Figure 9.1 revealed that remnants of sealer might hamper adhesion inside the post space preparation. Various studies analyzed in vitro the effect of different endodontic sealers on the retention of fiber posts (Demiryurek et al. 2010; Aleisa et al. 2012; Vano et al. 2012; AlEisa et al. 2013; Mesquita et al. 2013). In studies of one working group, eugenol-based sealers reduced the retention of fiber posts luted with resin cements compared to epoxy resin-based sealers (Aleisa et al. 2012; AlEisa et al. 2013). These effects may be attributed to remnants of phenolic components that can collect free radicals and delay the polymerization reaction when interacting with resin (Mayer et al. 1997). However, other researchers concluded that the chemical formulation of endodontic sealers did not affect the retention of posts luted with resin cements (Hagge et al. 2002; Kurtz et al. 2003) or the marginal seal of adhesively luted carbon fiber posts (Mannocci et al. 2001). Nevertheless, unobturated controls revealed significantly higher retentive post strengths compared to root-filled groups indicating that remnants of sealer might hamper adhesion inside the root canal (Boone et al. 2001; Hagge et al. 2002).

Another fact is the length of time that eugenol-containing materials remain in contact with dentin until removal; a longer contact time might reduce adhesion (Hagge et al. 2002). Therefore, it has been suggested that effective removal of eugenol-based sealers should be performed immediately after obturation. On the other hand, delayed post space preparation 24 h or 7 days after obturation using a eugenol-based sealer followed by adhesive luting of fiber posts demonstrated significantly higher bond strength compared to immediate post space preparation and adhesive post luting (Vano et al. 2012). Equal results were obtained for an epoxy resin-based sealer (Vano et al. 2008). The authors speculated that contamination of the post space might be minimized when the sealer is allowed to set completely prior post space preparation. In all referenced papers, no final irrigation protocol except rinsing using tap water after post space preparation was applied; consequently the effects of the sealer type on the adhesion properties inside the root canal might be reduced when using a final irrigation protocol that has positive effects on the cleanliness of the post space cavity.

In conclusion, the effects of different types of sealer on adhesion of resin cements inside the root canal and the retentive strength of posts remain controversial, although the literature revealed a tendency towards a negative effect of eugenol-based sealers. It has been reported that this problem might be avoided by thorough cleaning of the root canal walls (Schwartz and Robbins 2004); moreover, the adhesive procedure inside the root canal will benefit the most from a perfect clean root dentin surface that is not contaminated with any kind of sealer.

The smear layer is formed during post space preparation and consists of remnants of root canal dentin, gutta-percha, and sealer. Moreover, the smear layer may be plasticized because of the frictional heat of the drill (Khalighinejad et al. 2014). Besides mechanical cleaning methods that have been mentioned above, various chemical agents such as sodium hypochlorite (NaOCl), ethylenediaminetetraacetic acid (EDTA), chlorhexidine (CHX), ethanol, or combinations of these agents have been used to test their ability to remove the smear layer inside the root canal and their effects on bond strengths of fiber posts (Carvalho et al. 2009; Lindblad et al. 2010, 2012; Cecchin et al. 2011; Bitter et al. 2013; Bitter et al. 2014a; Khalighinejad et al. 2014). In this regard, it should be considered that NaOCl and EDTA are common endodontic irrigants, although their prolonged use at high concentrations may have negative effects on the physical properties of root canal dentin, such as reduced flexural strength, elastic modulus, and microhardness (Tang et al. 2010). These changes of the structural properties of dentin may affect the bond strengths between adhesives and luting agents to root canal dentin (Dogan Buzoglu et al. 2007). Consequently, irrigation after post space preparation and its effects on the bond strength of different adhesive strategies are a matter of interest, in particular because manufacturers’ recommendations vary from the use of sodium hypochlorite (NaOCl) to no recommendations at all. With this in mind, five different irrigation protocols after post space preparation were investigated in vitro to analyze their effects on bond strengths of fiber posts to root dentin using three different adhesive strategies (Bitter et al. 2013). Besides a control group using distilled water, the irrigation protocols were passive ultrasonic irrigation (PUI) using NaOCl 1 and 5.25 %, respectively. Additionally, irrigation using 18 % EDTA followed by 5.25 % NaOCl as well as 2 % CHX was tested after post space preparation. Fiber posts were luted using a self-etch and an etch-and-rinse adhesive system as well as a self-adhesive resin cement. The effects of the irrigation protocol on bond strengths were significantly affected by the adhesive strategy. For the etch-and-rinse adhesive system, irrigation using EDTA and NaOCl 5.25 % resulted in significantly lower mean bond strengths compared to the control group, whereas irrigation using NaOCl 1 % with PUI revealed the highest mean bond strengths. These results were supported by the analyses of the adhesive interface using confocal laser scanning microscopy (CLSM) in dual-fluorescence mode (Fig. 9.3 a–c), which demonstrated a deep infiltration of adhesive and luting cement into the strongly demineralized root canal dentin. Although the demineralizing effect of 18 % EDTA and 5.25 % NaOCl would be desirable in terms of smear layer removal, alteration of the chemical structure of the underlying dentin, especially in combination with phosphoric acid, might result in extensive demineralization with suboptimal adhesion. Consequently, this irrigation protocol cannot be recommended for etch-and-rinse adhesive systems.

