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The bond strength of root canal filling after calcium hydroxide removal with a simple apical negative pressure kit in oval-shaped root canal

1 Department of Restorative Dentistry and Periodontology, Division of Endodontics, Faculty of Dentistry, Chiang Mai University, Chiang Mai, Thailand
2 Dental Research Center, Faculty of Dentistry, Chiang Mai University, Chiang Mai, Thailand

Date of Submission05-Aug-2021
Date of Decision08-Jul-2022
Date of Acceptance23-Aug-2022
Date of Web Publication11-Nov-2022

Correspondence Address:
Anat Dewi,
Department of Restorative Dentistry and Periodontology, Faculty of Dentistry, Chiang Mai University, Suthep Rd, T. Suthep, A. Muang, Chiang Mai 50200
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/ccd.ccd_583_21


Background: The aim of this study was to evaluate the effect of a simple irrigating device, which produced from the apical negative pressure concept, as a final irrigating method after calcium hydroxide (CH) medication on the bond strength of epoxy resin-based sealer in the oval-shaped root canal. Methods: Forty-eight single-rooted premolars with oval-shaped canals were included in the study. The crown was decoronized and canal instrumented with Mtwo (VDW GmbH, Munich, Germany) up to size 40/04. The specimens were assigned to four groups according to the irrigation protocol after 1 week of CH medication: control group (no medication) and three experimental groups conventional needle irrigation (CNI), passive ultrasonic irrigation (PUI), and simple apical negative pressure kit (sANP). The teeth were obturated with a warm vertical technique using epoxy resin-based sealers. After 1 week, the roots were transversally sectioned at coronal, middle, and apical thirds. A push-out test was performed by a universal testing machine, and statistical analysis was performed using One-Way ANOVA with Duncan's post hoc test. Results: The bond strength in the control group was significantly higher than the CNI group in all root canal thirds (P < 0.05). At coronal third, sANP showed lower bond strength than PUI group (P < 0.05). However, the bond strength of sANP group was comparable to the control group (P > 0.05) which was significantly higher than the PUI group at apical third (P < 0.05). Conclusion: A sANP enhanced the bond strength of epoxy resin-based sealer in the apical third of CH-medicated root canal in the oval-shaped canal.

Keywords: Apical negative pressure, bond strength, calcium hydroxide removal, epoxy resin-based sealers, oval-shaped root canal

How to cite this URL:
Leelapornpisid W, Sastraruji T, Louwakul P, Dewi A. The bond strength of root canal filling after calcium hydroxide removal with a simple apical negative pressure kit in oval-shaped root canal. Contemp Clin Dent [Epub ahead of print] [cited 2023 Feb 2]. Available from:

   Introduction Top

The eradication of microorganisms from the root canal systems or root canal debridement is essential for improve endodontic outcomes.[1] However, the bacterial-free canal system is not adequate from the mechanical instrumentation technique used in the present day. Therefore, root canal medication is an extra method required for continue disinfection between endodontic appointments.[2]

To date, calcium hydroxide (CH) is the gold standard root canal medicament used in endodontics because of the effective antimicrobial activity.[3] However, the complexity within the root canal systems hinders the medicament-free environment before root canal obturation.[4] The oval canal is one of the root canal anatomy found in several types of the tooth.[5] A previous study on oval-shaped root canals found that there was residual CH after root canal irrigating in different debridement methods.[6] CH remnants at the canal walls might interfere with the adhesion quality of root filling materials.[7] Therefore, complete eradication of CH medicament from the root canal is the crucial step before root canal obturation to achieve good sealing ability.[8]

Syringe with the needle or conventional needle irrigation (CNI) is a commonly used root canal irrigation method. Previous studies reported that the intracanal medicaments were not completely removed by CNI.[9] Mechanical activation of chemical irrigants has been introduced to endodontics because it can bring irrigants to inaccessible root canal anatomy. Passive ultrasonic irrigation (PUI) is used as a gold standard of irrigant activation methods in several studies.[10] The acoustic microstreaming will be occurred surrounding the instrument after activating PUI which promoted the flow of irrigants to the root canal systems and CH can be removed effectively.[10]

Apical negative pressure system (ANP) has been developed to be used in root canal debridement procedures. The device can deliver root canal irrigant to the apical portion of the root canal with the evacuation of irrigant and organic/inorganic contents from the full length of the root canal. Previous studies reported that ANP had higher debridement efficiency compared to CNI.[11] A simple apical negative pressure kit (sANP) can be developed from the intravenous set which is a commonly used medical device.[12] Previous studies showed the ability of the sANP device in the removal of smear layer effectively at the apical third of the root canal after mechanical instrumentation.[12]

CH removal has an impact on root canal obturation quality. Therefore, the purpose of this study was to evaluate the efficacy of the final irrigation technique with the sANP kit on the bond strengths of epoxy resin-based sealer in the oval-shaped root canal.

