To evaluate the effect of cyclic loading on the bond strength of resin composite to zirconia framework material.
Bar shaped zirconia/composite specimens (2 mm × 2 mm × 25 mm) were prepared using three different resin cements and placed in a four-point bending test setup. The flexure strength ( F s ) was calculated by placing the bars ( n = 10) fixed between the four supports (at 10 and 20 mm) with the interface centered between the inner rollers and subsequently loaded (1 mm/min crosshead speed) until fracture. Rotating fatigue resistance (RFR) was determined in a rotating bending cantilever test setup (104, 1.2 Hz) with the highest stress located at the interface ( n = 20). The RFR was determined by the staircase method and the mean RFR was calculated using logistic regression analysis.
Resin cement composition had no significant influence on the bond strength value obtained by both F s ( F = 0.6, P ≥ 0.5) and RFR ( F = 1.1, P ≥ 0.3) tests. However, after rotating fatigue testing there was a significant reduction in bond strength between 46 and 50% of the three resin cements.
Zirconia resin bond strength is liable to deterioration under the influence of fatigue.
The good mechanical properties and the superior fracture toughness of zirconia made it a favorable framework material for the construction of all-ceramic restorations . On the other hand its chemical stability and surface inertness, made bonding to this material a challenging process. Previous studies proved that, neither hydrofluoric acid etching nor silanization resulted in a satisfactory resin bond to zirconia . Many studies reported that airborne-particle abrasion increased bond strength to zirconia by increasing the surface area, resulting in acceptable micro-mechanical retention .
Moreover, the size of aluminum oxide particles, pressure and distance used, and the time spent all had significant influence on the surface of zirconia. Using course particles at higher pressure resulted in rougher zirconia surface . It should also be noticed that increasing the surface roughness of zirconia had a negative influence on its flexure strength and long term clinical performance.
Tribo-chemical silica coating has also been proposed as an alternative conditioning method . Such technique is supposed to provide ultra-fine mechanical retention by embedding silica coated alumina particles in the treated surface. Atsu et al. showed that application of mixture of MDP and silane-coupling agent increased the shear bond strength between zirconium oxide ceramic and resin cement.
Up to date, the combination of airborne-particle abrasion by pure aluminum oxide particles in combination with MDP, an organofunctional phosphate monomer, is the most recommended method for bonding to zirconia frameworks. Several studies reported relevant bond strength stability after long term water storage .
In a recent study, application of MDP monomer on non-retentive (as-sintered) zirconia surface failed to produce any bond with zirconia as most of the specimens, demonstrated premature failure, which questioned the role of this MDP agent . Similar findings were observed for novel zirconia primers which demonstrated adequate bond strength to roughened zirconia surface but revealed low bond strength on as-sintered zirconia surface indicating that micro-mechanical retention is a pre-requisite for achieving reliable bond strength with zirconia. In a following study, these new zirconia primers demonstrated reduction in zirconia resin bond strength after 90 days of water storage which was attributed instability of the chemical structure of these primers . Yang et al. also reported initial chemical bonding between phosphate monomer containing primers and polished zirconia surface . However, these chemical bonds were not water resistant and bond strengths were reduced to 0 MPa after long term water storage. Such finding also questions the role of chemical retention with zirconia frameworks.
Selective infiltration etching of zirconia-based materials was introduced to create a retentive surface; here the adhesive resin can infiltrate and interlock in order to establish a strong and a durable bond with this framework material . In a previous study novel zirconia primers were used in combination with selective infiltration etching in order to enhance the bond strength. In that study there were significant differences in the bond strength between the primers used, while zirconia surface was always the same. The differences were related to the reactivity of these primers. In combination with the nano-rough surface created by selective infiltration etching, improved zirconia resin bond strength was achieved .
Establishing a strong bond with zirconia is only one part of the challenge as maintaining this bond under the circumstances of a relatively aggressive oral environment, fatigue, presence of water and other active chemicals, is the ultimate goal. In several studies investigating the influence of accelerated artificial aging using water storage, thermo-cycling, or fatigue reduction in the zirconia resin bond strength was observed .
