In vitro study to analyze reverse torque values of attachment screws with multiple loosening/tightening preload cycles.

Purpose: To determine the number of torque/reverse torque cycles attachment screws can withstand before fracture, and to record the magnitude of reverse torque values for each of the 10 cycles of screw tightening and loosening.

Materials And Methods: The testing assemblies consisted of 30 temporary cylinder/multiunit abutment/internal connection implants (10 each from Dentsply Sirona Astra, Biohorizons, and Zimmer Biomet Dental). The assemblies were inserted into resin blocks, and temporary cylinders were connected to the multiunit abutments with new attachment screws and torqued to the recommended preload (15 Ncm Dentsply Sirona Astra; 15 Ncm Biohorizons; 10 Ncm Zimmer Biomet Dental) using a digital torque gauge instrument (Model 3, MARK-10 Corporation, New York, USA) at room temperature without lubrication. After 24 h, the attachment screws were reverse torqued with the same instrument and the values were recorded. The torque/reverse torque cycles were repeated every 24 h for 10 days. Data were analyzed using mixed model analysis. A second test included torquing the used screws (30) until fracture or stripping occurred. The reverse torque values were recorded and compared with new screws that were torqued until fracture or stripping. A sample of used and new attachment screws from each assembly were viewed under both laser microscope (LEXT OLS4000, 3D Measuring Laser Microscope, Olympus) and scanning electron microscope (JEOL-JSM6510, Tokyo, Japan). Data were analyzed using t-test.

Results: Following 10 cycles of torquing and reverse torquing, the reverse torque values measured on day 10 were lesser compared with the reverse torque value measured on day 1 for 29/30 attachment screws (96.6%). According to the t-test used for the second test, the p-values among all three groups were ≥0.9 which were not statistically significant.

Conclusion: The results of this study indicated that after 10 cycles of torquing and reverse torquing attachment screws, the reverse torque values decreased. There were no statistical differences between the magnitude of torque necessary to fracture new and used attachment screws.