To evaluate and compare bond strengths of different primers and resin cements to silica-based and zirconia ceramics.
Silica-based and zirconia ceramic specimens were ground flat with #600-grit SiC paper. The ceramic surfaces were airborne-particle abraded and then divided into 11 groups of seven each: untreated (control); and conditioned with one of the six primers in combination with a resin cement from the same manufacturer as follows: Bistite II/Tokuso Ceramic Primer, Linkmax/GC Ceramic Primer, RelyX ARC/RelyX Ceramic Primer, Panavia F 2.0/Clearfil Ceramic Primer, and Resicem/Shofu Porcelain Primer and Resicem/AZ Primer. Stainless steel rods were bonded to the ceramic surfaces using one of the five resin cements. After 24-h water storage, the tensile bond strengths were tested using a universal testing machine and failure modes were examined.
Conditioning with primers containing a silane coupling agent (all the primers except AZ Primer) significantly enhanced bond strengths of resin cements to silica-based ceramic. For zirconia ceramic, Resicem/AZ Primer exhibited significantly higher bond strength than the other groups except Panavia F 2.0/Clearfil Ceramic Primer. The predominant failure mode of the groups conditioned with primers containing a phosphonic acid monomer (AZ Primer) or a phosphate ester monomer (Clearfil Ceramic Primer and Tokuso Ceramic Primer) was cohesive failure in cements whereas that with the other primers was adhesive failure at the zirconia surfaces.
The use of primers containing a silane coupling agent improved resin bonding to silica-based ceramic. On the other hand, the use of primers containing a phosphonic acid monomer or a phosphate ester monomer improved resin bonding to zirconia ceramic.
The popularity of all-ceramic restorations has increased in recent years due to superior esthetic appearance and metal-free substructure of these materials . All-ceramic restorations can be fabricated in many systems using different ceramic materials . Computer-aided design and manufacturing (CAD/CAM) has become an increasingly interesting alternative to the conventional casting or pressing techniques. The clinical success of CAD/CAM-fabricated densely sintered high-purity alumina ceramic relies on its high flexural strength and fracture resistance compared with other ceramics, such as feldspathic, leucite-reinforced, and glass-infiltrated alumina ceramics .
Yttria partially stabilized tetragonal zirconia polycrystalline (Y-TZP) ceramic, which is used as an orthopedic material , exhibits high flexural strength (>1000 MPa) and fracture toughness (>9–10 MPam 1/2 ) . Y-TZP ceramic has been recently introduced to restorative dentistry, based on these improved physical properties compared with alumina-based ceramics. CAD/CAM technologies have contributed to the ease of working with this high crystalline material, allowing the fabrication of frameworks for complete coverage all-ceramic crowns and fixed partial dentures .
Similar to any restorative material, successful cementation of Y-TZP ceramic is important for the clinical success of the restoration . Zirconia crowns and fixed partial dentures have high fracture resistance and can be cemented using conventional methods recommended by the manufacturers . However, resin bonding between a dental substrate and a restoration is advocated for improved retention, marginal adaptation, and inhibition of secondary caries .
Obtaining adhesion between resin cement and ceramic surface requires surface pretreatment . The use of a silane coupling agent is recommended for glasses and porcelains in order to form a siloxane network with the silica in the ceramic surface and improve the bond strength between the resin cement and the ceramic . However, this technique may not improve the bond strength of zirconia and alumina ceramics because such a chemical reaction is not possible with those substrates . Micromechanical interlocking of the cement and the ceramic surface may be improved on roughened restoration surfaces. High crystalline content makes zirconia and alumina ceramics resistant to roughening by hydrofluoric acid etching . Therefore, in order to increase the micromechanical interlocking, surface roughening by airborne-particle abrasion has been introduced as an alternative method for these high-strength ceramics .
Blatz et al. compared the bond strengths of different combinations of bonding/silane coupling agents and resin cements to zirconia ceramic. The authors concluded that conditioning with a bonding/silane agent, containing a phosphate ester monomer, MDP (10-methacryloyloxydecyl dihydrogen phosphate), yielded superior resin bonds to zirconia surface airborne-particle abraded with Al 2 O 3 particles.
A new primer (AZ Primer, Shofu, Kyoto, Japan) containing a phosphonic acid monomer, 6-MHPA (6-methacryloxyhexylphosphonoacetate), has been marketed for bonding to alumina and zirconia ceramics; however, no information has been reported in the literature regarding the adhesiveness of this primer to zirconia.
In vitro investigations are indispensable to indentify superior materials before their clinical evaluation, especially for comparative studies of primer/cement combinations. The purpose of this study was to examine the effect of primer treatment on tensile bond strengths of five resin cements to silica-based and zirconia ceramics. The null hypotheses proposed were: (i) the use of a primer did not improve the resin cement bond to silica-based ceramic, and (ii) the use of a primer did not improve the cement bond to zirconia ceramic.
Materials and methods
Materials used in the study
The ceramic materials used in the present study are shown in Table 1 . Seventy-seven silica-based ceramic specimens (GN-1 Ceramic Block; GC, Tokyo, Japan) were obtained from the manufacturer. The dimensions of these specimens were 13 mm × 17 mm × 21 mm. Also, 77 pre-sintered Y-TZP ceramic specimens were milled from ingots (Cercon Base; Degudent, Hanau, Germany) and sintered according to the instructions in the relevant equipment of the supplier (Cercon Heat; Degudent). The cylindrical specimens were approximately 15 mm in diameter and 2 mm in thickness.