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Finite Element Analysis to Determine Implant Preload

Abstract: The nature of the forces used to clamp implant components together is still a mystery. How the forces are generated and sustained is lacking in the literature. This study examined the dynamic nature of developing the preload in an implant complex using finite element analysis (FEA). The implant complex was modeled three-dimensionally in accordance with the geometric designs for the Nobel Biocare implant systems. A thread helix design for the abutment screw and implant screw bore was modeled in order to create the exact geometric design for these units in the implant systems. Modeling the threads to the exact machining specifications permitted simulation of screw tightening using specific finite element software. Using these models, the effect of the coefficient of friction on the development of preload amount in the implant complex during and after abutment screw tightening was determined. Using the software programs HyperWorks and LS3D-Dyna, two 3-dimensional finite element models of (1) a Brånemark System 3.75 X 10 mm titaniium Mark III implant, a CeraOne titanum abutment, a Unigrip gold alloy abutment screw, and (2) a Replace Select System 4.30 X 10 mm titanium implant, a Straight Esthetic titanium abutment, and a TorqTite titanium abutment screw were created. The abutment screws were subjected to a tightening torque in increments of 1 Ncm from 0-64 Ncm using ABAQUS software. In the first experiment, the coefficient of friction was set to 0.20 between the titanium bearing surface of the abutments and the implant bearing surfaces, and 0.26 between the gold abutment screw and the titanium implant screw bore. In the second experiment, the coefficient of friction was varied; the titanium implant and titanium abutment bearing surfaces were set to a coefficient of friction of 0.20 while the Mark III gold and the Replace Select titanium abutment screws and their respective titanium screw bores in the implants were set to 0.12. The preload amount in Newton’s was determined from the finite element analysis. The stress distribution pattern clearly demonstrated a transfer of preload force from the screw to the implant during tightening. A preload of 75% of the yield strength of the abutment screw was not established using the recommended tightening torques.
Conclusions: Using FEA a torque of 32 Ncm applied to 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. In order to reach the desired preload of 75% of the yield strength, using a torque of 32 Ncm applied to the abutment screws in the implant assemblies studied, the coefficient of friction between the implant components should be 0.12.

Abstract: A key factor in osseointegration of titanium implants has been attributed to the presence of surface contamination in the “as delivered state.” The purpose of this study was to examine the surface of implants from six major implant systems and to assess the presence, type and amount of surface contaminants following manufacturing. Five implant obtained from different manufacturing lots for the following systems formed the experimental population: 1) 3I, 2) Impla-Med, 3) Brånemark, 4) Steri-Oss, 5) Lifecore – Restore, and 6) Swede-Vent. Four sites 5 x 5 microns in area were surveyed from the flat bottom on each implant using Auger electron spectroscopy (AES). The elements present in the Auger spectra for each implant system were identified and the surface area of the contaminants measured. Contaminating elements included carbon, calcium, sulfur, phosphorus, silicon, boron, chloride, potassium, fluorine, silver, cobalt, aluminum, antimony, vanadium, nickel and iron. The Brånemark and Steri-Oss implants exhibited the highest average surface concentrations of titanium (31.7 and 32% respectively), and oxygen (31.2 and 34.9% respectively). These implant systems also demonstrated the lowest amounts of total contaminants (16.7 and 16.2%). The Lifecore – Restore implants had the highest amount of total contaminants (29.6%) and the lowest average concentration of titanium (16.8% and oxygen (22.7%). Swede-Vent implants had high amounts of surface fluorine (7.1%), while the Steri-Oss implants had high amounts of silicon (5.6%).
Conclusions: Significant differences were found between the implant systems evaluated for most elements (P>0.05) with repeated measures ANOVA and Scheffé’s S-test.