Numerical simulation of bone screw induced pretension: The cases of under-tapping and conical profile

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CHATZISTERGOS, Panagiotis ; Magnissalis, Evangelos A. ; Kourkoulis, Stavros K. (2014)

Even though screw induced pretension impacts the holding strength of bone screws,\ud its implementation into the numerical simulation of the pullout phenomenon remains\ud a problem with no apparent solution. The present study aims at developing a new\ud methodology to simulate screw induced pretension for the cases of: a) cylindrical\ud screws inserted with under-tapping and b) conical screws. For this purpose pullout\ud was studied experimentally using synthetic bone and then simulated numerically.\ud Synthetic bone failure was simulated using a bilinear cohesive zone material model.\ud Pretension generation was simulated by allowing the screw to expand inside a hole\ud with smaller dimensions or different shape than the screw itself. The finite element\ud models developed here were validated against experimental results and then utilized\ud to investigate the impact of under-tapping and conical angle. The results indicated that\ud pretension can indeed increase a screw's pullout force but only up to a certain degree.\ud Under-tapping increased cylindrical screws' pullout force up to 12%, 15% and 17%\ud for synthetic bones of density equal to 0.08 g/cc, 0.16 g/cc and 0.28 g/cc respectively.\ud Inserting a conical screw into a cylindrical hole increased pullout force up to 11%. In\ud any case an optimum level of screw induced pretension exists.
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