Chemo-mechanical instrumentation is of vital importance for the successful treatment outcome in non-surgical endodontic therapy . During the cleaning and shaping process, organic pulpal remnants and inorganic dentinal debris accumulate and adhere onto the radicular canal wall producing an amorphous irregular smear layer . The use of both hand and rotary instruments create smear layer of different thickness on the canal walls as a consequence of the denting cutting action .
The organic component of the smear layer might constitute a proper substrate for bacterial growth; in addition it might interfere with sealer adhesion onto the canal wall and allow leakage to take place . Some studies , have suggested that viable microorganisms in the dentinal tubules may use the smear layer as a reservoir for sustained growth and replication. If the smear layer is left intact, it may slowly dissolve with leaking obturation materials, or it may be removed by acids and enzymes that are produced by viable microorganisms left in the tubules or by bacteria that gain entrance via coronal leakage .
According to Orstavik et al. , the presence of a smear layer may also inhibit the action and effectiveness of root canal irrigants and inter-appointment medicaments. Oksan and coworkers , have concluded that removing the smear layer will allow better adaptation of obturation materials to the canal wall; this has also been demonstrated by Wennberg et al. , and by Leonard et al. , who demonstrated better adhesion and tubular penetration of sealers into dentin once the smear layer had been removed. Sodium hypochlorite (NaOCl), a deproteinizing agent has become the most popular irrigating solution in endodontics , and its alternate use with EDTA, a calcium-chelating agent, has been recommended for the efficient removal of the smear layer [12,13]. To improve cleanliness, irrigants should be in contact with root canals . The traditional needle irrigation technique delivers solutions no more than 0–1.1 mm beyond the needle tip . This is insufficient for complete cleaning of the complex anatomy of the root canal system (lateral canals, isthmus, fins, and accessory canals .
Different devices for irrigation delivery have been proposed to increase the flow and distribution of irrigating solutions within the root canal system . Recently the XP- Endo Finisher™ file (Brasseler USA®, Savannah, Georgia) has been introduced to the market. According to the manufacturer the file has a semi-circular shape with a 3mm diameter that enables it to transform into any canal shape and reach irregularities, fins and resorptive areas. It is designed to be used with irrigants after initial root canal instrumentation with the aim of removing vital and/or necrotic tissues, and dentinal debris accumulated during instrumentation and smear layer . A study by Elnaghy et al. , found that the utilizing the XP-endo Finisher file and the EndoActivator system resulted in less debris and lower smear layer scores than file agitation and the standard needle irrigation technique at the coronal, middle, and apical regions in curved canals.
Passive ultrasonic irrigation (PUI), first described by Weller et al. , uses a stainless steel file to activate the irrigant in the canal . PUI is able to alter and eventually disrupt the endodontic biofilm, facilitating better penetration of irrigants throughout the endodontic dentinal walls . Various studies have shown that ultrasonic activation of irrigating solutions as PUI promotes better removal of the smear layer in the apical and isthmus regions [23-25].
Currently, there are no studies between the XP-Endo Finisher File System and PUI for the evaluation of smear layer removal utilizing similar experimental protocols. A recent research study by Leoni et al.  found that the PUI technique and XP-endo Finisher instrument were associated with significantly lower levels of accumulated hard tissue debris compared with conventional irrigation and the modified SAF system protocol in mesial root canals of mandibular molars.
Therefore, the aim of this study was to compare the efficacy of both systems regarding smear layer removal after root canal instrumentation.
