In Vitro Evaluation Of The Effectiveness Of Intracanal Bacterial Removal Related To Terminal Working Widths Of Apical Preparation

Objective：The purpose of this study was to compare and evaluate the removal effectivenessof Enterococcus faecalis(E. faecalis) within root canal related to different terminalworking widths of apical preparation in vitro. It will provide a theoretical andexperimental basis for the selection of a proper terminal working width of apicalprepration in clinic.Methods：Fifty-two extracted human maxillary first permanent molar teeth were selectedfor this experiment, their crowns were removed and distobuccal root canals werestandardized to a length of12mm. The working length was11mm. All root canalswere enlarged with K-file to size20, and then the apical foramens were sealed byepoxy resin. All root canals were autoclaved at121℃,1.5MPa,for15min. Then, theywere kept in CO2incubator at37℃for48h to check the efficacy of the sterilizationtreatment. Each root canal was completely filled with the E. faecalis suspension, andincubated in CO₂incubator at37℃. The medium was changed every2days.Bacteriology characterization approaches were used to make sure that there were noliving contaminants. All samples were infected for21days. Four samples werechosen to evaluate how the dentinal tubule was infected by E.faecalis in the scanningelectron microscope. The other forty-eight infected canals were randomly and evenlydivided into six groups, which underwent different apical preparation sizes: group Awas size25,group B was size30, group C was size35, group D was size40, group Eand F were the control groups with root cananl irrigation and no mechanicalinstrumentation. Root canals were prepared with ProTaper rotary instruments andstainless steel K files using a crown-down technique. Group A,B,C,D,E wereirrigated with1%sodium hypochlorite and17%EDTA；Group F was irrigated with0.9%saline water. Microbiological samples were taken using paper points from the root canals before preparation and after preparation. After10-fold serial dilutions insaline, aliquots were plated onto agar plates and incubated in CO2incubator at37℃for48hours. The colony forming units (CFUs) were counted and analyzed. Thebacteria recovery experiment was used to observe the condition of bactera growth in72hours after postpreparation sampling.Results:1. The liquid medium containing all the samples kept clear after a48hoursincubation in CO2incubators at37℃, indicating samples had been sterilizedsuccessfully after autoclaved at121℃,1.5apm, for15min. By means of sorts ofbacteriology characterization approaches, the infection of root canal by E. faecaliscould be sure and results showed that there were no living contaminants. After a21-day incubation, bacterial colonization could be observed on root canal walls, andE. faecalis were into the dentinal tubules by using a scanning electron microscope.The root canal infection model could be successfully established in vitro.2. Before root canal preparation, the numbers of Enterococcus faecalis in rootcanals were no statistically significant diference between each of groups. After rootcanal preparation,the numbers of Enterococcus faecalis in root canals in each of thesix groups were effectively reduced (P <0.01), and group D showed maximumreduction in CFUs. There was statistically significant diference between any towamong six groups for the reduction in CFUs (P<0.05).3. In groups A,B,C and D, percentages of postpreparation negative cultureswere0%,12.5%,37.5%and75%, respectively；in the control groups E and F,noneof the specimens showed negative culture.4. Bacteria recovery experiments showed that there were no bacterialcontamination after an incubation for three days,but bacterial survival was stillobserved in all canals of each group.Conclusion:1. The model for root canal invaded by Enterococeus faecalis can be establishedsuccessfully in vitro.2. The effect of reducing intracanal bacterial was improved with progressively larger apical preparation size.3.The percentage of negative cultures after root canal preparation was improvedwith progressively larger apical preparation size.4. A complete elimination of the intracanal bacteria was not achieved whendistobuccal root canals of maxillary first permanent molars teeth were prepared toapical size40with antibacterial irrigation.