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 Table of Contents  
ORIGINAL ARTICLES
Year : 2019  |  Volume : 13  |  Issue : 2  |  Page : 34-38

Evaluation of the efficacy of Er:YAG laser–activated irrigation in a simulated accessory canal


1 Department of Endodontology and Operative Dentistry, Division of Oral Rehabilitation, Faculty of Dental Science, Kyushu University, Fukuoka, Japan
2 Department of Endodontology, Kyushu University Hospital, Fukuoka, Japan

Date of Web Publication14-Nov-2019

Correspondence Address:
Dr. Yoshito Yoshimine
Department of Endodontology and Operative Dentistry, Division of Oral Rehabilitation, Faculty of Dental Science, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582.
Japan
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jdl.jdl_4_19

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  Abstract 

Aim: The aim of this study was to evaluate the effect of laser-activated irrigation (LAI) on the removal of debris-mimicking hydrogel from a simulated accessory canal. Materials and Methods: The simulated accessory canal was located 3 mm from the apex, perpendicular to the straight main canal. Gelatin hydrogel as a substitute of debris was used to fill the simulated accessory canal. The root canals were irrigated for 20 or 40 seconds by LAI using an erbium: yttrium aluminum garnet (Er:YAG) laser (30 mJ, 20 pps) or for 40 seconds by syringe irrigation (SI). During LAI, the cone-shaped tip of the laser was positioned stationary at 3 or 10 mm from the apex. Irrigation was performed using 5% NaOCl. The distance over which the hydrogel was removed from the accessory canal entrance was measured and compared between the irrigation procedures. Results: Using NaOCl as the irrigant, a significant increase was observed in the distance over which the hydrogel was removed by LAI compared with that by SI. A longer irradiation period with LAI resulted in significantly greater amount of hydrogel removal. There was no significant difference in hydrogel removal when the laser tip was positioned at 3 and 10 mm from the apex. Conclusion: Within the limitations of this in vitro model, LAI removed more hydrogel from the accessory canal than SI, when using NaOCl as the irrigant. Furthermore, the irradiation time influenced the cleaning efficacy, but the tip position did not.

Keywords: Accessory canal, Er:YAG laser, irrigation, root canal
Key Message: Laser-activated irrigation using an Er:YAG laser may aid in the cleaning of complex anatomical structures such as an accessory canal of the root canal system.


How to cite this article:
Kihara T, Matsumoto H, Yoshimine Y. Evaluation of the efficacy of Er:YAG laser–activated irrigation in a simulated accessory canal. J Dent Lasers 2019;13:34-8

How to cite this URL:
Kihara T, Matsumoto H, Yoshimine Y. Evaluation of the efficacy of Er:YAG laser–activated irrigation in a simulated accessory canal. J Dent Lasers [serial online] 2019 [cited 2019 Dec 6];13:34-8. Available from: http://www.jdentlasers.org/text.asp?2019/13/2/34/271014




  Introduction Top


Mechanical debridement and chemical irrigation play a pivotal role in improving debris removal and root canal disinfection and are essential for successful endodontic treatments. However, because of the anatomical complexity of the root canal system, such as fins, isthmuses, and accessory canals, necrotic pulp tissue and bacteria tend to remain in these structures after instrumentation and conventional irrigation, particularly in the apical area.[1],[2] Irrigation using a syringe and needle still remains the most common method. In addition to the chemical-dissolving action of the irrigant itself, efficient irrigation mainly depends on active movement of the solution in the root canal.[3],[4]

Several irrigant activation techniques, including sonic and ultrasonic, and negative apical pressure, have been introduced to clean the root canal more effectively than syringe irrigation (SI).[5] Recently, laser-activated irrigation (LAI) using an erbium: yttrium aluminum garnet (Er:YAG) laser (2,940nm) or erbium, chromium: yttrium scandium gallium garnet laser (2,870nm) has been proposed as an efficient activation method of irrigant activation.[6],[7],[8],[9],[10],[11],[12],[13],[14],[15],[16],[17] In addition, to elucidate the cleaning mechanisms of LAI, several visualization studies have been reported using transparent root canal models and high-speed cameras.[18],[19],[20] Er:YAG laser radiation has extremely high absorption of water, and the effect of LAI is thought to result from the rapid fluid flow induced by the explosion and implosion of laser-induced bubbles when the pulsed energy irradiates the irrigant as well as from the extremely powerful shock waves.