Selection, Preparation and Classification of the Specimens
Thirty-six human single-rooted non-carious mandibular premolars extracted for orthodontic purposes were randomly selected from the same age group (14- to 22-year-old patients) with the approval of the Ethics Committee for Scientific Research of the University Inca Garcilaso de la Vega, Lima, Peru. Teeth selected had no cracks, endodontic treatments, and restorations. Only teeth with intact and mature root apices and roots longer than 14 mm were selected. Teeth were then x-rayed buccolingually and mesiodistally to confirm straight single canals and the canal space size. Teeth with root canal curvatures greater than 20° or calcified root canals were excluded. After extraction, teeth were stored in 2% thymol solution at room temperature and used within 1 week. Inclusion and exclusion criteria were verified under a 20X magnification laboratory microscope (AmScope SE308-P Binocular Stereo Microscope, Irvine, California USA). After the access cavity was created, a #10 K-file (Dentsply Maillefer, Ballaigues, Switzerland) was inserted into the canal until the instrument tip was barely visible at the apical foramen. The root lengths were standardized to 16 mm by decoronation of the tooth perpendicular to the long axis by means of a high-speed, water-cooled diamond disc. To simulate clinical conditions, apices were sealed with hot glue, and to prevent the glue from entering the canal, a #10 K-file was inserted before the apex was sealed. Root canal preparations were performed by the same operator using the Pro-Train endodontic training system for tooth preparation standardization (Simit Dental, Mantova, Italy). The specimens were shaped by means of the Wave One 40.08 nickel-titanium single file reciprocating system (Dentsply Maillefer, Ballaigues, Switzerland) according to the manufacturer’s instructions until the file reached the working length (WL). Each instrument was used to shape only 4 specimens. After each instrumentation and before the next, canals were rinsed with 2.5 mL 2.5% NaOCl at 37C° (Chematek SpA, Rome, Italy).The apical patency was checked after each instrument with a #10 K-file. Irrigating solutions were delivered by means of a 30-G syringe needle (NaviTip; Ultradent, South Jordan, UT) inserted deeply at 1 mm from the working length.
All specimens were then irrigated with 2.5 ml distilled water. The specimens were then dried with sterile paper points and assigned randomly to each of four groups for final activation (Table 1). All specimens were then irrigated with 2.0 ml distilled water and dried with sterile paper points.
Field emission scanning electron microscopy was used to evaluate endodontic smear layer removal from the instrumented root canals. To facilitate fracture into halves, all roots were grooved longitudinally on the external surface with a diamond disc without penetration into the root canals. The roots were then split into halves with a chisel with a Wave-One large gutta-percha cone in the root canal to limit tooth fragments covering endodontic canal walls. For each root, the half containing the most visible part of the endodontic wall was conserved and coded. The coded specimens were secured on metal stubs, desiccated, and viewed with field emission scanning electron microscopy (FEI COMPANY; Inspect S50, Hillsboro, OR, USA). The main operating parameters of the instrument were 5 KeV as gun voltage and a working distance of about 11 μm; both parameters were chosen to avoid an excessive charging of the specimens. Smear layer removal was evaluated by microphotographs taken at 3 and 7mm from the apex at an initial magnification of X1500.Using that magnification 3 areas were selected randomly at the same positions (3 and 7 mm from the apex) and evaluated at X5000 magnification (Figure 1).
Two observers performed blind evaluations independently after examining 12 specimens for calibration purposes.
Intra- and inter-examiner reliability for field emission scanning electron microscopic assessment was verified by the kappa test. The images taken at X5000 were evaluated by using Image J version 1.50e to enable identification and expression of the percentage of area of open dentinal tubules in relation to the total area of the analyzed image. Data were analyzed using the Kolmogorov-Smirnov test to analyze the normality of the continuous variables. The combination of variables: “groups” and “canal thirds” demonstrated a normal distribution. The significant differences in the amount of smear layer removal by the final irrigation protocol were analyzed by using two factor analysis of variance (ANOVA). The analysis of variance was then used for multiple comparisons, with Bonferroni correction, to isolate the differences, which reduced the P value to 0.001 in all groups, with the exception of groups 2 and 4 for both canal thirds ( apical and middle) (p>0.05).
The results of smear layer removal amount in each of the experimental groups appear in Table 2 in the form of percentage distribution. Figure. 1 shows examples of smear layer removal in the middle, and apical thirds.
In all groups, the middle third demonstrated a significant higher percentage of dentinal tubules free of smear layer compared to the apical third (p<0.05). When comparing the amount of smear layer removal by all 4 groups for each root third, group 1(XP Endo-finisher NaOCl/EDTA) showed a higher statistically significant difference in the results of the middle third (P < 0.001), and apical third (P <0.001). There was also no significant difference between groups 2 (XP Endo finisher NaOCl) and 4 (PUI NSK Varios 350 NaOCl) for both root thirds in terms of smear layer removal (p>0.05). The results also demonstrate that group 3 (PUI NSK Varios 350 NaOCl/EDTA) had a significant higher percentage of cleaner dentinal tubules at both root thirds than groups 2 (XP Endo-finisher NaOCl) and 4 (PUI NSK Varios 350 NaOCl) respectively (p<0.001).
