Stress Analysis of the Dental Surrounding Tissues of Different Degree Tilted Abutments and the Effects of Different Buccolingual Diameter of Pontic to the Abutment Periodontium: Three-dimensional Finite Element Analyses 不同角度的倾斜基牙以及桥体的不同程度颊舌减径对固定修复后基牙牙周受力影响的三维有限元研究