**4. Discussion**

352 Biomaterials – Physics and Chemistry

Fig. 2. SEM images (X1000) showing the fractured surface between the composite resin and dentin side. The fractured bonded surface of dentin by the single bond adhesive treatment as a result of the shear bond strength test followed by 37% phosphoric acid etching (A), and 1.4 W of power intensity irradiation followed by a Single bond procedure (B), and 2.25 W of power intensity irradiation followed by a Single bond procedure (C) were evaluated by SEM. The fractured bonded surface of the dentin by Clearfil SE bond adhesive treatment as a result of the shear bond strength test followed by 37% phosphoric acid etching (D), and 1.4 W of power intensity irradiation followed by the SE bond procedure (E), and 2.25 W of power intensity irradiation followed by the SE bond procedure (F) were evaluated by SEM.

Recently, Er:YAG and Cr:YSGG lasers were introduced in dentistry. For physical and medical reasons, they are used for the treatment of hard tissue. The advantage of an Erbium wave is that it is well absorbed by water and dental hard tissue. Although dentin contains a high proportion of water, the depth is shallow for laser transmission. The strong absorption of water reduces the level of heat during tooth preparation. As water absorbs laser radiation better than dental hard tissue, it reduces the increasing temperature of the tissue during the preparation. Water reaches the boiling point and causes microexposure of the tooth. This action breaks up the surrounding tissue into small pieces and dissipates them at the same time. As this explosion occurs in water, it is so-called a preparation induced by water. Although most radiation is absorbed in water, a certain amount of heat transmission is unavoidable. Therefore, a water spray is used for cooling. The proper amount of water spray prevents pulpal damage. As heat transmission normally depends on the pulse shape and pulse maintenance time, the amount of water varies according to the laser system and treatment itself. A lack of enough water spray causes pulpal damage.

In contrast to initial studies, there was no immunological difference between the laser system and conventional etching method. Moreover, the positive effect of the laser to regenerate pulp was discussed. Essentially, cavity preparation should be performed with the proper instruments with the "method of minimal pulse energy" according to the manufacturer's protocol. This is because the preparation should be performed with the capability to minimize additional damage.

The bond strength of the adhesive system is one of the major factors for the success of restorations. Therefore, this restoration parameter can be measured accurately by the bonding test. The bonding of resin composite material to the tooth structure prepared with different types of lasers has been reported (Visuri et al., 1996; Armengol et al., 1999; Martínez-Insua et al., 2000; Carrieri et al., 2007; Ekworapoj et al., 2007; Lee et al., 2007; de Carvalho et al., 2008; Gurgan et al., 2008; Chou et al., 2009). These studies reported variable results comparing the bond strengths of composite resin to a laser-prepared and acid etched dentin surface, and suggested that the laser preparation is more effective than etching in bonding strength of the composite resin to the tooth structure (Obeidi et al., 2010; Turkmen et al., 2010; Navimipour et al., 2011). This study compared the in vitro SBS of two different adhesive systems to dentin treated with an Er,Cr:YSGG laser. These results showed that the type of adhesive system had no effect on SBS, whereas the laser intensity was a criterion to be considered.

Generally, the manufacturer of Er,Cr:YSGG lasers recommended 2.25 to 2.5 W for laser etching but other studies used 2 to 3 W referring from their results of a pilot study to obtain the proper effects (Çalşkan et al., 2010). The data from this study showed that the shear bond strength (SBS) of dentin treated with 1.4 W, was higher than that of SBS with 2.25 W, which was recommended by the manufacturer. These results were different from other study results showing that the shear bond strength of laser etching (1.25 W or 3 W) was higher than the diamond bur (Gurgan et al., 2008). The distance between the laser tip and laser irradiated dentin surface may not be strict in this kinds of study designs. On the other hand, in this experiment, the sweeping motion of the laser irradiation 6 mm away from the dentin surface was used before the application of adhesive procedures to employ consistent defocused irradiation with lower intensity. Therefore, the difference in the effect between

Effect of the Er, Cr: YSGG Laser Parameters on

**5. Conclusion** 

**6. Acknowledgment** 

(NRF-20100023448).

ISSN 0894-8275

145-150, ISSN 0268-8921

3, pp. 165-170, ISSN 0268-8921

3, pp. 481-486, ISSN 1549-5418

2, pp. 117-124, ISSN 0268-8921

pp. 616-624, ISSN 0287-4547

**7. References** 

layer appears to be the main reason for the lower bond strength.

Shear Bond Strength and Microstructure on Human Dentin Surface 355

These results suggest that the acid etching of lased dentin can reinforce the hybrid layer and formation of resin tags, but the acidic monomer of SE bonds cannot function at its best due to the obstruction of dentinal tubules and denatured collagen fibrils network and the absence of smear layer. These results were not the same as expected. A self etching adhesive system and two steps adhesive system showed a similar SBS because the resin tags only contribute to the bond strength in small portions (15%) and the poorer quality of the hybrid

An Er,Cr:YSGG laser was used to determine if the laser can increase the shear bond strength between the composite resin and surface treated dentine surfaces. On the other hand, the pretreatment of dentin with an Er,Cr:YSGG laser does not affect the shear bond strength of the two different adhesive systems under these experimental conditions. In addition, the shear bond strength of two different adhesive systems in dentin treated with a 1.4W laser intensity was higher than that treated with 2.25W but the difference was not significant.

