الفهرس 
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Abstract
This in vitro study was performed on sixty extracted human maxillary central incisors to evaluate the fate of two bacterial species (Staphylococcus aureus and Escerechia coli) on the canal wall and within dentinal tubules after obturation of the root canals with guttapercha and two types of sealer (Apexit and RCS) for one week and one month time periods. Also, this study was conducted to evaluate the apical internal leakage of the tested bacteria through the apical foramina of the tested teeth after root canal obturation for different time intervals (3, 7, 14, 21, and 30 days). The selected teeth were prepared endodontically using stepback preparation technique and sterilized in an autoclave. The prepared and sterilized root canals were then sealed apically with sterile blue wax and inoculated with a mixed bacterial suspension of 1x107 bacteria/ml of both Staph. aureus and E. coli. The access openings were sealed with sterile blue wax to prevent seepage of the bacterial suspension. All teeth were then placed in sterile Eppendorf tubes which held vertically in a perforated tray and incubated at 37 oC for seven days. At the end of the incubation period the selected teeth were equally divided into three main groups of twenty teeth each according to whether the root canals were filled with guttapercha and Apexit sealer (group I) or guttapercha and RCS sealer (group II) or left without filling to act as a control (group III). Each group was further subdivided into two main subgroups (A and B) according to the time period of one week or one month. Root surfaces of each tooth were coated with 3 succissive layers of nail polish and covered with sterile pink wax leaving only the apical onemm uncovered. Each tooth was placed in a sterile Eppendorf tube containing 0.5ml. sterile nutrient broth, so that only the apical 1mm. was kept immersed in the nutrient broth. Evaluation of apical bacterial leakage from inside the filled or empty (unfilled) root canals was done at 3& 7days of obturation for the first subgroup A of all groups and at 3, 7,14, 21, & 30 days of obturation for the subgroup (B) of all groups. Bacterial leakage was evaluated by measuring the optical density of the nutrient broth by a spectrophotometer. Optical density was transformed into numbers using a mathematical equation. Types of bacteria used were confirmed morphologically by inoculation on agar plates and by Gram stained films. At the end of each time period (one week and one month), the crown of each tooth was removed at cementoenamel junction, the roots were sectioned into two halves and the root fillings were removed by sterile excavators. Evaluation of the fate of remaining bacteria on the canal wall was done by collecting dentin powder from the root canal walls of one half of the root sections in five rasping actions by Hfile in sterile Eppendorf tube containing 0.5 ml of sterile nutrient broth. Bacterial count and identification of the present bacteria was done as before. The other half of each root was used to check the evidence of any remaining viable bacteria within the dentinal tubules as follows: three different areas were selected to represent each apical, middle and coronal thirds of the root canal. Dentin powder was collected from each third in two depths 0.5 & 1 mm using ISO # 012 and 010 round burs respectively. Confirmation of the depth was done by graduated and sterile periodontal probe. Dentin powder from each third and from each depth was collected in separate, sterile Eppendorf tubes containing 0.5 ml of sterile nutrient broth and bacterial counting was made as before. from the results of this study the following conclusions can be drawn: 1?Both tested sealers used develop higher antibacterial properties on the walls of the root canal. 2?The antibacterial properties of both sealers decrease when the depth within dentin increase. 3?Apexit develops more antibacterial effect than RCS after one week with no difference after one month. 4?Apext shows similar antibacterial effect after one week and one month. 5?RCS develop higher antibacterial effect after one month than after one week. 6?The tested sealers do not completely render the dentinal tubules free of bacteria after obturation. 7?Root canal obturation with guttapercha and both Apexit and RCS sealers could have the ability to provide leakage proof sealing against internal apical bacterial leakage. 8?Internal apical bacterial leakage may occur transient from inside the root canals only within the first 3 days of obturation.