Yu and Yan

Int Chin J Dent 2004; 4: 56-60.

Eliminating effect of Er, Cr: YSGG laser irradiation on thesmear layer of dentin
Jingtao Yu, MD, DDS, and Lu Yan, MD, DDS
College of Stomatology, China Medical University, Shenyang, P. R. China
Purpose: To observe morphological changes and elimination of smear layer on the dentin exposed to Er,
Cr: YSGG laser using different energy and irradiating distance.
Materials and Methods: Twenty-two first premolars were selected and slices with 1.5 mm thickness wereprepared by horizontal sectioning through the middle third of the teeth crown. The occlusal surface having
been coated by smear layer were to be treated. Each dentinal slice was irradiated under the laser withdifferent irradiating distance (<2 and 3 mm) and different energy (1, 2, 3, 4, and 6 W). After routine
preparation for microscopic observation, the specimens were observed with a scanning electron
microscope.
Results: Laser with energy of 2-6 W and irradiating distance below 2 mm could effectively remove thesmear layer from the dentin. Dentinal tubule orifices were diminished or partly occluded. With the energyof irradiation increased, the elimination of the intertubular dentin increased; on the other hand, laser withenergy beyond 4 W and irradiating distance of 3 mm can also effectively eliminate the smear layer.
Conclusion: Energy of 2-3 W (irradiating distance below 2 mm) and energy of 4-6 W (irradiating distanceabout 3 mm) are recommended settings with which Er, Cr: YSGG laser can effectively remove the smear
layer from the dentin surface. (Int Chin J Dent 2004; 4: 56-60.)
Clinical Significance: As a new, valid, and safe method, Er, Cr: YSGG laser selected proper setting can beapplied in the clinical practice to remove the smear layer from dentin surface.
Key Words: dentin, Er, Cr: YSGG laser, scanning electron microscopy, smear layer.

Introduction
During the process of restoration for carious tooth, the mechanical preparation of dental cavities willproduces a thin smear layer that covers the dentin and enamel walls. The existence of smear layer, which isone of the most important factors to weaken the bond strength between filling materials and the cavity
walls, not only affect long-term result of restoration, but also easily result in the producing of secondary
caries.

Now most dentists adopt the chemical methods to remove this layer, but the operation iscomplicated and easily do harm to the hard tissue, particularly not well in the hypersensitive tooth. It has
been reported recently that erbium, chromium: yttrium-scandium-gallium-garnet laser (Er, Cr: YSGG laser)can validate to eliminate the smear layer. Furthermore, this laser seems have no apparent adverse thermaleffect on pulp or hart tissue, and almost doesn’t produce any pain and discomfort.1,2 Otherwise, at present
the reports about which settings of power output should be used to effectively remove the smear layer while
do not damage the histological structure of the dentin surface have not been seen.
The purpose of this investigation was to compare the morphological changes and elimination of thesmear layer on dentin surface after Er, Cr: YSGG laser’s irradiation with different power output andirradiating distances, then acquire the most suitable setting to be applied in clinical practice.

56Yu and Yan

Int Chin J Dent 2004; 4: 56-60.

Materials and Methods
Twenty-two freshly extracted, noncaries and intact first premolars were used.

Dentin slices wereprepared by horizontal sectioning through the middle third of the teeth crown using a low speed diamonddisk (SH-E, NSK, Tokyo, Japan) and the thickness of the slices was about 1.5 mm. The occlusal surface
having been coated by smear layer were experimental surfaces to be treated. The surfaces were washed for
1 minute with running distilled water and dried with air. One slice was randomly selected as the negativecontrol not to be treated and one slice was randomly selected as the positive control to be treated with 37%
phosphoric acid gel (ED550104CL, Tongsheng Technology, Inc, Tianjin, P. R. China) for 30 s. Anothertwenty slices were to be irradiated by laser.
Er, Cr: YSGG laser hydrokinetic system (HKS; Millennium System, Biolase Technology, Inc., SanClemente, CA, USA) was used. This laser system emitted photons at a wavelength of 2.78 µm, pulsed witha duration between 140-200 ms and a repetition rate of 20 Hz. The beam spot size was 0.75 mm. With theuse of air 55% and water 35%, dentin surfaces were perpendicularly irradiated by laser. Twenty slices
were randomly divided into two groups. The 10 slices of group 1 were irradiated at the distance below 2mm, and the irradiating distance of another group was beyond 2 mm (about 3 mm). Each specimen of
every group was irradiated for 6 s using different power output, viz. 1, 2, 3, 4, and 6 W, and the area to be
irradiated was 3x4 mm2.
The specimens treated by laser and phosphoric acid were dehydrated in the ascending alcohol series,
mounted on aluminum slabs, gold coated and examined the morphologic changes of dentin surface with an
SEM (SSX-550, Daojing, Tokyo, Japan).

