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 Table of Contents  
ORIGINAL ARTICLE
Year : 2022  |  Volume : 12  |  Issue : 1  |  Page : 106-113

Effect of sonic irrigation activation at different frequencies in smear layer removal; An in vitro experimental study


1 Department of Endodontics, Hamdan Bin Mohammed College of Dental Medicine, Mohammed Bin Rashid University of Medicine and Health Sciences, Dubai; Department of Dentistry, Zayed Military Hospital, Abu Dhabi, UAE
2 Department of Endodontics, Hamdan Bin Mohammed College of Dental Medicine, Mohammed Bin Rashid University of Medicine and Health Sciences, Dubai, UAE

Date of Submission31-Jul-2021
Date of Decision18-Aug-2021
Date of Acceptance28-Aug-2021
Date of Web Publication8-Jan-2022

Correspondence Address:
Mohamed Jamal
Department of Endodontics, Hamdan Bin Mohamed College of Dental Medicine, Mohammed Bin Rashid University of Medicine and Health Sciences, Building 14, Dubai Health Care City, P.O. Box: 505 055
UAE
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/sej.sej_164_21

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  Abstract 


Introduction: The aim of this study was to compare the vibration/oscillation frequencies of WaterPik and EndoActivator devices and their effectiveness in smear layer (SL) removal.
Materials and Methods: The root canal of 60 single-rooted extracted human premolars were prepared until ProTaper Universal F2 file, and randomly grouped according to the irrigation activation technique used: EndoActivator group; WaterPik group; manual dynamic agitation (MDA) group; and control group (conventional irrigation with no activation). The vibration/oscillation frequencies of EndoActivator and WaterPik devices were measured using a digital tachometer. Electron microscopy images of all specimens were evaluated for SL removal according to a modified Hülsmann scoring system. Data were statistically analyzed.
Results: The mean vibration frequency of the EndoActivator was significantly higher than that of WaterPik (200.6 ± 2.1 and 185.2 ± 2.744 Hz, respectively). A significant difference was detected in the SL removal along the full length of the canal between the experimental and control groups (P < 0.001). EndoActivator and MDA groups had significantly more samples with no to minimum SL at the coronal and middle thirds compared to the apical third, whereas there was no significant difference among the thirds in the WaterPik group. At the apical third, the WaterPik group had significantly more samples with no to minimum SL (60%) than WaterPik and MDA groups (20% and 26.7%, respectively).
Conclusion: WaterPik was as effective as MDA and EndoActivator in SL removal, with better performance at the apical third, probably attributed to the lower vibration/oscillation frequency of WaterPik.

Keywords: EndoActivator, irrigation activation, oscillation frequency, smear layer, WaterPik


How to cite this article:
Aalmohamed E, Ahmed F, Alfardan L, El Abed R, Khamis AH, Jamal M. Effect of sonic irrigation activation at different frequencies in smear layer removal; An in vitro experimental study. Saudi Endod J 2022;12:106-13

How to cite this URL:
Aalmohamed E, Ahmed F, Alfardan L, El Abed R, Khamis AH, Jamal M. Effect of sonic irrigation activation at different frequencies in smear layer removal; An in vitro experimental study. Saudi Endod J [serial online] 2022 [cited 2022 Jan 25];12:106-13. Available from: https://www.saudiendodj.com/text.asp?2022/12/1/106/335240




  Introduction Top


The principal aim of root canal treatment is to eradicate microorganisms in the root canal system to prevent and/or treat apical periodontitis. To achieve this aim, canal instrumentation and irrigation are performed to substantially reduce the microbial load.[1] However, during mechanical instrumentation, a considerable amount of debris is produced, forming a layer covering the root canal walls. In 1970, using an electron microscope, Eick et al. were the first to report the presence of this debris layer on root canal walls.[2] The term smear layer (SL) was later described in 1975 by McComb and Smith.[3] This layer is 0.5–2 μm thick and mainly comprised a mixture of inorganic and organic materials (such as pulp remnants, odontoblastic processes, necrotic pulp tissue, and microorganisms and their by-products).[3],[4],[5]

