Influence of screw diameter on the pull-out resistance of additively manufactured auxetic bone screws in cancellous and cortical bone.

Abstract

This study investigates the effect of screw diameter on the pull-out strength of a bone screw with a loosening-resistant auxetic structure manufactured using additive manufacturing. The variation in mechanical performance depending on screw diameter was examined experimentally and through finite element analysis in cancellous and cortical bone structures. First, screw designs with different diameters incorporating an auxetic cell structure were created in SolidWorks. The designed screws were analyzed using ANSYS Workbench finite element software, confirming that the structures exhibit a negative Poisson's ratio (NPR). The screws were manufactured from Ti6Al4V alloy using the direct metal laser sintering additive manufacturing process. The fabricated auxetic screws were inserted into synthetic cancellous and cortical bone (L4-L5 vertebral bone). The pull-out tests were performed under conditions conforming to the ASTM F543 standard. Force-displacement curves were obtained from the experiments, and tensile tests were also conducted to determine the mechanical strength of the screws. To evaluate the performance of the auxetic screws, identical pull-out tests were carried out on conventional bone screws. After the tests, SEM images were obtained to examine the damage mechanisms in detail. The results showed that auxetic bone screws exhibited approximately 4%-15% higher pull-out strength compared to conventional screws. This improvement is attributed to increased surface roughness and radial expansion under tensile loading, which enhance mechanical interlocking at the bone-screw interface. Finite element analyses further indicated that the magnitude of the NPR increased with increasing screw diameter, suggesting more effective mechanical behavior at larger diameters. In addition, the porous structure of the auxetic screw is expected to facilitate bone ingrowth, thereby supporting bone healing and reducing the risk of loosening. Finally, the developed auxetic bone screw has the potential to increase resistance to loosening. In addition, it may enhance biological integration with bone, which could contribute to improved surgical outcomes and greater patient comfort.