Effect of ultrasonic and Nd: Yag laser activation on irrigants on the push-out bond strength of fiber post to the root canal

Abstract Objective: This in vitro study aimed to compare the efficacy of irrigants using various irrigation activation methods to the push-out bond strengths of fiber post to root canal luted with self-adhesive resin cement (SARC). Methodology: Forty-eight decoronated human canines were used. The specimens were divided into four groups corresponding with the post-space irrigation process and were treated as follows: distilled water (DW) (Control) group received 15 mL of DW; sodium hypochlorite (NaOCl)+ethylenediaminetetraacetic acid (EDTA) group was treated with 5 mL of 5.25% NaOCl, 5 mL of 17% EDTA, and 5 mL of DW; passive ultrasonic irrigation (PUI) group was treated with 5 mL of 5.25% NaOCl, 5 mL of 17% EDTA, and 5 mL of DW, and each irrigant was agitated with an ultrasonic file; and laser activated irrigation (LAI) group was treated with 5 mL of 5.25% NaOCl, 5 mL of 17% EDTA, and 5 mL of DW, and each irrigant was irradiated with Nd: YAG laser. Fiber posts were luted with SARC, and a push-out test was performed. Data was analyzed using one-way analysis of variance and Tukey HSD test. Results: The bond strength values for the groups obtained were as follows: Control (10.04 MPa), NaOCl+EDTA (11.07 MPa), PUI (11.85 MPa), and LAI (11.63 MPa). No statistically significant differences were found among all experimental groups (p>0.05). The coronal (12.66 MPa) and middle (11.63 MPa) root regions indicated a significantly higher bond strength compared with the apical (9.16 MPa) region (p<0.05). Conclusions: Irrigant activation methods did not increase the bond strength of fiber post to canal.

Introductıon Endodontically treated teeth with excessive loss of coronal structure can be restored by using fiber posts. Some advantages of fiber posts include credible mechanical properties and lower modulus of elasticity, similar to that of dentin, and thus there exists little risk of causing root fracture. Another advantage is that it is post-translucent, which allows the passage of light which is necessary for cementing polymerization, allowing the post to be connected to the dentin wall. 1 When the post is luted to the root canal, two interfaces occur. One of them is between the post and the cement, and the other is between the cement and the root dentin. 2 Most commonly reported mode of clinical failure in fiber post is debonding from root dentin. 3 The main problem that is believed to adversely affect the bonding strength of fiber post is the blockage of cement adhesion by obstruction of the dentinal tubules by the Gutta-percha and sealer remnants, dentine debris and smear layer. 4 In addition to external factors such as irrigants, types of adhesives, and endodontic sealers, dentin-related factors such as dentin status and orientation of dentin tubules also affect these interfaces. 5 Irrigation after post space preparation procedures help remove the smear layer and may increase the strength between the cement bond and the root canal dentin 6 . Frequently suggested irrigation protocols are 5.25% sodium hypochlorite (NaOCl) and 17% ethylenediaminetetraacetic acid (EDTA). NaOCl is used to remove the organic content of the smear layer while EDTA is often used to remove the inorganic content of the smear layer. 7 Unfortunately, irrigants are unable to completely remove the filling material from the canals. 8 Therefore, several activation techniques were developed to more effectively remove pulp tissue and microorganisms, smear layer, and dentin debris from the root canal system, such as passive ultrasonic irrigation (PUI) and laser activated irrigation (LAI). 9 PUI is the ultrasonic activation of an irrigant in the root canal via an ultrasonically oscillating small file placed in the root canal after the root canal has been shaped. 10 Recently, LAI has been introduced as an activation method of irrigation solutions that uses transfer of pulsed energy by means of various laser systems. 11 Self-adhesive resin cement (SARC) systems are utilized to overcome the technical problems of multistep applications and to shorten the duration of clinical application. The main adhesive characteristic of SARC is attributable to a chemical reaction between phosphate methacrylates and hydroxyapatite; this cement presents limited infiltration into the tooth tissue. 12 The connection between post-dentin and cement is important for restoration stability and longevity. 13 Hence the effect of different irrigation methods on post-dentin bonding strength should be investigated after post-space preparation. Therefore, in this study, we aimed to evaluate the effect of irrigant activation techniques on the push-out bond strength of fiber posts. The null hypothesis tested was that irrigant activation techniques do not affect the push-out bond strengths of fiber posts to root dentin.