The mean bond strengths of the self-etch adhesive system used in this study were not affected by the different irrigation protocols. These results were confirmed by other in vitro studies (Zhang et al. 2008; Fawzi et al. 2010). Effective removal of the smear layer and deeper penetration of adhesive system and resin cement were also visualized using CLSM (Fig. 9.4a–c); however, this did not correlate with an increase in bond strength.

For the self-adhesive resin cement, smear layer removal with 18 % EDTA and 5.25 % NaOCl resulted in significantly higher-mean bond strengths compared to the control group (Bitter et al. 2013). These effects were visualized using CLSM (Fig. 9.5a–c). A deeper penetration of the resin cement into the dentinal tubules was evident. Consequently, each adhesive strategy need to be adapted to a specific irrigation protocol; however, irrigation using 1 % NaOCl applied with passive ultrasonic activation resulted in high-mean bond strength for all groups. Consequently, this protocol can be recommended for smear layer removal after post space preparation irrespective of the adhesive strategy.

Besides initial bond strength testing, durability of adhesion inside the root canal is an important factor for the longevity of postendodontic restoration using fiber posts and explained in detail in Chap.​ 8. One strategy to prevent degradation of resin dentin bonds is the application of matrix metalloproteinase (MMP) inhibitors to the demineralized collagen matrix prior to the application of dentin adhesives. For more details, the reader is referred to Chap.​ 8 . For final irrigation, CHX has been used as a nonspecific MMP inhibitor during adhesive application, and inside the root canal, it did not negatively affect immediate and long-term bond strength in post bond cementation (Lindblad et al. 2010, 2012; Cecchin et al. 2011). However, other studies revealed a significant decrease of fiber post bond strength to CHX-treated root canal dentin after thermocycling and storage (Bitter et al. 2014a) or thermomechanical loading (Cecchin et al. 2014).

A simplified procedure of the ethanol wet bonding technique, which is explained in detail in Chap.​ 8, is to irrigate the root canal using 99 % ethanol prior to application of the adhesive system or the luting agent. This procedure was able to prevent a decrease in bond strength after thermomechanical loading (Cecchin et al. 2014) as well as after thermocycling and storage for an etch-and-rinse adhesive as well as for a self-adhesive resin cement in the middle and apical part of the root canal (Bitter et al. 2014a). This leads to the assumption that insufficient moisture control in the depths of the root canal might be compensated with the application of ethanol probably rendering the collagen matrix more hydrophobic by replacing water with ethanol. However, the effects of ethanol pretreatment of the root canal dentin on fiber post bond strength seem to depend on the composition of the adhesive system used (Carvalho et al. 2009; Cecchin et al. 2011). Therefore, more research on this field is required until final recommendation can be given despite the present results on the effects of ethanol pretreatment of the root canal dentin seem to be promising.

SEM observations in Fig. 9.6 illustrate examples for possible failure modes of adhesively luted fiber posts, i.e., adhesive failure between root canal dentin and cement (a) and adhesive failure between post and cement (b).

Consequently, a reliable bond to the post surface, as well as to the root canal dentin, is mandatory for establishing durable post-endodontic restorations. Although failures at the interface between root canal dentin and luting agent seem to occur more often (Rasimick et al. 2010), numerous pretreatment procedures of fiber posts have been suggested to enhance the bond strength between fiber post surface and luting agent.

