Comparison of the effect of QMix and conventional root canal irrigants on push‐out bond strength of fiber post to root dentin

Abstract Objectives This study aimed to assess the effect of application of QMix and common root canal irrigating solutions on the bond strength of fiber post to root dentin. Material and Methods In this in‐vitro study, 60 extracted incisor teeth were decoronated such that 15 mm of root length remained. The canals were prepared with ProTaper rotary system to F5 and filled with gutta‐percha and AH26 sealer. Prior to post placement, the teeth were divided into four groups based on the type of final irrigating solution namely saline, 5.25% NaOCl, 2% chlorhexidine, and QMix. The fiber posts were then cemented with Panavia F2 resin cement. The roots were sectioned perpendicular to their longitudinal axis, and four sections with 1 mm thickness were made at the middle and coronal thirds of the roots. The push‐out bond strength of fiber posts was measured by a universal testing machine in megapascals. Data were analyzed by two‐way ANOVA and Tukey's test. Results The maximum and minimum bond strength values were noted in QMix and NaOCl groups in both the middle and coronal third of the root, respectively. But, there was no significant difference between the push‐out bond strength in the middle or coronal third of the root (p = 0.054). Adhesive failure was the most common mode of failure in all groups. Conclusion Use of QMix for post space irrigation does not compromise the bond strength of fiber post to root dentin, and can be used for final rinsing of the post space.


| INTRODUCTION
Fiber posts were introduced as an alternative to cast post and cores and metal dowels for restoration of endodontically treated teeth that have lost a great portion of their structure. These posts have advantages such as higher esthetics (Zicari et al., 2008). Also, they have a modulus of elasticity similar to that of dentin and therefore, significantly decrease the risk of vertical root fracture (Zicari et al., 2008).
Sodium hypochlorite (NaOCl) has long been used as one of the most common root canal irrigating solutions (Stojicic et al., 2012). Its popularity is attributed to its bactericidal and virucidal properties as well as its optimal tissue dissolution ability (Khalilak et al., 2011).
Moreover, NaOCl has a low viscosity and long shelf-life (Stojicic et al., 2012). On the other hand, NaOCl is incapable of elimination of the smear layer and therefore, it is often used along with a chelating agent such as ethylenediaminetetraacetic acid (EDTA) (Dibaji, Afkhami, et al., 2017;Gündo gar et al., 2018). Use of NaOCl as final root canal irrigant after EDTA may compromise the structural integrity of root dentin (Gündo gar et al., 2018). Also, evidence shows that use of NaOCl for final irrigation can decrease the bond strength of sealers and glass fiber posts to root dentin (Elnaghy, 2014;Gündo gar et al., 2018).
Chlorhexidine gluconate (CHX) is also commonly used as a root canal irrigating solution and an intracanal medicament due to its antimicrobial activity, long-term substantivity, and low cytotoxicity (Gündo gar et al., 2018). One advantage of CHX as root canal irrigating solution is that it does not compromise the bond strength of resin to root dentin (Afkhami et al., 2018). Nonetheless, evidence shows that NaOCl is superior to CHX in terms of antimicrobial activity and tissue dissolution ability (Afkhami et al., 2018). CHX cannot dissolve the mineral phase of the smear layer either (Gündo gar et al., 2018). Also, if used in combination with NaOCl, it results in formation of a toxic precipitate known as parachloroaniline in the root canal system, which negatively affects the root canal seal by the root filling materials (Gündo gar et al., 2018).
QMix is a new root canal irrigant for elimination of the smear layer, which has antimicrobial properties as well (Gündo gar et al., 2018). It contains EDTA, CHX, a detergent, and deionized water (Elnaghy, 2014). It was introduced as a final root canal irrigating solution, and should be used for 60 to 90 s according to the manufacturer's instructions. It is a ready-to-use clear solution and evidence shows that it can effectively remove the smear layer and bacteria such as Enterococcus faecalis (Elnaghy, 2014;Gündo gar et al., 2018). QMix solution increases the radicular dentin demineralization due to the chelating effects of EDTA and simultaneous disinfecting properties (Dai et al., 2011;Stojicic et al., 2012). The main reason behind addition of surfactant to QMix is to decrease the surface tension of the solution and simultaneously increase the wettability and flowability of the irrigating solution in the root canal system to enhance its contact with the smear layer and the underlying dentin (Giardino et al., 2006;Stojicic et al., 2012). These observations have been confirmed by scanning electron microscopic studies as well, and the optimal efficacy of QMix for elimination of the smear layer has been well documented (Elnaghy, 2014).  This in vitro experimental study evaluated 60 sound extracted teeth. Inclusion criteria included incisors with single canal, closed apex, and straight roots that had been extracted in the past 3 months for orthodontic treatment or due to periodontal disease. Exclusion criteria included the following: presence of any cracks, caries, resorption defects, decalcifications, or previous endodontic treatment.