SL removal is crucial for the success of endodontic therapy for a number of reasons, as highlighted by multiple in vivo and in vitro studies. Primarily, it aids in reducing the microbial load within the root canal system by eradicating bacteria and biofilms within the layer itself.[4],[6],[7],[8] It improves the efficacy of intracanal medicaments by enhancing its penetration into the dentinal tubules[9] and decreasing the time required for the medicaments to disinfect the canal wall surfaces.[10] Moreover, SL removal can significantly improve the apical seal by increasing the bond strength of the resin sealers to the root canal.[11],[12]

Various methods have been proposed for SL removal. These methods include using different types of irrigation solutions in different protocols and activation using ultrasonic/sonic devices and laser-based protocols.[7],[13],[14] Several irrigation agitation/activation techniques and devices are available, such as the use of Gutta-percha cones in a process called “manual dynamic agitation” (MDA) technique and the use of machine-assisted techniques, such as rotary brushes and sonic (such as EndoActivator and ultrasonic devices.[15],[16],[17] The WaterPik power flosser is a sonic device that has been tested as a root canal irrigation activation device owing to its ready availability and cost-effectiveness.[18],[19] Shenoy et al. found that irrigation activation with the WaterPik device resulted in a similar reduction in colony-forming units when compared to the EndoActivator device.[18] In another in vitro study, both sonic devices performed similarly in terms of debris removal.[19] Interestingly, in both studies, the WaterPik device performed better at the apical root third compared to the EndoActivator device. This better performance could be attributed to the possible difference in the vibration/oscillation frequencies between the two sonic devices. Studies have shown that the unintentional contact of the activation tips of sonic or ultrasonic devices to the canal walls may reduce the efficacy of irrigation activation and subsequently affect the effectiveness of SL removal.[20],[21],[22] This can be observed specifically at the apical third due to its small canal diameter.[23],[24] However, to date, data on the vibration frequency of WaterPik are not yet available. Therefore, this study aimed to compare the vibration/oscillation frequencies of WaterPik and EndoActivator devices and their effectiveness in SL removal at the different thirds of the root canal of extracted human premolars.


  Materials and Methods Top


This study was conducted according to the guidelines of the Declaration of Helsinki and approved by the Research Ethics Committee of Hamdan Bin Mohamed College of Dental Medicine, Mohammed Bin Rashid University of Medicine and Health Sciences (Protocol code EC1017-00).

Measuring sonic devices' frequency

Three EndoActivators (Dentsply Sirona) and three WaterPik (FLA-220 WaterPik Power Flosser) sonic devices equipped with new AA alkaline batteries (Energizer) to ensure equal electrical power supply were used. Size #25/.04 polymer activation tips (Dentsply Sirona) were fitted on each device and were operated at their maximum power (highest setting) in a dark room. A handheld digital tachometer (PinShang, China, DT2240B) was used to assess the vibration frequency of the activated tips in revolutions per minute (rpm). Briefly, the light emitted from the digital tachometer was flashed onto the activated tips. The final reading of the tachometer was recorded when the activated tip appeared static. The reading of each device was repeated thrice to ensure reproducibility. The rpm values were subsequently converted to Hz and tabulated for statistical analysis.

Specimen selection

Sixty intact single-rooted human maxillary and mandibular premolars extracted for orthodontic reasons were selected for this study. Briefly, teeth with fractured crowns, extensive restorations, resorbed roots, previous root canal treatment, and open apices were excluded from the study. Periapical radiographs from two views, buccolingual and mesiodistal, were obtained for all extracted teeth to assess the number of canals and their curvature. Image analysis software (ImageJ) was used to measure root canal curvature.[25] Teeth with a single canal and a curvature <20°were included in this study. The sample size of 60 teeth with 15 teeth per group was determined based on power analysis using Cochran's test, where the amount of SL in the previous study[26] was considered the relevant difference.