Specimen preparation
Forty-eight freshly extracted human maxillary canines were selected for this study. Teeth with a single straight root canal and developed apices fulfilled the inclusion criteria. All teeth were stored in 0.1% thymol until the experimental procedure. Periapical radiographs were taken from both the mesiodistal and buccolingual sides to ensure that there was only one straight canal in each tooth. Teeth that presented prior endodontic treatment and fracture lines were excluded from this study.
Each specimen was decoronated using a low-speed saw (Mecatome T180; Presi, Eybens, France) under water cooling to provide a uniform root length of 15 mm. All root canals were prepared to size R50 with the RECIPROC system (VDW, Munich, Germany). Irrigation was made using 5 mL of 5.25% NaOCl with 5 mL of 17% EDTA solution for 1 min and 5 mL of 5.25% NaOCl between instrument changes. Distilled water (DW; 5 mL) was used for a final irrigation. Finally, the canals were dried using sterile paper points. All instrumented teeth were obturated with gutta-percha cone and AH Plus sealer (Dentsply De Trey, Konstanz, Germany) with use of the cold lateral compaction technique, and the canals were covered with temporary filling material (Cavit-G; 3M ESPE, Seefeld, Germany). All specimens were stored at 37°C and 100% humidity for 7 days, after which the temporary fill was removed. The filling with mild pressure, and the SARC was polymerized with a light-emitting diode unit (Elipar S10, 3M ESPE, Neuss, Germany). All specimens were stored at 37°C and 100% humidity for 24 hours.
The specimens were buried in acrylic blocks to be cut with a low-speed saw. The samples were horizontally cut to obtain 1-mm sections. Six samples from each root were obtained. The samples were then separated into two pieces belonging to the coronal, middle and apical parts of the root. The 2 nd , 4 th , and 6 th slices were selected for a push-out test of the samples obtained from the coronal, middle and apical parts.

Push-out test
The push-out bond strength was measured using a universal testing machine (Autograph AGS X; Shimadzu Co, Japan). The push-out test was applied at 0.5 mm/min using a 1-mm diameter metallic plunger

Results
The means and standard deviations of the push-out bond strength values are presented in

Intragroup comparisons revealed that bond
strength values decreased in the coronal-to-apical direction, there was no significant difference in Control and LAI groups (p>0.05) but apical root regions were significantly lower than the coronal root region in NaOCl+EDTA and PUI groups (p<0.05) ( Table 1).
The frequency of each type of bond failure mode is given in Table 2. The most common failure mode was adhesive (51.4%), followed by mix failure between dentin and resin cement (41.7%) and cohesive failure in the resin cement (6.9%).

Dıscussıon
In many cases, treatment failure caused by cementation failure could be restored with a post after endodontic treatment. 1 The purpose of this in vitro study was to evaluate the effects of various irrigation methods on the push-out bond strengths of fiber posts to root dentin with SARC. Our results revealed that the examined irrigation methods slightly affected fiber post bond strength, but this was not found to be statistically significant. In accordance with the results, the null hypothesis was accepted.
After the post cavity was prepared, the presence of a smear layer consisting of sealer, gutta-percha, and debris was observed on the dentin surfaces examined with a scanning electron microscope. 20 Table 2-Mode failure percentages with respect to post space irrigation procedure and-rinse systems are used, removing the smear layer becomes essential in order to achieve a hybrid layer. 4 When endodontic treatment is applied for the first time or during retreatment, a combination of NaOCl and EDTA is considered as effective irrigation for smear removal but smear layer cannot be completely removed. 21 Therefore, self-adhesive cement does not require hybrid layer formation and is considered as an advantage for post retention. 12

Conclusıons
The findings of this study showed that irrigant activation methods did not increase the bond strength of fiber post to dentin luted with SARC, and apical root regions exhibited a significantly lower bond strength than the coronal and middle root regions.