Effect Of Hydrokinetic Erbium-Chromium: Ysgg Laser On Enamel And Dentin: In Vitro Evaluation P. Benazzato*, A. Stefani, R. Benvegnù, M. Calabrese (1), P.N. Mason (1) (1: Dept. Of Restorative Dentistry, Dental School, University Of Padova, Italy) The aim of this in vitro study was to evaluate the effects of a new hydrokinetic erbium.chromium YSGG on hard tissues of the tooth in comparision with traditional bur cavity preparation. It was used the laser “Millennium” (Biolase Tech. Inc.). This laser has two action mechanisms: first the direct impact of the beam against the tissues of the tooth, second, laser energy is absorbed by the water droplets of the spray and produces a microesplosion at the tooth surface (hydrokinetic system HKS). Water is also important for cooling the target surface. The wavelenght of “Millennium” is 2780nm pulsed at 20 Hz, 140 msec. When it was used, laser was setted at 6 Watts power for enamel and 4 Watts for dentin always with air-water spray. Were used 16 teeth divided in 6 groups:   GROUP 1: (2 premolars) for each tooth was prepared a class II° cavity with laser to observe dentin with S.E.M.;   GROUP 2: (2 incisors) for each tooth was prepared a class II° cavity with laser to observe dentin with S.E.M.;   GROUP 3: (2 premolars) for each tooth was prepared a class II° cavity with a traditional diamond bur to observe dentin with S.E.M.;   GROUP 4: (2 incisors) for each group tooth was prepared a class V° cavity with diamond bur to observe dentin with S.E.M.;   GROUP 5: (4 incisors) for each tooth was prepared the vestibular surface: half surface was treated with laser and half with bur and then etched with H3PO4 for 40 seconds, the teeth were observed with S.E.M.;   GROUP 6: (4 premolars) for each tooth was prepared a class II° cavity with laser. Two cavities were etched with H3PO4 for 20 seconds, rinse, dryed and restored.   Two cavities were not etched but only restored using Scotchbond 1 and Z250 (3M). After restoration, each tooth was immersed in a 6% basic fuchsin dye solution for 48 hours. To prevent dye penetration, the apical foramina of each tooth was occluded with wax and the root surfaces were coated with two layers of nail warnish to within 0,5mm from the restoration margins. After rinsing, specimens were sectioned: 3 longitudinal sections in a M-D direction were taken from each tooth. The dye penetration in enamel was measured under magnification. The S.E.M. morphology of the enamel treated with laser was very similar to that of the acid etched surface, without any smear layer or thermal altered tissue surface. This structure provides a strong micromechanical bonding of the resin composite and an absence of microleakage (samples group 6): in this way the etching of the enamel treated with laser is optional. The dentin cutted with laser showed open dentinal tubules with no smear layer and smear plugs. The surface was cleaned without thermal damages (denaturation, cratering, craking, fissuring, re-crystallization).