Investigations Of New Techniques In Prevention And Treatment Of Intra-canal Infections

During root canal treatment,the infected tissues are removed,and the root canal system is shaped,disinfected,and sealed with a core bio-inert material(e.g.,gutta percha)combined with an endodontic sealer.The elimination of infection within the canal is crucial for the success of a root canal treatment,which is currently carried out through a variety of chemo-mechanical techniques.However,due to the complicated anatomical structure of the tooth root,it is well known that the complete eradication of the bacteria in the canal and dentin tubules is virtually impossible,regardless of instrumentation and irrigation procedures.The residual bacteria can cause reinfection,leading to the failure of the endodontic treatment.This indicates that there is always a need of more and effective techniques for root canal disinfection.In addition,as the final step in a root canal treatment,it is desirable for the endodontic sealer to exhibit an antibacterial effect to eliminate or inhibit the growth of residual bacteria that are untouched by the chemo-mechanical procedure.Therefore,the primary goal of this project is the development of new techniques in prevention and treatment of intra-canal infections so that to improve the success rate of root canal treatment,maintain the long-term effectiveness of root canal treatment,and prevent the failure of root canal treatment.The attempts that we had tried are listed as follows: 1.Disinfection effect of laser assisted protocols in in vitro infected extracted root canalsFirstly,we established an in vitro model of infected extracted root canal with Enterococcus faecalis(E.faecalis),and evaluated the bactericidal effect of Nd:YAG,Er:YAG,Er,Cr:YSGG laser radiation and antimicrobial photodynamic therapy.Results showed that all the laser radiation protocols tested,especially Er:YAG laser activated sodium hypochlorite(NaOCl)irrigation,have effective bactericidal effect in experimentally infected root canals.However,at the same time,Er:YAG laser activated NaOCl irrigation could only eliminate E.faecalis located on root canal walls and invaded within 200 μm inside dentinal tubules,but showed limited bactericidal effect on E.faecalis deep(>300 μm)inside dentinal tubules.Therefore,we further investigated the bactericidal effect of Er:YAG laser activated NaOCl irrigation under different irradiation power(0.3,0.5,and 1.0 W)and time settings(20 and 30s),hoping to find an optimized parameter combination for the Er:YAG laser radiation.Results showed that Er:YAG laser activated NaOCl irrigation with the Er:YAG laser irradiated at 0.5 W for 30 seconds was the optimized combination,which can eliminate E.faecalis invaded around 500 μm inside dentinal tubules.2.Synthesis and characterization of quaternized silica bioactive nanospheres: a promising substrate for long-term antibacterial endodontic sealerFirstly,the silica bioactive nanospheres(SBG-NS)was synthesized by the sol-gel process,and a series of quaternary ammonium methacrylate salts(QAMs)with different lengths of alkyl chains were synthesized via the addition reaction of tertiary amines with organo-halides.Then,KH-570,a silane coupling agent which contains the carbon double bond,was introduced to the surface of SBG-NS and then got the intermediate product SBG-NS/ KH-570.Finally,the QAMs were grafted to the surface of SBG-NS/ KH-570 through a radical polymerization reaction,and generated the quaternized silica bioactive nanospheres substrate(SBG-QAPMs).Thereafter,we tested(1)the water solubility and ion releasing ability of the SBG-QAPMs,(2)the antibacterial effect of the SBG-QAPMs against persistent microorganisms associated with infected root canals,and(3)the bio-safety(in vitro cytocompatibility and in vivo inflammatory effect)of the SBG-QAPMs.Results showed that the SBG-QAPMs had excellent stability in PBS solution,could release mineralization related ions such as Si4+ and Ca2+,and had a long-term effective antibacterial activity and favorable bio-safety.In conclusion,we can draw the following conclusions:(1)Er:YAG laser activated NaOCl irrigation with the Er:YAG laser irradiated at 0.5 W for 30 seconds could be considered a new option for effective root canal disinfection,and(2)the SBG-QAPMs are promising substrates for the development of long-term antibacterial endodontic sealers.