Currently, several techniques and systems are available and reported to improve final irrigation before obturation . The recommended combination is a final rinse of 15% or 17% EDTA solution followed by 1%–6% of NaOCl [12,28]. How-ever, there is no consensus on volume [28,29], time of applica-tion , or activation method [31,32] of irrigating solutions. The aim of this study was to compare the effectiveness of a new instrument the XP-Endo finisher file (Brasseler USA®, Savan-nah, Georgia) with that of passive ultrasonic irrigation (PUI) in terms of smear layer removal. These activation techniques are different in their theory and mode of application.Results of this study showed that as regards of percentage of open dentinal tubules scores for all canal thirds, the XP-Endo Finisher/NaOCl-EDTA group had the highest scores, with sig-nificant difference when compared to the other groups. This was followed by the PUI-NSK Varios/NaOCl-EDTA group and finally by the PUI-NSK Varios/NaOCl and XP-Endo Fin-isher/NaOCl groups, which demonstrated the statistically low-est mean scores of open dentinal tubules. The efficiency of the XP-Endo finisher file might be attributed to several reasons. The first is its expansion capacity given its small core size- ISO 25 in diameter and its zero taper, the file exhibits good flexibil-ity and ability to contact several areas of the canal walls. Ac-cording to the manufacturer , the file can expand its reach 6mm in diameter or 100-fold of an equivalent sized file. The second is its shape-memory effect.
The file is straight in its M-phase, which is achieved when it is cooled. When the file is exposed to the body temperature (inside the canal) it will change its shape due to its molecular memory to the A-phase. The A-phase shape in the rotation mode allows the file to access and clean areas that are otherwise impossible to reach with standard instruments.
The relatively lower smear removal scores achieved with PUI/NaOCl-EDTA in comparison to the XP-Endo finisher/NaOCl-EDTA group might be due to the size of the ultrasonic tip used (20/01), which might be very small for these larger canals (40/08). Also, the power setting used (power setting of 4- approx. 28khz) might be weak to allow a proper acoustic streaming with this small tip. This is in agreement with Jiang et al. , who found that a higher ultrasonic intensity results in higher amplitude of oscillation and, consequently, enhances the cleaning efficacy of PUI.
Another factor that could have influenced the results of the PUI/NaOCl-EDTA group in the present study could have been the time of application, concentration and volume of the irrigants used. Some authors have shown that the use of PUI for 3–5 minutes with NaOCl concentrations of 3% or 5% [34,35] is sufficient for the complete removal of the smear layer in instrumented root canals. In contrast, a time of application less than 1 minute did not allow for complete removal of the smear layer with 1% NaOCl .
In this study, sodium hypochlorite without the activation of EDTA for both the PUI and the XP-Endo Finisher file groups did not remove smear layer, proving that irrigation with NaOCl alone is not effective. Baumgartner & Cuenin  also observed that ultrasonically energized NaOCl, even in higher concentrations such as 5.25%, did not remove smear layer from the root canal walls.
Although the ability of PUI to remove the smear layer has been reported previously [38,25], our results showed that PUI was not superior to the XP-Endo finisher file under similar experimental conditions. This is in agreement with other studies [39,40], which have demonstrated that PUI did not completely remove smear layers from the apical third of canal walls.
The results of this study are in accordance with previous studies [41-44], which have demonstrated that the most effective protocol for smear layer elimination might be related to the use of a final flush with 17% EDTA solution. Without a final activation with 17% EDTA solution, the smear layer was seen to cover the root canal surface in the middle and apical thirds, regardless of whether the XP-Endo finisher file or PUI was used.
Within the limitations of this study, the XP-Endo Finisher file in combination with 2.5% NaOCl and EDTA 17% did enhance the removal of smear layer as compared with PUI activation with 2.5% NaOCl and EDTA 17%. The removal of smear layer was more complete in the middle third than in the apical third for all groups. Further research entailing different solutions, volumes, and activation times of the irrigants would be necessary to fully evaluate the effectiveness of the XP-Endo Finisher file system for smear layer removal after root canal instrumentation.