Comparison of screw loosening between cement retained and screw retained restorations, using a dynamic non-Linear Finite Element Analysis

Abstract

AIM: To compare the abutment screw loosening between two commercially available types of abutments (cemented retained and screw retained abutments) at the location of left lower first molar, employing a finite element analysis. Methods: Implants, abutments, abutment screws, and connections were accurately measured using a profile projector. Implants and both type abutment components scanned by 3D scanner (rainbow Digital Dentistry) and digitally reconstructed which are imported into the finite element module (FEM). In a similar way, cemented retained suprastructures designed by EXOCAD software and fabricated by 3D scanner. Nonlinear dynamic analysis was used by FEM to collect the implant system’s response. We used von Mises stress as an analysis criterion to identified maximum and minimum displacement of the screw head and the internal part of abutment at their interface before loading and during vertical, oblique and concomitant vertical and horizontal loading. The penetration or gap between the head screw and its seat was considered as screw loosening. Results: The amount of preload displacement was almost the same for both types of abutments (7.7 μm on the screw head and 1.3-1.8 μm on the internal surface of abutment), in the other word the preload force (30 N/cm) caused the implants to move 7.7 μm toward the bone in the longitudinal axis. Applying force in any direction reduced the amount of downward preload displacement to the occlusal surface and decreased abutemnt-implant stability. In the cemented retained (CR) abutment under vertical, oblique and vertical-horizontal loads the amount of inner abutment displacement was always less than the screw head displacement, so the stability of the abutment-implant complex was maintained. In the screw retained (SR) abutment under vertical-horizontal load (100 & 50N respectively) the amount of displacement at the screw head and inner side of abutment was close to each other (12.8 μm & 12.3 μm respectively), and this put the screw on the verge of loosening. However, under oblique pressure of 100 N the inner abutment displacement exceeded the screw head displacement ( 14.7μm and 14μm respectively) and the screw was loosened. Under vertical loading of 100 and 150 N the stability of implant abutment was maintained. Conclusion: Screw loosening is less likely to occur under vertical load compares to oblique load. The cemented retained abutments have a higher biomechanical stability rather than screw retained abutments.