In silico biomechanical evaluation of an innovative intramedullary interphalangeal nail in Nitinol

Arthrodesis is a reliable alternative to conservative treatments in handling intractable arthritic pain and joint deformity. Temporary Kirschner wire are a widely adopted solution to perform arthrodesis, however they do not provide compression in the joint and do not control the phalanx rotation, with associated risk of infection, breakage, migration, and discomfort for the patient. To overcome these limitations, permanent intramedullary interphalangeal Nitinol implants (IINIs) have been recently developed. These innovative devices exploit the shape memory effect and superelasticity of Nitinol to change their shape once implanted in the body in order to anchor to the bone and provide compression at the arthrodesis site. Within this context, Finite Element (FE) method is an efficient tool to assist the devices design process and achieve better clinical outcomes. In silico simulations are playing an increasingly decisive role in evaluating the mechanical performance of medical devices and providing evidence for their certification. In this perspective, this work aims to define a numerical framework to evaluate the biomechanical behavior of an IINI. Two FE simulations were conducted on a three-dimensional model of the IINI Stryker Smart Toe II. The geometry was created by reproducing drawings and measurements publicly available, was meshed with tetrahedral elements and a superelastic behavior of Nitinol was used. The first test aims to reproduce in silico the push-off phase of gait in order to identify the most stressed areas and provide an estimate of the bending force and displacement the nail can support. Therefore, the proximal part of the device was constrained while the distal part was flexed upward. The second test allows to determine the contact forces between bone and implant once it has expanded into the medullary canal. The nail was initially compressed into its pre-implant form and then gradually expanded. The anchor force was consequently computed, predicting the risk of bone non-union and implant migration. The simulations conducted enabled to identify critical areas for structural failure, to predict the maximum supportable loads and to compute the anchorage forces. To conclude, the proposed FE analyzes could minimize the device development costs and times while improving the quality of the product and providing useful information for achieving certification.