The influence of different working lengths on apically extruded debris

Aim: To evaluate the influence of three different working lengths on the amount of apically extruded debris. Methodology: Thirty lower premolars with single roots and straight root canals were standardized at 17 mm. Then, they were inserted into Eppendorf tubes, and the 1.5% agar gel was introduced into the tubes surrounding the roots. The coronal section of the roots was kept visible. The set of tubes and agar gel was weighed 3 times and the average value was recorded. Then, the specimens were randomly distributed in 3 different groups according to the working length (CT) used for the instrumentation: Group (CT -1) the working length 1 mm below the major foramen (MF); Group (CT 00) the length was determined at the MF, and Group (CT +1) the CT was determined 1 mm beyond the MF. Instrumentation was performed with Reciproc Blue R25 (VDW, Munich, Germany) under irrigation with 0.9% saline. After preparation, samples were removed from Eppendorf tubes and weighed 3 times again. The difference between the mean values of the initial and final weight was recorded. The one-way ANOVA test (post-hoc Bonferroni) was used with P> 0.05. Results: The average weight of the extruded waste was 0.0134 ± 0.0157 for CT -1, 0.0075 ± 0.0062 for CT 00 and 0.0075 ± 0.0068 for CT +1, with no statistically significant differences between groups. Conclusion: There was no impact of the different CTs on the amount of extruded debris beyond the apex.


Introduction
The working length (WL) determines where the root canal should be instrumented and obturated. Traditionally, this determination is set 1 mm short of the radiographic apex, aiming the instrumentation solely at the dentinal canal rather than its full extension (Ricucci et al., 2016). Other studies, however, suggest that the WL should be set at the limit of the major foramen (MF), thus allowing the cleaning of the canal, encompassing both dentinal and cemental canals (De Souza Filho, et al., 1987).
By instrumenting the canal beyond the traditional 1 mm short of the apex, there is a cleaning of the MF and an enlargement of the preparation in the apical third. A more significant enlargement demonstrated decreased bacterial load, increasing the odds of a proper outcome (Card et al., 2002). One possible drawback of the foraminal cleaning is the risk of increasing postoperative pain, mainly when sodium hypochlorite is used as irrigant (Cruz Junior et al., 2016). Saline has been proposed as irrigant to prevent the possible disadvantage of NaOCl beyond the apex, associated with 2% chlorhexidine in a gel formulation (Ferraz et al., 2007). Therefore, the risk of the cytotoxic effects of NaOCl beyond the apex is minimized.
It has been suggested that instrumentation beyond the apical constriction might increase the risk of debris extrusion beyond the apex (Beeson et al., 1998). Another study, however, demonstrated that a different working length did not impact the extrusion of debris (Silva et al., 2016). The majority of these previous studies used a methodology that is not able to assess the extrusion of irrigants, underestimating the role of these in the total amount of material extruded beyond the apex.
Recently, a different methodology was adopted to assess extrusion beyond the apex, allowing the data collection of solid debris and liquid beyond the apex (Lu et al., 2013). This methodology better replicates the clinical condition that the clinician faces daily. Therefore, this study aimed to assess the debris and irrigants extrusion with different WL determinations.

Methodology
The Ethical Committee approved this research protocol of our Institution under the number 3.141.750. This research encompassed 30 mandibular premolars obtained from patients requiring either periodontal or orthodontic treatments. After radiographic images were obtained from buccolingual and mesio-distal directions, teeth presenting single roots and single root canals were initially selected. Further, an examination was done to ensure that the MF was smaller than 0.20 mm and that the roots presented straight root canals (curvature < 5°) according to a method established in a previous study (Schneider, 1971).
The exclusion criteria ruled out teeth presenting any signs of cracks, fractures, caries, pulp canal obliteration, internal or external resorption, or previous endodontic therapy.

Sample Size Calculation
The number of specimens required for this study was based on a previous study by Lu et al. (2013). The sample size was calculated for achieving an alpha-type error of 0.5, and a study power of 95% was input to test using a G*Power 3.0 for Mac statistical package. The results indicated a total of 30 specimens equally distributed in 3 different groups in order to observe statistically significant differences in the total of apically extruded debris.

Preparation of the Specimens
The teeth were decoronated using a diamond disk, and the roots were standardized in the same length of 17 mm. Under copious irrigation of 5% NaOCl, a size #10 k-file was gently used to remove the canal's content. All of the major foramen (MF) were standardized to a size of 0.15 mm using a size 15 k-file. The roots presenting with MF smaller than 0.15 mm were gently instrumented until this size was reached. The roots with an initial diameter of the MF larger than 0.15 mm were discarded.
The roots were covered with a teflon tape throughout the full extension, except the 1 mm of the apical third that was kept exposed. The tubes' lids (Eppendorf of Brasil, São Paulo, SP, Brazil) were pierced, the roots were inserted through these lids, and the gap between the roots and the lids were sealed using cyanoacrylate and gingival dam. The root and lid set were weighed on a scale with 10 -4 precision (Nowak, São José do Rio Preto, Brasil). Each set was weighted 3 times and the mean weight was registered in a spreadsheet. Then, 1.5% agar gel (Kasvi, São José dos Pinhais-PR, Brasil) was placed inside the Eppendorf tubes, and the set of root and lid were inserted in the tubes to have the roots completely involved in the agar gel. After 24 hours of sitting at room temperature, the whole set was again weighted 3 times and the mean weight was calculated. The difference between the 2 means was considered the initial weight.

