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

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