In Vitro Fracture Behavior Of Flared Root Canals Of Mandibular Premolars Restored With Different Post-and-Core Systems In Combination With Resin Rehabilitation

PURPOSE:To investigate the fracture strength and pattern of failure of mandibular premolar teeth with flared root canals restored with casting metal post-and-core(Co-Cr),and with resin composite in combination with three different post-and-core systems(metal cast post-and-core (Co-Cr);prefabricated glass fiber post;prefabricated metal post),to compare the fracture behaviors with those of mandibular premolar teeth with normal root canals restored with cast post-and-core(Co-Cr),and with or without ferrule.Methods:A total of 54 recently exacted human mandibular premolar teeth were endodontically treated,amputated partial crown 2mm above the amelocemental junction,and randomly divided into six groups.The teeth of two groups were remained normal canals,and the other teeth were enlarged the root canals at cervical region until the canal walls remained 1mm.Group A were teeth with normal root canals and 1.5mm ferrule restored with cast post-and-core(Co-Cr).Group B were teeth with normal root canals restored with cast post-and-core(Co-Cr) and without ferrule.Group C were teeth with flared root canals restored with cast post-and-core(Co-Cr). Group D were teeth with flared root canals restored with cast post-and-core(Co-Cr)after rehabilitating the canals by resin composite.Group E were teeth with flared root canals restored with resin composite in combination with prefabricated glass fiber post and resin core.Group F were teeth with flared root canals restored with resin composite in combination with prefabricated metal post and resin core.All teeth were also restored by casting metal crowns.Group B-F were without ferrule.Each specimen was mounted on the CMT5101 universal test machine and subjected to a load at 30 degrees angle to the long axis of the teeth until failure.The cross-head speed was 1mm/min.The maximum strength and the pattern of fracture were recorded.Repairable fracture were defined as adhesive failures,post fracture and root fracture above the 2mm deep of the entrance of the root canal.Nonrepairable fracture were defined as root fracture below the 2mm deep of the entrance of the root canal. Results were analyzed statistically with one-way ANOVA and Fisher exact probabilities.Results:Statistic results showed that Group A had the significant higher fracture strength[2.13±0.55KN]than the other groups[group B (1.31±0.56KN):group C(1.48±0.37KN);group D(1.21±0.30 KN);group E(1.39±0.56KN);group F(1.17±0.40KN)].There were no statistic difference among group B,C,D,E,F.The pattern of fracture in Group A-D were 100%nonrepairable.33.3% teeth in group E and 11.1%teeth in group F were repairable.The patterns of fracture were statistically different between Group A and group B,C,D,E,F(Fisher exact probabilitiesα=0.05,P=0.000297,P＜0.05;group A and group E:α=0.0033,P=0.00041,group A and group F:α=0.0033, P=0.00041).There were no statistic difference among group B,C,D, E,F.Conclusion:1.The results in this study suggested that different post-and-core systems had no influence on the fracture behaviors of flared root canals mandibular premolars restored with metal crowns under static loading.The rehabilitation of flared root canals by resin composite can reinforce thin root dentin wall in certain degree.2.According to the study,the fracture strength and failure patterns of the flared root canals teeth treated with prefabricated glass fiber posts is not statistically different with the group of teeth treated with prefabricated metal posts and cast post-and-cores,however they have a little more favorable fracture patterns,fracture models of the glass fiber post group remained more teeth structure,and fiber post can be removed from root canals easily and allowed the possibility of repairation. They are more preferable in restoration of flared root canal mandibular premolar teeth.3.The fracture strength of resin composite+cast post-and-core in restoring the flared root canals of mandibular premolar teeth was in the same as that of resin composite+prefabricated glass fiber post+resin core,but the fracture pattern was not as good as that of glass fiber post,and the treating method was complex,so we' dbetternot chose resin composite+cast post-and-core to restore flared root canals of mandibular premolar teeth.4.1.5mm ferrule can magnificantly increase fracture strength of teeth restored by post-and-core and crown.However,under the maximum strength,the teeth restored by cast post fractured nonrepairably.5.The more similar of the elastic modulus of post and dentin,the more favorable the fracture pattern is.But if the stiffness of post is small,the post itself is likely to fracture.