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Evaluation of cleaning efficacy-related properties of root canalirrigant activation using a computer-controlled hot tip poweredwith a diode laser
Tomoyuki Hongo, DDS, Satoshi Watanabe, DDS, PhD, Kanako Yao, DDS, PhD,
Kazuhisa Satake, DDS, and Takashi Okiji, DDS, PhD
Department of Pulp Biology and Endodontics, Graduate School of Medical and Dental Sciences, Tokyo
Medical and Dental University, Tokyo, Japan
Purpose: This study aimed to examine root canal irrigant activation using a computer-controlled hot tip(thermo-optically powered [TOP] tip) heated with a 980 nm diode laser system (Alta Modular Laser System),with regard to vaporized bubble formation and soft tissue dissolution capacity.
Materials and Methods: As a parameter of cleaning efficacy, the number and size of vaporized bubbles
generated in root canal models during TOP tip-induced activation were analyzed with high-speed digital video
imaging. To evaluate the soft tissue-dissolving efficacy of TOP tip-induced activation with a sodium
hypochlorite irrigant, dissolution of porcine soft tissues filled in lateral canals set at two levels in root canalmodels were morphometrically analyzed. Irrigant temperature during TOP tip-induced activation was measured
with thermocouples attached to the apical foramen and the lateral canals of root canal models.
Results: High-speed video imaging revealed that TOP tip-induced activation of the water that filled the root
canal models, required less time to achieve the first bubble formation and generated a larger number of bubbles
than a normal clear tip-induced activation using the same laser. NaOCl solution activated with TOP tip dissolved
more soft tissues filled in lateral canals of the canal models, than with passive ultrasonic irrigation and syringe
irrigation. Irrigant temperature during TOP tip-induced activation showed a maximum increase of approximately
50˚C near the lateral canal, as measured with thermocouples.
Conclusion: Under the present experimental conditions, TOP tip-induced irrigant activation exerted better
cleaning efficacy-related properties than the other methods tested.
(Asian Pac J Dent 2019; 19: 9-15.)
Key Words: diode laser, hot tip, irrigant activation

Introduction
Intracanal pathogens, such as necrotic soft tissue remnants and microorganisms, are the primary cause of apicalperiodontitis. To eradicate the pathogens, root canal treatment is conducted via a combination of mechanicalinstrumentation and root canal irrigation. However, complete elimination is challenging due to the presence ofcomplex root canal structures that do not allow effective debridement, such as lateral canals, fins, isthmuses andapical ramifications [1,2]. Thus, improvement of root canal cleaning procedures, aimed at more effective
removal of intracanal pathogens, is of significant importance to elevate the predictability of root canal treatment.
Mechanical instrumentation allows for the access of only 40-80% of the root canal area [1,3]. Therefore, root
canal irrigation serves as a critical procedure to facilitate the elimination of intracanal pathogens that are left inthe uninstrumented area [1,3]. Sodium hypochlorite (NaOCl) is a root canal irrigant of choice due to its effective
antimicrobial and organic tissue-dissolving capacities [4]. Hand irrigation using a syringe and a needle (syringe
irrigation; SI) is employed routinely, although it has limited effectiveness in the apical portion as well as thecomplexities of the root canal [4]. To enhance irrigation efficacy, sonic and ultrasonic activation have been
employed [5].
Laser-activated irrigation (LAI) has gained attention for its potential to efficiently clean the root canal system.
LAI exerts improved efficacy through generation of cavitation bubbles, high-velocity water flow, and shockwaves [6], and is similarly or significantly more effective in cleaning the root canal system compared with hand
irrigation and ultrasonic irrigation [7-9]. Moreover, LAI with NaOCl has been reported as significantly more

