Nakane et al.

Asian Pac J Dent 2019; 19: 33-37

Change of dentin surface by low-power irradiation of erbiumdoped yttrium aluminum garnet laser
Akira Nakane, DDS, Masayuki Otsuki, DDS, PhD, and Junji Tagami, DDS, PhD
Cariology and Operative Dentistry, Department of Restorative Sciences, Graduate School of Medicaland Dental Sciences, Tokyo Medical and Dental University, Tokyo, Japan
Purpose: The objective of this study was to evaluate the effect of low output power irradiation of erbium-dopedyttrium aluminum garnet (Er:YAG) laser on surface morphology, roughness, and microhardness of the root
dentin.
Materials and Methods: Flat root dentin surfaces were prepared from extracted human molars. The surfaces
were irradiated by an Er:YAG laser at 10 pps, 30 mJ, sweeping at approximately 2 mm/s by hand. Irradiatedpower was changed by means of the distance between laser tip and surface kept at 0, 5, 10, 15, 20, 25, 30, 35,
and 40 mm. No irradiated surface was employed as control. Those surfaces and crosscut surfaces were observed
by a scanning electron microscope (SEM). Specimens of irradiation distance at 0, 5, 10, 20, 30, and 40 mm andcontrol (no irradiation) were further prepared and then, surface roughness (Ra) and microhardness (Vickershardness) were measured.
Results: Ablation of dentin was observed at distance between 0-15 mm. At 20-30 mm, degenerated layer wasobserved without ablation. At 35 and 40 mm, ablation and degenerated layer were not found. Ra values of all
experimental groups (0-40 mm) showed statistically higher than that of control group (p < 0.05). Among theexperimental groups, 0 mm group showed statistically higher Ra values than the other groups (p < 0.05). For
Vickers hardness, no significant difference was found all groups except 0 mm group (p > 0.05).
Conclusion: It was concluded that specific low output power Er:YAG laser irradiation produced thindegenerated layer on the root dentin without ablation. This irradiated surfaces showed slight increase ofroughness without decrease of microhardness.
(Asian Pac J Dent 2019; 19: 33-37.)
Key Words: Er:YAG laser; low level laser therapy; root dentin

Introduction
The laser was first developed in 1960 [1] and this technology went through a continuous development and anincreasing utilization in many fields including medicine. The laser technology was soon introduced for dentistryin 1966 [2]. Recently, various lasers have been used for both soft tissues and hard tissues in clinical dentistry.
Among them, erbium-doped yttrium aluminum garnet (Er:YAG) laser can remove caries in tooth structures
together with sound enamel and dentin under minimal thermal effects in the adjacent hard and soft tissues [3].
The Er:YAG laser appears to be one of the best suited laser types for cavity preparation [4]. Since Er:YAG laser
with wavelength of 2.94 µm was introduced in dentistry [2,5], many basic studies and clinical researches on
Er:YAG laser have been reported. The Er:YAG laser is absorbed by water and hydroxyapatite and the reactionof Er:YAG laser and water causes “micro explosion” which ablates enamel and dentin [4]. Generally, the cavity
preparation using Er:YAG laser takes more time, compared to rotary cutting instruments [6,7]. However, that
preparation method advantages include low noise and vibration, eliminating the need for local anesthesia in mostcases [8]. A clinical study suggested that the application of the Er:YAG laser system was a more comfortable
alternative or adjunctive method than conventional mechanical cavity preparation [7]. For removing dental hardtissues by Er:YAG laser, high energy (more than approximately 150 mJ) is required. However, effect of low
output power irradiation of Er:YAG laser on dentin was not well-known.
In the early time, various dental lasers were used with high output power for incision or removing soft andhard tissues. And later, low level laser therapy (LLLT) was introduced as soft laser therapy or bio-stimulation.
The LLLT technique is used for the promotion of wound healing in dentistry including periodontitis [9],33Nakane et al.

Asian Pac J Dent 2019; 19: 33-37

temporomandibular disorder [10], denture stomatitis [11], peri-implant mucositis [12], and orthodontic pain [13],expecting soft tissue modulation and neural modulation. The LLLT has been also applied for dentin
hypersensitivity [14]. In many case of LLLT for dentin hypersensitivity, diode lasers with several wavelengths
and output power of 10-50 mW were used [14]. Also, Er:YAG laser was used for desensitizing of hypersensitive
dentin [15]. However, there are few studies on the morphological change of dentin by low output power
irradiation of Er:YAG laser. The objective of this study was to evaluate effect of low output power irradiation ofEr:YAG laser on surface morphology, roughness, and microhardness of the root dentin.