   Methods Top

Selection and preparation of specimens

This study was approved by the Faculty of Dentistry Human Experimentation Committee (Certificate of ethical clearance No. 43/2020, approved on June 19, 2020). The sample size was estimated by using the power of calculation to have adequate power to apply the statistical test of the research hypothesis. Forty-eight extracted human mandibular premolars were collected and kept in a 0.1% (v/v) thymol solution. Teeth with the carious lesion, filling, crack, fracture, or open apex were excluded. The oval canals, which were radiographically evaluated by B-Li and M-D dimension, canals with a maximum diameter of more than a minimum diameter of at least 1.5 mm × at 5 mm from root apex,[8] were included in this study. The teeth were decoronized with a diamond disc (Intensive SA, Montagnola, Switzerland) into 16-mm-long root blocks. The outer surface of the root blocks was covered with two layers of nail varnish to avoid dehydration. The blocks were stabilized in a vertical orientation and embedded firmly into putty-type silicone impression material (Henry Schein Dental, Cardiff, UK) to replicate a close system and easy for manipulation during the experiment. The root canals were then shaped with the rotary instruments (Mtwo, VDW GmbH, Munich, Germany) up to size 40/04 at the 1-mm from the root apex. During the instrumentation, the root canals were irrigated by using 20 ml of 2.5% sodium hypochlorite solution (Sigma-Aldrich, St. Louis, MO, USA) with a side-vented 30-G needle with the recapitulation by size 10 K-file. The smear layer was removed with 5 ml of 2.5% NaOCl and 2 ml of 17% ethylenediaminetetraacetic acid (EDTA). The root canals were then finally flushed with 5 ml of distilled water, and the canals were dried with paper points. The 12 root blocks were randomly assigned as a negative control (no medication). The other 36 root blocks were medicated with premixed CH (UltraCal; Ultradent, South Jordan, USA) by using lentulo spiral to confirm that CH was filled to the working length. The canal orifices were then sealed with a temporary restoration (Cavit; 3M ESPE, Seefeld, Germany). All specimens were incubated in 100% humidity at 37°C for 1 week.

Experimental protocol

After 7 days of incubation, the temporary restoration was removed by using a round bur with high-speed handpiece. The specimens were randomly assigned into three experimental groups (n = 12/each group) according to the irrigation system as followings.

Group 1: Conventional needle irrigation

The canals were irrigated with 4 ml of 2.5% NaOCl for 3 min by using 30 gauge side-vented needle, the needle was placed 2 mm short of the working length. The NaOCl was left in the canal for another 1 min before being aspirated, and the canal was dried with paper points. The canal was then irrigated with 2 ml of 17% EDTA for 1 min. After that, the canal was irrigated with 2 ml of 2.5% NaOCl for 1 min.

Group 2: Passive ultrasonic irrigation

The canals were irrigated with the irrigants, concentration, and time similar to group 1. The additional irrigation protocol was the activation with an Ultrasonic Generator (NEWTRON P5, Acteon) at 28000 hertz together with an IrriSafe tip size #25IRR (Satelec Acteon Group, Merignac, France). The tip was placed 2 mm from the working length and then ultrasonically activated in the 2–3 mm upward-downward motion.

