The porcelain fused to metal technique has been known as a reliable choice of treatment for fixed partial dentures (FPD) for almost four decades now, still representing the gold standard.[1–3] However, with the increasing interest in esthetic dentistry and the question of some dental metals and alloys being biocompatible or not, the development of other alternatives to metallic ceramic dental restorations has advanced.[4] In the early 1990s, yttrium oxide partially stabilized tetragonal zirconia polycrystal (Y-TZP) was introduced to dentistry as a core material for all-ceramic restorations and has been made available through the CAD/CAM technique. Y-TZP has proved to be superior in mechanical properties compared with other all-ceramic systems (flexural strength of 900-1200 MP a,[56] and fracture toughness of 9-10 MP a.m1/2[7]) due to a transformation toughening mechanism.

Long-term clinical results for zirconia all-ceramic restorations are not available at the present time. In short[8] and medium-term studies[9–11] the Y-TZP core ceramic had a high stability as a framework material. There have been no fractures of the zirconia framework reported to date.[10–12] However, delamination or minor chip-off fracture of veneering porcelain has been described as the most frequent reason for the failures of zirconia FPDs. The incidence of veneer fractures in zirconia FPDs was significantly higher compared with those in metal-ceramic FPDs.[12] Therefore, the bond between core and veneer or the veneer material itself is one of the weaknesses in layered zirconia-based restorations and plays a crucial role in their long-term success.[13]

Chip-off fracture rates of 15% after 24 months,[9] 25% after 31 months,[11] 15.2% after 60 months[14] and 8% and 13% after 36-38 months, were observed respectively.[10] A review of the literature for FPDs with metal framework, however, revealed either no fracture of the veneering ceramic[15] or substantially lower fracture rates ranging from 2.7 up to 5.5% for observation periods from 10 to 15 years.[1617] The cause of fracture of veneering ceramics on zirconia all-ceramic cores was reported to be multifactorial in clinical applications. Restoration geometry such as lack of proper veneering ceramic support, inadequate framework design and thickness of the ceramic layers seem to play a decisive role.[10] Moreover, direction, magnitude and frequency of the applied load other than the size and location of occlusal contact areas can lead to failures of the veneering ceramic.[11] Bond strength is determined by a series of factors including strength of the chemical bonds, mechanical interlocking, type and concentration of defects at the interface, wetting properties, and the degree of compressive stress in the veneering layer due to a difference in the coefficients of thermal expansion between zirconia and the veneering ceramic.[18–20]

Since the mechanical integrity and adhesion of the veneering ceramic to the ceramic substructure have proven to be key factors for the successful performance of veneer/core bilayered restorations, the initial bond strength and their reliability after thermocycling obtained from in vitro investigations can provide useful information for the behavior and predictability of Y-TZP all-ceramic systems in clinical application.[11]

The purpose of this study was to evaluate the shear bond strength (SBS) of two commercial zirconia core ceramics to their corresponding veneering ceramics and microscopic characteristics of bond failure at fracture surface.