Materials and Methods
Observation using scanning electron microscope (SEM)
Fifty seven extracted human molars were used for this study. This study was approved by Ethics Committee,
Tokyo Medical and Dental University, Faculty of Dentistry (D2013-022). The 5 × 5 mm specimen was obtained
from buccal root surface using a diamond bur and an air turbine handpiece under copious water. Obtained
specimen was embedded in a plastic tube with an epoxy resin (EpoxiCure2, Buehler, Lake Bluff, IL, USA).
After curing the epoxy resin, flat and smooth root dentin surface was obtained by slightly grinding and polishingusing silicon carbide papers (#180-2,000, Sankyo Rikagaku, Okegawa, Japan) and diamond paste (6, 3, 1, and0.25 µm, DP-Paste P, Struers, Ballerup, Denmark).
An Er:YAG laser (λ = 2,940 nm, Erwin AdvErl Evo, J. Morita, Kyoto, Japan) and a quartz contact tip (φ =600 µm, C600F, J. Morita) were used for this study. The laser was irradiated vertically on the whole of dentin
surface uniformly sweeping at approximately 2 mm/s by hand at 10 pps, 30 mJ without water splay for 90 s.
During the irradiation, the distance between the dentin surface and the contact tip was kept 0, 5, 10, 15, 20, 25,30, 35, and 40 mm. At 0 mm, the tip was slightly contact with the surface of the sample. For those distances,power intensity and power density were shown in Table 1. The samples without irradiation were employed ascontrol group. After irradiation, the specimen was cut through the center, and dried and gold-coated routinely.
The irradiated surface and crosscut surface of the specimen were observed by a scanning electron microscope(JSM-5310LV, JEOL, Akishima, Japan).
Table 1 Output power and power density of each experimental group and assignment of expedientDistance (mm)Output power (mJ)Power density (mJ/cm2)

SEM

Surface roughness

Vickers hardness

017.3

0.61

○

○

○

5

16.5

0.58

○

○

○

10

14.8

0.52

○

○

○

15

14.1

0.50

○

××

20

13.7

0.49

○

○

○

25

12.6

0.44

○

××

30

11.9

0.42

○

○

○

35

11.5

0.41

○

××4010.30.37

○

○

○

○

○

○

Control○: Experiment was assigned. ×: Experiment was not assigned.

Surface roughness and microhardness tests
Specimens of irradiation distance at 0, 5, 10, 20, 30, and 40 mm and no irradiation (control group) were further
prepared and were subjected to the surface roughness test and microhardness test (Table 1). Surface roughness34

Nakane et al.

Asian Pac J Dent 2019; 19: 33-37(Ra) was measured by a profilometer (SJ-210, Mitutoyo, Kawasaki, Japan) with cutoff value λc = 0.8 and λs =2.5 at 0.5 mm/s. Then, Vickers hardness was measured by a hardness tester (HM-102, Mitutoyo) with 0.49 N ofload and 15 s of loading time. The number of samples was five in each group (n = 5). Obtained data wasstatistically analyzed by one-way ANOVA and Tukey HSD test using a package software (IBM SPSS 23.0, IBM,Armonk, NY, USA). The confidential level was 0.05 (α = 0.05).

Results
SEM observation
Typical SEM images were shown Fig. 1. Although the acquired SEM images were varied among the samples
with same distances, following tendencies were recognized. For 0, 5, 10, and 15 mm distances, ablation of dentin
and opening of dentinal tubules were apparent. For 20, 25, and 30 mm distances, ablation was not found and thedegeneration was found on the surface of irradiated dentin by crosscut images. Some dentinal tubules wereoccluded. In 35 mm and 40 mm groups, any change in surface was not observed.

Fig. 1 Typical SEM images after irradiation by each condition of irradiation
S, Surface; C, Crosscut surface; Cont, control group (without irradiation)

Surface roughness and microhardness
The average values and standard deviations of surface roughness (Ra) of each group was shown in Table 2. Ravalues of all experimental groups (0-40 mm) showed statistically higher than that of control group (p < 0.05).
Among the experimental groups, 0-mm-distance group showed statistically higher Ra value than the other35

Nakane et al.

