Dynamic fatigue performance of implant-abutment assemblies with different tightening torque values

Abstract

Implant-abutment assemblies are usually subject to long-term cyclic loading. To evaluate the dynamic fatigue performance of implant-abutment assemblies with different tightening torque values, thirty implant-abutment assemblies (Zimmer Dental, Carlsbad, CA, USA) were randomly assigned to three tightening groups (24 Ncm; 30 Ncm; 36 Ncm), each consisted of 10 implants. Five specimens from each group were unscrewed, and their reverse torque values recorded. The remaining specimens were subjected to a load between 30N～300N at a loading frequency of 15 Hz for 5×10 6 cycles. After fatigue tests, residual reverse torque values were recorded if available. In the 24 Ncm tightening group, all the implants fractured at the first outer thread of the implant after fatigue loading, with fatigue crack propagation at the fractured surface showed by SEM observation. For the 30 Ncm and 36 Ncm tightening groups, a statistical significant difference (p<0.05) between the unloaded and loaded groups was revealed. Compared with the unloaded specimens, the specimens went through fatigue loading had decreased reverse torque values. It was demonstrated that insufficient torque will lead to poor fatigue performance of dental implant-abutment assemblies and abutment screws should be tightened to the torque recommended by the manufacturer. It was also concluded that fatigue loading would lead to preload loss.

In Vitro Evaluation of Manual Torque Values Applied to Implant-Abutment Complex by Different Clinicians and Abutment Screw Loosening

“…These phenomena cause complications such as inflammation/infection of the soft tissues and fracture of the screw. To prevent this, it is crucial to apply the optimum torsional force to the implant-abutment connection, ideally using a torque-calibrated ratchet wrench [4, 6, 10]. Screws can be tightened manually if a ratchet wrench is not available.…”

“…The risk of loosening is particularly high in abutment screws that are tightened with forces lower than the recommended tightening torque. However, tightening an abutment screw with high forces causes its yield strength to be exceeded and the screw to lose its mechanical characteristics due to plastic deformation [6]. …”

“…The loosening of abutment or prosthesis screws is more likely to be encountered in single-tooth implants and in the presence of parafunction and cantilevers [6, 7]. The screw loosening rate after 5 years has been reported to be 5.8–12.7% [8, 9].…”

“…Screw loosening can be caused by inadequate tightening torque, excess mechanical loads, inconsistencies in the material and design of the abutment screw, vibration during functional loading, temperature changes in the oral cavity, and misplacement of the implant [2, 6, 10]. …”

“…Screw loosening can cause implant and screw fracture, unbalanced distribution of occlusal forces, a microgap space between the implant and abutment that can allow bacteria in by causing a micromovement, peri-implant inflammation, and loss of osseointegration due to the microgap [4, 6, 10]. …”