Microtensile bond strength of a resin cement to glass infiltrated zirconia-reinforced ceramic: The effect of surface conditioning

Summary

Objectives

This study evaluated the effect of three surface conditioning methods on the microtensile bond strength of resin cement to a glass-infiltrated zirconia-reinforced alumina-based core ceramic.

Methods

Thirty blocks (5×5×4 mm) of In-Ceram Zirconia ceramics (In-Ceram Zirconia-INC-ZR, VITA) were fabricated according to the manufacturer's instructions and duplicated in resin composite. The specimens were polished and assigned to one of the following three treatment conditions (n=10): (1) Airborne particle abrasion with 110 μm Al₂O₃ particles + silanization, (2) Silica coating with 110 μm SiO_x particles (Rocatec Pre and Plus, 3M ESPE) + silanization, (3) Silica coating with 30 μm SiO_x particles (CoJet, 3M ESPE) + silanization. The ceramic-composite blocks were cemented with the resin cement (Panavia F) and stored at 37 °C in distilled water for 7 days prior to bond tests. The blocks were cut under coolant water to produce bar specimens with a bonding area of approximately 0.6 mm². The bond strength tests were performed in a universal testing machine (cross-head speed: 1 mm/min). The mean bond strengths of the specimens of each block were statistically analyzed using ANOVA and Tukey's test (α≤0.05).

Results

Silica coating with silanization either using 110 μm SiO_x or 30 μm SiO_x particles increased the bond strength of the resin cement (24.6±2.7 MPa and 26.7±2.4 MPa, respectively) to the zirconia-based ceramic significantly compared to that of airborne particle abrasion with 110-μm Al₂O₃ (20.5±3.8 MPa) (ANOVA, P<0.05).

Significance

Conditioning the INC-ZR ceramic surfaces with silica coating and silanization using either chairside or laboratory devices provided higher bond strengths of the resin cement than with airborne particle abrasion using 110 μm Al₂O₃.

Introduction

Etching the inner surfaces of ceramics with glassy matrix using hydrofluoric acid followed by the application of a silane coupling agent is an efficient conditioning method for bonding resin composite [1], [2], [3], [4], [5], [6]. However neither etching with this agent nor adding silane resulted in an adequate resin bond to some new high-strength ceramics [7], [8]. Particularly high-alumina [9], [10], [11], [12] or zirconia-reinforced ceramics [13], [14] cannot be roughened by hydrofluoric acid etching since such ceramics do not contain a silicon dioxide (silica) phase. Similarly, cement adhesion to glass-infiltrated zirconia–alumina ceramic (In-Ceram Zirconia-INC-ZR) is also not favorable since this ceramic presents the same characteristics due to its high crystal content (aluminum oxide: ±67 wt%; zirconium oxide: ±13 wt%) and limited vitreous phase (lanthanum aluminum silicate: ±20 wt%) [15]. For this reason, special conditioning systems are indicated for these types of ceramics [16].

Previous investigations revealed that most clinical failures have initiated from the cementation or internal surfaces. Failure rates due to high-strength ceramic fractures have been reported to range between 2.3 and 8% [17], [18], [19]. Therefore, the integrity of the luting cement to ceramic surfaces plays a major role in the longevity of the restoration and the failures originated from cementation surfaces identified the need for a reliable conditioning method to strengthen this critical area.

Modern surface conditioning methods require airborne particle abrasion of the surface before bonding in order to achieve high bond strength. One such system is silica coating. In this technique, the surfaces are air-abraded with aluminum oxide particles modified with silisic acid [20], [21]. The blasting pressure results in the embedding of silica particles on the ceramic surface, rendering the silica-modified surface chemically more reactive to the resin through silane coupling agents. Silane molecules, after being hydrolized to silanol, can form polysiloxane network or hyroxyl groups cover the silica surface. Monomeric ends of the silane molecules react with the methacrylate groups of the adhesive resins by free radical polymerization process. When a ceramic exhibits chemical states of silicon and oxygen, then siloxane bond will be achieved as these represent the binding sites for the coupling agent to the ceramic surface. Since silane coupling agents do not bond well to alumina, the bond strengths of resin composite to such ceramics could be affected [10].

Air-particle abrasion is a prerequisite for achieving sufficient bond strength between the resins and high-strength ceramics that are reinforced either with alumina or zirconia [22]. The air abrasion systems rely on air-particle abrasion with different particle sizes ranging from 30 to 250 μm [16], [23]. The abrasive process removes loose contaminated layers and the roughened surface provides some degree of mechanical interlocking or ‘keying’ with the adhesive. It can be argued that the increased roughness also forms a larger surface area for the bond. While these mechanisms explain some of the general characteristics of adhesion to roughened surfaces, it may also introduce physico-chemical changes that affect surface energy and wettability. Such conditioning systems could be applied either at the laboratory or chairside, using large or small size particles. However, there is limited knowledge as to whether micromechanical retention using large or small particle size increase resin bond to high-strength ceramics of different microstructures and chemical compositions.

A high and reliable resin bond to alumina and zirconia ceramics was also achieved with airborne particle abrasion and by using a phosphate monomer (MDP) containing resin composite luting cement. Although there are some studies on bond strength of resin cements to the zirconium-based ceramics [24], [25], [26], to the authors' knowledge, no study has investigated the bond strength of phosphate-monomer based resin cement to zirconium-reinforced ceramics in combination with conditioning methods that rely on chairside conditioning systems.

The aim of this study, therefore, was to evaluate the effect of three surface conditioning methods based on airborne particle abrasion, employing three types of sand particles, on the microtensile bond strength of the resin cement to a glass-infiltrated zirconia-reinforced ceramic.