Modelling of mechanical performance by finite element analysis of compressive bone staples made of superelastic NiTi and Ti-Zr-Nb-Sn alloys.

Abstract

Superelastic NiTi bone staples have become increasingly favoured for internal fixators by surgeons in clinical practice due to their high ability to maintain bone compression. They can adapt dynamically to structural changes and preserve bone alignment. Their insertion is easy, compared to other fixation methods. Due to the severe allergic reactions and cytotoxicity of nickel, Ni-free superelastic biomedical alloys recently benefit from special attention. Through a finite element analysis (FEA), this work firstly compares the mechanical performance of superelastic compressive staples made in NiTi with two recent superelastic metastable β-Ti alloys: Ti-24Zr-10Nb-2Sn and Ti-22Zr-11Nb-2Sn. The initial staple design is based on a classical protocol. The mechanical performance of the staple is evaluated through its clamping pressure, contact force, contact area with the bone, and minimum compressive strain into the bone. The first comparison concerns the three superelastic staples mentioned above with four different bone properties. For all the bone types, Ti-24Zr-10Nb-2Sn staples clearly outperform NiTi staples, making them a promising substitute for a wide range of bone conditions. The second part of the study aims to investigate the impacts of staple bridge widths and staple leg tapering shapes. Thanks to a parametric study, a staple bridge width of 15 mm is identified as an inflection point in the clamping pressure and maximum bone strain results. Additionally, moderately tapering the staple limbs is found to effectively increase clamping pressure while ensuring assembly safety, which provides valuable design insights, particularly for cases with limited space for staple insertion. This comprehensive study on the mechanical performance of compressive staples by FEA provides guidance for optimizing bone staple design, using Ni-free superelastic alloys and refining staple geometry dimensions.