Asian Pac J Dent 2019; 19: 33-37

groups (p < 0.05).
The average values and standard deviations of microhardness (Vickers hardness) of each group are shown inTable 2. For 0 mm group, it was impossible to measure the microhaedness because of rough surface. No
significant difference was found for other groups including control group (p > 0.05).
Table 2 Surface roughness and microhardness
Distance (mm)0510203040Control

Surface roughness

4.79a

0.15b

0.11 b0.10 b

0.09 b

0.09 b0.04c(Ra)

-0.07

-0.11

-0.01

0-0.02

-0.02

-0.03

not
available

53.5a

58.2a

57.5a

59.3a

60.6a

60.4a

Microhardness(Vickers hardness)
-1.2
-1.6
-1.6
-0.6
-2.6
-0.19
Mean values and standard deviations; Identical superscripts indicate no significant differences in a row (p > 0.05).

Discussion
Since Er:YAG laser was introduced in dentistry [2,5], many researches were reported. When the Er:YAG laser isirradiated and the molecules of the water in the tooth become superheated, micro explosion is occurred dividing
the surrounding tissue into very small pieces and blowing them apart [4]. The dentin surface irradiated byEr:YAG laser was irregular, scaly or flaky and dentinal tubules were opened without smear layer [16,17]. The
vaporization of peritubular dentin was greater than that of the intertubular dentin with a cuff-like appearance [4].
For removing dental hard tissues by Er:YAG laser, high energy (more than approximately 150 mJ) is required.
In this study, Er:YAG laser was irradiated on the dentin surfaces at much lower output power (10.3-17.3 mJ) as
shown in Table 1. At the distance of 0 mm (17.3 mJ) and 15 mm (14.1 mJ), dentin was ablated and irregular andscaly surface with opening dentinal tubules were observed as well as high-power lased dentin. However, at thedistance between 20 mm (13.7 mJ) and 30 mm (11.9 mJ), dentin ablation was not found and degenerated layer
was still existed.
The desensitizing of hypersensitive dentin with an Er:YAG laser irradiation (80 mJ/pulse, 3 Hz) was effective[15]. Dentin hypersensitivity can be defined as short, sharp pain in response to stimuli by exposed dentin with
open dentinal tubules [18]. Dentin hypersensitivity is one of the most common symptomatic condition whichcauses complaints of discomfort in patients. The hydrodynamic theory was proposed about the mechanism ofdentin hypersensitivity [19]. According to this theory, sensitive dentin is caused by thermal, physical or chemical
stimuli which induce fluid flow in the dentinal tubules and consequent nociceptor activation in the pulp/dentin
border area. This leads to the displacement of the pulp-dentin fluid, which trigger pulpal sensory nervous
terminations resulting in the characteristic short and sharp pain of dentin hypersensitivity. The application of an
Er:YAG laser would be anticipated to decrease these fluid movements by evaporating the superficial layers ofthe dentinal fluid [15]. In this study, degenerated layer without ablation could be produced by irradiation at thedistance between 20 mm (13.7 mJ) and 30 mm (11.9 mJ) with slight increase of surface roughness. It can bespeculated that Er:YAG laser irradiation with this condition may be effective for reducing of dentin
hypersensitivity with less morphological change of irradiated surface.
By irradiation at the distance between 20 mm (13.7 mJ) and 30 mm (11.9 mJ), occluded dentinal tables wereobserved. It may be also effective for dentin hypersensitivity. However, some dentinal tubules were still open.
Combination of desensitizing agent and Er:YAG laser irradiation for dentin hypersensitivity seems to beinteresting. The occluding effects of Er:YAG laser and dentifrice containing nano-carbonate apatite on the36

Nakane et al.

Asian Pac J Dent 2019; 19: 33-37

dentinal tubules was reported [20]. For this case, lower output power employed in this study may be also
effective and useful. Further study including clinical trial is required to determine the suitable parameter of
Er:YAG laser irradiation for the treatment of dentin hypersensitivity.
Within the limitation of this study, it could be concluded that specific low output power Er:YAG laser
irradiation produced thin degenerated layer on the root dentin. This irradiated surfaces showed slight increase ofroughness without decrease of microhardness.
Conflict of Interest
The authors do not have any financial interest in the companies whose materials are included in this article.

References1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.

