Assessment of preload, remaining torque, and removal torque in abutment screws under different frictional conditions: A finite element analysis.

STATEMENT OF PROBLEM: Contamination with salivary fluids or blood during the treatment process changes the preload, remaining torque, and removal torque of retained screws, which ultimately affects the resistance to screw loosening. In previous studies, no consensus has been reached as to whether contamination can be used as a lubricant.

PURPOSE: The purpose of this 3-dimensional (3D) finite element analysis study was to compare the preload, remaining torque, and removal torque under different frictional conditions in the processes of tightening, waiting period, and removal of abutment screws using a numeric method and finite element analysis.

MATERIAL AND METHODS: Three-dimensional finite element models of a single implant restoration including a crown, dental implant, abutment, and abutment screw, along with the surrounding bone, were constructed. The geometry of all threaded interfaces was designed as a threaded helix. The Abaqus software was used to perform the dynamic simulation of 3 steps such as tightening, waiting period, and removal. Three static and kinetic friction conditions were considered to determine the effect of different frictional conditions. The values of preload at the tightening step, remaining torque at the waiting period, and removal torque at the removal step were evaluated and compared with theoretically predicted values.

RESULTS: The amount of removal torque required to loosen the abutment screw was smaller than the insertion torque for all frictional conditions. By decreasing the coefficient of friction, the remaining torque and the preload increased, and the torsional relaxation and removal torque decreased.

CONCLUSIONS: Although the value of the removal torque decreased by decreasing the coefficient of friction, the resistance to screw loosening increased with the increase of the preload and the increase of the remaining torque in the implant complex assembly. Considering the biological complications of fluid lubricants, clinicians may contaminate the abutment screw with lubricants. Gold-coated screws should also be preferred to noncoated screws.