Two types of zirconia-based ceramics were selected for this experimental study: Biodenta (Biodenta Swiss AG, Bernek, Switzerland) and Cercon (Degudent, Hanau, Germany). With each zirconia system, 10 disk-shaped specimens of 7-mm diameter and 3-mm height were fabricated. Presintered zirconium oxide blocks were milled according to the manufacturers’ instructions. Then, they were cleaned, dried, and sintered according to the suggested firing schedules [Table 1]. The bonding surfaces of zirconia core specimens were polished with diamond paste (Lach Diamant, Hanau, Germany) to obtain standardized surface roughness of 3 μm. Then, airborne particle abrasion was applied on the bonding surfaces with 110-μm aluminum oxide (Al₂O₃) particles (Hasenfratz, Assling, Germany) for 15 seconds at 3 bar pressure and at 10-mm distance from the surface. Finally, the specimens were ultrasonically cleaned in 96% isopropyl alcohol for 3 minutes and steam-cleaned for 15 seconds. The specimens of Biodenta and Cercon zirconia systems were veneered with their manufacturer-recommended veneering ceramics, 2 in 1 ceramic (Biodenta Swiss AG, Bernek, Switzerland) and Cercon Ceram (Degudent, Hanau, Germany), respectively [Table 2]. Using a specially designed, separable stainless steel mold, a prepared zirconia disk specimen was placed in the mold where clearance of 5-mm diameter and 3-mm height was available above the core material for condensing the veneer ceramic. The veneering procedure was performed using the manual layering technique. First, two liner layers of porcelain were applied and fired independently, then the dentin porcelain was condensed using the vibration blotting technique, fired and finally a glaze firing was performed according to the manufacturer's instructions. By means of an autopolymerized acrylic resin (Meliodent, Heraeus Kulzer GmbH, Hanau, Germany), each specimen was embedded at the center of a T-shaped metal holder, with the core-veneer interface positioned at the top level of the holder [Figure 1]. All specimens were stored in distilled water at 37°C for 1 week. and then thermal cycled for 5000 cycles, 5°-55°C with a 30-s dwell time.

Then, these metal holders were mounted in universal testing machine (Type LFM-L, Walter+Bai AG, Löhningen, Switzerland). Specimens were tightened and stabilized to ensure that the 1-mm thick edge of the shearing device was in contact with the core surface and was positioned as close as possible to the veneer-core interface. Shear load was applied at a crosshead speed of 0.5 mm/min until fracture occurred. The ultimate load to failure was recorded in Newton (N). The average SBS (MPa) was calculated by dividing the load (N) at which failure occurred by the bonding area (mm²) as follows:

Shear stress (MPa) = Load (N)/19.625 (mm²);

These data were used to calculate the mean failure load and standard deviation for each group. The fractured surfaces were visually analyzed with a stereomicroscope (MBX–10, N9116734, St Petersburg, Russia) to determine the failure modes of specimens. Failure modes were classified as follows: cohesive fracture within the veneer, adhesive fracture between the core and veneer, or a combination of both. t-test was used to analyze the differences in SBS between zirconia ceramics and their veneering ceramics. An alpha-level of 0.05 was used for all statistical analyses, which were performed using statistical software (SPSS 15.0; SPSS, Inc, Chicago, IL).

Table 3 summarizes the mean values and standard deviations of SBS for the tested zirconia ceramics and veneering ceramics and presents the fracture analysis results in percentage. T-test revealed that there is no significant difference between two groups (P = 0.551). Upon examination under the stereomicroscope (×36), the Biodenta group exhibited mostly adhesive failures [Figure 2]; whereas, the Cercon group showed mixed cohesive/adhesive failures [Figure 3].

Bond strength measurement of metal ceramic systems was standardized by the International Organization of Standardization through the Schwickerath crack initiation test (three point bending test) and the mean debonding strength/crack initiation strength should be greater than 25 MPa to meet the ISO requirement.[21–23] Due to the brittleness of all-ceramic core materials this test setup cannot be applied to all-ceramic multilayered system.[24] An adequate standardized test set-up and a minimum required bond strength for bi-layered all-ceramic materials has not been determine yet.[25] Few articles have utilized various bond strength test methods for all-ceramic core and veneering ceramic, such as the SBS test,[25–31] three and four point loading test,[32] biaxial flexure strength test,[19] and the microtensile bond strength test.[2733–35] However, each test has a common limitation which is the difficulty in determining the core-veneer bond strength from applied force at failure on the sample in the specific test setup. In this study, the SBS test method was selected because of its simplicity, such as the ease of specimen preparation, simple test protocol and the ability to rank different products according to bond strength values. But the SBS test has some disadvantages; such as high standard deviations, occurrence of nonuniform interfacial stresses, and the influence from specimen geometry. Therefore, the standardization of specimen preparation, cross-sectional surface area and rate of loading application are important for improving the clinical usefulness of SBS test.[30] In metal-ceramic systems, excessive stresses arising from coefficient of thermal expansion (CTE) mismatch may be compensated to some extent by plastic or elastic deformation of the metallic framework.[36] However, unlike metals, the zirconia framework has a higher rigidity and this feature causes more destructive stress to be formed in the veneer layer of zirconia-based restorations.[37] In some studies, the SBS between metal alloys and porcelain has been found to range from 26.4 to 96.80 MPa.[22283839] For core-veneered all-ceramic restorations, previous investigations indicated that the core-veneer bond strength ranged from 9.4 MPa to 42 MPa.[252628–3133–3540] Mean SBS values obtained in the present study were 27.19 and 28.22 for Cercon and Biodenta groups, respectively, confirming the findings of previous studies.