This study was supported in part by a grant from Kyung Hee University in 2006 (KHU-20060930) and by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology

Abdalla AI, Davidson CL (1998) Bonding efficiency and interfacial morphology of one-bottle

Armengol V, Jean A, Rohanizadeh R, Hamel H (1999) Scanning electron microscopic

Çalşkan MK, Parlar NK, Oruçoğlu H, Aydn B (2010) Apical microleakage of root-end

Carrieri TCD, Freitas PM, Navarro RS, P. Eduardo C, Mori M (2007) Adhesion of composite

Chou JC, Chen CC, Ding SJ (2009) Effect of Er, Cr: YSGG laser parameters on shear bond

de Carvalho RCR, de Freitas PM, Otsuki M, de Eduardo CP, Tagami J (2008) Micro-shear

Dunn WJ, Davis JT, Bush AC (2005) Shear bond strength and SEM evaluation of composite

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cavities prepared by Er, Cr: YSGG laser. *Lasers in Medical Science,* Vol. 25, No. 1, pp.

luting cement to Er:YAG-laser-treated dentin. *Lasers in Medical Science,* Vol. 22, No.

strength and microstructure of dentine. *Photomedicine and Laser Surgery,* Vol. 27, No.

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bonded to Er:YAG laser-prepared dentin and enamel. *Dent Mater,* Vol. 21, No. 7,

the distance and irradiation time of the laser could not be performed and compared with other studies in this study (Chou et al., 2009; Obeidi et al., 2009; Obeidi et al., 2010).

Based on previous studies using Er,Cr: YSGG laser irradiation on the tooth structure, normal range of power intensity (2.5 W) was utilized on the surface of dentin while high power intensity of the laser (higher than 3 W) was more effective on the bonding strength of the enamel surface (Obeidi et al., 2010), (Dunn et al., 2005). On the other hand, Tagami *et al*. reported that 70mJ (1.75 W) of low laser intensity irradiation on the dentin surface followed by SE bond showed a higher micro shear bond strength than with 150mJ (3 W) (de Carvalho et al., 2008), which is similar to the present experiments. According to the manufacturer's recommendation for laser etching on dentin surface, it was decided to follow the laser protocol mode using two power intensities (2.5 W and 1.25 W). If a power intensity of 2.25 W and 1.4 W are converted to the mJ scale, they could be approximately 112.5 mJ and 70 mJ, respectively. In the present study, 1.4 W of laser intensity irradiation showed the highest shear bond strength between the composite resin and bonded dentin surface with the two types of adhesive systems (Single bond and Clearfil SE bond). Consequently, the SE primer of the SE bond probably could not remove the superficial layers of the irradiated dentin completely. On the other hand, phosphoric acid of three steps adhesive or two steps adhesive could remove most of the superficial layers of the irradiated dentin. In addition, one study reported the advantages of mechanical instruments or combining acid etching and mechanical instruments in removing the superficial layers (Obeidi et al., 2009) .

With the respect to the micromorphological changes after acid etching and laser etching, SEM revealed different characteristic features from those found in conventional acid etched surfaces. The dentin surfaces irradiated with the Er,Cr:YSGG laser had a scaly, irregular, and rugged appearance compared to the acid etched dentin surface (Chou et al., 2009). In addition, with the higher laser intensity or longer irradiation time, the condition was worse in terms of the irregularity of the dentin surface, which has a close relationship with the bonding strength of the composite resin to dentin.

The control groups showed a higher SBS than the 2.25 W of laser irradiated groups in all bonding systems, but, SEM evaluation revealed more adhesive chips remaining on the control group than on the 2.25 W laser irradiated group. On the other hand, thick, rough, and irregular collapsed composite resins were estimated on both the control and 2.25 W laser irradiated groups. SEM examinations of the two kinds of laser intensity irradiation (1.4 W and 2.25 W) revealed that the 1.4 W laser intensity group had more adhesive chips remaining on the dentin than the 2.25 W laser group in the Single bond adhesive system, which is opposite to that observed in the Clearfil SE bond system.

One possible explanation is that the thermomechanical effects of higher laser intensity probably have extended into the subsurface dentin and undermined the integrity of the resin–dentin interface resulting in a lower bond strength. The formation of fissures or cracks in the subsurface dentin might be a start point for the failure of resin-dentin adhesion. Obviously, all irradiated groups were affected by thermo-mechanical effect, but 1.4 W and 2.25W were all affected by thermomechanical effects. On the other hand, 1.4 W laser intensity might not be seriously affected. Laser irradiation with a high intensity may obstruct the dentinal tubules by melting and fail to produce a good hybrid layer. Whereas, laser irradiation with a low intensity may leave the dentinal tubule open and facilitate the infiltration of bonding agent. This may account for the lower SBS values with 2.25 W irradiated dentin than with 1.4 W laser intensity.

These results suggest that the acid etching of lased dentin can reinforce the hybrid layer and formation of resin tags, but the acidic monomer of SE bonds cannot function at its best due to the obstruction of dentinal tubules and denatured collagen fibrils network and the absence of smear layer. These results were not the same as expected. A self etching adhesive system and two steps adhesive system showed a similar SBS because the resin tags only contribute to the bond strength in small portions (15%) and the poorer quality of the hybrid layer appears to be the main reason for the lower bond strength.
