Comprehensive Finite Element Analysis of a new osteosynthesis screw designed for variable fixation of bone fractures

The main purpose of this thesis work is the analysis of a new bone fracture device that allows variable fixation treatment. It is an osteosynthesis screw, called Variable Fixation Locking Screw (VFLS), which aims to ensure variable behaviour of the fixing structure according to the different needs of the bone tissue during the healing process. The variable fixation locking screw is characterised by a polymeric sleeve on the screw rod made of a material with a specific degradation profile over time. In this way, in the initial steps, it will tend to limit movements by acting as a standard locking screw, ensuring contact with the bone both proximal and distal to the plate. However, when the callus formation begins, the sleeve degradation allows increasing the range of inter-fragmentary micro-movements of the structure by favouring the bone healing process. In the first part of this project, it is carried out a finite element analysis of a sheep tibia model previously reconstructed from a preclinical investigation. It shows a fracture characterized by an internal fixation treatment. More specifically, a stabilization is simulated in which there is a mixed configuration of osteosynthesis screws, consisting of three standard locking screws in the distal fragment and three variable fixation locking screw in the proximal area. This model is designed to compare the stability of this treatment with that of two limit configurations discussed in previous studies. The first is based on a standard locking technology with six locking screws, while the second is a variable fixation obtained with six VFLS. Transverse and oblique fractures characterise the models developed in this thesis work. The comparison is made for different fracture gap widths to test which is the most appropriate configuration in the specific application cases. The compared parameters are deformation, displacement and stress state obtained by increasing polymer degradation. As expected, the results show that the partial variable fixation pattern registers an intermediate behaviour between the two limit configurations in terms of both deformation and stress. To underline this performance, a three-dimensional map for transverse fractures is used, which reports the deformation depending on the weeks and the amplitude of the fracture gap. It allows stressing a variable and intermediate trend again. In fact, the results show that the model studied in this project has a more flexible behaviour compared to the configuration with standard locking screws. Still, it is also stiffer than the structure characterized by six VFLS. In the second part of the project, a human femur model and an osteosynthesis plate one are developed from acquired CT scans. Different experimental data are considered to choose the proper bone geometry that allows a better fit with the reconstructed plate. Subsequently, the structure is tested by finite element analysis using a system of experimental loads and constraints to prove the solidity of the model. In conclusion, a comprehensive model is obtained that can be further developed to simulate faithfully the mechanical behaviour of a fractured human femur treated by variable fixation.