Efficacy of different irrigation techniques on calcium hydroxide removal from the root canal

Summary Introduction Calcium hydroxide (CH) is a medicament widely used in endodontic treatment due to its antibacterial, regenerative and biocompatible properties. Studies have shown that remaining CH on root canal walls and dentinal tubules can compromise sealer penetration, leading to its weak adhesion, volume change and, consequently, apical leakage. The aim of this study was to compare the efficacy of four different techniques in removing calcium hydroxide from the root canal. Material and Methods 32 extracted single-rooted teeth with one canal were used in this study. The canals were prepared using BioRaCe system (FKG Dentaire, Swiss) BR5 40/.04 with sodium hypochlorite irrigation after each instrument. Longitudinal grooves were formed on the proximal root surfaces. All canals (except negative control) were filled with aqueous CH suspension. After seven days of incubation, the teeth were allocated into the four groups (n=7), plus positive and negative control. Four techniques (systems) for CH removal were tested: conventional syringe irrigation (CSI), passive ultrasonic irrigation (PUI), XP-endo Finisher (FKG Dentaire, Swiss) and Canal Brush (Roeko, Coltene) with irrigation of 5 ml 2% NaOCl and 5 ml 10% citric acid. All the roots were then split into the two halves with chisel and observed under the stereomicroscope (Boeco, Germany) at magnification of 20x. The area with remaining CH on the root canal wall surface was then divided with the total root canal surface area (%). The obtained results were statistically processed using One-way ANOVA and Tukey post-hoc test (p<0.05). Results The most efficient system was XP-endo Finisher with 91.33% of clean surface, followed by PUI 88.36%, Canal Brush 87.83%, and CSI with 66.92%. Conclusion None of the systems completely removed the traces of the medicament from the root canal. For optimal clinical success, it is necessary to combine various systems with copious irrigation.


INTRODUCTION
Canal instrumentation and irrigation is not sufficient to complete full cleaning of root canals. Despite technological advancements in the instrumentation technique and irrigation systems, canal medication is still necessary phase in some cases.
Calcium hydroxide (CH) is the most often used intracanal medicament due to its antibacterial, biocompatible and regenerative properties. Before obturation, it has to be completely removed from the root canal walls in order to allow sealer adhesion and prevent intracanal microleakage as a consequence. Numerous studies have dealt with the problem of CH removal from the root canals and the role of different irrigation substances [1] or different irrigant activation techniques: laser activated irrigation (PIPS) [2,3], sonic and ultrasonic irrigant activation [4,5], RinsEndo system [6], EndoVac system [7], SAF (Self adjusting files) [8], Gentle Wave system [9] and others.
The most described method for CH removal from the root canal walls is instrumentation of the canals with a master instrument along with excessive irrigation [3]. Research indicates that with this technique only the main part of the canal can be cleaned and that the depth of needle plays a crucial role [10]. Passive ultrasonic irrigation (PUI) increases the efficacy of canal disinfectants by agitation of the solution, which was beforehand placed in the canal [8]. XP Endo Finisher (FKG Dentaire, La-Chauxde Fonds, Switzerland) is a NiTi canal instrument with the size of ISO #25, and without taper (25/.00). This instrument increases the penetration of the solution for irrigation in irregular root canals [11,12]. CanalBrush (CB) (Roeko Canal Brush TM Coltene/Whaledent, Langenau, Germany) is flexible endodontic micro brush, made of polypropylene and used with endodontic motor providing efficient canal cleaning just before the obturation [8,11].
The aim of this study was to compare the efficacy of conventional irrigation technique, passive ultrasonic irrigation, Canal Brush and XP Endo Finisher in CH removal from the root canal walls.

