Mechanics of the taper integrated screwed-in (TIS) abutments used in dental implants

The tapered implant-abutment interface is becoming more popular due to the mechanical reliability of retention it provides. Consequently, understanding the mechanical properties of the tapered interface with or without a screw at the bottom has been the subject of a considerable amount of studies involving experiments and finite element (FE) analysis. This paper focuses on the tapered implant-abutment interface with a screw integrated at the bottom of the abutment. The tightening and loosening torques are the main factors in determining the reliability and the stability of the attachment. Analytical formulas are developed to predict tightening and loosening torque values by combining the equations related to the tapered interface with screw mechanics equations. This enables the identification of the effects of the parameters such as friction, geometric properties of the screw, the taper angle, and the elastic properties of the materials on the mechanics of the system. In particular, a relation between the tightening torque and the screw pretension is identified. It was shown that the loosening torque is smaller than the tightening torque for typical values of the parameters. Most of the tightening load is carried by the tapered section of the abutment, and in certain combinations of the parameters the pretension in the screw may become zero. The calculations performed to determine the loosening torque as a percentage of tightening torque resulted in the range 85-137%, depending on the values of taper angle and the friction coefficient.