia specimens ranged from 30.03 (9.49) MPa to 47.18 (12.99) MPa, (P < 0.05). The result of this study was not in accordance with Ishibe et al. who reported a higher bond strength value for layered samples when compared to pressed specimens.

Delamination can be due to a weak bond between the zirconia core and the ceramic veneer.[17] SEM examination of the core veneer interface of sound specimens shows that pressable veneers had better surface contact with zirconia. This can be due to an improved surface contact between the two materials due to the applied external pressure, especially at the microscopic surface roughness level. As they cool below glass transition point under pressure, there is less chance for micro gap formation as a result of deformation and cooling stresses. However, in layering multiple firing of veneer ceramic over zirconia core, relaxation of residual stresses and tetragonal monolithic transformation of zirconia may occur.[18] This may not only affect the strength of the structure, but may also affect core veneer bond strength. Surface lifts could occur during the tetragonal monolithic transformation. These uplifts, which occur at the microscopic level, can reduce the core veneer bond strength.[1917] The limitations of this study include the fact that the design of the specimen does not report the clinical situation and limited sample size.

Conclusion

Within the limitations of this study, it is conclusive that the mean shear bond strength of specimens veneered on to zirconia core by pressing technique performed significantly better than the layered specimen (P = 0.001). The mean shear bond strength of pressed samples was 12.54 ± 1.63 MPa and layered samples were 8.458 ± 0.845 MPa. The clinicians and dental laboratory technicians should consider the use of pressed ceramics as an alternative to traditional layering of ceramics on to zirconia core which would reduce the chances of chipping and delamination of the veneered ceramics with other advantages such as excellent accuracy and better marginal adaptation.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.