| Objectives:This study is to compare stress conditions during ratory instrumentation in a curved root for five Ni Ti file with different design based on the construction of three-dimensional finite models,and to analyses the potential relationship between design of NiTi rotary instruments and vertical root fractures and is a preliminary exploration of individual design on root canal preparation.Methods:1 Micro-CT was used to scan the intact maxillary first molar and five kinds of NiTi rotaty files(ProTaper Universal F2,Mtwo #25/0.06,TF #25/0.06,Hyflex CM#25/0.06,ProTaper Next X2).The scan results was imported to Mimics 17.0software for the construction of 3D models.With the method of forward engineering and reverse enginereering,the 3D finite element(FE)models was created using Geomagic Studio 2012,Unigraphics NX 8.0,CATIA P3 V5R21 and HyperMeshs14.0 softwares.2 The stress conditions resluting from the simulated shaping action with240 rpm were evaluated at the apical third and the curved part of the MB root canal of the maxillary first molar using Abaqus 2016 software.Results:1 The fine 3D FE models of the MB root and canal of the maxillary first molar,ProTaper Universal F2,Mtwo #25/0.06,TF #25/0.06,Hyflex CM #25/0.06 and ProTaper Next X2 was constructed successfully.After 3D meshing,the final FE model of MB root consists of 14303 nodes and 69231 tetrehedral elements,ProTaper Universal F2 consists of 3666 nodes and 13584 tetrehedral elements,Mtwo #25/0.06 consists of 3410 nodes and 12457 tetrehedral elements,TF #25/0.06 consists of 2922 nodes and 10206 tetrehedral elements,Hyflex CM #25/0.06 consists of 4258 nodes and 16051 tetrehedral elements and ProTaper Next X2 consists of 2685 nodes and 9254 tetrehedral elements.These models can provide a basis of further biomechanic research.2 The FE analysis simulated the stress condition along the root canal wall during the NiTi files rotating.Observation area was at the apical portion and the curved part.In the five groups,the highest Von Mises stress captured was at the outer aspect of curvature of the canal.Near the canal constriction,ProTaper Universal F2 induced the highest Von Mises stress concentration(189.4190MPa)followed by ProTaper Next X2(156.7400MPa),Mtwo #25/0.06(30.4984MPa)and Hyflex CM #25/0.06(30.2221MPa),and TF #25/0.06 induced the smallest Von Mises stress concentration(23.7153MPa).At the curved part,ProTaper Universal F2 induced the highest Von Mises stress concentration(431.0100MPa)followed by ProTaper Next X2(112.9760 MPa),Mtwo #25/0.06(95.2230MPa)and Hyflex CM#25/0.06(44.5673MPa),and TF #25/0.06 induced the smallest Von Mises stress consentration(26.1847MPa).At the apical portion,the highest Von Mises stress induced by ProTaper Universal F2 was beyond dentine's ultimate tensile strength,and at the curved part,the highest Von Mises stress was beyond dentine's ultimate tensile and compression stress.The highest Von Mises stress induced by ProTaper Next X2 was beyond dentine's ultimate tensile strength and in the range of dentine's ultimate compression stress.In other conditons,the highest Von Mises stresses were all in the range of dentine's ultimate tensile and compression stress.Conclusions:1 The fine 3D FE models of the MB root and canal of the maxillary first molar,ProTaper Universal F2,Mtwo #25/0.06,TF #25/0.06,Hyflex CM #25/0.06 and ProTaper Next X2 can be constructed successfully using Micro-CT,forward and reverse enginereering technic and CAD softewares.2 The design of NiTi rotary files would affect stress distribution in canal during root shaping.Taper and the size of cross-section of Ni Ti rotary files are the main factors.The number of contact points and screw pitch may also affect stress distribution on canal wall.Increasing taper,increasing cross-section size,reducing contact points or widening screw pitch may induce dentine defects which could later propagate into VRFs if the tooth is under repeated stresses from endodontic,restorative procedures or after long-term functional stresses like chewing. |
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