Specimen preparation

After inserting the teeth into silicon molds, a conventional access cavity was prepared for all the teeth. Working length (WL) was determined by inserting a K-type file #10 (Dentsply Maillefer, Ballaigues, Switzerland) into the canal until it was just visible at the apical foramen under a dental operating microscope (Leica, Germany) at ×25. The length was measured, and 1 mm was subtracted from the recorded length at that point. The root canals of all teeth were cleaned and shaped using ProTaper Universal System (Dentsply Maillefer, Ballaigues, Switzerland) until the F2 rotary files, following a protocol recommended by the manufacturer and several in vivo and in vitro studies.[27],[28],[29] During cleaning and shaping, the canals were irrigated with 2 mL of 2.5% sodium hypochlorite (NaOCl) using a syringe with a 30- G side-vented needle that was inserted as far as it can go (NaviTip; Ultradent, South Jordan, UT) after each instrument was used. At the end of the preparation procedures, the canals were flushed with 5-mL ethylenediaminetetraacetic acid (EDTA) followed by 5 mL of 2.5% NaOCl using a syringe and a 30G needle at 1 mm short of WL[30] and irrigation was activated during the final irrigation regime according to each experimental group, as described below.

Irrigation activation and experimental groups

The experimental teeth were randomly divided into three experimental groups and a control group (n = 15/group) according to the final irrigation protocol.

In the EndoActivator group, 5 mL of 2.5% NaOCl was introduced into the canal using a 30-gauge side-vented needle inserted 1 mm short of the WL. Subsequently, it was activated using an EndoActivator device operated at its highest setting, with a size #25/.04 polymer tip for 30 s. The activator tip was fitted loosely within 2 mm of the WL as per the manufacturer's instructions.[31] Next, it was followed by 5 mL of 17% EDTA, which was similarly activated for 1 min. A final flush with 2.5% NaOCl was performed without activation.

In the WaterPik group, activation of the irrigation solution was performed using a WaterPik device, in which the EndoActivator tip fitted and snapped in perfectly. The same irrigation protocol (activation tip and time) as in the EndoActivator group was implemented.

In the MDA group, MDA was performed. Briefly, irrigants were introduced using a 30-gauge side-vented needle, 1 mm short of the WL. It was then manually activated (agitated) using size 25/0.04 gutta-percha points with gentle manual strokes, applying the same irrigation sequence and activation time as the previous groups.

In the control group, the same irrigation sequence and time were implemented, except that no activation was performed.

Scanning electron microscopy analysis

Teeth were split longitudinally into two equal halves and prepared for scanning electron microscopy (SEM) using standard and widely published protocols.[32],[33] Briefly, after irrigation activation is completed, canal dried with paper points, and Gutta-percha cones were fitted in the canal to avoid introducing any debris during teeth splitting. Then two longitudinal groves were made (without exposing the canal) and teeth were split using orthodontic wire. One of the two halves was selected randomly for SEM analysis. The selected halves were dehydrated by immersion in graded ethanol series 30%, 50%, 75%, and 100%. After all, samples have been dehydrated, they were mounted on an aluminum stub and coated with gold sputtering (Quorum Technology Mini Sputter Coater, SC7620: Gold/Palladium (80% and 20%); 57 mm Ø × 0.1 mm thick. Then, each sample was examined with a scanning electron microscope (Tescan VEGA XM variable pressure SEM) and observed at low (×500) and high (×1000) at 20.0 kV. The SEM images were obtained randomly from each third: Coronal, middle, and apical. Evaluation and analysis of the images and the scoring of the SL were performed according to the modified criteria described by Hülsmann et al.[34] [Table 1] and [Figure 1].
Table 1: Modified Hülsmann criteria[34] for scoring the smear layer

Click here to view
Figure 1: Scanning electron microscopy images of the root canal dentin with different amounts of smear layer representing the four scores of the modified Hülsmann criteria for scoring the smear layer. (a) Score 1, (b) score 2, (c) score 3 and (d) score 4. The scale bar is 25 µm

Click here to view


The SEM images were analyzed and scored by two independent blinded examiners (endodontists) who were previously trained on the scoring procedures and system. After a month, a second reading of the images was performed by the same two examiners who were blinded to the previous readings. To check the reproducibility and reliability of the data, the data obtained were subjected to Cohen's kappa test, and Altman's scale was used for interpretation.[35] In case of disagreements between the observers, a third evaluator (expert endodontist) was consulted to resolve the disagreement and finalize the score.