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effective than conventional irrigation techniques for soft tissue dissolution [10,11]. Several kinds of lasers havebeen reported to achieve the laser-induced cavitation and streaming effect, including erbium chromium, yttrium
scandium gallium garnet (Er,Cr:YSGG) [6,7], erbium-doped yttrium aluminum garnet (Er:YAG) [7-9],
neodymium-doped yttrium aluminum garnet (Nd:YAG) lasers [12], and diode lasers at 940 and 980 nm
wavelengths [13,14].
A "hot tip" or "laser-activated thermal probe" that transforms the laser energy into thermal energy is widely
employed in various laser-assisted surgeries, and is formed by adhesion of certain flammable materials to the
distal end of the laser tip, such as tissue fragments, paper, wood, and carbon materials as an optical-thermal
converter [15,16]. A diode laser with a 970 nm wavelength and carbon-coated hot tip activation in water was
reported to exert hydrodynamic effects; it generates vaporized bubbles and high-speed streaming up to 100 mm/s
resulting from explosive boiling [17,18].
Recently, a novel 980 nm diode laser, the Alta Modular Laser System (Alta MLS; Dental Photonics, Walpole,
MA, USA), featuring a system to create a computer-controlled hot tip (thermo-optically powered tip: TOP tip),
was developed [15]. Studies have reported its efficacy in soft tissue operations and a reduction of thermal
collateral damage to the surrounding tissues compared with the performance of a normal clear tip [15,16]. In thissystem, the distal end of the cylindrical quartz glass fiber tip is converted into a highly light-absorbent
particle-impregnated layer with a computer-controlled initiation process to achieve a consistent hot tip. The fiber
tip is first pressed into a carbon-based target and activated at a preset power level, resulting in the deposition ofparticles onto the distal end of the fiber tip. The TOP tip emits incandescent thermal radiation heating to
500-1,200˚C at 1,400-11,000 nm broadband wavelength, which differs from a laser [15].
Application of the Alta MLS with a TOP tip-induced irrigant activation may have potential clinical benefits,
since the system may induce rapid heating of irrigants, resulting in efficient vaporized bubble formation andhigh-temperature water streaming. However, the root canal cleaning efficacy of this system when used for
irrigant activation has not yet been investigated. Thus, the aim of this study was to examine TOP tip-induced
irrigant activation with regard to selected properties related to cleaning efficacy, i.e., vaporized bubble formationand soft tissue dissolution capacity.

Materials and Methods
Experiment 1: Evaluation of TOP tip-induced vaporized bubbles with high-speed digital video imaging
First, generation and kinetics of vaporized bubbles during root canal irrigation were examined as parameters ofcleaning efficacy; the time taken to the first bubble formation and number of bubbles were measured.
Twenty-one plastic root canal models with lateral branches (Thermafil Training Bloc #1, apical diameter 0.30mm, 0.04 taper; Dentsply Sirona, Ballaigues, Switzerland) were used. These models simulated lateral canals
located 6 and 12 mm from the apical foramen. Distilled water, rather than NaOCl solution, was used as theirrigant to protect the experimental devices. It has been reported that the light transmission spectrum of NaOCl
solution follows the spectrum of water to a large extent [19].
The samples used were divided into 3 groups (n = 7, each). In Group Clear, the Alta MLS (Fig. 1) was used at
2 W (45 mJ, 44 pps) with a normal "clear tip" formed as a result of a mechanical cleave (DS1-200, Dental
Photonics). In Group TOP, the Alta MLS with a TOP tip converted from the DS1-200 tip, which emits thermalradiation, was used at 2 W (45 mJ, 44 pps). A group without laser irradiation served as the control group. After

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filling the root canal with distilled water, the laser tip was positioned 9 mm from the apical foramen andirradiation was performed seven times for 10 s each.
Vaporized bubbles in the distilled water were recorded with a high-speed camera (VW-9000, Keyence, Osaka,
Japan). The frame rate and the exposure time were set at 1,000 fps and 1/3,000 s, respectively. The camera wasplaced horizontally at a distance of 28 mm from the sample at 75× magnification. The time taken fromcommencement of irradiation to the first bubble formation and number of bubbles on the captured images wereanalyzed with video analysis software (VW-9000 Motion Analyzer 1.4.0.0, Keyence) at every 0.1 s for 5 s. The
first moment of irradiation was designated as 0 s.