Bond strength can be adequate when measured immediately, but could deteriorate with time, causing loss of retention, and microleakage. Two main mechanisms of deterioration of the established bond strength have been proposed; mechanical fatigue and hydrolytic degradation . Fatigue can result from stresses placed on the bond due to, thermal expansion and contraction, occlusal forces, or pre-stress induced by the polymerization shrinkage of the resin composite . To improve bond stability, there is a need to understand the mechanisms of bond degradation. Hydrolytic or chemical bond degradation was assumed to be related to the diffusion phenomena of liquids, which is time dependent, e.g. it takes time to penetrate the bonded interface and to cause chemical breakdown .
To improve bonding to zirconia, it is important to understand which mechanism of degradation is most important. If it is hydrolytic degradation, then strategies should be directed towards making the bonded interface more chemically stable especially in the presence of saliva. On the other hand, if it is fatigue dependent then toughening of the interface and inhibition of crack propagation should be considered. The purpose of this study was to test the zirconia resin bond strength and stability under the influence of rotating fatigue. The proposed hypothesis was that mechanical fatigue will decrease the ultimate zirconia resin bond strength.
Materials and methods
Preparation of the specimens
Bar shaped zirconia specimens (2 mm × 2 mm × 12.5 mm) were prepared by cutting zirconia milling blocks (Procera zirconia, Nobel Biocare, Göteborg, Sweden) using a precision cutting instrument (Isomet 1000; Buehler, Lake Bluff, IL, USA) and a diamond-coated cutting disc (Diamond Wafering Blade, No 11-4276; Buehler). The location of the cuts was controlled using a traveling stage and a horizontally displaced digital micrometer (ID-C1508; Mitutoyo Corp, Utsunomiya, Japan). The cut specimens were sintered using the relevant electrical induction furnace (Cercon Heat, DeguDent, Hanau, Germany), which uses a 6.5 h sintering program with a maximum temperature of 1350 °C. The sintered bars were polished using an ascending stepwise approach starting with 120-grit and ending with 800-grit silicon carbide paper (Microcut; Buehler); using a rotating metallographic polishing device (Ecomet; Buehler) under a fixed load (200 g) and water cooling. The bonding surface (the cross-section surface of the bar) of each specimen was airborne-particle abraded (P-G 400; Harnisch & Rieth, Winterbach, Germany) with 120-μm aluminum oxide particles (S-U-Alustral; Schuler-Dental, Ulm, Germany) at 0.35 MPa pressure for 25 s/cm 2 at a distance of 2 cm followed by ultrasonic cleaning first in 90% alcohol then in distilled water for 15 min.
Hereafter the zirconia bars were placed in a split mold and three different adhesive resin cement systems, Panavia F 2.0 (Kuraray Co Ltd, Tokyo, Japan), RelyX Unicem (3M ESPE, St. Paul, MN, USA), and Multilink Automix (Ivoclar-Vivadent, Schaan, Liechtenstein) were used independently coated on the abraded zirconia surface according to the manufacturers’ instructions ( Table 1 ). The remaining part of the split mold was filled with composite resin (Filtek Z250, shade A2; 3 M ESPE) according to the ISO 4049 standard. After light polymerization (Elipar FreeLight 2; 3M ESPE) and 24 h water storage the zirconia-composite resin beams were finished on 600 grit wet abrasive paper to remove exceed composites. Great care was taken during the polishing to avoid generation of any undue stresses on the bonded specimens. Fig. 1 shows a step to step approach for preparing the specimens.
|Multilink Automix a||The monomer matrix is composed of dimethacrylate (DMA), HEMA, Ba-glass fillers, ytterbium fluoride, spheroid mixed oxide||K54378|
|A primer: aqueous solution of initiator|
|B primer: HEMA and phosphoric acid and acrylic acid monomers|
|Metal/zirconia primer: phosphoric acid acrylate and methacrylate cross-linking agents in an organic solution|
|Panavia F2.0 b||A paste: silica, dimethacrylate monomer, functional acid MDP, photo initiator, accelerator||41233|
|B paste: brown color, barium glass, sodium fluoride, DMA monomer|
|RelyX UniCem c||Powder: glass powder, initiator, silica sil., pyrimidine, calcium hydroxide, peroxy compound, pigment||288018|
|Liquid: methacrylated phosphoric ester, DMA, acetate, stabilizer, initiator|