Politecnico di Torino

G. Putame, M. Terzini, C. Bignardi, P. Costa, E. Zanetti, A. Audenino

Department of Mechanical and Aerospace Engineering, Politecnico di Torino, Torino, Italy

Intrauma S.p.A, Rivoli (TO), Italy

Department of Engineering, University of Perugia, Perugia, Italy

Background - Intramedullary nail fixation is a gold standard treatment for long bone diaphyseal fractures. Compared to inter-locking nails, self-locking nails allow for reduced soft tissues injuries in the distal area thanks to expandable mechanisms. Although different self-locking mechanisms have been proposed, they still show limitations in terms of implant stability and reversibility. In order to predict the in vivo implant stability, static analysis using finite element method is generally adopted. However, static analysis cannot describe interaction between bone and nail during dynamic loads. This study investigates the suitability of the numerical multibody analysis as an alternative approach to evaluate the biomechanical performance of an intramedullary self-locking nailing device under dynamic loads.

Methods - A 3D standard model of the human femur was modified in order to reproduce the geometry of the fractured medullary canal that surrounds the implanted device. Therefore, the self-locking nailing device model was created and joined with the femur model. Hence, torsional, compressive and bending dynamic loads were simulated for three different opening cases.

Results – Finally, bone-device contact forces and mean stiffness for each studied loading case are presented and discussed. In general, results show that the device stiffness always increases when the slider is at its distal position.

Conclusion - Even though the present study is based on a specific nailing device, the findings suggest that the multibody analysis may be a valid alternative approach to the finite element analysis in order to assess the biomechanical performance of complex models that involves large deformations and many contacts.

Figure 1. (a) Nail model at the beginning of the closing step; (b) Nail model at the end of the closing step; (c) Nail model at the end of the opening step inside the fractured femur model (slider at proximal position).

Figure 2. Models for compressive and torsional tests (a) and four-point bending test with four supporting cylinders (in green) (b). White arrows indicate applied forces and moments.

Figure 3. Contact force (a) and pressure (b) between wire tips and intramedullary canal during the opening phase.[/caption]