Variable Fixation: a Finite Element Analysis of new technology for orthopaedic traumatology

Aim: The primary objective of this work is to introduce the advantages and limitations of a new medical device for internal fixation of bone fractures: the Variable Fixation Locking Screw. This new solution has a dual behaviour, thanks to the presence of a degradable polymeric sleeve that changes its mechanical properties during the early phases of fracture healing. Initially it acts like a traditional screw (Locking Screw) and, once this element is completely degraded, it behaves like a competitor screw called Far Cortical Locking Screw. According to the developers, its characteristics have the chance to enhance the bone fractures healing process, potentially reducing the occurrence of complications. Methods: The stability provided by Variable Fixation technology has been compared with that provided by standard locking technology reproducing different facture patterns on a realistic reconstruction of the tibia of one sheep included in a preclinical investigation. Transverse, oblique and comminuted fractures were investigated. Parameters of interest were observing differences in fracture gap strains and displacements among different time points. A stress analysis completed the investigation providing a comparative image of the tensional status of the different constructs. Finally, a tool was realized to help surgeons to quantitatively recognize the strain of the fracture gap, in order to understand what mechanical stimuli bone cells are subjected during the VFLS degradation cycle. Findings: Results have shown a potentially positive effect in case of transverse and oblique fractures, except in cases where the fracture gap was very small. In particular, a three-dimensional map was created for transverse fractures to help surgeons quantify the fracture gap strain during the weeks after the implant, until the sleeve is completely degraded. With regard to the example of comminuted fracture, it is suggested to improve the model to be able to stimulate even the innermost regions of the fracture gap, reaching a good bone regeneration, which otherwise cannot be completed. The most interesting aspect of this work was definitely the comparison of the Variable Fixation Locking Screw with the traditional one; with all the obtained results we have shown that, for fracture cases treated, this device, over time, allows to achieve greater uniformity in the strain of the fracture gap.