Testing Methods to Determine the Shear-Bond-Strength of Veneering-Porcelain to Zirconia

Objectives: This study evaluated different testing methodologies for measurement of zirconia to veneering porcelain bond strength.

Methods: Three different methodologies evaluated with all specimens fabricated with CAD/CAM zirconia (Zeno, WielandImes) and veneering porcelain [(VP) Zirox, Wieland]. The zirconia surface was treated as per manufacturer's directions (110 mm alumina abrasion @2 bars). 30 specimens were fabricated according to Schwickerath bending test (SBT). Zirconia bars (25x3x0.5mm) with veneering porcelain (5x3x1.1mm) in the center of tension were loaded according to the three-point-bending-test. Specimens were loaded until failure of the VP and further until fracture of the zirconia substructure. The flexural-strength of zirconia was calculated additionally. The second method was the testing of a standard crown according to Voss (VSC). Twelve central incisor crowns were tested at a 30°angle noting the first crack and complete failure levels. 30 specimens were made for the shear test according to Schulmeyer (STS) consisting of VP (5.8x3x3mm) fired to zirconia (5.9x5.9x2mm). The specimens were loaded at VP with a 45°-indenter and tested until fracture of VP. Descriptive and Weibull analyses were carried out for all groups. One-way ANOVA was employed to detect differences between the testing methods.

Results: SBT-specimens exhibited a mean bond-strength of 57.0 (±20.7)MPa showing mostly adhesive fracture patterns. The flexural strength of the zirconia bar was 974.1 (±125.2)MPa. VSC-specimens showed the first crack at 276.6 (±99.8)N, while further loading led to fracutre into multiple pieces. STS-specimens exhibited a mean bond-strength of 20.0 (±7.1)MPa showing mostly cohesive fracture patterns. Results from the different tests were statistically significant (p<0.001), however, comparison of VSC to the other tests was not possible.

Conclusions: The SMT appears the best method for evaluating shear bond strength since adhesive fracture patterns occurred and the influence of surface-treatments prior to veneering to the flexural-strength of the zirconia substructure can be evaluated.