A Computational Approximation to Model Variation in Cancellous Bone Screw Pull Out

Abstract

Cancellous bone screws are used to achieve good pullout characteristics when connected to cancellous bone. This study examines some screw characteristics, including pitch and inner diameter, using a model of cancellous bone with a range of bone apparent densities (ADs). This was achieved using bone geometry based on microCTscanned cancellous bone and converted into a geometric model using mimicsآ® software. The finite element (FE) models were produced in ansysآ®. The calculated reaction force for pullout of 0.2 mm shows the influence of design parameters. Change in the proximal half angle increased the stiffness by about 15% in line with the experimental findings of others. An increase in pullout reaction force with an increase in bone AD was also observed. However, when a particular screw geometry in lower AD bone was modeled and then rotated through 180 deg on plan, a significant reduction in reaction force was noted. Further models with screws of similar geometry in the same location showed similar reductions in reaction force and hence pullout stiffness. Examination of the geometry of the bone/screw interface indicates that in certain positions there is little cancellous bone to support the implant—leading to low pullout reaction forces, which is very difficult to predict. The study also examined the effect of increasing the bone stiffness adjacent to the implant, and concluded that, even in bone of low AD, increases in pullout stiffness might be achieved.