Mean SBS value of Cercon zirconia to Cercon Ceram has been reported to be 20.19 MPa by Ozkurt et al.,[29] 25.43 MPa by Choi et al.[30] and 9.4 MPa by Guess et al.[28] In our study this value was higher than the previous studies. In a study by Aboushelib et al.,[35] Cercon Ceram Express (press-on veneering ceramic) exhibited a bond strength value of 37.9 MPa with the Cercon framework, which was higher than the 27.19 MPa obtained in this study and the values of SBS in previous studies for Cercon Ceram (layering veneer ceramic). Maybe, a key reason for this difference in bond strength lays in the use of press-on veneering ceramic versus the layering veneer ceramic. We could not find any studies evaluating the mechanical properties of Biodenta Zirconia System.

In the present study, specimens of Biodenta group revealed predominantly adhesive failure between the zirconia cores and their veneering ceramics, but in Cercon Group, the most failure pattern was combined. Adhesive failure does not occur in the presence of a good bond between compatible ceramic core and veneering materials,[25] so it seems that the cohesive strength of Biodenta ceramics was higher than the bond strength between Biodenta zirconia and the veneering ceramic. In Cercon group the weakest link was the veneering ceramic. Some studies showed that the bond strength of veneering ceramics to zirconia core seems to be higher than the SBS of the ceramic itself. In SBS tests with zirconia/veneering ceramic composites, adhesive failures were the least failure modes seen in these studies,[25303133] whereas others claimed that the SBS of veneering ceramics were higher than SBS between core and veneering ceramics and the failure mode observed was mainly combined as adhesive at the interface and cohesive in the veneering ceramic.[28344142]

Ozkurt et al. showed 80% adhesive and 20% combined failure modes for Cercon zirconia ceramics in their study, and Guess et al. showed that the intrinsic SBS of Cercon Ceram S (33.6 MPa) was significantly higher than the SBS between Cercon zirconia core and its corresponding ceramic (9.4 MPa) which was not consistent with the findings of our study. We could not find any studies about Biodenta zirconia system. In order to compare the strength of the adherence zone and the mechanical properties of the veneering ceramic, determination of the intrinsic SBS of the veneering ceramic is recommended in future studies.

In the present study, all the test specimens were stored in distilled water at 37°C for one week before testing and thermal cycling was also performed for each specimen, which is important in the simulation of clinical conditions, but some critical aspects must be taken into account when using an in vitro method to estimate the clinical performance of materials. First, in-vitro information cannot be used as a direct, straightforward prediction for the clinical situation. Secondly, large variations exist in in vitro test results.[43] The specimens investigated do not represent clinical shape conditions of dental restorations, but provide a geometry that permits SBS measurement. This was another limitation of our study.

Within the limitations of the current study it can be concluded that (1) SBS of veneering porcelain to zirconia core for both Cercon and Biodenta systems did not show significant difference and (2) failure mode for Biodenta system was mostly adhesive while it was mostly combined (adhesive and cohesive) for Cercon system.