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 Table of Contents  
Year : 2019  |  Volume : 13  |  Issue : 2  |  Page : 39-43

Fluid flow analysis of laser-activated irrigation in the simulated root 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.
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jdl.jdl_18_18

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Aim: The aim of the study was to evaluate the efficacy of laser-activated irrigation, the streaming pattern, and its relevance to laser-induced bubbles in a transparent simulated root canal. In addition, the effects of tip position on irrigant flow were examined. Materials and Methods: Particle image velocimetry with a high-speed camera was used, which enabled tracking of buoyant tracer movement. Recorded images were analyzed using two-dimensional fluid analysis software. The tip of an erbium: yttrium aluminum garnet (Er:YAG) laser was placed either in the coronal chamber or in the root canal. Results: On laser irradiation within the coronal chamber, rapid streaming appeared in the entire root canal immediately after the advent of the vapor bubble. Peak flow velocity was observed when the vapor bubble grew to peak size. In contrast, when the tip was placed in the root canal, rapid streaming was related to the appearance of secondary cavitation bubbles and confined to the apical region. Conclusion: The streaming pattern in laser-activated irrigation was affected by the tip position in the root canal. In addition, there was a close relation between the formation of laser-induced bubbles and rapid streaming.

Keywords: Er: YAG laser, high-speed camera, irrigation, laser-induced bubble, root canal
Key message: The streaming pattern in laser-activated irrigation is affected by the tip position in the root canal. There is a close relation between the formation of laser-induced bubbles and rapid streaming.

How to cite this article:
Kihara T, Matsumoto H, Yoshimine Y. Fluid flow analysis of laser-activated irrigation in the simulated root canal. J Dent Lasers 2019;13:39-43

How to cite this URL:
Kihara T, Matsumoto H, Yoshimine Y. Fluid flow analysis of laser-activated irrigation in the simulated root canal. J Dent Lasers [serial online] 2019 [cited 2024 Feb 26];13:39-43. Available from:

  Introduction Top

The success rate of root canal treatment can be maximized by complete removal of necrotic tissues, bacteria, and their by-products.[1],[2] However, because of the complex structures in the root canal system, such as the isthmus, fin, and collateral root canals, bacteria can often survive even after thorough root canal debridement using mechanical instruments.[3]

Consequently, endodontists have focused on different irrigation techniques for the root canal in the recent past in a bid to seek the most effective chemical irrigation technique.[4],[5],[6] Although conventional syringe techniques have been widely used for irrigating the root canal, it is difficult to efficiently deliver the irrigant to the apical area, particularly in a narrow, curved canal.[7],[8],[9]

To improve the cleaning effects in the apical third of a root canal, sonic and ultrasonic activation have been used, and more recently, laser-activated irrigation (LAI) has been endorsed as an innovative technique.[10],[11],[12] erbium: yttrium aluminum garnet (Er:YAG) and erbium, chromium: yttrium scandium gallium garnet (Er,Cr:YSGG) lasers, which have high absorption properties in water, have been reported to be suitable for activating irrigation solution in the root canal.[13],[14] Moreover, the irrigation efficiency and mechanism of action of LAI have been shown through various ex vivo studies. Arslan et al.[15] revealed that the photon-initiated photoacoustic streaming technique, which uses a low-energy laser and a hovering tip at the coronal orifice during irrigation, was more effective than the conventional, sonic, and ultrasonic irrigation techniques in removing apically placed dentinal debris. In addition, we[16] tried to elucidate the mechanisms of LAI using a high-speed camera and concluded that Er:YAG laser irrigation may be an outcome of rapid streaming of the fluid caused by the formation and collapse of laser-induced bubbles. The physical mechanisms underlying this cleaning technique, however, are still not well understood.

On the contrary, fluid flow analysis may be helpful for the evaluation of the irrigation efficiency inside the root canal. Chen et al.[17] analyzed irrigant dynamics during syringe irrigation, negative pressure irrigation, and passive ultrasonic irrigation using a computational fluid dynamics model. Koch et al.[18] investigated the velocity of fluid flow around a polymer rotary finishing file using particle image velocimetry (PIV). However, the streaming pattern during LAI is so complex and rapid that only limited information has been revealed to date. For example, de Groot et al.[19] measured the velocity profile and estimated shear stress during LAI using high-speed imaging. However, the fluid flow caused by laser activation in the root canal remains unclear.

In this study, we visualized the streaming patterns during LAI, using the PIV technique with tracer particles in a simulated root canal. Furthermore, we examined the effects of tip position on the irrigation flow and velocity.

  Materials and Methods Top

Transparent root canal model

To simulate and visualize the phenomena occurring in the root canal during LAI, we used a transparent acrylic straight root canal model, which consisted of the artificial canal and coronal chamber-like and apically closed structures. The dimensions of the model were as follows: diameter at the apex, 0.4 mm; taper, 5%; canal length, 18 mm; and chamber height, 8 mm. The chamber space served as a reservoir for irrigation solution.

Laser equipment and parameters

The Er:YAG laser (Erwin AdvErl; Morita, Osaka, Japan) was equipped with a cone-shaped tip (R200T; Morita), which was capable of emitting the laser beam approximately 80% laterally.[20],[21] The outer diameter was 300 µm, the transmission rate was 37%, the top angle was 84°, and the length was 18 mm. Laser light was applied with a pulse energy of 30 mJ at a repetition rate of 20 pulses per second (pps) and a pulse duration of approximately 75 µs. Coaxial water and air sprays were turned off during irradiation.

