The Effect Of Root Canal Therapy On The Cracked Maxillary Second Premolars:a Three-Dimensional Finite Element Study

Objective: Using three-dimensional(3D)finite element method based on the CBCT scanning images,the study aims to build models of cracked maxillary second premolars after root canal therapy by different methods of root canal preparation,simulate the occlusal movements in the oral cavity,analyze the stress of tooth tissue,periodontal tissue and the crack part,and provide the theoretical basis of mechanics for the etiology of cracked tooth and the selection of treatments.Methods:(1)From the CBCT images of the patients treated in the Department of Stomatology of the Affiliated Hospital of Qingdao University from July 2019 to October2019,the image data of the maxillary second premolar in accordance with the Chinese standard were screened and collected.The image data were imported into the Mimics software in DICOM format.According to the grayscale thresholds of different densities of each part of the tooth and periodontal tissue,the boundary was divided,and the triangular surface models of the tooth and alveolar bone were established,which was imported into the Geomagic Studio software in STL format for surface optimization and the periodontal membrane model was constructed.The 3D solid models of enamel,dentin,pulp,periodontal membrane,cortical bone and cancellous bone were obtained by Boolean operations.The models were imported into Hyper Mesh software in IGS format for topology optimization and mesh division,and the material parameters and attributes were set in Abqus software.The stress analysis of the tooth and periodontal tissues was carried out under the conditions of vertical and oblique loadings of 200 N respectively.(2)Three kinds of cracks with different lengths and depths were constructed in the stress concentration area from the occlusal surface of the maxillary second premolar model.The crack front was designed,and the same vertical and oblique loadings were carried out again to analyze the stress of the tooth body,periodontal tissue and the cracked part.(3)Fifteen isolated maxillary second premolars extracted for orthodontic treatments were collected.The "1-1" type single root canal teeth were screened by pulp opening and root canal exploration.The teeth were randomly divided into three groups,which were treated with conventional K file,Pro Taper and TF-A respectively.After warm vertical condensation,3M resin was used directly to repair the crown defect.After the isolated teeth scanned by CBCT,the pulp cavity and root canal models were established according to the above steps and assembled with the cracked tooth models.The material properties were set and the resin core was constructed.The same vertical and oblique loadings were carried out to analyze the stress distribution and general rules of the models.Results:(1)Fine,complete and high quality three-dimensional finite element models of the cracked maxillary second premolar after root canal therapy by different root canal preparation methods were constructed.The models of each part of the teeth,periodontal membrane,and alveolar bone conform to the standard anatomical morphology of the real oral tissues,which can be rotated and accurately measured at will.(2)The results of stress analysis on the intact tooth and periodontal tissue models showed as follows.Under vertical loading,the red stress concentration area was mainly distributed at the loading point and the tissues around the buccal and palatal position of the tooth neck,and the stress distribution was uniform in the tooth crown.Under oblique loading,the red stress concentration area was distributed at the loading point and around the palatal tissue of the tooth neck.The occlusal third of palatal side of the tooth root had a large compressive stress while the buccal side had a large tensile stress.There was a stress difference on both sides of the mesial groove,and the stress peak values of each part of the model were greater than those of the vertical loading.(3)The stress analysis results of tooth models with different degrees of cracks,periodontal tissues and cracked parts were as follows.Under vertical and oblique loadings,the stress peak values of the whole tooth model and the dentin model increased with the crack deepening.The stress concentration zone appeared at the crack tip,and the stress peak value increased with the crack deepening.The stress concentration also gradually appeared in the distal groove area.(4)The results of stress analysis on the models of cracked tooth after root canal therapy by different root canal preparation methods showed as follows.Under the two loading modes,there was a stress concentration area on the mesial junction between resin core and tooth tissue,and with the increase of crack depth,the stress concentration also gradually appeared at the distal junction.Under oblique loading,there was a stress difference between the resin and the surrounding tooth tissue.The stress peak value at the crack of Pro Taper group was the highest,K file group was the second,TFA group was the third,and the control group was the lowest.Conclusion: It is feasible and effective to establish a fine 3D finite element model of a single tooth and its surrounding tissues based on CBCT images.During occlusal movements of the maxillary second premolar,the stress is mainly concentrated in the buccal and palatal tissue of the tooth neck.The periodontal membrane and alveolar bone have a certain role in the distribution of occlusal force.The stress of maxillary second premolar in lateral occlusal movement is greater than that in vertical occlusal movement,and cracks are more likely to occur near the mesial groove.With the increase of the depth and length of the crack,the resistance to fracture of the tooth decreases.Under the action of occlusal force,the crack has a tendency to expand along the crack tip and the grooves.The mechanical nickel-titanium files with small taper can preserve the fracture resistance of the cracked second maxillary premolar after root canal treatment.