Finite Element Analysis Of Cracked Mandibular First Molar With Zirconia Crown

Objective:Finite element analysis was used to study the stress distribution of enamel and dentin of cracked mandibular first molar with full crown restoration under vertical and oblique loads.The difference in stress distribution was also be studied when the restoration completely covered the crack or not.It is hoped that the results will provide some reference for the selection of clinical treatment options.Materials and Method:1.The dentition image was obtained by CBCT scan,and the three-dimensional model of the tooth was constructed by using reverse engineering software.The model of zirconia full crown restoration without cracked line was set as the control group.2.A cracked tooth model was constructed based on the control group.According to the location of the crack,the model was divided into A(Crack runs through the mesial and distal edges of the occlusal plane.One side ends at the mesial edge and the other end passes far distal edge,extending towards the gingival side),B(Crack extends through the mesial and distal edges of the occlusal plane and extends toward the gingival side)and C(Crack starts from the central fossa and extends far beyond the distal edge extending toward the gingival side).According to the end of the crack,it was divided into 1(The end is above the edge of the crown),2(The end is under the crown edge)two groups.The final experimental group consisted of six groups: A1,A2,B1,B2,C1 and C2.3.After the mesh was divided,a complete three-dimensional finite element model of the tooth was constructed,and three sets of working conditions wereapplied to each group of models.Condition 1: A force of 600 N was applied in the vertical direction on the central fossa to simulate the tooth stress under the condition of maximum bite force;Condition 2: The force of 200 N was applied on the occlusal surface of the model in the vertical direction on the central fossa to simulate the force of the teeth in the middle of the occlusal movement under the normal bite force;Condition 3: A force of 200 was applied to the model in the oblique direction on the buccal incline of mesial and distal buccal cusp,simulating lateral movement.Observed the size and distribution of the maximum principal stress in the remaining tooth tissue of the 7 groups of models.Results:1.Oblique 45°loading:When the end of the crack was above the crown edge,the peak stress of enamel was slightly lower than that of the control group,and the stress distribution was consistent with that of the control group,which appeared in the mesial and distal neck region.When the end of the crack was below the crown edge,the peak of enamel was significantly higher than that of the control group,concentrated at the end of the neck crack.The peak and distribution of dentin of the two forms were not significantly different from those in the control group.2.On vertical loading:The peak stress of the enamel under two forms of crack was higher than that in the control group,and the stress concentration area was also different: when the crack end was above the crown edge,the stress was mainly concentrated at the crack edge of the occlusal surface;the other was concentrated at the end of the neck crack.The peak and distribution of dentin of the two forms were not significantly different from those in the control group.Conclusion:1.When the end of the crack is completely covered by the crown,the full crown can provide better protection to the dental tissue under the oblique load,however,the crack end on the occlusal surface will be subject to greater tensile stress than other parts under vertical load,and more likely to failure.2.When the crack end is exposed outside the crown edge,the stress concentration area is always at the crack end of neck,and the stress peak is larger under the oblique load.That is,the oblique load is more likely to lead to the crack end growth than the vertical load.3.When the prosthesis do not completely cover cracks,it is most likely to fracture that the cracks will cross the mesial and distal edges at the same time,followed by one ends at the central fossa and one extends distally,and the last is that one ends at the mesial edge and one extends to the distal.4.Compared with enamel,both vertical and oblique loads have little effect on dentin near the crack.