Statistical analyses

Data were subjected to Cohen's kappa, Chi-squared, Student's t-test, and Mann–Whitney U tests for statistical analyses. Categorical variables were cross-tabulated to examine the independence between variables, and comparisons between means were performed using the Mann–Whitney U test. A value of P < 0.05 was considered statistically significant.


  Results Top


Frequency analysis

The frequency analysis showed that the mean vibration frequency of the Endo-Activator device was 200.6 ± 2.1 Hz, whereas it was 185.2 ± 2.744 Hz for the WaterPik device [Figure 2]. Statistical analysis showed a significant difference between the two groups (P < 0.001).
Figure 2: Scatterplot showing the mean vibration frequency of EndoActivator and WaterPik. ***Statistically significant difference between the two groups (P < 0.001)

Click here to view


Smear layer removal

As planned, the two blinded observers independently reviewed all SEM images. The kappa test indicated good intra-examiner reliability (κ =0.61) and moderate inter-examiner reliability (κ =0.44). Disagreements between the observers were resolved by consulting a third evaluator (expert endodontist). Expert opinion was required in 46 out of 180 images.

The analysis of SL removal along the full length of the canal and comparisons of the four groups showed that most of the specimens (except in the control group) scored 1 or 2 (no to minimum SL) [Figure 3]. In the EndoActivator group, 60% of the samples scored 1 or 2. Similar results were observed in WaterPik and MDA, in which 77.8% and 73.3% of the samples scored 1 or 2, respectively. However, only 26.7% of the samples scored 1 and 2 in the control group, and a significant difference was detected in the SL removal along the full length of the canal between the experimental and control groups (P <. 001). Furthermore, the WaterPik group showed the best performance among all experimental groups in achieving a score of 1 or 2 along the full length of the canal; however, the difference between the experimental groups was not statistically significant (P = 0.087)
Figure 3: Bar chart showing the overall performance of different groups in smear layer removal along the full length of the canal. The percentage indicates the number of samples with either no to minimum smear layer (score 1 + 2) or moderate-to-heavy smear layer (score 3 + 4)

Click here to view


Within the different experimental groups, SL removal was more effective in the coronal and middle thirds than in the apical third [Figure 4] and [Figure 5]. Specifically, there were significantly more samples with scores of 1 or 2 in the coronal and middle thirds in the EndoActivator and MDA groups than in the apical third (P < 0.001 for both groups) [Figure 4]. In contrast, there was no significant difference in the number of samples with scores of 1 or 2 among the different root canal thirds in the WaterPik and control groups.
Figure 4: Bar chart showing the performance of different groups in smear layer removal at different canal thirds (coronal, middle, and apical thirds). The percentage indicates the number of samples with either no to minimum smear layer (score 1 + 2) or moderate-to-heavy smear layer (score 3 + 4)

Click here to view
Figure 5: Scanning electron microscopy images of the most common score achieved at different thirds by the different groups. The scale bar is 25 µm

Click here to view


Within the different thirds of the canal, at the apical third, the WaterPik samples scored either 1 or 2 in 60% of the samples, whereas the percentages of SL removal in MDA and EndoActivator were 26.7% and 20%, respectively [Figure 4] and [Figure 5]. The WaterPik group performed significantly better in SL removal than the other experimental groups at the apical third (P = 0.04). In the middle and coronal thirds, all experimental groups performed similarly, with no significant differences among the experimental groups (P = 0.089 for the middle third and P = 0.360 for the coronal third).