Accessory canals are minute canals that extend in a horizontal, vertical, or lateral direction from the pulp to the periodontium, with the majority of canals found in the apical third of the root.[21] Ricucci and Siqueira[22] have histopathologically shown that when pulp necrosis reached the level of the accessory canals, the tissue within was partially or completely necrotic. Accessory canals serve as avenues for the passage of necrotic irritants, which are probably one of the major causes of persistent apical periodontitis.[23]

Scanning electron microscope studies using extracted human teeth have elucidated the cleaning effects of LAI on the main root canal,[11],[24] and other studies have reported the removal of dentin debris from the artificial root canal wall groove using LAI.[8],[9],[25] These results have revealed that LAI can clean the main root canal better than other irrigation procedures using a syringe or ultrasonic techniques. To the best of our knowledge, however, there have been no studies on the efficacy of LAI in the accessory canal. Therefore, the aim of this study was to measure the removal rate of debris-mimicking hydrogel from simulated accessory canals using LAI, with SI acting as a control for comparison, using NaOCl as the irrigant. The effect of the laser tip position on the efficacy of cleaning the accessory canals was also investigated.


  Materials and Methods Top


Fabrication of the root canal model with an accessory canal

Transparent acrylic root canal models with an accessory canal were created [Figure 1]. The main canal with a length of 18 mm was simulated, using a #40 finger spreader (MANI, Tochigi, Japan). This had a length of 25 mm, a tip diameter of 0.40 mm, and a 0.05 taper. The accessory canal was made using a 0.2-mm stainless-steel wire and was located horizontally 3 mm from the apex.
Figure 1: A schematic illustration of the simulated accessory canal model (A) and a red gelatin-filled accessory canal (B)

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Experimental hydrogel preparation

The debris-mimicking hydrogel contained 3g of gelatin (type A; MP Biomedicals, Santa Ana, CA), 0.25g of red food dye (Kyoritsu Foods, Tokyo, Japan), and 0.06g of hyaluronic acid sodium salt (Wako, Tokyo, Japan) in 45mL of water at 50°C.[26] The hydrogel was injected using a needle into the accessory canal without air, where it completely solidified after 60 minutes at room temperature of 21–23°C under moist conditions. Then a #20 reamer was used to break the surplus hydrogel in the main canal into pieces, which was carefully removed using paper points. Finally, the exit of the accessory canal was sealed using a silicon rubber. An accessory canal filled with the red-dyed hydrogel is shown in [Figure 1B]. Each of the following irrigation procedures was repeated eight times.

Laser-activated irrigation

LAI used an Er:YAG laser (Erwin AdvErl; Morita, Kyoto, Japan) equipped with a cone-shaped tip with a core diameter of 200 µm. The main root canal was filled with 5% NaOCl (Wako, Tokyo, Japan) without air, and the laser tip was positioned stationary at a depth of 10 mm from the apex. Laser irradiation involved a pulse energy of 30 mJ, a pulse length of 100 µs, and a repetition rate of 20 pulse/s for 20 or 40 seconds, without air or water cooling.

To investigate the effect of the laser tip position, LAI with the tip positioned 3 mm from the apex was also performed under identical irradiation conditions. This position corresponded to the accessory canal entrance.

Syringe irrigation

For SI, the canal was irrigated with 2mL of 5% NaOCl using a 2.5-mL syringe (JMS, Hiroshima, Japan) and a blunt open-ended 25-G needle (Rootclin needle; Nippon Shika Yakuhin, Yamaguchi, Japan). The needle was inserted 5 mm from the apex, and the canal was irrigated for 20 or 40 seconds at a flow rate of 1mL per 10 seconds with an up and down motion.

Quantitative measurements of the hydrogel removal

Immediately after each irrigation procedure, the samples were photographed with a camera (AxioCam ERc 5S; Carl Zeiss, Germany) attached to the stereomicroscope (Stemi 2000-CS; Carl Zeiss, Germany), using eyepiece lens magnification of ×0.5 and objective lens magnification of ×1.6. The photos before and after irrigation were analyzed using ImageJ software (NIH, Bethesda, MD) to measure the distance over which the hydrogel had been removed, that is, the distance from the accessory canal entrance to the closest hydrogel–irrigant interface in the simulated accessory canal.

