Mechanical Comparison of Experimental Conical-Head Abutment Screws with Conventional Flat-Head Abutment Screws for External-Hex and Internal Tri-Channel Implant Connections: An In Vitro Evaluation of Loosening Torque

Purpose: This study compared the loosening torque of experimental conical-head abutment screws to that of conventional flat-head screws of implants with external-hex (EH) and internal tri-channel (IT) connections before and after mechanical loading. Materials and Methods: Forty-four implant/screw/abutment assemblies were divided into four groups: EH/flat-head screw (EH), IT/flat-head screw (IT), EH/conical-head screw (EHCS), and IT/conical-head screw (ITCS). Three assemblies from each group were analyzed in a stereoscopic magnifier and then returned to their respective groups. One assembly was removed from each group and transversely sectioned for optical microscopy analysis. Abutments were tightened at 32 Ncm of torque; after 10 minutes, loosening torque was measured. The same abutments were then retightened with 32 Ncm of torque; after 10 minutes, they were mechanically loaded for 300,000 cycles and loosening torque was again measured. Data were collected and an exploratory analysis was performed. Comparisons were made by orthogonal contrasts using a linear mixed effects model (random and fixed effects). Results: Comparisons before mechanical cycling showed significant differences between groups except for EH vs IT and EHCS vs ITCS. After mechanical cycling, only the comparisons EH vs IT and EHCS vs ITCS showed no significant differences. Considering the same group, before and after mechanical cycling, all comparisons showed significant differences. Conclusions: The shape of the abutment screw head significantly influenced loosening torque: conical-head screws showed higher loosening torque values than conventional flat-head screws before and after loading. The implant/abutment connection design exerted no significant influence on loosening torque.