Biomedical Engineering Reference
In-Depth Information
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.
