A finite element model for assessing the fatigue behavior of Nitinol orthopedic staples according to the standard ASTM F564

Fractures are one of the most common injuries globally. Over the years, the treatment of these injuries has been subject of continuous study, leading to significant advancements. Various devices, including plates, screws, pins and wires, have been employed. Orthopedic bone staples, particularly those made of Nitinol, have gained attention as promising alternatives for internal fixation, due to their simplified attachment to bones, decreased surgical duration, and minimized trauma in respect to the previously mentioned methods. Given that these devices undergo repetitive biomechanical stresses and strains, a thorough understanding of Nitinol's resistance to fatigue and fracture becomes imperative. In this context, this study focuses on assessing the mechanical response, including generated forces and fatigue behavior, of two commercial super-elastic Nitinol staples characterized by two and four legs (staple 1 and staple 2), respectively. Existing finite element (finite element) models of both staples are being employed, along with same models but with inverted materials. This approach aims to distinguish the influence of geometry from that of material properties on the mechanical behavior of the devices. The assessment involves a virtual 4-point bending test aligned with the ASTM F564 – 17 standard, implementing a FE model of the 4-point bending load apparatus in accordance with the regulation. Initially, generated forces vs. displacement curves were evaluated for the four models, underscoring the significant impact on generated forces of both geometric features (with peak values approximately 50% higher for staple 2 geometry) and material properties (with peak values approximately 20% higher for staple 1 material). Following that, the fatigue behavior of both staples was analyzed: first, a constant life mean versus alternating first principal strain diagram was utilized for all FE models considered; second, using the same models and boundary conditions, the fatigue safety factor was evaluated for every point of the models. This evaluation aimed to identify any potential critical zones. Two different cycles were applied for both methods, a more conservative one and a more realistic one. The results from both methods were comparable, and, like generated forces, fatigue behavior was found to be dependent on both geometry and material characteristic, although to a lesser degree. Additionally, an experimental 4-point bending test was conducted on another group of three commercial Nitinol staples to gauge the forces generated and compare them with those of the previous two staples. The obtained curves not only differed from the previous ones but also varied among themselves, showing differences in peak force values that varies in a range from 20% to 45% between the three tested staples. In conclusion, this dual approach provides a comprehensive understanding of the mechanical characteristics of super-elastic Nitinol staples, combining computational simulations and experimentation. It also underscores how small differences in geometries or material properties can have significant effects on the mechanical behavior of the device.