Effects of Self-Expandable Pedicle Screws with Shape Memory Alloy Structures on Spinal Fixation Strength under Cyclic Loading using a Numerical Simulation

In many spinal diseases, the surgeon has to use pedicle screws to keep one or more vertebrae stable. Various factors can gradually reduce the strength of the pedicle screws inside the vertebrae, which can cause adverse effects. One of the solutions to deal with this phenomenon is the use of self-expandable screws (standard screws with nickel-titanium memory sheets), which have been investigated recently due to less damage and biocompatible properties. In this research, the purpose of which is to compare standard and self-expandable screws, three screw samples were examined using finite element method. Standard screw, simple self-expandable screw (with smooth surface) and self-expandable screw with porous surface. Each of the screws was examined under cyclic loading and compared with each other. For this purpose, 3D models of standard and openable bone and screw were designed and evaluated using Abaqus software. After the simulation, the amount of equivalent plastic strain recorded for standard, simple self-expandable and porous screws was equal to 1.78, 0.15 and 0.06, respectively, which indicates less bone destruction in self-expandable models, especially porous ones. Also, the external work is maximum for the standard screw and minimum for the porous self-opening screw, which indicates less displacement and, as a result, greater strength of the porous self-expandable model. The results of this research have potential applications in orthopedic clinical research, which helps to develop appropriate therapeutic approaches.