Research On Crack Propagation Of Cracked Mandibular First Molar After Full Crown Restoration Based On XFEM

Preface:Cracked tooth is the third leading cause of tooth loss after caries and periodontal diseases.When cracks are hidden in the enamel and superficial dentin,patients usually have no obvious abnormal sensation,but when they develop further,bacterial invasion along the crack as a channel may lead to pulpal symptoms,cracked tooth syndrome（CTS）and tooth loss.For some patients who need crown restoration due to CTS,there is still no clear answer to the question of whether full crown restoration can play a protective role and contain the crack expansion for different types of CTS,and the theoretical basis of such treatment options still needs further research.In this paper,we will conduct an extended finite element study of the mandibular first molar,which has the highest incidence of CTS,to evaluate the possibility of further expansion of CTS after full-crown restoration.Objective:To apply the extended finite element method under different loading conditions to analyze the expansion of the cracked mandibular first molar after zirconia full-crown restoration.To reveal the expansion of crack in the dentin when loaded,and to provide some theoretical basis for the prognosis of cracked teeth restored by zirconia full crowns.Methods:The original Dicom 3 D data of the volunteer mandibular first molar was obtained by CBCT,and after reverse modeling with Mimics software,models were refined and smoothed with Geomagic Studio software.Finally,the structure of alveolar bone,periodontal ligament,zirconia full crown,resin bonding cement,dentin preparation and cracks was modeled in Solidworks to form the final 3D model.The Abaqus finite element solver was applied for simulation.In this experiment,the mandibular first molar model after full crown restoration was used as the control group;According to the different position of the mesial end point of the crack,the experimental group models were divided into three groups: A,B,and C with a crack depth of 1mm:In group A,the crack originated from the central fossa of the occlusal region and ended in the mesial proximal surface;In group B,the crack originated from the distal marginal ridge and ended in the mesial proximal surface;In group C,the crack originated from the distal proximal surface and ended in the mesial proximal surface.The groups A,B and C were further divided into groups A1,B1,C1 and A2,B2,C2 according to the relationship between the proximal end point of the crack and the cervical margin of the crown.Among them:The end points of groups A1,B1,and C1 were located 0.5 mm coronal to the cervical margin of the crown,and the cracks were completely covered by the crown;The end points of groups A2,B2,and C2 were located in the root side of the cervical margin of the crown 0.5mm,and the cracks were not completely covered by the crown.The above model was fixedly restrained at the bottom of the alveolar bone and its crown occlusal points were loaded vertically to simulate the normal occlusal force of200 N,the maximum occlusal force of 600 N,and the short-time cyclic occlusal force of 200 N.The maximum principal stress distribution,the maximum peak principal stress,the initiating crack load,the area of the fracture unit,and the crack expansion of each group were analyzed.Results:（1）The maximum principal stresses to dentin in all groups were mainly concentrated at the distal root bifurcation,and the peak stresses increased with the increase of load.The peak stress under short-time cyclic loading was smaller than the peak stress under equal static loading.The longer the crack,the larger the stress peak,but when the end of the crack is outside the crown,the stress peak is smaller than that of the inner crown group.（2）The crack initiation load of each experimental group containing cracks was lower than the value of the control group.The cracks were basically in the form of "Z" or "一" shape.With the extension of the distal end of the crack,the crack initiation load gradually decreased and the area of the rupture unit gradually increased.The crack initiation load at the end of the crack outside the crown was smaller than that of the inner crown group.Conclusion:For cracked teeth,zirconia full-crown restorations provide better protection.However,the fracture resistance gradually decreases as the crack grows and the fracture initiation load gradually decreases.The fracture resistance was higher when the crack was completely wrapped by the zirconia full-crown restoration;when the crack extended to the root side of the crown margin and was not completely covered,the fullcrown protection was diminished.Regardless of whether the end of the crack is located inside or outside the crown,the dentin danger zone is the distal root bifurcation,where the crack emerges after the applied load reaches the crack initiation load,and with the increase of the load,the crack expands in a "Z" or "一" shape to the mesial and distal direction.