Effect Of Different Tilting Angels Of Third Molar On The Stress Distribution Of The Rigid Fixed Bridge In Deletion Of First Molar And Second Molar In Mandibular

Objective: To construct a three-dimensional(3-D) finite element model of three abutments of the rigid fixed bridge in deletion of first molar and second molar in mandibular and evaluate the influence of different tilting angles of the mandibular third molar on the stress distribution in the restoration, odontal tissue and periodontal membrane.Methods: (1) Choose a a healthy volunteer, get the images scanned from imfraorbital margin to chin margin by CT technique and reconstruct a 3-D model of three abutments of the rigid fixed bridge in deletion of first molar and second molar in mandibular by a software named Mimics. (2) Based on the 3-D model, a 3-D finite element model including teeth, restorations, odontoperiosteum and alveolar bone was reconstracted by a software named Ansys. (3) Define different parts with different Young's moduluses and Poisson's ratios, mesh the model ,and set the boundary restraints and constraints. Apply a force about 300N on the lingual incline surface of the buccal dental cusps of the restoration vertically or 45°tilted, and evaluate the influence of the mandibular third molar with different tilting angles of 0°,10°,20°,30°and 40°on the stress distribution in the restoration, odontal tissue and periodontal membrane.Results: (1) A ideal 3-D finite element model of the fixed bridge was constructed, including 4260835 elements and 739986 nodes. (2) When the force was applied vertically, the region of the maximum stress was on the surface of the connecter of the fixed partial denture,the cervix of the tooth and the periodontal membrane and the mesial apex of third molar. When the tilting angle of third molar was increasing, the maximum Von Mises stresses were increasing. When third molar tilted at 30°, the Von Mises stress peak value on the peridental membrane of mesio-dental cervix increased sharply and was 2.4 times than that of the normal third molar. (3)When the oblique load was simulated, the region of the maximum stress was on the surface of the connecter of the fixed partial denture too. The regions of the maximum stress of odontal tissue were on the middle of the roots of premolars, the disto-buccal cervix and the mesial apex of third molar. The regions of the maximum stress of periodontal membrane were on buccal cervix and the middle of disto-buccal tooth of third molar. When third molar tilted at 30°, the Von Mises stress peak value on the peridental membrane of a third of disto-buccal tooth of third molar increased sharply. (4) Under the same force application, the Von Mises stresses of odontal tissue and periodontal membrane with the oblique load was larger than that of the vertical load.Conclusion:(1)The feasible and effective finite element model could be reconstructed by combining CT technique with two softwares named Mimics and Ansys. (2) The region of the maximum stress was on the surface of the connecter of the fixed partial denture whether the force was applied vertically or 45°tilted. (3)The tooth tilted more than 30°could not be selected as an abutment in the restoration of fixed partial denture. (4)The clinicians should lower the height of the cusp to reduce the oblique load of the abutment when they used fixed bridge as a treatment.