Fig. 1 Scanning electron microscope images of the clear tip (a cylindrical fiber tip; left) and the TOP tip (right)
Fig. 2 Representative images showing porcine soft tissues (black) remaining during TOP tip-induced irrigant
activation (a), PUI (b), SI (c), and control (d)Experiment 2: Soft tissue-dissolution during irrigant activation
This experiment was conducted to examine whether the TOP tip-induced cavitation and streaming effect iscapable of enhancing the soft tissue dissolution capacity of NaOCl solution compared with passive ultrasonicirrigation (PUI) and syringe irrigation (SI). Twenty-eight plastic root canal models, identical to those used inExperiment 1, were used. The samples used were divided into 4 groups (n = 7, each). Porcine soft tissue stainedwith a 1.0% methylene blue solution to aid visualization was packed into each lateral canal. An NaOCl solution(6%, Dental Antiformin, Nippon Shika Yakuhin, Shimonoseki, Japan) was used as the irrigant.
In Group TOP, the Alta MLS set to 2 W (120 mJ, 16 pps) with the TOP tip converted from the DS1-200 tip
was used. After filling the root canal with the NaOCl solution, the tip was positioned 9 mm from the apicalforamen and activated three times for 20 s each (i.e., 60 s). In passive ultrasonic irrigation (Group PUI), theENAC SE10 ultrasonic system (Osada, Tokyo, Japan, 30 kHz) with an ultrasonic tip (SC4, Osada) was used.
After filling the root canal with the NaOCl solution, the tip was positioned 9 mm from the apical foramen andactivated at the highest power within the recommended setting range by the manufacturer for the tip (setting: 3,
5.6 W) three times for 20 s each (i.e., 60 s). Conventional syringe irrigation (Group SI) was conducted with asyringe (Nipro Syringe; Nipro, Osaka, Japan) and a 27-gauge (ø 0.40 mm) irrigation needle (Nipro Blunt Needle,
Nipro). After filling the root canal with the NaOCl solution, the needle tip was positioned at 9 mm from theapical foramen and 1 mL of the irrigant was delivered manually for 20 s, three times (i.e., 60 s). In the control
group, the NaOCl solution was left stationary for 60 s.
Following the irrigation with NaOCl solution, the root canals were irrigated with 1 mL of distilled water anddried with paper points. All procedures were recorded with a dental operating microscope (OPMI Pico with
MORA interface, Carl Zeiss, Oberkochen, Germany) equipped with a digital video recorder (DATA Gen PRO
for Dental; Seventh Dimension Design, Kobe, Japan) at a 21.3× magnification (Fig. 2). Captured images were11

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analyzed for dissolved soft tissue area before and after the experiment via image analysis software (Photoshop
CS5; Adobe, San Jose, CA, USA), according to the method described in a previous study [10].
Experiment 3: Measurement of the irrigant temperature during TOP tip-induced irrigant activation
To examine whether TOP tip-induced irrigant activation exerts thermal effects related to the cleaning efficacy,
irrigant temperature rise was measured during the activation. Plastic root canal models identical to those used inExperiment 1 were used. Three K-type thermo-couples (ø 0.15 mm, Okazaki, Kobe, Japan) were inserted into
the canal through the apical foramen and the simulated lateral canals, and set at 2, 6, and 12 mm from the apicalforamen (Fig. 3). Distilled water was used as an irrigant and experiments were done in a thermostat air box
maintained at 37˚C. The TOP tip was positioned at 10 mm from the apical foramen and activated as inExperiment 2, and the irradiation was repeated 7 times for 20 s. Temperature changes were recorded
continuously in real-time with a recording device (NR-1000, Keyence) and data logging software (Wave Thermo
1000, Keyence) with a 0.1-s sampling interval.
Statistical analyses
Statistical analyses for all experimental results were performed using one-way analysis of variance (ANOVA)
and the Tukey-Kramer post-hoc test. Significance was set to 5% for all analyses.

