The Effect of Contamination of Implant Screws on Reverse Torque

Purpose: To determine if contamination of abutment screws with titanium nanoparticles affects the preload by measuring the reverse torque after multiple cycles of screw closing-opening. Materials and Methods: The study included 30 internal hex implants, titanium abutments, and titanium abutment screws. Fifteen abutment screws were contaminated with 60- to 80-nm titanium nanoparticles (contamination group), and the remaining 15 screws did not receive titanium nanoparticle contamination (noncontamination group). Each abutment screw was initially tightened to 25 Ncm with a digital torque gauge to stabilize the abutment to the implant. The second torque, 25 Ncm, was applied 10 minutes after the initial torque. After an additional 5 minutes, the screw was loosened to measure the reverse torque. Ten cycles of screw insertion and removal were conducted, and reverse torque values were measured in each cycle. Repeated measures analysis of variance and the Student t test were used for statistical analysis. The level of significance was set at α = .05. After 10 cycles of closure-opening, abutment screw threads were observed under ×40 magnification. Results: Abutment screws at every cycle generally showed preload values less than the initial applied torque. The mean reverse torque values in both groups had a tendency to decline as the test cycle progressed, except at the 6th, 9th, and 10th cycle in the contamination group. The noncontamination group always had higher mean reverse torques than the contamination group at the same test cycle with significant differences at the 1st, 2nd, 4th, 5th, 7th, and 8th cycles. The preload loss in percentages were 7.83% and 12.57% after the 1st cycle, 14.48% and 19.77% after 5 cycles, and 18.83% and 19.83% after 10 cycles in the noncontamination and contamination group, respectively. Abutment screws in both groups showed various degrees of wear and metal debris on screw surfaces. Conclusion: Contamination of abutment screws with titanium nanoparticles decreased screw reverse torque values because of a settling effect, though this effect seemed minimal after five cycles. A future clinical study is indicated to validate if cleaning of contaminated screws before the delivery of the prosthesis will increase preloads. Parole chiave: preload, screw loosening, settling effect