Current fiber posts consist of unidirectional fibers (glass or quartz) that are embedded in a resin matrix. Different matrices are used by the manufacturers, i.e., epoxy resin, methacrylate resin, or a proprietary resin (Zicari et al. 2012a). It has been demonstrated that different glass fiber posts may vary in flexural properties and micromorphology and that flexural properties may be affected by mechanical properties of the resin matrix and interfacial adhesion between fiber and matrix (Zicari et al. 2013). Moreover, differences in micromorphology, surface texture, and composition may have an impact on the effects of post-pretreatment on fiber post bond strength. Chemical and micro-mechanical pretreatment protocols of fiber posts have been analyzed with the aim to increase bond strength between post surface and resin luting agent. The most common micro-mechanical post-surface pretreatment methods include sandblasting and application of the Cojet System (3M ESPE). The goal of sandblasting is to remove the top layer of resin in order to expose glass fibers for possible chemical interaction as well as roughening the surface. Data in the literature indicate that sandblasting enhanced bond strength of fiber posts (Balbosh and Kern 2006); however, the effects of sandblasting on bond strength of fiber posts with and without additional silane application seem to depend on the post type and the luting cement (Magni et al. 2007; Radovic et al. 2007). Moreover, sandblasting resulted in undesirable alterations of the post surface with a disruption of the interface between matrix and fiber that lead to fractures of superficial fibers after mechanical loading (Soares et al. 2008). Consequently, a weakening effect of sandblasting on long-term fiber post stability cannot be excluded. The Cojet System uses silica-coated alumina particles resulting in a tribochemical coating of the post surface, which can then be silane-treated (Zicari et al. 2012a). Again, controversial data exists on the effects of fiber post-pretreatment using the Cojet System (Bitter et al. 2006; Zicari et al. 2012a), and the effects were also significantly affected by fiber post type and luting cement. Fiber damage has also been observed (Fig. 9.7a, b).

Clinically, silanization of the post surface is the most often employed chemical pretreatment procedure. However, this pretreatment procedure has resulted in contradictory results (Goracci et al. 2005; Perdigao et al. 2006; Bitter et al. 2007; Zicari et al. 2012a). The main effects of silane pretreatment are based on improved wettability of the surface as well as chemical bridge formation between filler particles and fibers of the post and the resin matrix of adhesive or resin cement (Zicari et al. 2012a).

The effects of this pretreatment seem to be strongly dependent on the composition and micromorphology of the post surface (i.e., reachable fibers and filler particles at the surface of the post) and the composition of the applied adhesive or resin cement. Moreover, it has been speculated that the interface produced between resin cements and silanized posts might be affected by the phenomenon of hydrolytic weakening hampering the proper interaction between these materials (Machado et al. 2015). The application of a hydrophobic resin adhesive on a previously silanized post increased fiber post retention in vitro probably due to a reduction of hydrolysis (Machado et al. 2015).

Further combined chemical and micro-mechanical pretreatment procedures include the application of hydrofluoric acid followed by silanization (Monticelli et al. 2008a; Schmage et al. 2009a) that resulted in an increased fiber post bond strength dependent on the luting agent used. However, the effect of the acid on the post surface has been proven to be time dependent and affected by the post composition (type of matrix and/or fibers). With regard to the application time of the acid, this technique produced substantial damage to the glass fibers and affected the integrity of the post (Valandro et al. 2006).

Other pretreatment procedures of fiber posts, such as immersion into hydrogen peroxide (Vano et al. 2006) or the use of sodium ethoxide (Monticelli et al. 2006a), aimed to dissolve the epoxy or methacrylate-based resin matrix of the post surface and concomitant exposure of undamaged fibers, thus leading to increased bond strengths to resin luting agents after application of silane. Etching of the post surface for 20 min using 10 % hydrogen peroxide increased bond strength between resin luting agent and fiber post (Monticelli et al. 2006b); however, this procedure seems to be not suitable for daily clinical practice.

Recently, surface polydopamine functionalization was found to be effective in improving the bond strength between resin luting agents and fiber posts without damaging the fiber post surface (Chen et al. 2014). More research is needed to evaluate whether this surface modification also leads to long-term bond stability.

To simplify the clinical procedure, posts with a pre-coated surface have been introduced onto the market (Fig. 9.8