Objective:To study the adhesive strength between fiber posts and four different composite resin core materials.Methods:Four resin core materials (Multicore Heavy Body、LuxaCore、 LIGHT-CORE and Z100) were placed around the fiber posts. The adhesive strengths of the samples were tested by the universal electrical testing machine. The interface between fiber post and resin core was observed by SEM. Results:Different resin core materials exhibited significant different influences on adhesive strengths(P<0.05). The LuxaCore reached the maximum of (314.31±15.36) N and the Z100 was the minimum of (267.39±10.54) N.Conclusion:Resin core material exhibits a significant influence on adhesive strength. The dual-curing composite core material with low viscosity has the highest bond strength in all groups.

Objective To evaluate the clinic effect of dental operating microscope ( DOM) and ul-trasonic root canal technique in removing fractured root canal instruments .Methods Eight cases with frac-tured instruments located in different parts of root canals were treated with ultrasonic technique under DOM and the results were analyzed .The fractured instruments were divided into stainless steel file , Ni-Ti file and post according to material type .Results Twenty-seven fractured instruments were removed and the total success rate was71.1%.Among them, the success rate of stainless steel file was 71.4%,of Ni-Ti file was 37.5%and of post was 100%.No root canal perforation occurred .According to the place fractured instru-ments located , the success rate of stainless steel file located at upper 1/3 root canals was 100%,at middle 1/3 was 81.8%and at apex 1/3 was 33.3%;the success rate of Ni-Ti file located at middle 1/3 was 60%and at apex 1/3 was 0.In bent root canals , the success rate of stainless steel file located at bent part crown square was 86.7%and at bent part apex square was 20%;the success rate of Ni-Ti file located at bent part crown square was 60%and at bent part apex square was 0.Conclusion DOM combined with ultrasonic root canal technique in removing fractured instruments is effective , but the success rate is relatively low for Ni-Ti files and the cases fractured instruments located at root canal apex 1/3 or bent part apex square .