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

Unlabelled: Information. 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.

Purpose: 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.

Material And Methods: Twenty implants were divided into 2 groups absed on diameter: 3.7 mm(Group A) and 4.5 mm(Group B), with 10 implants in each group. Each group was further 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, while 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(SEM). The data were analyzed using repeated measures two-way analysis of variance(α=0.05).

Results: Torque loss showed a significant upward trend across all groups with increased tightening cycle(P<0.05). Implant diameter significantly influenced torque loss, with smaller diameters exhibiting greater torque loss(P<0.05). Besides, the angular deviation of the central screw relative to the implant was not affected by different diameters (P>0.05) but was affected by the closing torque and the cycles of multiple tightening and loosening procedures(P<0.05). 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 values of 16.40 N·cm for 3.7 mm implants and 12.42 N·cm for 4.5 mm implants.

Conclusion: Repeated tightening and loosening procedures increased both torque loss and angular deviation.The diameter of the implant may impact the torque loss, with larger diameter showing less torque loss. It is suggested to limit the number of tightening cycles for narrow-diameter implants to reduce the risk of potential complications.