The Effect Of The Number Of Disinfection And Sterilization On Cyclic Fatigue Of Stainless Steel Files

Objective:Dental root canal instruments is non-disposable equipment, need to be cycle used after disinfection and sterilization. However, disinfection and sterilization should certain influence on cyclic fatigue of dental instruments, there existing broken risk during root canal preparation. This subject is to study the change of the cyclic fatigue of stainless steel K files after different times of disinfection and sterilization. At the same time, it will lead the changes of the surface appearance and broken position. The result will provide essential reference of the number of disinfection and sterilization of stainless steel files and reduce the occurrence of instruments fracture.Methods:1. Consulting relative references, stimulating clinical root canal morphology and fabricating a cyclic fatigue testing device of dental root canal instruments with a metal incline of 45°bending angle.2. 70 instruments of 25# and 70 instruments of 30# stainless steel K files were randomly divided into 7 groups, which included 10 for each, and gave the number 1-7 for each group. Group 1: The control group, neither disinfecting nor sterilizing; Group 2-5 : Taking 1, 3, 4, 5 steam sterilization cycles; Group 6-7 : Taking 2, 5 disinfection cycles(1) Group1-5 of 25# and 30# stainless steel K files were subjected to 0, 1, 3, 4, 5 times of steam sterilization cycles, then recorded the time from the beginning, and calculated the number of cycles to failure(NCF) by the motor Speed. Microstructure and metallographic-structure of the specimen’s fracture surface was analyzed by Scanning Electron microscope(SEM) and Metallographic microscope(MM).(2) Group 6-7 of 25# and 30# stainless steel K files were subjected to 2 and 5 times of disinfection cycles, then repeated the above experimental process and recorded the testing results.(3) Observing the broken position of the fracture specimens.Results:1. The whole experimental process of cyclic fatigue testing device was easy operated under direct vision. The expanding machine made the data standardization and results better repeatability.2. After 0, 1, 3, 4, 5 times of steam sterilization cycles of 25# and 30# stainless steel K files, the differences of cyclic fatigue among Group 1-5 were significantly different(P <0.05). After 4 or 5 times of steam sterilization, cyclic fatigue decreased which compared with control group(P < 0.05). After 1 or 3 times of steam sterilization, cyclic fatigue didn’t change which compared with control group(P > 0.05).3. After 0, 2, 5 times of disinfection cycles of 25# and 30# stainless steel K files, there’s no statistical significance of cyclic fatigue among Group1,6,7(P > 0.05). So, after 2 and 5 times of disinfection, cyclic fatigue didn’t change which compared with control group(P > 0.05).4. Fracture surface in Group 1, 2, 3, 6, 7 was tough dimple structure and large numbers of regular, deep dimples were distributed on the surface. You could also see micro cavities formed by fracture. Fracture surface in Group 4-5 was dimple structure in brittle intergranular morphology. It was characterized by the presence of thin brittle precipitates, clean and smooth crystal interface, and a great sense of polyhedral stereo. Metallographic analysis showed the fracture specimen’s grain of Group 4 was more obvious than Group 1, there were thicker and darker strip black belts just liked grooves around grain boundary, they were mainly chromium carbide, which made the grain boundary form “Poor Cr zone” and were more likely to be intergranular corrosion. But the fracture specimen’s grain of Group 7 wasn’t more obvious than Group 1, neither saw the strip black belts around grain boundary.5. There’s no statistical significance of the broken specimen’s blade length after repeated disinfection and sterilization(P > 0.05), the means were14.38±0.31 mm and 15.02±0.12 mm, the broken position was located at maximum bending part.Conclusions:1. The self-developed cyclic fatigue testing device of dental root canal instruments was easy operation and the experimental results had better repeatability, which could be used for testing the cyclic fatigue of dental root canal instruments.2. After 3 times of steam sterilization, the cyclic fatigue of 25# and 30# stainless steel K files declined, the surface morphology and organization structure existed brittle deformation, which possessed the potential risk of fracture.3. Disinfection didn’t effect the cyclic fatigue strength of 25# and 30# stainless steel K files.4. Disinfection and sterilization had little relationship with broken position of stainless steel instruments. The broken position was located at maximum bending part.5. 25# and 30# stainless steel K files had similar results in the study.