Fig. 3 Schematic diagram of the setup for Experiment 3. Three K-type thermo-couples were inserted into the canalthrough the apex and the lateral canals of the root canal model, and positioned at 2, 6, and 12 mm from the apicalforamen.
Fig. 4 Representative images showing bubbles generated in the root canal model during TOP tip-induced irrigant
activation. Numbers show the time (µs) from the onset of the laser-pulse. The arrow shows the first bubble after
irradiation.

Results
Representative high-speed camera images of the TOP group are shown in Fig. 4. As shown in Fig. 5, the mean
number of bubbles detected in the TOP groups was significantly greater than that in the Clear and control groups(p < 0.05). In addition, observable bubbles in the TOP group appeared significantly faster than those in the Clear
group (p < 0.05). It took 191 ± 44 and 3,549 ± 2,060 ms to detect observable bubbles in the TOP and Clear
groups, respectively. No bubbles were observed within 5 s in the control group (Fig. 5).
The results of soft tissue-dissolution during irrigation are shown in Fig. 6. In the lateral canal at the 12 mm
level, the TOP and PUI groups showed significantly larger areas of dissolution than the other groups (p < 0.05;
Fig. 6, upper panel). In the lateral canal at the 6 mm level, however, the TOP group exhibited a significantly

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larger area than the other groups (p < 0.05; Fig. 6, lower panel).
The mean maximum temperature rise of the irrigant by location during TOP tip-induced irrigant activation
corresponded to 12 mm > 6 mm > 2 mm (p < 0.05; Fig. 7). An approximately 50˚C increase in temperature was
detected at the 12 mm level (2 mm from the laser tip).

Fig. 5 Time-course changes (upper panel) and means (lower panel) in the number of bubbles formed during
irrigant activation. n = 7 in each group. Different letters indicate statistically significant differences (p < 0.05).
Fig. 6 (Upper panel) Average of dissolved tissue area at the lateral canal positioned 12 mm from the apicalforamen during each irrigation. (Lower panel) The average of dissolved tissue area at the lateral canal positioned 6
mm from the apical foramen during each irrigation. Different letters indicate statistically significant differences (p <0.05).

Fig. 7
Means of the maximum temperature rise of
the irrigant at different levels from the apex
(0 mm) during TOP tip-induced irrigant
activation. n = 7.
Different letters indicate statisticallysignificant differences (p < 0.05).

Discussion
The mechanism underlying the TOP tip involves the fast conversion of laser energy into thermal energy at thesurface of the distal end of the fiber tip, which contains a thin, light absorbing layer as a hot tip. The distal end of
the TOP tip heated to 500-1,200˚C emits thermal radiation [15]. As a consequence, it can rapidly boil the wateraround the distal end of the tip, resulting in evaporation and bubble formation. The water absorption property ofeach wavelength is also associated with the generation of bubbles in the root canal and subsequent water
streaming [6,8,9,13,14,19,20]. The values of absorption into water of 980 nm wavelengths are relatively small(0.243) [19]. This fact explains the results of Experiment 1, in which only a few bubbles were observed in the980 nm diode laser (Clear) group. In the present study, it took about 3 s in the 980 nm diode laser (Clear) groupto acquire observable bubbles. This is consistent with earlier studies showing that a 980 nm diode laser(Sirolaser, Sirona, Bensheim, Germany) can generate cavitation bubbles within 5 s after irradiation at 2.5 W, 25
pps [13] or about 13 s after irradiation at 2 W, 20 pps [14]. In contrast, the TOP group required significantly
shorter time (only 0.2 s) until initial bubble formation and generated significantly more bubbles compared with13

