| PrefaceSince the discovery of the ruby laser by MainMan in 1960,lasers have been widely and increasingly used in medicine and dentistry. However,most lasers are unable to effectively cut biocalcified tissues. C02 and Nd: YAG lasers induce surface changes in enamel, yet tend to cause fissuring, cracking, recrystallization or crateriform foci of melting. They also can cause significant temperature rise on the irradiated tooth surface and harm dental pulp. Some of the eximer lasers are able to ablate carious material,but they can not effectively cut sound tooth structure and therefore they are not amenale to cavity preparation. So, most lasers are used in soft tissues, prevention caries, root canal treatment and dental sensitive prevention.Recently, with the development of the laser technique applied, the animal experiments and the clinical studies of the abroad scholars show that there are different characters between Er,Cr;YSGG laser, Er:YAG laser and other lasers. The Er,Cr:YSGG laser and ErrYAG laser, while used with an air - spray, can cut enamel, dentin, cemen-tum and bone efficiently and cleanly. In many reports, the irradiation of these lasers can improve dental shear bond strength and cause little or no temperature rise. But, in other reports, there are no significantdifferences on shear bond strength between laser - cut and etched bur - cut enamel.This study assessed the dental shear bond strength with an Er, Cr: YSGG laser - powered hydrokinetic system ( HKS, Millennium system) cutting dental tissues. In this study, the structural, morphological and atomic composition of dental surfaces had been analyzed to assess if there are differences between laser - cut and bur - cut.Materials and MethodsThe forty - six vitro bovine incisors were divided into two groups. The first groups were cut by Er, Cr: YSGG laser and the second groups were cut by high - speed dental handpiece ( KaVo). The ten teeth of each group were boxed in acrylic resin before preparation for shear bond strength testing. The shear bond strength tests were performed on the EHF - FB Material Testing Machine ( JAPAN). The two teeth of each group were analyzed with eyes and SEM(1000 ) to compare the morphological changes. The five teeth of each group were tested by e-lection microprobe analysis of dispersive energy (EDX) and five teeth of each group were tested by X -ray diffraction(XRD).The data of shear bond strength were analyzed by t - test.ResultThrough the observation of scanning election microscope (SEM) , the dental surfaces of both groups were illustrated in Figures 1 and 2. Laser - cut surfaces were clean and devoid of a smear layer, also had no fissuring, recrystallization or crateriform foci of melting. Bur - cutsurfaces masked by a diffuse smear layer. There were significant difference between two groups. On the structural and atomic composition of dental surfaces, there were no significant difference between two groups. The analysis of data demonstrated that shear bond strength was significantly higher for the laser - cut surfaces than for the bur -cut surfaces (p <0. 001) . But the ablation rate of the laser group was lower.DiscussionThe Er,Cr;YSGG laser ( =2.78 m) ,when used with an air - warter spray, its energy absorbed by the water microdroplets immediately and the pressure produced causes microeplosion and vaporization. Through this process,the enamel,dentin,cementum and bone can be efficiently and cleanly cut off without significant thermal and structural damage. And because of this character, Er,Cr: YSGG laser can safely be used in cutting dental tissues.In this study, the results of EDX and XRD showed that the structural and atomic composition of dental surfaces had no significant differences between two groups. The most structure of crystals in dental surfaces were hydroxyapatite ( HA). The SEM analysis showed that laser - cut surfaces were beneficial to composite bonding. Because the laser - cut surfaces are clean and have no fissuring, recrystallization and smear layer. Topograp...
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