Er:Yag-Laser Assisted Root Canal Treatment M. Frentzen*, A. Braun, S. Esser (University Dental Clinic, Department Of Operative Dentistry And Periodontology Welschnonnenstraße 17, D-53111 Bonn, Germany) The aim of the study was to determine the cleaning and shaping ability of an Er:YAG-laser system in root canal preparation.   The canals of 40 freshly extracted single rooted human teeth were instrumented to an apical size of ISO 40 using manual Kerr® files and NaOCl (5%). The samples were randomly separated into 2 groups of 20 teeth each. Following canal preparation the root canals of the test group (n = 20) were irradiated using the KaVo KEY LASER 1242, the handpiece E 2055 and an optical fibre plug (40/28) with 0.37 mm in diameter. The selected laser parameter (50 mJ / pulse at the center surface of the fibre, 2 Hz) were adjusted by external control prior to canal cleaning. The application of 30 laser pulses/canal was repeated 4 times. After treatment the samples of the test and control group were fixed in formalin (10%). For light microscopic investigation a plastic embedding technique was used to cut the undecalcified teeth in sections of 15 µm thickness at defined levels (2, 4, 8 mm from the apex). These sections were then stained with toluidine blue.   The laser-assisted treatment of the root canal could be carried out without fractures in the delivery system. No canal blockages or excessive removal of canal dentin (via falsa) occured. Following laser application, the quantity of residual pulpal soft tissues decreased significantly compared to the control group.   Although laser induced side effects (cracks, carbonization) were not detectable in the histological sections, some areas of the canal walls were roughened by the laser ablation.   Er:YAG-laser assisted preparation of root canals significantly increased the removal of pulpal tissues.