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Traumatic aortic rupture (TAR) is one of the leading causes of morbidity and mortality in motor-vehicle accidents with the majority of injuries occurring in the peri-isthmus region. To date, the mechanisms of aorta injury are poorly understood as this injury cannot be replicated reliably in cadaver crash tests. Due to inconclusiveness of the experimental tests, finite element (FE) modeling is often used to gain a better insight into the mechanisms of TAR. However, the FE models are also hindered by many unknowns particularly the soft tissues biomechanical responses. A crucial step to improve the FE models of blunt chest trauma is to advance our understanding of the local mechanical properties of aortic tissue subject to high loading rates associated with TAR. The objective of this dissertation was to investigate the effects of tissue rate dependency and inhomogeneity in the modeling of loading conditions that lead to TAR. The material properties of human aorta in large deformations and high loading rates were characterized based on oscillatory biaxial tests. It was shown that a quasilinear viscoelastic (QLV) model with the instantaneous elastic response of the second order and the reduced relaxation function with one exponentially decaying term could describe the experimental results between 20 Hz and 130 Hz. The obtained decay rates (in the range of 70 to 550 s-1) were 10 to 100 folds higher than previously reported values and showed significant rate dependence within 10 ms after the loading. It was shown that the rate dependent properties, similar to the elastic properties, were anisotropic with generally higher decay rate and stiffness observed in the circumferential direction compared to the longitudinal direction. The inhomogeneity of porcine descending thoracic aorta was characterized in three dimensions using a nano-indentation technique and QLV modeling approach. The tests were conducted in the axial, circumferential, and radial orientations with about 100 micrometer spatial resolution. Aortic tissue was divided into 10 regions across the thickness, 4 quadrants in the circumferential direction, and 3 sections in the longitudinal direction. While across the thickness, the results in different orientations were significantly different, four distinct layers were identified that were matched with the anatomical features. In the axial direction, the medial quadrant, and in all directions, the proximal DTA had the lowest stiffness. The results predict that under equal stresses, the inner layers of the medial quadrant in upper DTA would undergo more strains and will be therefore more prone to failure. This prediction is in agreement with clinical observations. The inhomogeneity and rate dependency of aorta were implemented in the Global Human Body Models Consortium full-body FE model. It was demonstrated that in a simulation of blunt chest impact, both features significantly affected the tissue strain levels particularly in the isthmus, arch, and ascending aorta. Accurate quantifications of these features are essential to assess the risk of aortic injury based on FE models. / Mechanical Engineering

Pilon fractures are a result of high energy impacts to the ankle joint causing comminution to the tibia. Open reduction and internal fixation is the current method of treatment, which involves reducing the fracture and placing an anterolateral or medial plate along the tibia to secure the bone fragments during the bone healing process, but these have frequent complications for patients. No previous studies have investigated the biomechanical performance of these plates using a model that consists of the tibia, fibula and the syndesmotic tissue. The purpose of this study was to evaluate the biomechanical effectiveness of these plates using cadaveric specimens. Eight pairs of cadaveric specimens were anatomically aligned and potted proximally. A typical fracture pattern was created in each specimen and then one from each pair treated with an anterolateral and the other with a medial plate. A materials testing machine applied an axial load to the specimen to determine the construct stiffness, followed by a ramp load to failure. An optical tracking system was configured to track the motion of the bone fragments in 3D space. The medial plates tended to provide superior results when compared to the anterolateral plate; however, no statistical difference was found. This represents the most complex fracture and comprehensive evaluation of the plating options available for this type of injury and may inform surgeons to help reduce the poor outcomes for patients. / Thesis / Master of Applied Science (MASc) / Severe distal tibial fractures (pilon fractures) occur during motor vehicle collisions and falls from a height and are typically treated with one of two fracture fixations. The evidence supporting the use of the fracture fixations is limited and research has yet to be done using cadaveric specimens. To assess the fracture fixations, six cadaveric specimens were subjected to compressive loading to determine the stiffness. An orthopaedic surgeon then simulated a pilon fracture and repaired each specimen with one of the two fixations. The specimens were then subjected to compressive loading to determine the repaired stiffness and then loaded until failure. During failure testing, an optical tracking system was used to assess the overall motion of the fracture fragments. Based on preliminary results from stiffness, strength, and relative motion evaluations, the medial plate showed superior results; however, the differences were not found to be statistically significant.