| Background Pulpo-periapical diseases have high prevalence among population. Of these diseases, chronic apical periodontitis(CAP) is considered as one of the more popular and serious lesions. Etiological studies in recent years have proved that the mixed microflora dominated by black pigmented bacteroides of gram negative anaerobes in the infected root canal is the mainly causative agent for them. The destructive process of bacteria is generally known as three stages in order:dental caries, pulptitis and apical periodentitis. It is conventionally accepted that root canal therapy(RCT) might be the optimal and effective approach of non-surgical endodontic therapy. However, disinfection of the root canal is difficult to accomplish by conventional method of intracanal medicament dressing and the remaining bacteria in the root-filling canal may cause inflammation which accounts for the failure of RCT. In addition, it is usually required multi-visit treatments . To avoid such limitations, many efforts have been done worldwide to explore the best way in which bacteria in the root system could be eliminated more efficiently, quickly and safely. Fortunately, the pulsed Nd:YAG laser was introduced in RCT. Studies have revealed that the laser had the abilitities of removing smear layer, sterilizing microorganism, fusing or glazing hard tissue on the root canal wall. It is also reported that the canal preparation with the laser resulted in a good sealing effect and reduced microleakage between canal wall and filling materials. But these studies were more in vitro experiments in which irradiation parameters and experimental condition varied greatly and providedlittle information for in vivo application. Few investigation at present has been found on the safety and feasibility in clinical treatment. Objective A pulsed Nd:YAG laser was used to establish the relationship between energy parameters irradiation in root canal and their effects in vitro, their safety to periodontal tissue in vivo, then the proper parameters may obtain and apply for clinical RCT to evaluate the feasibility, reliability and superiority in the healing of CAP.Methods and Materials1 Determination of the bactericidal effect-energy parameter relationship105 extracted human single-rooted teeth were used and prepared firstly in the routine way with their anatomical crown longer than 14mm removed. All of the roots wall were irradiated by different laser parameters after two bacteria strains including gram positive or negative one at certain concentration were inoculated into the sterilized roots. Bacteria cultivations were conducted and colony forming unit(CFU) were counted to calculate the bactericidal rate of each group.2 Measurement of temperature elevation on the root surface at every parameter settings of the laser 110 extracted human single-rooted teeth were prepared as above mentioned way and mounted in an alginate material at 37℃ water bath which was used simulated physiological periodontal condition. Temperature elevation was measured during lased in the root canal at different energy parameters with thermocouple converters. The relationship between temperature rising and frequency and power levels were established and the roles of the related factors were explored as well.3 Morphological observation of the root canal wall after lased at different parametersAfter previous procedures, two samples were selected randomly fromeach parameter levels of the laser, with or without initiators. Then they were split longitudinally for preparation of scanning electron microscopic(SEM) examination.4 Safety assessment to the periodontal tissue in dogs during irradiation by the laserBased on above data from experiments, the likely safe and effective scope was confined and was induced into RCT in healthy adult dogs. The histopathological changes in perioapical region were observed by light microscopy and the ultrastructures were examinted by transmission electron microscopy(TEM) at period of 1 d, 15 d, 30 d, 60 d res...
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