| Root canal therapy (RCT) is currently the most effective treatment for pulp and periapical diseases. Root canal preparation, which includes cleaning and shaping of the root canal systems, is one of the key steps of RCT procedure. The aim of root canal instrumentation is to forming an ideal canal shape (continuous tapering), which is essential for complete root canal irrigation and obturation.The conventional preparation of curved root canals with stainless steel (SS) instruments may produce a series of procedural errors such as: root canal straightening, zipping, elbow and step formation, root canal transportation etc, which may reduce the success rate of the RCT. This can be due to the restoring force developed by the file in the curved canals. Scholars found that, in addition to the root canal geometry, mechanical properties of the files, the angle and radius of the root canal curvatures are important factors that contribute to the separation of the root canal instruments, whereas the detailed mechanism is still unclear.The purpose of this study was to analyze the stress distribution of the root canal wall and endodontic instruments during preparation of the curved root canals by photoelastic and finite element methods. A full understanding the mechanism of development of procedural errors and instrument separation is essential for successful RCT on curved root canals.This investigation included 2 parts:Part 1 Photoelastic stress analysis of root canal preparation of curved canalsExperiment 1 : Preparation of epoxy resin models of curved root canalsSilicone rubber impression of a rectangular block (10mm×10mm×30mm ) was made to copy the shape of a resin simulated root canal (Densply, Swiss). A #20/.02 spreader (Densply, Swiss). was bent into an arc with an central angle of 60 degrees and a radius of 5 mm, then the spreader was fixed in the silicone rubber impression. Maleic anhydride, epoxy resin, dibutyl phthalate, and dimethyl were mixed together with a proportion of 30:100:5:0.05, and poured into the impression. After solidification the spreader was pulled out from the resin block, and a total of 20 epoxy resin models were prepared by above method.Experiment 2: Root canal preparation and photoelastic analysis A total of 20 epoxy resin models of curved canals were divided randomly and equally into 2 groups. One group was prepared with SS K file (SS group) and the other with nickel-titanium K file group (NiTi group), both groups using step-back technique (the sequence of file size was:15 #â†'20 #â†'15 #â†'25 #â†'20 #â†'30 #â†'25 #â†'35 #â†'30 #â†'40 #). Each file was retained in the root canal as preparation was completed, and the model was placed in 409-â…¡polarized light instrument to take photoelastic moire photos by a EOS 350D digital camera (Canon, Japan). The digital photographs were then input into software Image Pro plus 6.0 (Media Cybernetics, USA) to measure the area of stress (AS).The results showed that, for 15 # and 20 # files, obvious stress areas were not observed in both groups; after 25# file, however, the AS values of the SS group were significantly (P < 0.05) greater than the NiTi group for the file of same size. A 25# SS file produced similar amount of AS as that of a 35 # NiTi file (P> 0.05), and a 30# SS file had a equal AS value to a 40# NiTi file (P> 0.05). In SS group, AS of the outer wall of canal curvature was significantly larger than that of the inner wall (P < 0.05); whereas in NiTi group, the difference of AS values between the outer and inner canal walls had no statistical significance (P > 0.05).Part 2 Finite element analysis of endodontic instruments during preparation of the curved root canalsExperiment 1: Establishment of finite element models of 3 types of root canal files and 4 types of curved root canals.By using CAD/CAM software Pro/Engineer 5.0 (PTC, USA), three-dimensional finite element models of a 25# K file, a 25 # H file and a Protaper F1 file were created according to their manufacture geometrical parameters. Four mathematical models of curved root canals, which respectively had a radius of 2 mm and 5 mm, and a central angle of 30°and 45°, were also established. Experiment 2 Analysis of the stress distribution of the instruments under the bending and torsion loadsAfter defining the material properties for 3 models of endodontic instruments, and applying the loads and the constraints, Pro / MECHANICA FEA analysis software was used to analyze the stress distribution. The results showed that, the stress distribution pattern under clockwise torque was similar to that under the counter-clockwise torsion. In the curved canal of 45°/2mm, stress concentration of a K file presented at the cutting edge. In root canals of 30°/ 5mm and 45°/ 2mm, the maximum stress of the H file was situated at the bottom of the ?ute, that was closest to the center point of the cross section; and Stress position and reverse the direction had no significant relationship; in F1 Group: In root canals of 30°/2mm,30°/5mm and 45°/2mm,the maximum stress position are at the cutting edge, in addition to (45°/ 5mm) group acting on the cutting edge of the bottom of the ?ute. In conclusion,PSA and FEA could efficiently analyze and predict the locations of the procedural errors and instrument separation,could accurately and vividly analyze the interaction between the file and the root canal wall.The results were valuable for guiding proper use of endodontic instruments during preparing curved root canals. |
PDF Full Text Request