Dynamic Analysis on the Effect of the Increased Vertical Height of the Crown on the Maximum Stress Imposed on the Abutment Screw and the Bones Surrounding Short Implants in the Posterior Mandibular Region: A Finite Element Analysis

Short implants are used in the posterior mandible where the bone is insufficient. In these cases, the clinical crown is often made to match the level of the occlusal. As a result, the length of the lever arm and the stress of the implant complex, as well as the surrounding bone, are increased leading to the biomechanical problems. The purpose of this study was to evaluate dynamically the effect of increasing crown height space on the maximum stress induced in the abutment screw and the surrounding bone of the short dental implants in the posterior mandible using finite element analysis. Mandibular bone resorption in the posterior region, second premolar with three-crown lengths of 8.8, 11.2, and 13.6, two implants of 4 and 8 mm, two abutments, and two abutment screws were modeled using Solidworks software in this laboratory experimental study. In addition, Abacus software was used to the dynamic reconstruction of screw tightening and external load at an angle of 75.8 degrees with the occlusal plane. The stress values were calculated for the screw, abutment, fixture, and bone. In both 4- and 8-mm implants, the increased vertical height of the crown decreased the stress on the abutment screw and increased the stress on the abutment and fixture. For a 4-mm implant, the stress to the abutment screw at all three heights was less than 8 mm. At all heights, the stress values to the abutment and the fixture were more than 8mm for the 4-mm implant. The increased vertical height of the crown resulted in an increase in compressive and tensile stress in the surrounding bone for both 4-mm and 8-mm implants. The magnitude of these stresses in the 4-mm implant was more than 8 mm.   Increased vertical height of the crown and crown-to-implant ratio reduced the stress on abutment screws as the weakest member of the implant. However, it probably increased the failure due to fatigue in the abutment and fixture as well as bone resorption.