MATERIAL AND METHODS
Thirty-two extracted single-rooted teeth were used in this study. Water-cooled round diamond bur was used for access cavity preparation on the palatal surface of the crown. Canal instrument K-15 (Dentsply Maillefer) was used to check patency. Working length was determined 1 mm shorter of the apical foramen. The canals were prepared using BioRaCe system (FKG Dentaire, Swiss) BR5 40/.04 with 2% NaOCl irrigation after each instrument. On the buccal and lingual surfaces of the root, 1 mm deep longitudinal grooves were created using a diamond disk, taking care not to endanger the integrity of the root canal. After instrumentation and irrigation, canals were dried with paper points and filled with aqueous suspension of CH (Ca powder and distilled water), and closed with temporary filling Citodur hard (DoriDent-Dr.Hirschberg, Austria). All samples were wrapped in wet gauze soaked with distilled water and kept in an incubator at 37º C. After seven days of storage, the teeth were randomly divided into the four groups (n=7). Two teeth were used as positive and negative control. Positive control consisted of teeth filled with CH that was not removed from the root canal. Negative control consisted of prepared teeth with empty canals (without CH paste).
I group: Conventional syringe irrigation (CSI) -Medicament was removed using manual instruments (files) from K-15 to K-40 (master apical file-MAF) and irrigation.
II group: Passive ultrasonic irrigation (PUI) (PB-323,W&H Dentalwerk Bürmoos, Austria) -Ultrasonic needle was placed into the canal 1 mm shorter than the working length without contact with canal walls and activated 3 times for 20 sec (the frequency of 25-30 kHz). For each cycle fresh irrigant was inserted.
III group: XP-Endo finisher (XP) -The instrument was used with an X-smart endodontic motor (Dentsply Sirona, Ballaigues, Switzerland) at a speed of 800 rpm and torque of 1 N/cm. The instrument was placed in the canal 1 mm shorter than the working length, and used with gentle movements up and down in three cycles of 1 minute with constant irrigation.
IV group: CanalBrush (CB) -The canal brush size M was placed into the canal 1 mm shorter than the working length, and activated with an endo-motor at a speed of 600 rpm during 1 min for each irrigant. Gentle brushing movements with constant irrigation were used. A new brush was used for each canal.
All groups were irrigated under the same conditions, continuously irrigated with 5 ml of 2% NaOCl for 1 min, and 5 ml of 10% citric acid for 1 min. Finally, all samples were irrigated with 5 ml of distilled water.
All teeth were cut into the two halves with a chisel and observed under the stereomicroscope (Boeco BSZ-405, Germany) with an integrated digital camera at 20X magnification. Images were processed and measured in Scope Image 9.0 program (Teleskop, Austria). The total surface area of the canal (P s ) was measured, from the enamel-dentin junction to the apical foramen (expressed at 100%). Surfaces with residual medicament (P ch ) were measured in the same program. The percentage of clean surface of the root canal (P c ) was calculated by subtracting the obtained values (P s -P ch =P c ). The obtained results were statistically analysed using One-way ANOVA and Tukey post-hoc tests. P value <0.05 was considered significant.

RESULTS
The results are shown in Figures 1-5.  The mean value of clean surface of the root canals was 91.33% for XP EndoFinisher, 88.36% for passive ultrasonic irrigation, 87.83% for Canal Brush and 66.92% for conventional syringe irrigation (CSI). The third group with XP showed the highest efficacy in removing CH from the canal walls with maximum of 99.40% clean surface. For the samples from the fourth group, CB, the intrusion of CH in the apical third was observed. The lowest efficacy in the removal of CH from the canal walls was shown by conventional technique, CSI, where only 47.58% of the surface was cleaned in some samples. CSI was significantly less effective than XP and PUI (p<0.05), while there was no significant difference between XP and PUI. No irrigation technique was able to completely remove medication from the canal walls.