Results
Dentin Morphology of Control Group
Negative control group showed that the dentin surface was coated with smear layer and the tubular
structure was difficult to see (Fig. 1). Positive control group showed comparatively smooth dentin surface.
Smear layer was completely removed, and dentinal tubules were clearly visible, the diameter of the tubules
was slightly widened. Most orifices were opened, but few of them could be observed the white deposits(Fig. 2).
Dentin Morphology of Group with Irradiation Distance below 2 mm
1 W: Dentin surface showed more irregular than the control group. Part of the smear layer was removed.
Dentin tubules exposed in the areas whose smear layer had been removed, and the diameter had no obvious
changes (Fig. 3).
2 W: Dentin surface was irregular. Part of the smear layer was removed, most of tubules were opened
and a few of them were occluded.
3 W: Most smear layer was removed. Some dentin surfaces were not smooth and showed cascadingstructure. The diameter of tubules was slightly reduced and part of tubules had deposits. Intertubular
dentin seems to ablate more than the peritubular dentin, showing a protrusion of the tubules (Fig. 4).

57

Yu and Yan

Int Chin J Dent 2004; 4: 56-60.

12Fig. 1. Negative control group showed the dentin surface was coated with the smear layer.
Fig. 2. Positive control group showed the smear layer was completely removed and most orifices
were opened.

34Fig. 3. Specimens irradiated at 1 W below 2 mm showed more irregular appearance and part ofthe smear layer were removed.
Fig. 4. Specimens irradiated at 3 W below 2 mm showed cascading structure and the diameter oftubules was slightly reduced. Intertubular dentin ablate more than the peritubular dentin.

56Fig. 5. Specimens irradiated at 4 W below 2 mm showed more irregular appearance like squama
and the diameter was obviously reduced.
Fig. 6. Specimens irradiated at 6W about 3 mm showed most the smear layer was removed andsome intertubular dentin melted with peritubular dentin.
4 W: The smear layer was almost completely removed. The dentin surfaces showed more irregularappearance like squama. Most of dentin tubules were exposed and the diameter was obviously reduced(Fig. 5).

58

Yu and Yan

Int Chin J Dent 2004; 4: 56-60.

6 W: The morphological changes of dentin surface was comparable with the 4 W group, nevertheless, itshowed more irregular than it and had microcracks. Intertubular dentin was ablated more than abovegroup.
Dentin Morphology of Group with Irradiation Distance about 3 mm
Most of the smear layer of dentin surface was not removed for specimens that were irradiated at 1-3 W.
Part of the smear layer was removed at 4 W and we obtained a relative smooth surface. The dentin tubules
were partly exposed and some intertubular dentin melted with peritubular dentin. The morphologicalchanges of dentin surface at 6 W and at 4 W were alike except that most of orifices could be observed
occluded (Fig. 6).

Discussion
Smear layer is a loose structure made of dentin debris produced during the cavity preparation progress,
metamorphic collagen, saliva, germs and the etc, and the thickness of which is about 0.5-5 µm. The layer
can firmly attach the tooth and not be easily removed by flushing and wiping. Most scholars consider it animportant way to induce the edge microleakage of restoration removed, furthermore, the elimination of thesmear layer can also benefit to decrease the silver-amalgam edge microleakage.3
The debris and smear layer removal are thought to have occurred by the Er, Cr: YSGG laser irradiation
emitted being absorbed by water, thereby inducing a rapid rise in temperature causing the heated material
to be explosively removed.4 Therefore we observed the morphological changes in dentin surface using
different power with the water spray. In our study, when the irradiation distance was controlled below 2mm, the irradiation with the output power beyond 2 W objected to dentin surface could effectively remove
the smear layer. Compared to 4 W and 6 W, dentin surface irradiated at 2 W and 3 W showed more smoothappearance similar to the control group, that is most of dentin tubules were exposed and the orifices did not
be occluded or reduced. At 4 W and 6 W, dentin surfaces showed obviously scraggly appearance likesquama and clouds, and the intertubular dentin was ablated too much. Our study indicated that when weused lower output power, this laser would exert a simple effect just to remove the smear layer and would
not destroy the normal dentin. With the energy of irradiation increased, although the smear layers werealmost completely eliminated, the normal dentine beneath the layer would also been partly destroyed orablated. In fact, most former researcher used Er, Cr: YSGG laser for the ablation of dentin at the energy
output of 4-5 W,2,5 therefore, if our aim is simply to remove the smear layer of dentin, the power output
should be controlled between 2 W and 3 W. Moreover, we also found the intertubular dentin seemed toablate more than the peritubular dentin when high energy is used. The proper reason we presume is that Er,
Cr: YSGG laser has an energy wavelength peak of 2.78 µm that corresponds to the absorption coefficientof water and hydroxyapatite which are contained higher in the intertubular dentin.
The laser with the irradiation distance exceed 2 mm will alleviate the ablation effect, and therefore we
can regulate the distance between fiber head and tissue to control the effect of ablation.2 Our observations
indicate that the laser with energy above 4 W at the irradiation distance of 3 mm will also effectively59

