Adhesion/cementation to zirconia and other non-silicate ceramics: Where are we now?

Abstract

Non-silicate ceramics, especially zirconia, have become a topic of great interest in the field of prosthetic and implant dentistry. A clinical problem with use of zirconia-based components is the difficulty in achieving suitable adhesion with intended synthetic substrates or natural tissues. Traditional adhesive techniques used with silica-based ceramics do not work effectively with zirconia. Currently, several technologies are being utilized clinically to address this problem, and other approaches are under investigation. Most focus on surface modification of the inert surfaces of high strength ceramics. The ability to chemically functionalize the surface of zirconia appears to be critical in achieving adhesive bonding. This review will focus on currently available approaches as well as new advanced technologies to address this problem.

Introduction

Developments over the last 10–15 years in ceramic materials science for dental applications have led to a class of high strength materials (i.e., zirconia-based ceramics) which potentially provide better fracture resistance and long-term viability when compared to porcelain and other inorganic, non-metallic alternatives. There is a wealth of information in the scientific literature regarding the use of zirconia (ZrO₂) in dental applications [1], [2], [3]. Although superior in terms of mechanical performance (strength, toughness, fatigue resistance), there are some inherent problems associated with ZrO₂. One problem is with adhesion to the variety of substrates (synthetics or tissues) that can be encountered in dental or other biomedical applications. Conventional cementation/attachment techniques used with ZrO₂ components do not provide sufficient bond strength for many of these applications [4], [5], [6]. It is important for high retention, prevention of microleakage, and increased fracture/fatigue resistance, that bonding techniques be improved for zirconia-based systems. Strong resin bonding relies on micromechanical interlocking and adhesive chemical bonding to the ceramic surface, requiring surface roughening for mechanical bonding and surface activation for chemical adhesion. In some instances, high strength ceramic restorations do not require adhesive bonding to tooth structure and can be placed using conventional cements which rely only on micromechanical retention. However, resin bonding is desirable in many clinical situations—e.g., when the prepared tooth structure is unusually short or tapered. In addition, it is likely that strong chemical adhesion would lead to enhanced long-term fracture and fatigue resistance in the oral environment. Non-destructive methods for treating inert ceramics to produce an activated/functionalized surface are desirable in such cases. These methods could also be used in endodontic and implant applications, where ZrO₂ has become a prominent material for fabricating posts and implant components [7], [8], [9], [10], [11], [12], and where resin-based endodontic filling materials are often employed, and adhesive bonding is desired.

Bonding to traditional silica-based ceramics, generally employing both mechanical and adhesive retentions, has been well researched, and bond strengths are predictable. A strong resin bond relies on chemical adhesion between the cement and ceramic (by way of silane chemistry), and on micromechanical interlocking created by surface roughening. Current roughening techniques are: (1) grinding, (2) abrasion with diamond (or other) rotary instruments, (3) air abrasion with alumina (or other) particles, (4) acid etching (typically HF), and (5) a combination of any of these techniques. Unfortunately, the composition and physical properties of ZrO₂ differ from conventional silica-based materials like porcelain. Zirconia is not readily etched by HF, and requires very aggressive mechanical abrasion methods to increase surface roughness, possibly creating strength reducing surface flaws [13], [14], [15]. Therefore, in order to achieve acceptable cementation in a wide range of clinical applications, alternate attachment methods, ideally utilizing chemical adhesion in addition to mechanical retention, are required for zirconia ceramics. Various approaches to this problem will be discussed in this review.