Statement of problem: Implant abutment screw joints tend to loosen under clinical conditions. During impression and prosthesis fabrication, repeated clinical closing and opening of abutment screws may cause component wear and decrease frictional fit of the mating parts, resulting in altered resistance to opening and potential for loss of preload in function.
Purpose: This study recorded changes in opening torque values due to multiple consecutive closures at a constant torque within and between different abutment/implant (A/I) systems.
Material and methods: Repeated opening-closing cycles were used to simulate in vitro embedment relaxation and component wear of 7 A/I systems from 5 manufacturers. Screw opening torque values were recorded up to 200 consecutive closures at 20 N/cm.
Results: A progressive decrease in opening torque values was measured in all implant systems. Significant differences were found between A/I systems. Systems with morse tapered and spline connections consistently maintained a higher resistance to opening force. Percentage torque loss ranged from 3% to 20% on immediate opening, and from 4.5% to 36% for average of first 30 opening/closing cycles.
Conclusion: Repeated opening and closing of implant abutment screws caused progressive loss of torque retention with variations between systems. This was probably due to a decrease in the coefficient of friction between the mating components.