| Disinfecting approaches of infected dentin in root canals include instrumentation, irrigation and interappointment medication. In the first part of this study, the influence of operator's experience on the quality of instrumentation with WaveOne Primary Files by an experienced and an inexperienced operator was evaluated using micro-computed tomographic scanning (micro-CT). The results showed that the changes in canal volume and surface area after treatment, percentages of uninstrumented surface area, and the canal transportations were similar between experimental groups. The inexperienced operator without training used significantly more time for instrumentation than the experienced operator, after training for one month the instrumentation time by the same inexperienced operator was shorter, close to the experienced operator. WaveOne instrumentation technique required a short learning curve for inexperienced user in order to master this technology.Interappointment intracanal medication is a common chemical disinfection method. But failure to remove all Calcium hydroxide [Ca(OH)2], a most common used interappointment medicament, from the root canal before the canal is filled with a permanent root filling can interfere with the sealing ability of the root filling and result in leakage, even root canal therapy (RCT) failure. Therefore, in the second part of current study we evaluated the removal of Ca(OH)2 from root canals of mandibular molars by means of the GentleWave system, conventional instrumentation with passive ultrasonic activation, and conventional instrumentation with needle irrigation using micro-CT imaging as the assessment approach. The study confirmed the difficulty to remove Ca(OH)2 from root canals using conventional methods. The GentleWave system removed the paste within 90 seconds using water irrigation alone. Although most of irrigation methods couldn't remove Ca(OH)2 from canals completely, which may lead to RCT failure, improvement of root canal disinfection effectiveness would facilitate to achieve eventual RCT success.Bacteria colonizing in dentinal tubules may survive from root canal mechanical and chemical preparation, which is thought to be a main cause of RCT failure. A standardized in vitro dentinal tubule infection model could be aiding in the effectiveness evaluation of various dentinal canals disinfecting ways. It is important to take bacterial diversity and biofilm maturation into consideration when establishing the infected dentine model. In previous studies, the centrifugation has been proved to achieve homogenous and dense presence of bacteria in different specimens deep in dentin, but only one strain of bacteria was used in each investigation. Therefore, the present project established monospecies dentin canal biofilm model with two Enterococcus faecalis (E. faecalis) strains separately and multispecies dentin canal biofilm model with plaque bacteria using centrifugation. Furthermore, the effect of the source of biofilm bacteria and the level of biofilm maturation on their susceptibility in dentinal tubules was also evaluated.In the third part, E. faecalis VP3-181 and Gel 31 were introduced into dentinal tubules individually to form monospecies biofilms by centrifugation. After 1-week and 3-week incubation, the samples from each group were subjected to sterile water,2% chlorhexidine (CHX), and 2% sodium hypochlorite (NaOCl) respectively. Following 3-minute exposure, the proportions of killed bacteria in dentinal canals were assessed by LIVE/DEAD viability staining and confocal laser scanning microscopy (CLSM). The proportion of killed bacteria in mature (3 weeks) biofilms was lower than in young biofilms (1 week) after treatment (P <0.05), except no significant differences were detected for young and mature Gel 31 biofilms exposure to 2% NaOC1. E. faecalis Gel 31 biofilms were more resistant than VP3-181 biofilms except exposure to 2% NaOC1 of 3-week-old biofilms (P<0.05). No significant difference was detected between 2% CHX and 2% NaOC1 against the E. faecalis strains. The susceptibility of the monospecies E. faecalis dentin biofilm showed strain related differences. Mature monospecies biofilms in dentinal tubules were more resistant to disinfectants than young biofilms.In the fourth part, three specimens of pooled plaque bacteria collected from different donors were centrifuged into dentin to grow multispecies biofilms in dentin canals. And the following experimental procedures were the same as those in the third part. The results indicated that 3-week-old biofilms were more resistant than 1-week-old biofilms (P< 0.05). No differences in the susceptibilities to the disinfecting agents of the three multispecies biofilms were detected. The 2% NaOC1 was more effective against multispecies biofilms in dentin than 2% CHX (P<0.05). The multispecies biofilms from different donors showed similar susceptibility. Mature multispecies biofilms in dentinal tubules were more resistant to disinfectants than young biofilms. |
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