DISCUSSION
Irrigation and medication play an important role in the root canal infection control. The most often-used intra-   canal medicament is CH. Considering necessity of removing CH from the canal there are contradictory research results [13,14,15]. However, the necessity of removing CH was accepted, due to its influence on dentin adhesion, and adhesion of endodontic sealers [1]. Previous studies indicated difficulties in complete removal of CH paste from the root canal system, especially from the apical third [1, 13,16].
The current study was designed to compare the ability of different methods in removing CH from the root canal. To obtain precise results, the area of CH remnants on the canal walls was calculated in relation to the entire surface of the canal. Numerous studies used different scoring systems of 0-3 [1, 8,11] or 1-5 [5] to calculate residual medicament, where lower values indicated clean canal, and higher values indicated canal filled with CH. These methods actually represent subjective assessment of the amount of residual medicament or clean surface of the canal. Ma and al. agreed with this and highlighted that this scoring method was not sensitive enough for comparing samples and interpretation of the results [9]. The other described method is measuring the volume of CH before and after removal from the root canal, which often requires expensive and sophisticated equipment or usage of radioactive isotopes [3,9,17]. In our study, quite exact method of calculating the surface of cleaned canal walls was used. The main advantage of this method is repeatability [2, 16].
No technique of irrigation and cleaning the root canal, tested in our study, completely eliminated CH from the canal, and that is in compliance with findings of other studies that found at least 2-4% residual medicament on the canal walls [3,7,9].
The results of our study showed that conventional technique of removing medicament with MAF and constant irrigation was the least effective. The most efficient system was the XP Endo finisher. Silva et al. found that the amount of residual CH could be between 3 to 20% [18]. In our study, with conventional technique, the canal was cleaned only about 24%, while larger parts of canal walls remained unclean, which could have a negative effect on the outcome of endodontic therapy. In this paper, XP showed the best results with over 99% clean surface of the canal. The reason for such good result could be the design of the instrument that is placed into the canal after instrumentation and with gentle extrusion and intrusion movements extended the cleaning effect up to 6 mm in diameter. An additional reason for effective canal cleaning with XP was the corresponding dimension of the apex preparation 40 /.04. CH and smear layer were removed efficiently due to the physical contact of rotating instruments and canal walls. Findings of Haman et al. indicate that XP was superior, especially in the apical third, just because of the contact with walls, and more efficient than ultrasonic irrigation [16]. At PUI sonic energy and frequency up to 30 kHz transmitted through ultrasonic extension formed cavitation bubbles. This agitation of irrigant increased its penetration, although passively without touching the walls of canal. Ultrasonic tip was placed 1 mm from the working length, and in authors' opinion, the apical segment remained without direct effect of ultrasonic activation. This might be the reason of lagging medicament in the apical segment of the root canal [18]. Also, the efficiency of PUI does not depend only on the duration of irrigant activation, but also on the constant addition of fresh solution [5].
There was statistically significant difference between the conventional technique for CH removal and other irrigation systems in this study. Between CU, PUI and XP there was no statistically significant difference, although the Canal Brush did not prove to be effective enough, especially in the apical third. This was also pointed out by other authors [8,17]. Canal Brush failed to effectively remove medicament from all canal walls which was even supressed toward apex, especially those widely prepared. This could be a special problem in narrow and curved canals. The findings of our study are in accordance with other studies reporting XP and PUI as the most efficient methods for cleaning canal, without a significant difference between them [4,8,19,20]. It is known that canal irrigation with NaOCl alone is not sufficient to remove CH from the root canal [21]. In addition, chelating agent (citric acid or EDTA) alone is not efficient either [22]. Topçuoğlu et al. pointed out that combination of these irrigants (NaOCl, EDTA) improved their effectiveness in removal of CH [8]. Effective removal of medicament did not depend only on irrigation technique and volume of irrigant, but also on chemical activity of irrigation agents and the size of apical preparation. The amount of used irrigant was inversely proportional to the residual medicament [17]. On the other hand, Ma et al. pointed out the importance of duration time of irrigation (up to 7 min per canal) [9].
In our paper, simple root canals were used in order to assess the efficacy of the techniques themselves, without the influence of the complexity of the root canal. The efficacy of CH removal from complicated and irregular canals is subject to further testing.