Surface Treatment Prior to Cementation of Y-TZP Based Ceramics

Review Article

Austin J Dent. 2014;1(2): 1009.

Surface Treatment Prior to Cementation of Y-TZP Based Ceramics

Jefferson Ricardo Pereira1*, Alan Camilo Possamai Della2, Marcelo Sperandio3, Janaina Salomon Ghizoni4, Vitor Guarçoni de Paula5 and Fabio César Lorenzoni6

1Department of Prosthodontics, University of Southern Santa Catarina, Brazil

2Department of Prosthodontics, University of Southern Santa Catarina, Brazil

3Private Practice Clinic, Campinas-SP, Brazil

4Department of Clinical Practice, University of Southern Santa Catarina, Brazil

5Department of Prosthdontics, University São Paulo-USP, Brazil

6Department of Prosthodontics, University of Southern Santa Catarina, Brazil

*Corresponding author: Pereira JR, Department of Prosthodontics - Dental School - University of Southern Santa Catarina - UNISUL, Tubarão - SC, Brazil

Received: August 04, 2014;Accepted: September 02, 2014; Published: September 04, 2014


A progressive improvement in the mechanical properties of dental ceramics has led to an increase in all-ceramics restorations. The introduction of yttrium-stabilized tetragonal zirconia (Y-TZP) as a ceramic core material extended the design, and application limits of all-ceramic restorations [1-4]. The superior mechanical properties of this polycrystalline material, like high strength, fracture toughness, hardness, wear resistance, it combination with CAD/CAM technology have allow for fabrication of long-span and complex restorations with high accuracy and success rates [1,3,5,6].

In contrast to conventional dental ceramics (veneer ceramic), Y-TZP does not present the glassy phase in its crystallite structure [3,7]. In this way, the absence of a glassy phase impairs the effectiveness of conventional adhesive luting procedures, which include etching ceramic surfaces with hydrofluoric acid and applying silanes coats prior to the use of a resin cement [2,3,5,7-9].

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 [10]. 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 several clinical situations-e.g., when the prepared tooth structure is 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.

The longevity of an indirect restoration is closely related to the retention quality. High retention is important to maintaining integrity of the cement at the margin (prevention of micro leakage) [11], and increasing the fracture/fatigue resistance [11,12]. A 3 years follow-up of zirconia single crowns related that 7% of all Y-TZP crowns placed in the posterior region de bonded [13]. This draw back may suggest that promoting a durable bond between the Y-TZP and tooth structure is still challenging the adhesive dentistry.

Although the loss of retention has never been a topic of interest with Y- TZP restorations (because it has never been reported), efforts have been undertaken to establish ways to achieve a reliable bond between Y-TZP crowns and luting agents [14-19]. Comparatively, data from a literature review [20] showed a 2.8% loss of retention rate in all-ceramic systems (except for zirconia) after five years, while for a metal-ceramic system the loss of retention rate decreases to 0.7% after 10 years [21]. Thus, the effective bond strength between the substrate and ceramic plays an important role in enhancing the longevity of restorative treatments [17,18,22] and in preventing micro leakage [19].

A great diversity of materials for luting ceramics restorations is commercially available. These include zinc phosphate cements, conventional and resin-modified glass ionomer cements, resin cements and self-adhesive resin cements [23]. Conventional cementation techniques do not provide sufficient bond strength for some clinical applications [10]. Resin cements possess some advantages compared with the other classes of materials, since they have lower solubility, higher resistance and better esthetic characteristics [23,24]. In addition, the adhesive interface between the resin cement and ceramic might increase the restoration's resistance during occlusal loads [10,23].

The shear bond strength of 11 different types of cements to a Y-TZP ceramic was evaluated [23]. The results indicated that zinc phosphate and conventional and resin-modified glass ionomer cements were not able to form a durable bond to Y-TZP [23]. In another study, the authors stated that the bond strength of glass-ionomer cements and that of a conventional Bis-GMA-based resin-composite to Y-TZP ceramics is significantly lower, especially after thermal aging [24].

Due to the difficulties in choosing an adequate surface treatment and cementation for zirconia ceramic systems, the aim of this literature review was to investigate the treatment modalities for reinforced ceramic surfaces bonded to resin cements.

Mechanical Bonding - Roughening

Bonding to traditional silica-based ceramics, generally employing both mechanical and adhesive retention, has been well reported and bond strengths are predictable [10]. A strong resin bond relies on micromechanical interlocking created by surface roughening and chemical adhesion between the cement and ceramic [10]. Current techniques are: (1) grinding, (2) abrasion with diamond rotary instruments, (3) surface abrasion with alumina particles, (4) acid-etching (typically hydrofluoric acid [HF]), and (5) a combination of these techniques.

Unfortunately, the chemistry and microstructure of high strength ceramics (specifically alumina and Y-TZP) differ from those of conventional silica-based materials [3,10]. Phosphoric acid (H3PO4 ) or hydrofluoric acid (HF) etching are commonly recommended methods used to surface roughen silica-based ceramics [12]. This creates a rough and clean surface, which improves wet ability and increases surface area available for mechanical interlocking. Unfortunately, H3PO4 and hydrofluoric acid cannot be used effectively on non silica-based ceramics, like Y-TZP and alumina, making it difficult to roughen the surface for mechanical retention [25,26]. The lack of silica also removes the possibility of chemical bonding between silica-silane necessary for silanization.