Maiman TH. Stimulated optical adiation in ruby. Nature 1960; 187: 493-4.
Kinersly T, Jarabak JP, Phatak NM, DeMent J. Laser-induced microperforations in teeth sections. J Dent Res 1966; 45:
199-203.
Keller U, Hibst R. Experimental studies of the application of the Er:YAG laser on dental hard substances: II. Light
microscopic and SEM investigations. Lasers Surg Med 1989; 9: 345-51.
De Moor RJG, Delmé KIM. Laser-assisted cavity preparation and adhesion to erbium-lased tooth structure: part 1.
Laser-assisted cavity preparation. J Adhes Dent 2009; 11: 427-38.
Hibst R, Keller U. Experimental studies of the application of the Er:YAG laser on dental hard substances: I. Measurement ofthe ablation rate. Lasers Surg Med 1989; 9: 338-44.
Aoki A, Ishikawa I, Yamada T, Otsuki M, Watanabe H, Tagami J, et al. Comparison between Er:YAG laser and conventional
technique for root caries treatment in vitro. J Dent Res 1998; 77: 1404-14.
Keller U, Hibst R, Geurtsen W, Schilke R, Heidemann D, Klaiber B, et al. Erbium:YAG laser application in caries therapy.
Evaluation of patient perception and acceptance. J Dent 1998; 26: 649-56.
Liu J, Lai Y, Shu W, Lee S-Y. Acceptance and efficiency of Er:YAG laser for cavity preparation in children. Photomed Laser
Surg 2006; 24: 489-93.
Mokeem S. Efficacy of adjunctive low-level laser therapy in the treatment of aggressive periodontitis: A systematic review. JInvestig Clin Dent 2018; 9: e12361.
Brochado FT, Jesus LH de, Carrard VC, Freddo AL, Chaves KD, Martins MD. Comparative effectiveness ofphotobiomodulation and manual therapy alone or combined in TMD patients: a randomized clinical trial. Braz Oral Res2018; 32: e50.
Davoudi A, Ebadian B, Nosouhian S. Role of laser or photodynamic therapy in treatment of denture stomatitis: A systematicreview. J Prosthet Dent 2018; 120: 498-505.
Albaker AM, ArRejaie AS, Alrabiah M, Abduljabbar T. Effect of photodynamic and laser therapy in the treatment ofperi-implant mucositis: A systematic review. Photodiagnosis Photodyn Ther 2018; 21: 147-52.
Deana NF, Zaror C, Sandoval P, Alves N. Effectiveness of low-level laser therapy in reducing orthodontic pain: A systematic
review and meta-analysis. Pain Res Manag 2017; 2017: 1-18.
Junior AB, Garrini AE, Pinheiro A, Helena D. Laser therapy in the treatment of hypersensitivity. A histologic study andclinical application. Laser Therapy 2000; 12: 16-21.
Schwarz F, Arweiler N, Georg T, Reich E. Desensitizing effects of an Er:YAG laser on hypersensitive dentine: A controlled,prospective clinical study. J Clin Periodontol 2002; 29: 211-15.
Nishimoto Y, Otsuki M, Yamauti M, Eguchi T, Sato Y, Foxton RM, et al. Effect of pulse duration of Er:YAG laser on dentinablation. Dent Mater J 2008; 27: 433-9.
He Z, Otsuki M, Sadr A, Tagami J. Acid resistance of dentin after erbium:yttrium-aluminum-garnet laser irradiation. Lasers
Med Sci 2009; 24: 507-13.
Machado AC, Viana ÍEL, Farias-Neto AM, Braga MM, Eduardo CP, Freitas PM, et al. Is photobiomodulation (PBM) effectivefor the treatment of dentin hypersensitivity? A systematic review. Lasers Med Sci 2018; 33: 745-53.
Brännstrom M, Lindén LA, Johnson G. Movement of dentinal and pulpal fluid caused by clinical procedures. J Dent Res
1968; 47: 679-82.
Han SY, Jung HI, Kwon HK, Kim BI. Combined effects of Er:YAG laser and nano-carbonate apatite dentifrice on dentinal
tubule occlusion: In vitro study. Photomed Laser Surg 2013; 31: 342-8.

Correspondence to:Dr. Masayuki Otsuki
Cariology and Operative Dentistry, Department of Restorative Sciences,
Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University
1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, Japan
Fax: +81-3-5803-0195 E-mail: otsuki.ope@tmd.ac.jp
Accepted January 4, 2019.
Copyright ©2019 by the Asian Pacific Journal of Dentistry.
Online ISSN 2185-3487, Print ISSN 2185-3479

37