  Discussion Top


Sonic devices are commonly used to activate irrigation solutions. However, studies have shown that with high vibration frequencies, the unintentional contact of the activation tips to the canal walls may reduce the efficacy of irrigation activation specifically at the apical third.[20],[21],[22],[23],[24] Very few studies have reported the vibration frequency of the EndoActivator sonic device and data on the vibration frequency of WaterPik are not yet available. Therefore, in this study, we investigated and compared the vibration/oscillation frequency of two sonic devices: EndoActivator and WaterPik and their effectiveness in SL removal at different root levels. This was done in comparison to other activation techniques; MDA that is not sonic-based and to a control (no activation at all).

The inter- and intra-rater reliability was classified as moderate, indicating difficulties in the performance of the SEM assessments which might be due to the insufficient background of the two evaluators and the long time gap between the first and second reading. This problem was solved with the help of the third experienced endodontist.

The results of the present investigation showed that all experimental (irrigation activation) groups performed significantly better than the control group (conventional needle irrigation without activation) in all root thirds. The control group had a moderate to heavy SL (scores 3 and 4) in all thirds in 73% of the samples, compared to an average of 30% of all activation groups. These results clearly indicate the importance of irrigation activation in SL removal, regardless of the activation method used. This finding is in agreement with the findings of other studies, who concluded that irrigation activation was significantly more efficient in SL removal than conventional irrigation.[20],[36] Similar to the current study, Caron et al. compared the effect of irrigation activation using EndoActivator and MDA in comparison with conventional irrigation. Their results showed that activation of the irrigating solution significantly improved the performance of SL removal when compared to no activation.[18]

The results of this study showed that irrigation activation was significantly effective in SL removal in the coronal and middle thirds compared to the apical third. This finding is in agreement with the findings of Saber et al.'s study, who reported that in straight, single-rooted premolar, the apical third had the highest amount of SL, regardless of the activation system used.[37] A similar finding was reported by Abarajithan et al., who investigated the amount of SL remaining after irrigation activation in maxillary central incisors. They reported that the apical third contained significantly more SLs in the apical third than in the coronal and middle thirds.[38] Rödig et al.[21] reported similar findings in curved canals. These findings indicate that solution delivery and penetration to the apical third is always a challenge regardless of the needle insertion depth, mode activation, or use of delivery devices, such as EndoVac.[37] The vapor lock phenomenon could be one possible explanation for the difficulty in irrigation penetration to the root apical third. This phenomenon has been described by Senia et al.[39] as an air bubble that could be formed because of the interaction of NaOCl with organic material and prevent irrigation solution penetration to the apical third. Even activation with ultrasonic/sonic devices may be a challenge to break such a bubble. However, further studies are needed to prove this theory.

MDA was introduced as an alternative irrigation activation technique owing to its availability and cost-effectiveness. Several studies have shown a similar performance of EndoActivator and MDA in removing the SL at different root levels. In a study by Uroz-Torres et al.,[40] EndoActivator had no to minimum SL in 100%, 50%, and 10% of the samples in the coronal, middle, and apical thirds, respectively. Similar findings were reported by Khaord et al.[26] who stated that there was no statistically significant difference between EndoActivator and MDA, although MDA performed better in SL removal. Caron et al.[20] compared the efficacy of SL removal using EndoActivator and MDA on curved mandibular molars. Their results indicated no significant difference between the two modes of activation in the middle and apical thirds. However, EndoActivator performed better. Our study showed a similar finding to those of the above-mentioned studies. In the coronal third, both the EndoActivator and MDA groups had no to minimum SL in 100% of the samples. In the middle third, EndoActivator and MDA had no to minimum SL in 60% and 93% of the samples, respectively. At the apical third, only 20% and 26% of the samples had no to minimum SL in EndoActivator and MDA, respectively. There was no statistically significant difference between the two groups at any of the thirds. The results of our study, along with the abovementioned studies, clearly indicate that MDA can be considered a potentially cost-effective alternative to EndoActivator; however, it did not perform significantly well at the apical third.

