Study on Effects of an Extracoronal Direct Retainer's Arm to the Periodontal Ligament of an Abutment Tooth Utilizing Finite Element Method. Part 1. The Introduction of Three-Dimensional Finite Element Analysis.

Direct retainer's arm is placed on an abutment tooth when an edentulous space is restored by prosthetic treatment with a removable partial denture. Various attempts have been made to investigate the effects of the retainer of a partial denture on an abutment tooth during functional loading. However, no analysis seems to have been reported about the effects of retainers on the abutment teeth when the denture base rotates toward and away from an edentulous ridge. Attention was paid to the lower second premolar tooth, and the three-dimensional finite element method was used.The results were as follows:1. When only a retentive force was applied, the von Mises stress in the periodontal ligement increased.2. When both retentive and reciprocal forces were properly applied, the von Mises stress in the periodontal ligament was less than that with the retentive force only.3. When both retentive and reciprocal forces were properly set, and a reciprocal force was one-third of a retentive one, the value of the von Mises stress was within 2.4 MPa. Thus, it was suggested that the reciprocation effectively released the stress in the periodontal ligament of an abutment tooth.4. Effective factors for stress relaxation of the periodontal ligament were suggested to exist in the design of a retainer's arm.