The Effect of Repeated Closing Torque on Torque Loss and Angular Deviation: An In Vitro Study

Loosening and fracturing of the central screw are common mechanical complications after implant restoration. However, the relationship between these complications and the repetitive tightening and loosening of the central screw during the fabrication and maintenance of the implant-supported restorations remains unknown. The purpose of this study was to evaluate the torque loss after repetitive tightening and loosening of the central screws on implants with different diameters as well as the changes in the angle deviation of the central screw relative to the implant. Twenty implants were divided into 2 groups based on diameter: 3.7 mm (group A) and 4.5 mm (group B) with 10 implants in each group. Each group was subdivided into 4 subgroups: A1, A2, B1, and B2 (n = 5). A closing torque of 15 N.cm was applied to groups A1 and B1, whereas a closing torque of 35 N.cm was applied to groups A2 and B2. Reverse torque measurements were taken 10 times for each group. The angular deviation of the central screw relative to the implant was recorded, and the surface wear of the central screw was observed under a scanning electron microscope. The data were analyzed using repeated measures 2-way analysis of variance (α = 0.05). Torque loss showed a significant upward trend across all groups with increased tightening cycle (P < .05). Implant diameter significantly influenced torque loss with smaller diameters exhibiting greater torque loss (P < .05). In addition, the angular deviation of the central screw relative to the implant was not affected by different diameters (P > .05). Still, it was affected by the closing torque and the cycles of multiple tightening and loosening procedures (P < .05). Under a 35 N.cm closing torque, initial torque loss ranged from 9.12 N.cm to 10.98 N.cm. Peak torque loss occurred at the 10th cycle with 16.40 N.cm values for 3.7-mm implants and 12.42 N.cm for 4.5-mm implants. Repeated tightening and loosening procedures increased both torque loss and angular deviation. The diameter of the implant may impact the torque loss with a larger diameter showing less torque loss. To reduce the risk of potential complications, it is suggested that the number of tightening cycles for narrow-diameter implants be limited.