Quantitative analysis of stability after fracture fixation in relation to the altering of different fixation parameters

Abstract

Throughout history, the treatment of bone fractures has always included some form of reduction of the fracture gap between the bone fragments, followed by splinting in order to support the injury. In Neolithic times, humans have attempted to correct the deformities from fractured bones, with evidence of good unions between the bone fragments. Society has always improved the technology and innovation in order to obtain the perfect union for fractured bones which are at an increasing rate, as mal-union could produce economic and social disability. The use of internal fixation in the treatment of bone fractures is a common method to aid the healing process which is highly dependent on the stability of the mechanical environment around the fracture area. Fixation techniques aim to provide sufficiently rigid stabilization of the fracture to allow loads to be applied to the injured limb whilst allowing the biological process of fracture healing. The primary objective for this study is to analyse the stresses and strains present under load in a fractured femur fixed with a standard fracture-fixation plate and screws. Furthermore, the study looks at and quantifies how varying the parameters such as plate thickness and different screw configuration affects the mechanical stability at the fracture site. In order to achieve the analysis, an FE model was created on Ansys. A cylindrical model was created in order to simulate the femoral diaphysis and the appropriate material properties were assigned to the model. A typical 6mm fracture fixation plate, and a typical 60mm bi-cortical screw were also modelled and placed on the lateral side of the bone. Four different screws configurations were assigned to the plate, following the format of 2 screws near the fracture, 2 screws set far from the fracture, 2 screws per segment, and 3 screws per segment. Furthermore, different plate thicknesses were also created for the same screw configurations, with the 6mm plate acting as the standard, followed by a 3mm, 9mm, and a 12mm plate. Each construct was set as a load-sharing construct and loaded up to 342N. The model was verified by simulating the results on a bovine femur sample. From the analysis performed it was observed that the first principal strains in the 6mm plate were found to be in the range between 5% and 7%, which allows secondary healing through callus formation. The third principal strains were found to be in the range of 14% to 22% on the un-plated medial side of the bone. This strain would then be reduced as the callus forms from the plated side and travels to the medial side until the ideal mechanical conditions are reached. Furthermore, the strain magnitude with 6 screws was found to be the smallest for each respective plate while the 2 screws closest to the fracture showed the largest strains. The plate thickness with the least overall strain was found to be the 6mm, followed by the 9mm which showed similar values. The 12mm plate showed the largest strain] values for each construct.