The main retention force that holds fixed restorations in place is friction, which is the force generated between two objects that are in contact with each other and are moving relative to each other. The size of the friction force and the contact area between the two objects, close area and surface roughness. To obtain a large friction force in the process of restoration system must be: 1, restorations and preparation of the affected tooth contact surface should be very well matched; 2, restorations and preparation of the affected tooth contact surface should be appropriate roughness; 3, the axial surfaces of the body of the teeth are parallel to each other, the jaw to the polymerization of not more than 5 °; 4, the depth of the cavity and the height of the crown to be sufficient. Next is the adhesive force, which refers to the intermolecular bonding force at the interface between the adhesive and the bonded object, and the size of the adhesive force is related to the performance of the adhesive material, the bonding area, the surface condition of the bonding surface and the adhesive operation technique, in order to obtain a larger adhesive force, it is necessary to achieve the following: 1) the performance of the adhesive is good; 2) the adhesive area should be large; 3) the adhesive surface should be as close as possible and the surface should have an appropriate roughness; 4) the adhesive Thinness of the adhesive should be appropriate; 5, the bonding surface should be clean and dry.