Replace® Select Screws

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Finite Element Analysis to Determine Implant Preload
A FEM Evaluation of the Magnitude and Distribution of Preload When Tightening the Abutment/Implant Joint

Abstract: Any attempt to understand the biomechanics and structural properties associated with the dynamic nature of loading must first begin with understanding the load applications in the assembly of the implant complex prior to the implant functioning in the presence of any external forces. The implant complex is an assembly of multiple components that forms a mechanical screw joint. The forces applied in the assembly process are essential in the maintenance of the functional capacity of the implant system prior to supporting a dental prosthesis. Although the assembly process is well understood, the nature of the forces used to clamp the implant components together and how they are generated and sustained is not. Therefore, a study was initiated to examine the dynamic nature of loading in the development of the implant complex prior to supporting any external load related to the dental prosthesis. Two implant designs were selected for finite element modeling to represent both the external and internal designs in the screw joint assembly. The geometry for the designs were obtained from mechanical drawings provided by Nobel Biocare, AB. Using the software program HyperWorks®, investigators modeled a Brånemark® System 3.75 X 10 mm Mark III implant (SDCA 065), a CeraOne® abutment, (SDCA 068), a Unigrip abutment screw (DCA 1045-0) for the external screw joint design. A Replace Select™ System 4.30 X 10 mm tapered implant (61084), a Straight Esthetic abutment (61117), and a TorqTite abutment screw (62190) were modeled for the internal screw joint design. Most previous publication involving FEA have modeled screw threads and the screw bore using concentric rings because of the difficulty in modeling the thread helix. However, because the abutment screw would be tightened in order to induce the preload during the finite element analysis (FEA) by simulating screwing the abutment screw into the screw bore and clamping the abutment onto the implant, the exact geometry of the thread helix was necessary. Concentric ring simply cannot be screwed into a screw bore. Other essentials of the FEA are the properties of the materials used in the implant complex. These include the density, Young’s modulus, and Poisson’s ratio, for the implant, abutment, abutment screw, and bone. A torque wrench head was modeled to fit into the head of the abutment screws. The abutment screw was subjected to a tightening torque moment in increments of 1 Ncm from Zero (0) to 64 Ncm using the modeled wrench and ABAQUS® and LS3D-Dyna® software for both implant complex assemblies. The FEA was conducted in two experiments. For the first experiment (1), the coefficient of friction was set to 0.20 between all titanium components of the implant models, and 0.26 between gold and titanium as referenced in the literature for these materials when used in implant systems. In the second experiment, the coefficient of friction was varied. In both models, the titanium implant and abutment bearing surfaces was set to 0.20 while all other contacting surfaces involving the screws and the components was set to 0.10 During experiment 1, the preload force at the abutment/implant interface was 381.7 N for the Mark III complex at a torque of 32 Ncm (the recommended tightening torque for the Unigrip screw). For the Replace Select tapered implant the preload at this interface was 492.6 N at a tightening torque of 32 Ncm and 532.7 N at a tightening torque of 35 Ncm (the recommended tightening torque for the TorqTite screw). During experiment 2, the preload force at the abutment/implant interface was 677.6 N for the Mark III complex at a torque of 32 Ncm. For the Replace Select tapered implant the preload force was 722.9 N at a tightening torque of 32 Ncm and 805.8 N at a tightening torque of 35 Ncm.
Conclusions: Modeling the implant complex to determine the relationship between preload and torque delivery is only possible by modeling the exact geometry of the implant components, including the thread helix. Duriing this study, it was determine that a torque of 32 Ncm applied to the Mark III/ CeraOne assembly using the Unigrip screw resulted in a preload that was short of the optimum preload recommended. It was determine that a torque of 35 Ncm applied to the Replace Select Tapered assembly using the TorqTite screw resulted in a preload that was less than the optimum preload recommended but was 30% higher that the preload achieved for the Mark III implant assembly. If the coefficient of friction value was set to 0.12 for the interface between the abutment screw thread and the implant screw bore, the preload amount difference produced by the same torque applied to both models became small.

Abstract: There is little information in the dental literature on the amount and distribution of the preload when the abutment screw is tightening during the assembly of the abutment onto an implant. This study examined the development of the preload stress in the implant complex during and after abutment screw tightening. Using the software program HyperWorks®, two three-dimensional finite element models of: 1) a Brånemark® System 3.75 X 10 mm Mark III implant, a CeraOne® abutment, and a Unigrip abutment screw, and 2) a Replace Select™ System 4.25 X 10 mm implant, a Straight Esthetic abutment, and a TorqTite abutment screw. The abutment screws were subjected to a tightening torque in increments of 1 Ncm from 0-32 Ncm using ABAQUS® software. The coefficient of friction was set at 0.26 between all components in the implant models. The von Mises values in each element of interest were measured and converted to the preload in Newtons. Preload measurements were determined for the 1) base of the screw head, 2) screw head and shank junction 3) the screw shank, and 4) screw threads. The preload distribution pattern clearly demonstrated a transfer of stress from the screw to the implant and bone during tightening. A preload of 75% of the yield strength of the abutment screws suggested as optimum was not established.
Conclusions: Using FEA a torque of 32 Ncm applied in tightening the abutment screws in the implant assemblies studied in the presence of a coefficient of friction of 0.26 resulted in a lower than optimum preload for the abutment screws.