In silico framework for the certification of Nitinol staples

Fractures are the most common traumatic injuries associated to large-organs. The development of internal fixation devices such as plates, screws, pins and cerclage wires has led to improved surgical and clinical outcomes. In recent years, shape memory alloy staples have been introduced in orthopedic surgery as alternative fixation devices, with benefits in terms of time of the surgical procedure and improved bone healing. In detail, Nitinol staples dynamically adapt to changes in the bone structure and guarantee the bone-on-bone apposition during the healing process. Further studies are currently conducted to evaluate the staples design and to increase the treatment effectiveness and safety. A variety of Nitinol staples are available, but their mechanical properties compared to standard devices are not yet completely known. In this regard, Finite Element (FE) method is a powerful tool for evaluating the biomechanical response of fixation implants under different loading conditions, thus supporting the design phase to characterize and optimize the mechanical performance. In silico modelling of standard mechanical tests plays a fundamental role in supporting the medical device development and testing, enabling to reduce time and costs compared to a pure experimental approach. Within this context, this work focuses on the development of a numerical platform to support the characterization and the regulatory submission of staple fixation devices. A FE model of the commercially available DynaNite staple (Arthrex) was implemented. The geometry of the device was resembled from literature and manufacture's specifications, 3D tetrahedral elements were adopted to realize the mesh and a super-elastic material model was implemented to characterize the mechanical behaviour of Nitinol. Within the FE framework, the ASTM F564-17 standard was accounted by considering two separate mechanical tests, namely the elastic bending test and the constant amplitude bending test. Accordingly, numerical simulations of the two tests were carried out, implementing an FE model of the four-point bending load apparatus, in accordance with the regulation. The load of testing machine in function of the axial displacement of the actuator and the peak value of the maximum principal strain of the staple were considered to evaluate the mechanical performance of the device. The developed FE models allow to obtain the mechanical output quantities required by the ASTM standard. In addition, the study enables to identify critical zones for the structural integrity of the device, that are not experimentally obtainable. Once validated, the model may replace the corresponding experimental tests during the device certification phase.