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

Cancellous bone screws are used to achieve good pull-out 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 microCT-scanned cancellous bone and converted into a geometric model using MIMICS (R) software. The finite element (FE) models were produced in ANSYS (R). The calculated reaction force for pull-out of 0.2mm 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 pull-out 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 pull-out 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 pull-out 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 pull-out stiffness might be achieved.