Three-D finite element analysis of the stress distribution in periodontal tissue of abutments with or without extra-coronal retainer

Objective To compare abutment's periodontal stress distribution of two types of extra-coronal retainer(ECR) restoration(key-key-way attachment denture and telescopic crown denture) in the case of the lower first molar missed along the lower second molar with 30 or 45 degree mesial tilting.Methods Threedimensional finite element models of lower first molar missed along lower second molar with 30 or 45 degree mesial tilt with or without ECR restoration were constructed by combination of the computed tomographic(CT) scanning and Mimics,Freeform and ANSYS softwares.Axial occlusal force and oblique occlusal force of 200 N were applied to the lower second molar on the central fossae in each case.The stress distributions in the surrounding tissues of the lower second premolar and lower second molar were compared.Results In the cases of lower second molar with 30,45 degree mesial tilting,without restoration the maximal Von Mises stress values of lower second premolar were 2.80 MPa and 3.47 MPa,with key-key-way attachment those were 19.26 MPa and 25.18 MPa,and with telescopic crown 19.47 MPa and 24.48 MPa respectively.Without restoration the maximal Von Mises stress values of lower second molar were 20.45 MPa and 20.50 MPa,with key-key-way attachment those were 15.02 MPa and 11.84 MPa,with telescopic crown 18.04 MPa and 12.18 MPa respectively.With ECR the stress of periodontal tissue of lower second premolar was significantly increased,and that of lower second molar was significantly decreased.Conclusion Both key-key-way attachment and telescopic crown ECR can ameliorate the stress distribution in the periodontal tissue of the tilted abutment.It is necessary to add mesial abutment when the degree of lower second molar mesial tilting is too large or the periodontal condition of lower second premolar is not in good condition.