| Preparation and root canal treatment of the teeth
The teeth were cleaned with a periodontal curette and immersed in 10% formalin for 1 week for disinfection. The teeth were then stored in saline at room temperature (37 C) until the experiment. The teeth were decoronated perpendicular to the longitudinal axis of the root using a diamond bur and high-speed handpiece under copious water irrigation such that the remaining root length was 15 mm. Next, #10 and #15 K-files were passed through the canal orifice to ensure patency. In case of obstruction, RC-Prep was used to achieve patency.
Working length was determined by introducing a #10 K-file (Maillefer-Dentsply, Ballaigues, Switzerland) into the root canal until its tip was visible at the apical foramen; 1 mm was subtracted from this length to obtain the working length. The root canals were instrumented by ProTaper Universal rotary system to F5 (Dentsply Maillefer, Ballaigues, Switzerland) using the single-length technique.
After using each ProTaper file, the canal was rinsed with 1 ml of 2.5% NaOCl followed by 1 ml EDTA 17% for 60 s, after final irrigation the root canals were dried with paper points. The root canals were obturated using a #45 gutta-percha point (Aria Dent, Tehran, Iran) as the master cone and #20 gutta-percha accessory points (Aria Dent, Tehran, Iran) and AH26 sealer (Dentsply DeTrey, Konstanz, Germany).
The teeth were radiographed to ensure adequate quality of root filling. Next, G-Cavit temporary restorative material (3 M ESPE, Seefeld, Germany) was used to seal the orifice.

| Placement of prefabricated posts
The obturated root canals were incubated at 37 C and 100% humidity for 1 week. Next, 10 mm of gutta-percha was removed from each canal by #2 and #3 Gates-Glidden drills (Dentsply, Maillefer, Switzerland) such that 5 mm of gutta-percha remained at the apex to preserve the apical seal. Next, the drills present in the D.T. Light Post kit (D.T #3, Bisco Inc., Schaumburg, IL) were used for post space preparation. Eventually, the teeth were radiographed with a standard parallel angulation to ensure absence of gutta-percha and sealer on the canal walls. The teeth were then randomly divided into four groups (n = 15) according to the final irrigation protocol: • Group 1: (control group): 5 ml of 17% EDTA +5 ml of 5.25% NaOCl +5 ml of saline as final irrigant for 60 s.
• Group 2: Control group protocol +5 ml of 2% CHX as final irrigating solution for 60 s.
• Group 3: Control group protocol +5 ml of 5.25% NaOCl as final irrigating solution for 60 s.
• Group 4: Control group protocol +5 ml of QMix as final irrigating solution for 60 s.
The root canals in each group were dried with paper points. After rinsing the post space, the ED primer was applied on root canal walls with a microbrush. The primer was gently air-thinned for 30 s, and the for 20 s. After cementation to exposed dentin, the coronal part of the roots was covered with composite resin (Filtek Z250; 3 M ESPE, St. Paul, MN) and they were incubated at 37 C and 100% humidity for 1 week. The roots were radiographed after cementation of the posts.

| Measuring the bond strength
The roots were sectioned by a high-speed cutting machine (Mecatome T201A, Presi, Grenoble, France) under copious water irrigation perpendicular to the longitudinal axis of the root. Four sections with 1 ± 0.1 mm thickness (Elnaghy, 2014), were made of each root, two from the coronal third and two from the middle third of the root.
The coronal sections were made at 2 mm distance from the coronal margin of the root. The thickness of sections was measured by a digital caliper (Mitutoyo, Tokyo, Japan). A universal testing machine (Z050; Zwick Roell, Ulm, Germany) was used to measure the push-out bond strength of fiber post to root dentin. Load was applied apicocoronally at a crosshead speed of 0.5 mm/min by a cylindrical stainless steel piston with 0.7 mm diameter to the center of fiber post in each section with no contact with the root dentinal wall. Maximum load applied right before debonding was measured and recorded in Newtons (N). The push-out load was applied apico-coronally and pushed the post towards the wider root cross-section. Thus, there was no limitation for dislodgment of the post.
To calculate the push-out bond strength in megapascals (MPa), the debonding force in Newtons (N) was divided by the crosssectional area using the formula P=F/A where A is the post-dentin surface area, which was calculated using the formula below: After measuring the push-out bond strength, the mode of failure of each specimen was determined under a stereomicroscope (Olympus, SZ61, Olympus Optical Co., Tokyo, Japan) at x30 magnification. The failure mode was categorized as: • Adhesive at the dentin-cement interface.
• Adhesive at the post-cement interface.
• Cohesive within the cement.
• Cohesive within the post.
• Mixed. According to the Tukey's test, the bond strength of fiber post to root canal in 5.25% NaOCl group was significantly lower than that in the CHX (p < 0.02) and QMix (p < 0.004) groups but had no difference with saline group (p = 0.36).   compared with other irrigation protocols (Elnaghy, 2014).

