Design and Characterization of Mg Alloy Pedicle Screws for Atlantoaxial Fixation

Abstract

To investigate the feasibility of using absorbable Mg alloy pedicle screws for atlantoaxial dislocation fixation, four types of Mg alloy pedicle screws of different thread forms were designed, and simulation analysis of the pull-out force was performed using the finite element method. Stress and displacement distributions of the atlantoaxial fixation model were obtained. Subsequently, screw samples were prepared using the WE43 Mg alloy for extraction, torsion, and immersion corrosion tests. Finite element analysis results showed that the pull-out forces of triangular, rectangular, trapezoidal, and zigzag thread screws were 552.61, 540.91, 546.4, and 542.74 N, respectively, and the stresses on the screws were 146, 185, 195, and 265 MPa, respectively, when they were pulled out. In other words, the triangular thread screw had the largest pull-out resistance and smallest stress peak. The average corrosion rate of Mg alloy screws in vitro was 0.46 mg·cm−2·day−1. Compared with that before corrosion, the extraction resistance of the corroded screws did not change significantly; however, the torsional strength decreased, but it was still greater than the torque required for screw implantation. It can be concluded that triangular thread Mg alloy pedicle screws have good extraction resistance and mechanical stability and can meet the load-bearing requirements for atlantoaxial dislocation fixation. The degradation of the Mg alloy reduced the mechanical strength of the screws, but the triangularly threaded screws can still maintain their effectiveness.