A novel auxetic structure based bone screw design: Tensile mechanical characterization and pullout fixation strength evaluation

It was supposed that auxetic structure with negative Poisson's ratio (NPR) expands under stretch and could enhance the screw-bone fixation. In this study, the novel auxetic structure based bone screws were designed, and mechanical properties and fixation strength were evaluated. Auxetic unit cells (A1–A6) were introduced into the design of screw bodies after a mechanical evaluation. Tubular auxetic structures (TA1–TA6), auxetic screws (AS1–AS6) and one non-auxetic screw (NS) were manufactured using 3D-printing. The fabrication process well reproduced the original designs despite the some mismatch in the macro and micro morphologies. Tensile tests on specimens were conducted experimentally and computationally. The relationship between NPR and fixation strength of the screws was investigated by computationally bone-pullout test. Among all screw designs, AS2 generated the largest stiffness and strength, and better NPR, AS5 produced the highest NPR, and smallest stiffness and strength. Maximal pullout force within low-, mid- and high-density bone was shown in AS5 (399.39 N), AS6 (561.07 N) and AS2 (1185.93 N) respectively. It was concluded that varying auxetic structures altered the screw's mechanical properties especially its functional properties. The bone-screw fixation could be improved by auxetic structures while other design factors should also be taken in account.