Effects of self-expandable pedicle screws with shape memory alloy structures on spinal fixation strength: A finite element study

In many spine surgeries, pedicle screws are commonly used to stabilize vertebrae, however, loosening can be a complication. Different designs have shown improvements in fixation strength, with self-expandable screws featuring shape memory alloy (SMA) structures being of particular interest. This study aimed to assess the fixation strength of self-expandable pedicle screws made with SMA (specifically Nickel-Titanium) sheets. Three types of screws were evaluated: self-expandable screws with a smooth SMA surface, self-expandable screws with a porous SMA surface, and standard design screws. Each screw underwent pullout tests for comparison. Following the tests, the self-expandable screw with a porous surface exhibited the highest pullout force (1141.83 N), compared to 1056.86 N for the smooth self-expandable screw and 1104.25 N for the standard screw. The dissipated plastic strain energy differed among the screws, with values of 0.073 J for the porous self-expandable screw, 0.065 J for the smooth self-expandable screw, and 0.089 J for the standard pedicle screw. Notably, the porous self-expandable screw showed reduced stress on the bone-screw interface. Improving the mechanical design of pedicle screws could significantly enhance screw-bone fixation strength. The utilization of self-expandable pedicle screws with porous surface SMA sheets demonstrates superior performance, potentially mitigating complications like loosening.