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the Clear group. These findings clearly indicate that the TOP tip is more efficient in heating water to generate
vaporized bubbles and is different from diode laser-induced activation. Since the generation of bubbles gives riseto the circulation of liquid [17,18], TOP tip-induced irrigant activation may be beneficial in improving root canalcleaning efficacy.
In Experiment 2, we demonstrated that the amount of soft tissue dissolution by the TOP group was similar at
12 mm and higher at 6 mm compared with the results obtained with PUI (Fig. 6), indicating that the TOP group
was more effective than PUI over a longer distance. It has been reported that LAI with the tip at 5 mm from theapical foramen removes dentin debris plugs as efficiently as PUI with the tip at 1 mm [9]. Our results of TOP
tip-induced irrigant activation were similar, supporting the notion that laser-induced cavitation and streaming
cleans the root canal over a longer distance from the tip than PUI. In other words, shallower insertion depths
during TOP tip-induced irrigant activation may be sufficient.
Among several factors that influence soft tissue dissolution rates, including irrigant concentration,
temperature, agitation, and surfactant factors [21], temperature rise may have some influence on the difference
between TOP and PUI. Regarding the effects of temperature rise when NaOCl irrigant is used, it has beenreported that the dissolved tissue rate for 1.0% NaOCl solution heated at 60˚C is significantly higher than thatfor 5.25% NaOCl solution at 20˚C, and that the rate reached a plateau when the temperature was 60˚C [22].
Moreover, bactericidal rates for NaOCl solution may more than double for each 5 degrees rise in temperature in
the range of 5 to 60˚C [23]. As demonstrated in Experiment 2, the TOP tip-induced irrigant activation may havean irrigant heating ability sufficient to enhance its cleaning efficacy; in the present study, the mean maximum
irrigant temperature rise during TOP tip-induced irrigant activation was largest at the 12 mm level (2 mm fromthe laser tip), where approximately 50˚C of temperature rise was detected (Fig. 7).
While root canal treatment with a laser may offer therapeutic benefits, careful consideration of potential
complications, including irrigant extrusion outside the apical foramen [9] and thermal damage to theperiradicular tissues [24,25], is required. In particular, apical extrusion of NaOCl solution can cause severe
damage to the periradicular tissues [26-28]. As for thermal side-effects, excessive temperature increases on theroot surface can cause irreversible bone and periodontal ligament damage as well as dehydration effects to dentin,
resulting in external resorption [24,29]. The safety threshold of the temperature increase is considered to be 7˚C[25]. In this regard, we have preliminarily found that the TOP tip-induced irrigant activation caused root surface
temperature increase of less than 7˚C (data not shown), suggesting low risk of thermal damage. However, further
investigation on the safety of TOP tip-induced irrigant activation seems necessary.
Under the experimental conditions described in this study, results can be summarized as; (i) TOP tip-induced
irrigant activation required less time to produce the first bubble formation and generated a larger number ofvaporized bubbles than the normal clear tip-induced irrigant activation using the same 980 nm diode laser; (ii)
TOP tip-induced irrigant activation exerted better soft tissue dissolving capacity over a longer distance from thetip than PUI; and (iii) approximately 50˚C of maximum irrigant temperature rise during TOP tip-induced irrigant
activation was detected near the lateral canal. Thus, within the limitations of this study, it can be concluded that
TOP tip-induced irrigant activation exerted similar or better cleaning efficacy-related properties compared withthose of the other irrigation methods tested.
Acknowledgments
This study was supported, in part, by Grants-in-Aid for Young Scientists B from the Japan Society for the Promotion of

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Science (No. 25870195 to W.S. and No. 15K20398 to S.K.).
Conflicts of Interest
The authors declare no conflicts of interest.

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Correspondence to:
Dr. Satoshi Watanabe
Department of Pulp Biology and Endodontics, Graduate School of Medical and Dental Sciences,
Tokyo Medical and Dental University
1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, Japan
Fax: +81-3-5803-5494 E-mail: s.watanabe.endo@tmd.ac.jp
Accepted December 22, 2018.
Copyright ©2019 by the Asian Pacific Journal of Dentistry.
Online ISSN 2185-3487, Print ISSN 2185-3479

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