Comparative study of the shear bond strength of zirconia-based dental ceramics to resin cements after chemo-mechanical surface modification

Introduction
The loss of retention of zirconia-based fixed partial dentures (FPDs) is one of the most frequently reported failures.

Objectives
The purpose of this study was to evaluate the shear bond strength (SBS) of four resin cements to zirconia after air-borne particle abrasion with alumina of different grades and application of respective primers incorporating organophosphate monomers.

Material and methods
The 443 cuboid-shape zirconia specimens (Ceramill Zi; Amann Girrbach AG) were divided into 16 research groups, according to the chemo-mechanical modification: air-abrasion with alumina particles of 50 μm, 110 μm or 250 μm grade, no treatment and application of primers and resin cements: Clearfil Ceramic Primer/Panavia F2.0 (Kuraray Medical), Monobond Plus/Multilink Automix (Ivoclar Vivadent AG), AZ-Primer/ResiCem (Shofu), Z-Prime Plus/Duo-Link (Bisco). After storage in 37 ± 1°C distilled water for 24 hours, the SBS test was conducted (Zwick/Roell Z005; Zwick GmbH&Co.KG) at 1 mm/min crosshead speed to failure. Specimens were subjected to fractographic analysis at ×2.5 and ×35 magnification (Seliga Auctus; Seliga Microscopes Sp.z o.o.; SEM S-4700; Hitachi High-Technologies Corporation). Statistical significance was established with the Wilcoxon signed-rank test and the Kruskal-Wallis one-way analysis of variance (p = 0.05).

Results
The highest SBS values of zirconia to resin cement were obtained for Z-Prime Plus/DuoLink (8.24-14.15 MPa) and after 50 μm alumina air-abrasion (6.87-14.15 MPa). However, the alumina particle grade was insignificant. A significant decrease in the SBS was observed for untreated specimens (4.60-8.24 MPa). The failure mode in untreated specimens was mainly adhesive, while in sandblasted it was cohesive and mixed.

Conclusions
The chemo-mechanical treatment of zirconia surface before adhesive cementation is crucial to obtain satisfactory values of shear bond strength to resin cements.