Evaluating shear bond strength of zirconia cores and veneering porcelain with different test designs

In all-ceramic systems, with high mechanical properties zirconia based ceramics are distinguished. Unfortunately, zirconia is not as translucent as natural teeth. To improve the esthetics, zirconia cores are veneered with feldspathic ceramic. The most common failure faced with zirconia based ceramic restorations are delamination of the veneering porcelain whole or in the form of layers from the core material. To improve the zirconia core - veneering porcelain bonding mechanism, researchers have been made plenty of laboratory tests. Shear bond strenght (SBS) tests are commonly used to measure the bond strength. Small differences in the SBS tests can make significant differences on the bond strength values. The aim of this study is to investigate the influence of using the wire loop (WL), notched rod (NR) and knife edge chisel (NEC) knife designs on SBS values. Three zirconia core ceramics (Zirkonzahn Translucent, Upcera Super Translucent, Rainbow Trans) and their manufacturer recommended veneering ceramics (Zirkonzahn ICE Porcelain, Cerabien ZR Porcelain, Rainbow Porcelain) was used to perform SBS tests with different knife designs. Metal ceramic system (EOS CobaltChrome SP2 – GC Initial Porcelain) was used as a control group. All specimens was thermocycled (5–550 C, 3,000 cycles) for demonstrated clinical use. There were 90 specimens (n=10) in zirconia core test groups. Control group contained 30 samples (n=10). SBS tests were performed with WL knife design in first group, with NR knife design in second group and with KEC knife design in third group. Shear force applied to all specimens in a universal test machine with a cross head speed of 0,5 mm/min. Statistical analyses were performed Kuruskal Wallis test and Conover's post hoc test were used for multiple comparision. Fractured surface specimens were first evaluated with microscope and second with Scanning Electron Microscope (SEM). Results of statistical analyses shoved that there was no significant difference between WL, NR and KEC knife designs for zirconia specimens. But, for metal-ceramic control group, there was statistically significant difference between knife designs. WL group showed higher SBS values than NR and KEC groups. For the shear strength values, metal ceramic groups values were significantly higher than zirconia ceramic groups for all knife designs. SBS is a commonly used test which effected by the application type and applicator's skills. Further standardization is required to provide more accurate results and comparisons with previous studies.