Effects Of Multiple Use On The Centering Ability Of Three Reciprocating Instruments In Simulated Curved Canals

Reciprocating nickel-titanium instruments had been widely used clinically.Patients with multi-rooted teeth or multiple teeth requiring treatment are often encountered in clinical practice,the safety of single-file reciprocating systems for endodontic treatment of them is a concern of clinicians.Aim: To investigate the effects of multiple use on the centering ability and surface alteration of three reciprocating instruments in simulated curved canals.Methods: Three groups of 15 reciprocating instruments were tested: Reciproc R25(RPC),WaveOne Primary(WO)and WaveOne Gold Primary(WOG).Each file was subjected to prepare up to 15 J-shaped canals unless it was broken off during the instrumentation.The number of canals that each file had prepared was recorded.Canals in the training blocks were filled with black ink before and after preparation,pre-and post-preparation images were captured by using the dental operating microscope(Leica M320,F12).Following data were recorded by using Photoshop CS6 Extended software: centering ratio at 4 levels(0,2,4,and 6 mm from the tip)along the canal,the fragment lengths and the position of the fractured tip in the canal.The surface alterations of files after use 0,1,5,10,15 times and the feature of fractured files were examined under the scanning electron microscope(SEM).Statistical analysis was performed by using Generalized Estimating Equations(GEE).Results: There was no significant effect of multiple use or file type on centeringratio(P<0.05),When the centering ratio was considered at different levels,significant difference was found(P <0.05).All the files demonstrated the highest centering ability at 0 mm level and the lowest at 4 mm level(P < 0.05).Instrument separation started to occur in RPC and WO group after the files were multiple used in 10-11 canals.All the WOG files could be repeatedly used in 15 canals.The fragment lengths(mean ± standard deviation,mm)was 3.143±0.3322 in RPC,and 2.786±0.3185 in WO.The straight line distance(mean ± standard deviation,mm)from the broken end to the apex was 3.717±0.994 in RPC,and 3.225±0.340 in WO,showing that the broken tip located at the maximum curvature of the artificial canal.Fracture mostly occurred at the point of maximum flexure of the canal.Under SEM,defects were found on WO instrument after its fifth use and on RPC after multiple used 10 times.It was not until fifteenth times that the presence of surface cracks on WOG was detected.The fractured surfaces of RPC and WO featured cyclic fatigue,including crack initiation areas,numerous dimples and fatigue striations.Conclusion: Within the limitations of this study,heat-treated single-file NiTi reciprocating systems(M-wire for RPC and WO;Gold wire for WOG)could preserve well shaping ability after being repeatedly used within limited times;WOG showed better fatigue resistance and higher safety when preparing multiple roots compared with RCP and WO.And they highlight the potential of multiple uses of these instruments;however,the degree of canal curvatures must be taken into consideration,and the number of root canal preparation should be controlled in clinical use.