Objective: The preload induced in the implant complex during abutment screw tightening is fundamental to our understanding of the biomechanical principles in load transfer. The objective of this study was to measure the magnitudes of the preload contact force using the finite element analysis method (FEA).

Methods: A (FEA) model of a Brånemark® System Mark III implant, a CeraOne® abutment, a Unigrip abutment screw and its torque wrench were created with a thread helix using the software programs HyperWorks and LS3D-Dyna. The implant model was positioned within a bone model. Using the torque wrench, a 320 Nmm torque moment was applied followed by removal of the wrench using the software ABAQUS. The contact force (N) was measured at the interfaces between the: 1) Abutment Screw and Abutment Bearing surface (ASAB), 2) Abutment and Implant Bearing Surface (AIBS), and 3) Abutment screw Thread and Implant internal Thread bore (ATIT). The contact force direction of ATIT was normal to the thread contact flank. The preload was calculated from the ATIT contact forces.

Results: The contact forces after 320 Nmm torque tightening were: (ASAB) 554.4 N, (AIBS) 554.4 N, (ATIT) 573.9 N and the preload 522.17 N. Following After torque wrench removal, the contact force were reduced: (ASAB) 549.0 N, (AIBS) 549.0 N, (ATIT) 538.2 N and the preload 489.7 N.

Conclusions: The horizontal motion of the horizontal torque moment applied by the torque wrench lead to a vertical motion by the helix thread geometry of the ATIT interface. At the point of contact between the thread surfaces, the horizontal moment force was transformed to the contact forces at ASAB, AIBS and ATIT. The torque-contact forces demonstrated a simple regression during the moment application (R2=1.000) but changed to a polynomial regression after wrench removed. [Supported by Nobel Biocare/University of Michigan, Center For Excellence].

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