Adhesion/cementation to zirconia and other non-silicate ceramics: Where are we now?
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.
Section snippets
Zirconia as a biomaterial
Zirconium oxide (ZrO2), or zirconia, is a metal oxide that was identified as a reaction product of heating the gem, zircon, by the German chemist Martin Heinrich Klaproth in 1789 [16]. Zirconia is polymorphic in nature, meaning that it displays a different equilibrium (stable) crystal structure at different temperatures with no change in chemistry. It exists in three crystalline forms: monoclinic at low temperatures, tetragonal above 1170 °C and cubic above 2370 °C [17], [18]. A characteristic of
Mechanical bonding
Bonding of ZrO2 to tooth structure or other substrates requires a strong resin bond. The success of resin bonding relies on mechanical bonding through micromechanical interlocking from surface roughening, and if possible, chemical bonding between ceramic and cement. Phosphoric acid (H3PO4) or hydrofluoric acid (HF) etching are commonly recommended methods used to surface roughen silica-based ceramics [75]. This creates a rough, clean surface, which improves wettability and increases surface
Chemical bonding—silane coupling agents
Organo-silanes, generally referred to simply as “silanes” in dentistry, are compounds that contain a silicon (Si) atom or atoms, are similar to orthoesters in structure, and display dual reactivity. Their use in clinical dentistry and affect on adhesive bonding has been described in detail in the scientific literature [4], [48], [58], [59], [87], [88], [89], [90], [91], [92], [93], [94]. One end of a silane molecule is organically functional (e.g., vinyl–CHCH2, amino–NH2), and can polymerize
Primers—silica coating
Due to the lack of silica in ZrO2, silica-coating techniques have been explored to utilize the chemical bonding provided by silanization. The use of a tribochemical silica coating is a common practice for coating metal alloys and alumina- and zirconia-based dental ceramics with silica [12], [47], [51], [54], [79], [98], [99], [100], [101], [102], [103], [104] with the CoJet and Rocatec systems (3M ESPE, Seefeld, Germany) being the most heavily favored commercial products utilized for applying
Luting of zircoinia
Resin-based composite cements are the standard material used in luting a ceramic prosthetic to tooth structures [116]. Resin-based composite cements have compositions and characteristics similar to conventional restorative composites and consist of inorganic fillers embedded in an organic matrix (e.g., Bis-GMA, TEGDMA, UDMA). Retention of a dental restoration to tooth structure and sealing of the marginal gap between the restoration and tooth are dependent on the luting agent's ability to bond
Bonding of veneering material to zirconia
The ability to accurately fabricate ZrO2 sub-structures (copings) has improved dramatically in recent years. However, ZrO2 copings for crowns or multi-unit frameworks still require application of veneering ceramic, usually specialized porcelain, to achieve suitable esthetics. A high percentage of clinical failures of ZrO2-based dental prosthetics reported in the literature are attributed to debonding and/or fracture of veneering ceramic. Failure rates due to veneer debonding and/or fracture as
Summary
Although the science and technology applied to adhesion/bonding issues with ZrO2 have improved, there is still much to be learned to make this a predictable behavior for clinical use.
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