**5. Conclusion**

354 Biomaterials – Physics and Chemistry

the distance and irradiation time of the laser could not be performed and compared with

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

other studies in this study (Chou et al., 2009; Obeidi et al., 2009; Obeidi et al., 2010).

and mechanical instruments in removing the superficial layers (Obeidi et al., 2009) .

bonding strength of the composite resin to dentin.

irradiated dentin than with 1.4 W laser intensity.

which is opposite to that observed in the Clearfil SE bond system.

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

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,

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 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.