Yu and Yan

Int Chin J Dent 2004; 4: 56-60.

remove the dentin smear layer. However, concerning the distance and project angle is not easily controlled
in actual clinical application, we think this method be applicable to the tooth surface with the need of wider
irradiation scope and lower accurate degree.
At present, dentists primarily use phosphoric acid to remove the smear layer, but this method can enlarge
the diameter of dentin tubules, increase the permeability of tubules, then cause fluxion of the liquid insidewhich stimulate the pulpo-dentinal complex.

Furthermore, this method sometime will aggravate thediscomfort of the patients with hypersensitive tooth and even will induce the pulp inflammation. Aim at
the above weakness, lasers have characteristics better than and traditional chemistry method which havebeen accepted by lots of researchers. In recent years, Er, Cr: YSGG laser has already been confirmed tohave a series of advantage compared to the other lasers: the roughness of the dentin surface may be
obviously increased then consequently enhance the bond strength,6 dentin can obtain increased acidresistance and somewhat defending capacity for caries,1,7 this laser does not increase the pulp temperatureand 98.5% patients have no discomfort after the clinical application of this laser for cavity preparation.1,2
Therefore, as a new, validate, and safely methods to do away with the smear layer of dentin surface, thislaser have extensive applied foreground. Our observations in vitro proved that energy of 2-3 W (irradiatingdistance below 2 mm) and energy of 4-6 W (irradiating distance about 3 mm) were proper settings withwhich Er, Cr: YSGG laser can effectively remove the smear layer from the dentin surface, but considerable
additional research should be necessary around the influence of the irradiation on bond strength between
materials and tooth.

References
1. Hadley J, Young DA, Eversole LR, Gornbein JA. A laser-powered hydrokinetic system for caries removal and cavity
preparation. J Am Dent Assoc 2000; 131: 777-85.
2. Rizoiu I, Kohanghadosh F, Kimmel AI, Eversole LR. Pulpal thermal responses to a erbium, chromium: YSGG pulsed
laser hydro-kinetic system. Oral Surg Oral Med Oral Pathol 1998; 86: 220-3.
3. Chen XM, Duan HY. Influence of smear layer and adhesive resin on theadaptation of amalgam restoration to cavitywalls. West China J Stom 1997; 15: 303-5.
4. Yamazaki R, Goya C, Yu DG, Kimura Y, Matsumoto K. Effects of erbium, chromium: YSGG laser irradiation on root
canal walls: a scanning electron microscopic and thermographic study. J Endod 2001; 27: 9-12.
5. Lin S, Caputo AA, Eversole LR, Rizoiu I. Topogrphical characteristics and shear bond strength of tooth surfaces cut
with a laser-powered hydrokinetic system. J Prosthet Dent 1999; 82: 451-5.
6. Hossain M, Nakamura Y, Yamada Y, Suzuki N, Murakami Y, Matsumoto K. Analysis of surface roughness of enameland dentin after Er, Cr: YSGG laser irradiation. J Clin Laser Med Surg 2001; 19: 297-303.
7. Hossain M, Kimura Y, Nakamura Y, Yamada Y, Kinoshita JI, Matsumoto K. A study on acquired aced resistance ofenamel and dentin irradiated by Er, Cr: YSGG laser. J Clin Laser Med Surg 2001; 19: 159-63.

Reprint request to:
Dr. Lu Yan
College of Stomatology, China Medical University
Shenyang 110002, P. R. China
Email: luluxiaoyan@163.com
Received April 19, 2004. Revised May 23, 2004. Accepted June 1, 2004.
Copyright ©2004 by the Editorial Council of the International Chinese Journal of Dentistry.

60