The Effect of Abutment Screw Length on Screw Loosening in Dental Implants with External Abutment Connections After Thermocycling

Purpose: The aim of this study was to investigate the effects of abutment screw length on screw loosening after thermocycling. Materials and Methods: Twenty-one grade 5 titanium (Ti) abutments were connected to grade 4 Ti implants with grade 5 Ti abutment screws. These implant/abutment screw/abutment assemblies were divided into seven groups according to the length of the abutment screw; each group included three assemblies. The screws were tightened to 30 Ncm and subjected to 2,000 thermocycles in water baths at 4°C and 60°C, with 60 seconds of immersion at each temperature. Removal torque values (RTVs) were measured before and after thermocycling, and differences in RTVs according to abutment screw length pre- and postthermocycling were analyzed statistically. Results: Mean RTVs before and after thermocycling were as follows: 23.0 ± 4.4 Ncm and 25.5 ± 0.9 Ncm for 1.4-mm-long abutment screws, 26.7 ± 2.5 and 24.5 ± 2.6 Ncm for 1.8-mm screws, 26.0 ± 1.3 and 25.0 ± 1.8 for 2.2-mm screws, 25.0 ± 2.0 and 25.3 ± 0.8 for 2.6-mm screws, 22.5 ± 4.4 and 22.5 ± 0.5 for 3.0-mm screws, 26.5 ± 2.3 and 25.2 ± 1.3 for 3.4-mm screws, and 25.5 ± 4.8 and 23.2 ± 0.8 for 3.8-mm screws, respectively. As the screw length increased, no statistically significant differences were found in the mean RTVs among the groups before or after thermocycling. Additionally, none of the groups showed significant changes in RTVs after cyclic thermal stress. Conclusion: Abutment screw length had no significant effect on screw loosening under thermal stress. From the results of this study, daily temperature changes in the oral cavity may have no negative influence on the integrity of an implant-abutment joint when a minimum of three and a half threads of an abutment screw are engaged. Keywords: abutment, fracture, implant, removal torque value, screw length, thermocycling