| Statistical analysis
According to the current results, the bond strength values were almost the same in CHX and QMix groups. High bond strength in the CHX group may be related to absorption of CHX by the root dentin and subsequently enhanced resin infiltration into dentinal tubules due to its non-oxidizing property (Erdemir et al., 2004). CHX has a surfactant in its composition, which increases the surface energy of dentin and subsequently its wettability. Thus, it enhances the bond strength of fiber post to root dentin in use of resin cements (Hashem et al., 2009). CHX prevents or decreases the destruction of exposed collagen fibrils and preserves a stable hybrid layer, resulting in improved bond strength (Komori et al., 2009). It also prevents the host protease activity and preserves the morphological properties of the hybrid layer as such (Hebling et al., 2005).
According to the current results, minimum bond strength value was noted in NaOCl group. Sodium hypochlorite has extensive applications in endodontic treatment and its favorable efficacy for efficient debridement, root canal disinfection, lubrication, and tissue dissolution has been previously confirmed (Santos et al., 2006). The current results regarding the adverse effect of root canal irrigation with NaOCl on the bond strength of fiber post to root dentin are similar to the findings of a previous study (Elnaghy, 2014 F I G U R E 1 Error bar of the mean and 95% confidence interval of push-out bond strength of fiber post to root dentin in the coronal and middle thirds The main reason for reduction of push-out bond strength in the NaOCl group is production of free oxygen radicals by NaOCl, which serve as a barrier against the penetration and polymerization of adhesive resin (Hashem et al., 2009). Sodium hypochlorite oxidizes some of the constituents of the dentin matrix and leads to formation of free radicals derived from protein materials. These free radicals compete with the free radicals that are generated during the light-curing process of resin, and resultantly, formation of polymer chains is stopped and the process of polymerization cannot be well completed (Lai et al., 2001;Morris et al., 2001). Also, evidence shows that root canal irrigation with sodium hypochlorite decreases the calcium and phosphorous contents, as well as the mechanical properties of dentin such as its modulus of elasticity, microhardness, and flexural strength. This can be related to decreased micromechanical reactions between the adhesive resin and dentin following the use of sodium hypochlorite (Santos et al., 2006).  (Ebrahimi et al., 2014).
In this study, most failure modes were adhesive failure at the resindentin interface which was similar to the results of Elnaghy et al, (Elnaghy, 2014) Bouillaguet et al, (Bouillaguet et al., 2003), and Afkhami et al. (Afkhami et al., 2018) However, in this study, similar to that of Abreu et al, the mode of failure in most specimens was adhesive at the resin cement-dentin interface (Abreu et al., 2020). In the study by Bouillaguet et al, (Bouillaguet et al., 2003) higher frequency of adhesive failure at the resin cement-root dentin interface was attributed to polymerization stresses in this region, due to the canal geometry. Optimal bonding of cement to root dentin is difficult to achieve (Bouillaguet et al., 2003). Thus, the C-factor is high in this region, which would generate a high stress due to shrinkage of resin cement against the canal wall, that would compete with the bond strength at the adhesive interface . Moreover, according to Abreu et al. (2020), higher frequency of adhesive failure at the resin cement-root dentin interface is due to chemical compatibility of the resin matrix of fiber post with cement and also adequate wetting of the post with the cement when bonding the fiber post margins.

| CONCLUSION
Considering the maximum push-out bond strength of fiber post to root dentin achieved following final root canal irrigation with QMix, it seems that it does not interfere with the bond strength of fiber posts to root dentin, and can be safely used for final irrigation of the root canal system.

ACKNOWLEDGMENT
We would like to thank all people who participate in this study. This study was part of a D.D.S. thesis supported by Tehran University of Medical Sciences.

CONFLICT OF INTEREST
The authors declare no potential conflict of interest.

AUTHOR CONTRIBUTION
Farzaneh Afkhami, Mona Sadegh and Maryam Amirmoezzi planned the study and research design and also data acquisition. Farzaneh Afkhami and Aidin Sooratgar performed the data analyze/ interpretation and prepared the manuscript. All the authors read and approved the final manuscript.

DATA AVAILABILITY STATEMENT
Data available on request from the authors.