Statistical analysis

The differences between LAI and SI were nonparametrically compared using the Steel–Dwass test. The comparison of the tip positions in LAI was analyzed using Mann–Whitney U tests. P values of <0.05 were considered statistically significant.


  Results Top


There were statistically significant differences (P < 0.05) between LAI and SI regardless of the duration of irrigation (20 or 40 seconds) [Figure 2]. LAI removed significantly more hydrogel in 40 seconds than in 20 seconds (P < 0.05). In contrast, the hydrogel was hardly eliminated by SI.
Figure 2: Graphic representation of the distance in millimeters over which the hydrogel had been removed after irrigation (A). Representative photographs (B). n = 8, *P < 0.05, Steel–Dwass test. SI = syringe irrigation, LAI = laser-activated irrigation

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Performing LAI at two different tip positions from the apex produced no significant difference in hydrogel removal from the accessory canal, irrespective of duration of irrigation (P > 0.05) [Table 1].
Table 1: Distance in millimeters over which the hydrogel had been removed after LAI at two different tip positions (3 and 10 mm from the apex)

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  Discussion Top


It is well known that it is difficult to completely clean the accessory canal after chemomechanical preparation.[27] Any remaining necrotic tissue or bacterial biofilm in the accessory canal may cause root canal reinfection and treatment failure. As indicated by the results of previous studies on the cleanliness of accessory canals,[26],[28] it is difficult to reproduce the accessory canal of natural teeth using the simulated experimental model. For example, the acrylic resin used in this study is entirely different from human dentin. Furthermore, the simulated accessory canals used in this study were straight and 0.2 mm in diameter because of technical difficulties, although, using a microcomputed tomography technique, Xu et al.[23] reported that the average diameter of the accessory canals of human teeth is 67 µm and that the canals are tortuous rather than straight. Therefore, it may not be possible to draw conclusions reflecting exact clinical situations from the results of this study.

In evaluating the cleaning efficacy of the accessory canal in vitro, what is filled in the accessory canal is an important problem. In previous studies, Al-Jadaa et al.[28] used necrotic bovine pulp tissue. On the other hand, Macedo et al.[26] used gelatin with sodium hyaluronate as a biofilm-mimicking hydrogel. In this study, according to the latter, gelatin hydrogel was used because of its homogeneity and ease of use.

NaOCl possesses broad-spectrum antimicrobial activity against endodontic microorganisms and also dissolves organic material such as the necrotic pulp tissue.[29],[30] In this study, SI hardly removed the hydrogel in the accessory canal on using 5% NaOCl as the irrigant. This indicates that the fluid flow resulting from SI is restricted to the main canal and does not extend to the accessory canal. In contrast, LAI in conjunction with NaOCl sufficiently cleaned the accessory canal. In our preliminary study on LAI, the hydrogel removal rate using distilled water as the irrigant was approximately half of that using NaOCl (data not shown). These findings indicate that the physical movement of the fluid by LAI is not enough to clean the accessory canal and may require the synergistic effect of the dissolving ability of an irrigant, such as NaOCl, and the rapid fluid flow resulting from LAI. This is probably because secondary cavitation bubbles in the space created by the dissolution of hydrogel at the entrance of the accessory canal cause fluid flow so that the hydrogel removal advances to deeper parts of the accessory canal.

It is generally recommended that the laser tip should be positioned in the pulp chamber during LAI for cleaning the root canal.[11],[24] However, the effect of the tip position on the cleaning ability of LAI remains unclear. Recently, using a resin canal model with a 0.5-mm-wide, 1-mm-deep, and 4-mm-long groove, in which dentin debris were packed, Meire et al.[25] showed that the position of the tip in LAI significantly affected debris removal from the groove. They reported that placement of the tip close to the groove resulted in better debris removal because of the intense fluid motion near the point of the tip. In contrast, in this study, when the removal rate of hydrogel from the accessory canal was compared at two different tip positions (3 and 10 mm from the apex), no significant difference was found. This suggests that vapor bubble expansion and fluid flow near the point of the tip did not greatly influence the hydrogel removal rate, probably because only a small area (0.2 mm in diameter) was in contact with the irrigant.


  Conclusion Top


In conclusion, within the limitations of this in vitro study, we found that LAI possessed higher hydrogel-removal capacity from the accessory canals than SI when NaOCl was used as the irrigant. A longer irradiation period influenced cleaning efficacy, but the laser tip position did not.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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