Group 3: Simple apical negative pressure kit

The design of a simple ANP system was previously described by Upara et al.[12] and the schematic representation of the sANP apparatus is shown in [Figure 1]. The canals in the ANP group were first irrigated with 4 ml of 2.5% NaOCl for 3 min. The irrigant delivery tip was placed at the access cavity with the use of CH applicator tip for aspirating the irrigant from the middle portion of the root canal. The tip was moved in upward-downward motion to prevent the stuck of the instrument to the canal. After that, 2 mL of 17% EDTA was delivered from the irrigant delivery tip, and the side-vented 30- G irrigating needle was used to draw the irrigant back from the apical part of the root canals for 1 min. Following this, the 2.5% NaOCl 2 ml was used for 1 min with the use of the side-vented needle to draw the irrigant back from the working length.
Figure 1: The schematic representation of the sANP apparatus, sANP: Simple apical negative pressure

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Root canal obturation

After root canal irrigation, all samples were dried with paper points. The root canals were then obturated with a warm vertical technique using a 4% gutta-percha cone (Dentsply Sirona, Ballaigues, Vaud, Switzerland) and AH Plus sealer (Dentsply Sirona). The down pack was done at 4 mm from the working length by heat carrier tip, and the warm gutta percha was injected into the cementoenamel junction by obturation gun (B and L Biotech, Gyeonggi-do, South Korea). The canal orifices were sealed with a temporary restoration, and the teeth were stored in an incubator at 37°C and 100% humidity for 1 week.

Push-out bond strength analysis

Teeth were embedded in acrylic resin, then the specimens were cut perpendicular to their long axis using a precision saw (IsoMet 1000; Buehler, Lake Bluff, IL, USA). Three slices (1 mm thickness) were obtained from each tooth at 3 mm, 8 mm, and 13 mm from the root apex (apical, middle, and coronal section). The thickness of slices was calibrated with a digital caliper to avoid thickness variation. The push-out test was performed with a universal testing machine (Instron 5566 Universal Testing Machine, Instron Engineering Corporation, Norwood, MN, USA) at a constant crosshead speed of 0.5 mm/min. Cylindrical pluggers in 0.3 mm, 0.5 mm, and 0.7 mm were used to test at the apical, middle, and coronal sections, respectively. The maximum load applied to the filling material before failure was recorded in Newtons (N) and converted to mega Pascals (MPa) according to the following formula: MPa = F/A; F = bond strength (N), A = bonded area (mm2). The bonded area of the root canal filling to the oval-shaped canals was calculated following the study of Coniglio et al., 2011.[13]

After the push-out test was accomplished, each section of the samples was visually examined under an invert phase contrast microscope (Olympus, Tokyo, Japan) at 40× to determine the failure modes. Three types of failure were classified: adhesive failure (failure at sealer/dentin wall interface), cohesive failure (failure within filling material), and mixed (a combination of cohesive and adhesive failure).

The data of push-out bond strength from different irrigation techniques were analyzed using One-Way ANOVA with Duncan's post hoc test. The level of significance was set at P < 0.05. All calculations were performed with 17.0 (SPSS software, SPSS Inc., Chicago, IL, USA) statistical software.

   Results Top

The push-out-bond strength (in MPa) and standard deviation of control and all experiment groups are demonstrated in [Table 1]. The push-out bond strength of the control group was significantly higher than the CNI group in all root canal sections (P < 0.05). In the coronal third, the push-out-bond strength value in PUI was not statistically different from the control group (P > 0.05). The value in the CNI and sANP were comparable (P > 0.05), but lower than in the control group (P < 0.05). In the middle third, the push-out-bond strength value in CNI was significant lowest from the others (P < 0.05). In the apical third, the push-out-bond strength value in the sANP was not statistically different from the control group (P > 0.05). The value in PUI was significantly lower than in the control group (P < 0.05) [Figure 2].
Figure 2: Bar chart demonstrates the push-out bond strength values (MPa) for all groups at coronal, middle, and apical thirds, MPa: Mega Pascals

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Table 1: Mean (standard deviation) of the push-out bond strength values (MPa) for all experimental groups at different levels of root canal thirds

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The failure pattern distribution is shown in [Figure 3]. Mixed failure type and cohesive failure type were predominated in all irrigation protocols. However, adhesive failure was predominated in the CNI group than the others.
Figure 3: Failure mode distribution in all groups (in %) (C: Coronal third, M: Middle third, A: Apical third)

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   Discussion Top

CH medication reduced the bond strength of root canal filling materials, the standard irrigation protocol was not sufficient to remove CH remnant before obturation.[14],[15] The present study showed that the use of syringe and needle irrigation systems in CH treatment groups had significantly lower bond strength than the nonmedicated canal in all root canal thirds. This could be explained by the inference of CH remnants to the interfacial covalent bonds between AH plus sealer (epoxide rings) and dentin (exposed amino groups).[16] Moreover, the dentinal tubule penetration of the sealer was also interfered which compromised to the bond in the sealer-root canal wall interface as reported in previous studies.[17]