WaterPik sonic devices have been used recently as irrigation activation devices because of their ready availability and cost-effectiveness. However, few studies have investigated their effectiveness in SL removal, and no clear data are available on their vibration/oscillation frequencies.[18],[19] Our results showed that activation with the WaterPik sonic device resulted in no to minimum SL in 93%, 80%, and 60% in the coronal, middle, and apical thirds, respectively. There was no statistically significant difference among WaterPik, EndoActivator, and MDA in the coronal and middle thirds. However, the WaterPik group performed significantly better at the apical third compared to the EndoActivator and MDA groups. Similar results have been reported by Chaudhari et al., in which WaterPik and EndoActivator had almost the same performance in SL removal among the different thirds, with relatively better performance (not statistically significant) of WaterPik at the apical third.[19] In an in vitro study, and after contaminating the root canal with Enterococcus faecalis, Shenoy et al. reported that activation of 3% NaOCl by WaterPik resulted in a relatively lower number of colony-forming units at the apical third when compared to EndoActivator.[18] These results indicate that activation with WaterPik sonic device can result in similar SL removal to EndoActivator and MDA, with potentially better performance at the apical third. However, it is not completely clear why WaterPik performed better in the apical third. One possible explanation could be attributed to the frequency power and oscillation amplitude of WaterPik. It has been shown that frequent contact between the activated tip and canal walls at the apical third could decrease the efficiency of irrigation activation and produce new SLs.[20],[21],[22] This dampening phenomenon could explain why the lower frequency sonic devices performed better in SL removal at the apical third compared to activation with ultrasonic devices.[23],[24] Our results showed that the mean vibration/oscillation frequency of EndoActivator in its highest setting (200 Hz) was significantly higher than that of WaterPiK (185 Hz). This indicates that the lower vibration frequency of WaterPik could result in a less dampening effect on the activation tip at the apical third and subsequently a better performance in SL removal. However, this hypothesis is yet to be confirmed, and future studies are required to test the effect of different vibration/oscillation frequency powers on irrigation penetration and SL removal at the apical third. Interestingly, Jiang et al. reported a similar vibration frequency of EndoActivator as in the current study. However, their results showed no differences between the lowest (160 Hz) and highest (190 Hz) vibration/oscillation frequency settings of EndoActivator in debris removal.[41] The difference between their results and ours could be attributed to the larger apical preparation in their samples, which may reduce the dampening effect by providing a larger space for the activation tip to oscillate without touching the canal walls.

In this study, the EndoActivator activation tip of 25/0.04 in size was used in both EndoActivator and WaterPik groups. This size was chosen as all samples have been prepared till ProTaper F2 (final size of 25/0.08) ensuring enough space for the activation tip to fit loosely within 2 mm of the canal without binding. Moreover, the final preparation and activation tip size was chosen with an aim of producing results that are comparable to other similar studies.[18],[19],[20] Furthermore, no significant difference has been detected in the oscillatory pattern, wall contact, and effectiveness of irrigation activation between activation tips 25/.04 and 15/.02.[41] However, decreasing the activation tip size with lower vibration/oscillation frequency could decrease the dampening effect on the activation tip even further. Yet, this hypothesis needs to be investigated by further studies.

One of the limitations of this study is that it was performed on almost straight canals, and it is unknown how effective “low vibration frequency” in decreasing the dampening effect in curved canals. Therefore, future studies are required to test the effect of the degree of canal curvature and vibration frequency on the irrigation activation efficiency. Another limitation is the low level of agreement between the two evaluators, despite performing the required training and calibration. Although, this was resolved by consulting a third expert endodontist, this might also indicate the need to develop new tools to score the efficiency of SL removal more objectively.


  Conclusion Top


Our study showed that the WaterPik sonic device was as effective as MDA and EndoActivator in SL removal, with better performance at the apical third. This could be attributed to the lower vibration/oscillation frequency of WaterPik, although this hypothesis has yet to be confirmed in further studies.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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