Shear bond strength of veneering porcelain to zirconia and metal cores

Abstract

Statement of problem: Zirconia-based restorations have the common technical complication of delamination, or porcelain chipping, from the zirconia core. Thus the shear bond strength between the zirconia core and the veneering porcelain requires investigation in order to facilitate the material's clinical use.

Purpose: The purpose of this study was to evaluate the bonding strength of the porcelain veneer to the zirconia core and to other various metal alloys (high noble metal alloy and base metal alloy).

Material and methods: 15 rectangular (4×4×9mm) specimens each of zirconia (Cercon), base metal alloy (Tillite), high noble metal alloy (Degudent H) were fabricated for the shear bond strength test. The veneering porcelain recommended by the manufacturer for each type of material was fired to the core in thickness of 3mm. After firing, the specimens were embedded in the PTFE mold, placed on a mounting jig, and subjected to shear force in a universal testing machine. Load was applied at a crosshead speed of 0.5mm/min until fracture. The average shear strength (MPa) was analyzed with the one-way ANOVA and the Tukey's test (α= .05). The fractured specimens were examined using SEM and EDX to determine the failure pattern.

Results: The mean shear strength (± SD) in MPa was 25.43 (± 3.12) in the zirconia group, 35.87 (± 4.23) in the base metal group, 38.00 (± 5.23) in the high noble metal group. The ANOVA showed a significant difference among groups, and the Tukey's test presented a significant difference between the zirconia group and the metal group. Microscopic examination showed that the failure primarily occurred near the interface with the residual veneering porcelain remaining on the core.

Conclusion: There was a significant difference between the metal ceramic and zirconia ceramic group in shear bond strength. There was no significant difference between the base metal alloy and the high noble metal alloy.

Keywords: Core-veneer ceramic; Delamination; Failure mode; Shear bond strength; Zirconia ceramic.

Fig. 1

Final dimensions of specimens.

Fig. 2

Schematic diagram of specimen embedded in PTFE molds.

Fig. 3

Schematic diagram of SBS test.

Fig. 4

SEM image of zirconia-veneer group (Group I). (A) The arrow indicates the direction of load. The loaded side demonstrates cohesive failure within the veneering porcelain (original magnification × 30), (B) Note many pores within veneering 'porcelain (arrow), where fracture originated. The fractured Cercon Ceram kiss veneer demonstrates multiple cracks extending in a vertical direction (Hackle patterns) (original magnification × 250), (C) High magnification SEM image exhibited a very thin layer of porcelain covering zirconia grains (original magnification × 1000).

Fig. 5

SEM image of base metal alloy-veneer group (Group II). (A) The arrow indicates the direction of load. The loaded side demonstrates cohesive failure within the veneering porcelain (original magnification × 30), (B) Interface of the veneering porcelain and the metal core (original magnification × 250), (C) High magnification SEM image exhibited an opaque layer and an oxide layer (original magnification × 1000).

Fig. 6

SEM image of high noble metal alloy-veneer group (Group III) (A) The arrow indicates the direction of load (original magnification × 30), (B) Predominance of cohesive failure (original magnification × 250), (C) High magnification SEM image exhibited an opaque layer and oxide layer (original magnification × 1000).

Fig. 7

EDX results of Group I, II, III. (A) EDX results of Group I showed the presence of a thin porcelain layer over the zirconia, (B) EDX results of the fractured base metal alloy surface (Group II) demonstrated an exposed metal surface with some ceramic remainder, (C) EDX results of the fractured high noble metal alloy surface (Group III) presented no exposed metal surface.