PUI has efficiency in CH removal because the ultrasonically oscillating instrument can activate irrigants which provide acoustic microstreaming of solutions in the root canal system.[18] This study showed that PUI had similar push-out bond strength to the control group at the coronal and middle third of root canals which is in agreement with another study by Ghabraei et al.[14] However, bond strength values at the apical third of the root canal in the PUI group was significantly lower than control and sANP group. This might be the effect of the close system of experiment which was set to mimic clinical scenarios. The close system limited the flow of irrigants at the apical root third and caused the vapor lock phenomenon.[19] The tip of the PUI instrument could not be effectively activated in an air phase. Moreover, the tip could not move freely in the root canal at the apical third which resulted in inadequate of energy transmission to the root canal irrigant. Therefore, the efficiency of PUI instrument to activate the solution was reduced.[20] The previous study performed Micro-computed tomography to examine CH eradication in the PUI-activated group from the root canal and found that there was CH remnant at the apical third than the coronal third of the root canal.[21]

sANP kit which was assembled in this study had high push-out bond strength values at the apical root canal as the control group. Previous studies reported that the ANP system was more effective in medicament removal from the apical root canal compared to CNI[22],[23] because the needle of ANP, which was placed at the WL of specimens, was connected to the high-speed suction. The air entrapment or vapor lock at the apical part of the root canal was eradicated and the active solutions can flow throughout the root canal systems. Therefore, sANP was more effective in CH removal from an apical root canal than other experimental groups.

However, sANP had lower push-out bond strength values than PUI and the control group at the coronal third of specimens. A previous in vitro study[24] showed that the CH removal ability of ANP was comparable to CNI at coronal parts of the canal. The crown of the tooth, which was a reservoir of irrigants in the ANP system, was removed in the methodology. Therefore, the replenishment of solutions in the root canal was limited.

SEM study by Yücel et al.[9] found that PUI and ANP had similar efficiency in CH removal at all root canal levels. Therefore, the bond strength values in PUI and ANP were higher than in CNI. However, there was the discrepancy in methodologies from several studies. Therefore, the relevance of CH remnants in each root canal level to the bond strength of root filling materials should be further studied.

The oval-shaped root canal was selected in this study because the medication removal in this anatomy was challenging.[5] Moreover, it is difficult to achieve the three-dimensional obturation in the oval-shaped root canal.[25],[26] Therefore, the warm vertical compaction technique was selected in the experiment to promote the flow of gutta-percha. Previous studies showed that there was better adaptation and homogeneity of root canal filling material in the warm vertical compaction technique compared to the cold lateral compaction technique.[27]

The push-out bond strength values were significantly influenced by the root canal third, the values were increased from the coronal to the apical direction. This finding was in line with the previous studies.[27],[28] The dentinal tubule diameter or tubule density at the apical third was lower than other root canal levels, but this might not influence to the bonding ability of the obturation materials to the root canal walls.[29] The other factor which improved bonding ability was the root canal's cross-sectional shape, the canal was rounder at the apical third of the root than the coronal or middle thirds. In addition, the use of the warm vertical compaction technique caused more hydraulic force at the apical third which could promote the adaptation of filling materials to the canal walls at this level.

This study focused on the final rinse protocol with a simple ANP kit after CH medicament in comparison to PUI, which was the most presently used activating irrigation method. We found that the use of CNI alone was not improve the bond strength of root canal filling material after CH medication. PUI group showed higher bond strength values at the coronal third than the sANP group. However, sANP and the control group showed higher bond strength than PUI at apical third. The results of the efficacy of final irrigation techniques to the bond strength at different root thirds can be applied in the clinical setting. The combination of irrigating strategies to improve bond strengths of root canal filling materials can be modified in endodontic treatment. In addition, the push-out bond strength might be different on variated root canal anatomy. Therefore, the push-out bond strength on the curved canal, long oval canal, or C-shaped canals should be further study.

   Conclusion Top

Within the limitations of this study, CH medication showed a negative effect to the bond of epoxy resin-based sealer. A sANP promoted the bond strength of epoxy resin-based sealer in the apical third of CH-medicated root canal in the oval-shaped canal.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

   References Top

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