Abstract: Abstract: Objective To investigate the selection of cooling water in minimally invasive exodontia using high speed handpiece by evaluating the effect of sodium hypochlorite disinfection method which is a common approach to sterilize dental unit waterlines (DUWLs).Methods Ten dental units were randomly selected．Water samples were taken from their water outlets of highspeed handpiece at three different time points:8∶00 (the time after sterilization with 700 mg/L sodium hypochlorite and before receiving patients),11∶30 (the time right after semi day′s therapy) and 17∶30 (the time right before the end of the daily clinical work),respectively.Then,Colony Forming Units (CFU) were counted.The data were analyzed by generalized estimating equation with SPSS 19.0 software.Results The CFUs of water samples at different time points were significantly (P <0.05).No microorganisms were detected in samples of 8∶00.Microorganisms were detected in samples of 11∶30 and 17∶30,and the CFUs observed in samples of the latter were more than those of the former.Conclusions Sodium hypochlorite has a good effect on DUWLs infection control.However,during the use of the dental unit,the waterlines can be contaminated,which does not cater for the requirements of exodontia which should be an aseptic operation.Therefore,it is necessary to apply other cooling water system for high speed handpieces to cooling water systems in minimally invasive exodontia.

Key words: minimally invasive exodontia, dental unit waterlines, sodium hypochlorite, high-speed handpiece