Photographing and fluid analysis

Photographic recording was performed using a high-speed camera (Phantom v711; Ametek, New Jersey) using a halogen light (KL2500LCD; Carl Zeiss, Oberkochen, Germany) at a rate of 5000 frames per second. To capture the water flow, fluorescent polystyrene microparticles (mean diameter = 31 µm; Fluoro-Max; Thermo Scientific, Waltham, Massachusetts) were used as tracers. The canal model was carefully filled with distilled water containing 5% microparticles to avoid formation of air bubbles that might disturb the irrigant flow. The laser tip was placed either in the coronal chamber (20 mm from the apex) or in the root canal (10 mm from the apex), and was kept stationary [Figure 1].
Figure 1: Schematic illustrations of simulated root canals and tip positions. Tip was held stationary in the coronal chamber (A) or in the root canal 10 mm from the apex (B)

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Recorded images were analyzed using two-dimensional fluid analysis software (Flownizer; DITECT, Tokyo, Japan). The PIV technique was used in this study. Tracer motion was assessed as the velocity vector and equivalent surface. The velocity vector illustrates the direction and speed, whereas the equivalent surface reveals gathered points of a vector having the same speed.

  Results Top

[Figure 2] and [Figure 3] show the images recorded when the laser tip was positioned in the coronal chamber and in the root canal, respectively. Upper panels show a series of laser-induced bubbles after single-pulse irradiation from left to right, and lower panels reveal the corresponding equivalent surfaces. On the equivalent surfaces, rapid water flow is depicted in red, whereas slow flow in blue.
Figure 2: Photos of the time series. Laser-induced bubbles (upper) and equivalent surfaces (lower) during single-pulse irradiation when the tip was located in the coronal chamber. Red bar shows a laser tip. Yellow arrow indicates the largest vapor bubble. The velocity scale is indicated by the color bar on the left

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Figure 3: Photos of the time series. Laser-induced bubbles (upper) and equivalent surfaces (lower) during single-pulse irradiation when the tip was located in the root canal. Red bar shows a laser tip. Yellow and white arrows indicate the largest vapor bubble and secondary cavitation bubbles, respectively. Velocity scale is indicated by the color bar on the left

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When the tip was located in the coronal chamber [Figure 2], the peak flow velocity reached approximately 1.0 m/s at the moment when the laser-induced vapor bubble reached the maximum size (yellow arrow in the upper panel). The red equivalent surface emerged in the entire root canal immediately after the formation of vapor bubbles (lower panel). In contrast, when the tip was placed in the root canal [Figure 3], the maximum flow speed (ca 1.6 m/s) was seen 1.4ms after single-pulse irradiation simultaneously with the appearance of secondary cavitation bubbles (white arrow in the upper panel). The red equivalent surface was confined to the apical region in the root canal (lower panel).

  Discussion Top

In previous studies, the efficiency of root canal irrigation with the laser has been frequently evaluated. For example, Sahar-Helft et al.[22] compared the efficiency of smear-layer removal by LAI with that of ultrasonic or syringe irrigation. The authors showed that smear-layer removal was most effective when the root canal was irrigated using the Er:YAG laser, and this effect was not observed with ultrasonic or syringe irrigation. Alternatively, Deleu et al.[23] showed the high efficacy of LAI in removing dentin debris from simulated root canal irregularities. Furthermore, De Moor et al.[24] reported that LAI for 20s is as efficient as passive ultrasonic irrigation for 60s in removing artificially placed dentin debris in the groove at 2 mm from the apex.

Therefore, in this study, we used the PIV technique using a high-speed camera to evaluate the irrigation efficacy of LAI, which enabled tracking of buoyant tracer movement in the transparent root canal model and also allowed us to visualize complex and rapid fluid flow. Generally, it is difficult to analyze the three-dimensional movement of the tracer accurately in two dimensions. However, the depth of field in this study is only 0.5 mm, which allowed even a two-dimensional analysis to capture the movement velocity of the particles to a certain extent.

The shape of laser-induced vapor bubbles was influenced by differences in tip position. Within the chamber space, single-pulse irradiation enabled the free growth of vapor bubbles. Subsequently, water in the root canal moved vigorously up and down with expansion and implosion of the vapor bubble. Because such phenomena were repeated 20 times a second under the present conditions, the root canal may have been cleaned entirely.

When the tip was located in the root canal, the vapor bubble grew upward along the root canal wall following single-pulse irradiation. Consequently, it took longer for the vapor bubble to implode as compared to irradiation within the coronal chamber. Rapid fluid flow was predominantly observed in the apical region, which might be dependent on the formation and disappearance of secondary cavitation bubbles.

Although a cone-shaped tip of 200 µm in diameter was used in this study, thicker tips (300–600 µm) have been used in previous reports on LAI.[25],[26] When using thicker tips, care should be taken to avoid the apical extrusion of irrigant,[27],[28],[29] particularly when irritating solutions such as sodium hypochlorite are used, because much faster fluid flow may develop due to higher energy.

In ultrasonic irrigation, high-speed oscillation of an ultrasonic tip tends to damage the root canal wall as the tip is generally placed 1 or 2 mm from the apex.[17],[24],[30] Furthermore, it is difficult to position the ultrasonic tip deeply in a curved root canal. On the contrary, LAI can irrigate the root canal with a tip placed in the coronal chamber even in a narrow, curved canal. Another advantage of LAI is that there is no risk of perforation owing to its low-power irradiation.

In conclusion, when the water is irradiated by the Er:YAG laser even at a low-power level with a cone-shaped tip placed in the coronal chamber, rapid streaming of irrigant occurs in the entire root canal, as compared with that in the intra-canal tip position. Therefore, LAI may allow cleaning of the root canal without inserting the laser tip deeply into the canal, which would be very useful for irrigating root canals.

Financial support and sponsorship

This work was supported by Grant-in Aid for Scientific Research GAG5K11118 in Japan.

Conflicts of interest

There are no conflicts of interest.

  References Top

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