Root Canal Shaping
The root canals were shaped using a reciprocating instrument, Reciproc Blue (VDW, Munich, Germany), size 25, taper 8 in the first 3 mm. The file was used in a dedicated endodontic motor (VDW Silver, VDW, Munich, Germany) in the specific "Reciproc ALL" configuration. The instrument slowly moved to the determined WL in small increments from 3-4 mm of amplitude. After 3 movements, the instrument was removed, cleaned with gauze, and the canal was again irrigated and flooded with 0.9% saline. After each removal of the reciprocating instrument, the foraminal patency was re-checked with a size 10 kfile.
The specimens were assigned to 3 different groups according to the groups previously determined. The randomization of the specimens was achieved using a computerized algorithmrandom.org. The canals were instrumented in 3 different groups according to the WL of choice: Group (WL -1)instrumentation was done 1 mm short of the MF; Group (WL 00)instrumentation at the MF; and Group (WL +1)instrumentation performed 1 mm beyond the MF. The MF position was considered the point at which the tip of a size 10 k-file was visible under 10x magnification of a dental operating microscope.

Post-Instrumentation Analysis
After the root canal shaping procedures, the lids with the roots were removed. The set of Eppendorf tubes with the agar gel, at this moment impregnated with debris and irrigants, was weighted 3 times. Once more, the mean weight was calculated and registered. The final weight included both irrigants and debris extruded was the difference between this final weight and the previously calculated initial weight of the whole content.

Statistical Analysis
The data was submitted to the Kolmogorov-Smirnov test that assumed the normality of the results. Therefore, the ANOVA, post-hoc Bonferroni, was used for statistical analysis for differences among the groups. The Statistical Package for Social Sciences (SPSS 20.0 for Windows) was chosen, and the P < 0.05 was considered for statistical differences. Table 1 shows the mean weight, in grams, of the extruded debris and irrigants was 0.0134 ± 0.0157 for WL -1, 0.0075 ± 0.0062 for WL 00, and 0.0075 ± 0.0068 for WL +1. There was no statistically significant difference among the groups P > 0.05. Table 1. Mean and standard deviations (SD) of apically extruded debris by different working length (WL). p*: Same lowercase letters indicate that there was no statistical difference between the groups (p <0.05). Source: Authors.

Discussion
The most common methodology for assessing apically extruded debris is based on a study by Myers and Montgomery (Myers & Montgomery, 1991). In this methodology, the collected material extruded beyond the apex is dried to evaporate the liquid content so the amount of debris extruded can be measured. The findings of that methodology are limited to the solid content and do not permit the assessing the irrigant solution used during root canal shaping. The methodology used in this study included a 1.5% agar gel for collecting the apically extruded material (Lu et al., 2013). Therefore, the solid and the liquid material extruded beyond the apex could be assessed. Instead of an empty flask, the agar gel also simulates in vitro the counter-pressure promoted in vivo by the periapical tissues (Lu et al, 2013;George & Walsh, 2008). Therefore, it is our understanding that this in vitro methodology can better mimic the clinical situation.
The instrumentation protocol adopted in this study is based on a single-file reciprocating system. This system demonstrated to promote less extrusion than a full sequence of rotary instruments (De-Deus et al., 2010); likewise, it is well established that hand instrumentation is more prone to promote extrusion than rotary instrumentation (Ferraz et al., 2001, Toyoglu & Altunbas, 2017. Interestingly, a previous study showed that patency beyond the apex promoted more debris extrusion when hand instrumentation is adopted. That study also showed that when using rotary instrumentation, files beyond the apex did not increase the amount of extruded debris (Tinaz et al, 2005). A classical study using hand instrumentation also demonstrated that, using hand files, instrumentation beyond the apex with or without ultrasonic activation increases apically extruded debris (Martin & Cunningham, 1982). The present study results showed that, under reciprocating instrumentation, the determination at or beyond, the MF did not increase the amount of apically extruded debris. Therefore, these findings suggest that the instrumentation technique plays a more critical role in apically extruded debris than the WL determination.
A recent study showed that instrumentation at the apex produced the same amount of extruded debris as instrumentation 1 mm short of the apex (Silva et al., 2016). Also, further instrumentation up to the size 40.06 was not related to an increase of apically extruded debris. The present study's findings follow with that previous research; similarly, the present study also applied a reciprocating size 25.08 instrument. Based on the present study's findings, one may infer that instrumentation 1 mm beyond the MF is also safe in regard to apically extruded debris and irrigants.
Previous studies presented controversial results regarding postoperative pain after root canal preparation at the apex or 1 mm short of the apex. While Cruz-Jr et al. (2016) found that instrumentation at the apex resulted in higher postoperative pain levels, Silva et al. found no difference in postoperative pain assessing the same WL as a variable (Silva et al., 2013). It is essential to mention that both studies used NaOCl as irrigants. It is our understanding that, when instrumentation at or beyond the MF applies an irrigant other than NaOCL, such as saline, then either distilled water or chlorhexidine (CHX) should be considered.
The results of the present study showed the same amount of apically extruded debris and irrigants beyond the apex, the same amount of postoperative pain should be expected. Therefore, further clinical studies using instrumentation at or beyond the MF should consider using fewer cytotoxic irrigants.

Conclusion
The results of the present study conclude that instrumentation at the MF, 1 mm short or 1 mm beyond the MF, leads to the same amount of apically extruded debris and irrigants.