Preload and Friction of Carbon-Coated Screws in Implant-Abutment Connections

Objectives: The preload in an implant-abutment-screw arises when the tightening-torque leads to elongation of the screw. Preload depends on friction and the applied tightening-torque. For steel screws, it is presumed that the major part of tightening-torque is used to overcome friction while less than 10% is transferred to preload. For this reason and because titanium alloys have greater friction than steel, some manufacturers use carbon-coated screws to reduce friction and thereby achieve higher preload at the same torque. To calculate preload for a specific tightening-torque the coefficient of friction (COF) is needed.
The aim of this in vitro study was to measure preload and to calculate COF for an implant-abutment complex with a coated screw.
Methods: A custom load assembly was used to measure the preload after applying tightening-torque. COF was then calculated using these measurements.
Preload was measured for 25 units of unused titanium-implants, abutments and carbon-coated screws (Replace-Select-Tapered implants, Temporary Abutments, Abutment Screw Nobel Replace, Nobel Biocare AG, Zürich, Switzerland). A tightening-torque of 25 Ncm was applied three times to each screw.
Results: For the coated screws, there is an apparent and significant decrease of preload and increase of COF after each repetition. The table below depicts the results for preload and COF.
Conclusions: Even with coated implant-abutment-screws, the COF is high (COF steel: 0.08-0.16). Repeated tightening lead to decrease of preload and increase of COF. These effects can be attributed to increasing friction by cumulative micro-damages in the surfaces. The results suggest that it can be advantageous to use a new screw for the final insertion of the denture.