Effect Of Different Root Canal Filling Techniques On The Biomechanics Of Roots With Oval Canals

Objective: This study evaluated the effects of different filling techniques on fracture resistance and stress distribution of roots with oval and circular canals by in vitro fracture resistance experiment and three-dimensional finite element analysis method.The biomechanics of roots with oval canals were studied in order to choose the proper root canal filling techniques and improve the success rate of root canal therapy.Methods: Experiment One : Eighty fresh extracted single-canal mandibular second premolars with oval-or circular-shaped canal were randomly divided into 3 experimental groups(N=10)and 2 control groups(N=5),respectively.After root canal preparation and sterilization,the experimental groups were filled with different techniques: cold lateral(CL)condensation,warm vertical compaction(WVC),or single-cone obturation(SC).No instrumentation or obturation was performed in the negative control group and the samples of the positive control group were prepared without filling.Fracture resistance was tested through a universal testing machine.Carried through SPSS software,results were analyzed by one-way ANOVA followed by the SNK(Student-Newman-Keuls)test.The significance level was 0.05.The cracks were observed by a microscope.Experiment Two: A standard tooth was scanned by CT and the finite element models were established by reverse engineering.The shapes of root canal were modeled as circular and oval.The root was obturated as in vitro experiment.A 100-N load was applied vertically and von Mises stress was evaluated.Stress distribution of roots during compaction filling was also researched.A 30 N lateral load was applied on the whole canal wall uniformly to represent the lateral pressure during CL.To simulate the pressure of a plugger during WVC,the canal was filled by gutta-percha and MTA within 5 mm to the apex,and a vertical force of 30 N was loaded onto the surface of gutta-percha.Results: Experiment One: In vitro experiment: the fracture resistance of positive control group was the lowest,while that of negative control group was the highest.In the round root canal group,there was no significant difference between the negative control group and WVC and SC(P > 0.05),while CL had the least fracture resistance(P < 0.05).For the oval root canal group,SC had the greatest fracture resistance,followed by WVC and CL(P < 0.05).Within the same filling groups,the fracture resistance of round root canal group was generally higher than that of oval root canal group(P < 0.05).More cracks were observed in the buccal-lingual than in the mesial-distal direction.Experiment Two: Considering root stress distribution,the SC group had smaller highstress areas than the WVC or CL groups did.During the process of filling,roots with a circular canal showed relatively more uniform stress distribution than those with an oval canal,whereas the maximum stress of circular canal-root was greater.Conclusion: 1.Root canal preparation could reduce the fracture resistance,whlie the fracture resistance were improved after root canal filling.2.In terms of fracture resistance and stress distribution,the SC technique is more beneficial to the biomechanics of oval canal roots.3.Compared with the circular root canal,the oval root canal is more prone to root fracture.4.The buccal-lingual cracks are more common than mesial-distal cracks.