Biomechanical evaluation of the new intramedullary system II for treating reverse obliquity trochanteric fractures

Intramedullary nails are recommended for treating reverse obliquity trochanteric fractures (ROTFs) in recent years. Yet, the rate of fixation failure caused by traditional intramedullary nails for managing such fractures is high. To solve this issue, the New Intramedullary System-II (NIS-II) was created. Three variations of ROTF model (AO/OTA 31-A3) were built using finite element method. Four lengthened implants were depicted and assembled on the ROTF models, comprising the Proximal Femoral Nail Antirotation (PFNA), the InterTAN nail (ITN), the Proximal Femoral Bionic Nail (PFBN), and the NIS-II. The mesh size of finite element was determined by a convergence test. The models were validated via testing axial stiffness and comparing it with published data of biomechanical experiments. Two kinds of von Mises stress and displacement were evaluated under axial loads of 2,100 N. Among the four fixation models, the NIS-II model exhibited the best biomechanical stability. Specifically, in comparison with the PFNA models, the NIS-II models demonstrated a 9.0-11.2% reduction for maximal displacement and a 9.9-12.4% reduction for maximal displacement of fracture surface (MDFS). Besides, the NIS-II models indicated a 50.1-63.7% reduction for maximal stress on implants and a 32.6-38.9% reduction for maximal stress on femurs, compared with the PFNA models. The indicators, including maximal displacement, MDFS, and maximal stress on femurs, showed statistical difference between the PFNA and NIS-II groups (p < 0.05). The New Intramedullary System-II showed the most superior stress distribution and the overall mechanical stability, followed by PFBN, ITN, and PFNA in fixing ROTFs under axial loads. Accordingly, the design of NIS-II is feasible and this implant could be a new choice for the treatment of ROTFs.