The roughening surface for the micromechanical interlocking can be achieved, on Y-TZP ceramics, with use of sandblasting with aluminum oxide (Al2O3) particles [12]. The advantage of this surface grinding method is that they are generally easy to apply in a dental environment. However, research has shown that surface grinding techniques alone, using tradition resin cements, have no significant effect on increasing the bond strength of Y-TZP to resin cements [23,25-27].

While sandblasting seems to be a mandatory condition to achieve a reliable bond strength to zirconia [28,29], it can create surfaces flaws, which may act as crack initiation sites decreasing the strength and fracture toughness [12,30,31]. Surface grinding also results in a tetragonal to monoclinic phase change on the surface of zirconia. This can theoretically produce a compressive stress layer that counteracts the flaw-induced reduction in strength [4,30-32]. An in vitro study shows that sandblasting produced the most effective tetragonal to monoclinic phase change when compared to fine polishing, grinding with an abrasive wheel, or grinding using a diamond bur [32]. It was determined that sandblasting was able to induce transformation at low temperature, with minimal surface damage. Care has to be taken with the amount of surface grinding, as an excess amount can diminish the strength enhancing effect.

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.

Sandblasting with Al2O3 particles [33], tribochemical silica coating [17], selective infiltration etching [14] and experimental hot etching solution [34] have enhanced the zirconia bond strength. However, the concept employed for all of these methods relies on the micromechanical interlocking between the zirconia and the cement. Whereas sandblasting has been associated with micro cracks, which may decrease the mechanical properties of Y- TZP [30, 31], neither selective infiltration etching or experimental hot etching solution has not been examined with regard to this defect to date. Furthermore, these procedures are invasive methods and often require special equipment. Moreover, there is no consensus about the most appropriate or reliable method with which to bond Y-TZP restorations [22].

Chemical / Mechanical Bonding

Other types of micromechanical adhesion is the association of silica particles in sandblasting, which promote a silica coating on acid-resistant ceramic structure by interlocking silica particles in the ceramic surfaces. Thus, the silane is able to act on the treated surface and promote adhesion chemistry enhancing micromechanical bond [12].

Silanes are compounds that contain silicon (Si) atom or atoms, are similar to orthoesters in structure, and display dual reactivity. Their use in clinical dentistry and effect on adhesive bonding has been described in detail in the scientific literature [15,25,35-37]. One end of a silane molecule is organically functional (e.g., vinyl-CH CH2, amino-NH2), and can polymerize with an organic matrix (e.g., a methacrylate). The other end is generally comprised of alkoxy groups (e.g., methoxy-OCH3, ethoxy-OCH2 CH3), which can react with a hydroxylated surface, like porcelain. Silanes are commonly used in dentistry to coat glass filler particles in polymer matrix composites, to achieve adhesive bonding of porcelain (or other silica-containing ceramics) to resin luting cements for restorative applications, and with certain ceramic or ceramic-containing composite posts for endodontic applications employing resin-based filling materials. Silanes are also believed to promote surface wetting, which enhances potential micromechanical retention with low viscosity resin cements [37,38]. Traditional silane chemistry is not truly effective with Y-TZP, as it possesses a relatively non-polar surface, is more chemically stable than silica-containing ceramics, and not easily hydrolyzed.

Aboushelib et al. [39] showed that application of silane alone on Y-TZP resulted in low bond strength. The use of five silanes (MPS (3-methacryloyloxypropyl-trimethoxysilane)ACPS (3-acryloyloxypropyl-trimethoxysilane) and ICS (3-isocyanatopropyl-triethoxysilane) along with styrylethyltrimethoxysilane and 3-(N-allylamino propyltrimethoxysilane) to aid in luting of as-received Y-TZP resulted in bond strengths that were significantly less than when using the silanes on SIE (selective infiltration etching) Y-TZP. It was shown that MPS produced greater bond strength when used on SIE Y-TZP compared to the other silanes. However, bond strength of SIE Y-TZP using the silanes decreased significantly after long-term storage [15]. This decrease in bond strength demonstrates that use of silanes does not aid in producing a hydrolytically stable bond with Y-TZP. Although bond strength decreased after time, SIE does create a retentive surface for mechanical bonding. It is also possible that SIE could chemically modify the surface to improve bonding between the silane and Y-TZP.

Due to the absence of silica in Y-TZP, 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 [10,23,26,27,40-42] with the Co Jet and Rocatec systems (3M ESPE, Seefeld, Germany) being the most heavily favored commercial products utilized for applying the coating. The tribochemical technique air-abrades the ceramic surface with alumina particles that have been coated with silica, embedding/coating the surface with silica [43,44]. It results not only in preparing the surface for silanization, but also to creates the micromechanical retention [10]. Research has shown that the application of a tribochemical coating before silanization significantly enhances bond strength between a treated substrate and resin cement [10,23,26,27,40-42]. The bound silica particles serve as reactive sites for conventional organo-silane monomer primers, however residual bond strengths tend to be lower than those obtained on conventional dental porcelain surfaces [10]. Furthermore, there can be significant loss in bond strength over the long-term when using traditional resin cements used for silica-based ceramics. This might be a result of a low concentration of silica on the surface due to difficulty in particle abrasion caused by the high hardness of Y-TZP [12].