Adhesion of adhesive resin cements to dental zirconia ceramic and human dentin
In this work, the long-term bond strengths of adhesive resin cements to zirconia ceramic and human dentin were evaluated, and resin-ceramic and resin-dentin bonding mechanisms were investigated. In chapter 3, the influence of surface pre-treatment on the bonding durability of three resin cements (Super-Bond C&B resin cement : SB, Clearfil™ Esthetic cement: CEC, Chemiace II: CH) to zirconia ceramic was studied. Most importantly, the influence of chemical reactions of functional monomers in primer on the resin-ceramic bonding were investigated. Within the limitation of this study, priming pre-treatment with primers can increase the bonding durability of three resin cements to airborne-particle abraded ziconia ceramic. Only silane is important for the initial resin-ceramic bonding, but are not able to produce a strong durable resin-ceramic bonding. The pre-treatment with MDP or 4-META-containing primers make SB and CEC to produce a durable bond to airborne-particle abraded zirconia ceramic. Due to the decomposition of CH resin cement, the bonding durability of CH without pre-treatment and with pre-treatment with Monobond S and Metal/Zirconia Primer are not testable due to the spontaneous debond during storage. Without surface priming pre-treatment, only SB showed acceptable long-term bond strength to airborne-particle abraded zirconia ceramic. These conclusions can be supported by the combination of XPS, TBS and SEM results. In clinical try-in procedures of zirconia ceramic restorations, to remove contaminations prior to bonding is very important to realize a long-term durable resin bond clinically. In chapter 4, XPS and TBS were combined together to investigate whether there are contaminations on the zirconia ceramic surface left after try-in simulation, and the influences of contamination and cleaning methods on zirconia ceramic bonding durability with MDP-containing resin cements. After saliva immersion and using a silicone disclosing agent, airborne-particle abraded ceramic specimens were cleaned with acetone, 36% phosphoric acid, additional airborne-particle abrasion, or only water spray. Chemical analyses of specimen surfaces were done using XPS. Our results showed that contamination, existing after try-in simulation as confirmed by chemical analysis with XPS, significantly reduced resin -ceramic bonds. Airborne-particle abrasion was the most effective method to remove contaminants and to create a clean zirconia ceramic surface suitable for long-term durable resin bonding. The combination of chemical surface characterization with XPS and long-term TBS performance was very useful to determine the effects of the contamination and cleaning methods on ceramic bonding. Understanding the degradation mechanism of dentin bonding is of significant importance for improving the long-term dentin bonding of adhesive resin cements and the longevity of bonded zirconia ceramic restorations. In chapter 6, mTBS was used to evaluate the bonds of three resin cements (Super-Bond C&B resin cement: SB, Panavia F 2.0: PF, and RelyX Unicem: RU) to different regions of dentin, SEM and TEM were used to investigate the resin-dentin bonding interface and fractographic analysis after mTBS testing. ANOVA results showed that mTBS to superficial dentin was significantly higher than to deep or cervical dentin for all three resin cements. For groups SB and PF, with the highest mTBS to superficial dentin, failed primarily cohesively in resin cement. mTBS of SB to deep and cervical dentin were significantly higher than those of PF and RU. RU, with the lowest regional mTBS, failed mostly within demineralized dentin. SEM and TEM showed that adhesive failures in SB and PF occurred at the top of hybrid layer (HL), but no obvious hybrid layer was observed in RU. In chapter 7, the effect of structural changes of collagen fibrils on the bonding durability of a total etch resin cement (SB) and a self-etching resin cement (PF) to dentin was investigated. AFM was used to observe structural changes of intact dentin collagen fibrils after acidic conditionings of two bonding systems. After 90 d water storage and 15,000 thermal cycles (TC) as artificial aging, μTBS was utilized to evaluate the bonding durability of the two bonding systems to dentin. μTBS after 1 d or 90 d water storage without TC were separately measured in control groups. A cross-banding periodicity of about 67 nm along collagen fibrils was seen on demineralized intertubular dentin surfaces in AFM images. For both resin cements, thermal cycling decreased (p < 0.05) μTBS of 1 d and 90 d, compared to controls. Scanning electron microscope and transmission electron microscopic examinations revealed that the top and bottom of HL were weak links in the bonding interface over time. The results suggest that the top of HL contains disorganized collagen fibrils from the smear layer which degrade over time. AFM results indicate that the demineralized intact collagen fibrils beneath the smear layer were not denatured during acidic conditioning. However, these collagen fibrils may be structurally unstable due to poor-infiltration by resin or loss of resin protection within the HL over time, reducing the long-term μTBS. Artificial aging using water storage with thermal cycling accelerates the degradation of adhesive resin cement-dentin bonding and therefore serves as aging model. This study is very important to provide the evidence that the decomposition of dentin collagen fibrils in the resin-dentin bonding interface over time, although the short-term application of the acidic conditioners used in the two bonding systems do not denature the intact dentin collagen fibrils, is one main factor decreasing the dentin bonding.