Browsing by Subject "Soft tissue"

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    Colles’ fracture creation on cadaveric arms by impact loading.
    (2010-12) Magnuson, Thomas David
    Among the most common causes of upper extremity fracture is a fall on an outstretched hand. The result of this is often a fracture of the distal radius, and the most common of these is the two part Colles’ fracture. Understanding the fracture in greater detail can lead to better clinical treatment of the injury. Much study has been done in an attempt to understand the biomechanical factors that lead to the fracture and to recreate the event in multiple media with varying boundary conditions. In this study, we focused our efforts on the impact loading of forearm cadavera in an attempt to recreate, as best as possible, the physiologic events of a forward fall onto an outstretched hand. Fluoroscopic data was taken prior to testing to dimension the cortical bone of the radius, and afterwards to assess and characterize fracture patterns. A custom drop tower was designed and constructed to dynamically load cadaveric forearms to failure. Furthermore, impact mechanics theory was used to estimate the effective load applied with each test. It was determined that 3046 N ± 650 N, with a ratio of axial to bending loads of 2.8 ± 1.0, was necessary to impart a Colles’ fracture in completely intact cadaveric forearms. This load fell within published ranges associated with distal radius fractures. Testing included 15 cadaveric samples (11 female and 4 males). Though considered at first, no direct correlation was found between the body mass index of each donor and the load required to fracture the radius. Results also indicated that females were more susceptible to Colles’ fractures than were males. This may be due in part to the higher average age of the female donors. We concluded that it is possible to produce Colles’ fractures with a relatively high rate of success. However, uncontrollable biological factors provided variation such that a completely repeatable fracture pattern across the sample population was not achieved. Furthermore, while an impact model could be the most accurate recreation of a forward fall, more research is needed to truly validate the results obtained through this impact based body of work.
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    Form from function: generalized anisotropic inverse mechanics for soft tissues.
    (2011-08) Raghupathy, Ramesh
    Elastography, the imaging of soft tissues on the basis of elastic modulus has gained popularity in the last few decades and holds promise for application in many medical areas. Most of the attention has focused on heterogeneous materials that are locally isotropic, the intent being to detect a stiff tumor within a compliant tissue. Many tissues of mechanical interest, however, are anisotropic, so a method capable of determining material anisotropy would be attractive. This work presents a method, named GAIM (Generalized Anisotropic Inverse Mechanics), to determine the mechanical anisotropy of heterogeneous, anisotropic tissues, by directly solving the finite-element representation of the stress balance in the tissue. GAIM divides the sample into subdomains assumed to have uniform properties and determines the material constants in each subdomain. Use of a linear material model led to rapid computation with statistical confidence levels as performance metrics. Multiple tests, asymmetric loading and strain heterogeneity are needed to address the ill-posedness of the inverse problem, and represent a paradigm shift in the testing of soft tissues. Simulated experiments of fibrous soft-tissues demonstrated the ability of the method to capture anisotropy qualitatively even though only a linear model is used. Results from the tests on soft-tissue analogs demonstrated the success in identifying regional differences in anisotropy based on full-field displacements and boundary forces obtained from multiple biaxial extension tests. The method’s success in capturing regional anisotropic changes associated with growth and remodelling in fibroblastpopulated cruciforms is a significant achievement, and holds promise for determining structural information of tissues from the mechanical response, since the structural and mechanical anisotropy are correlated. The linear GAIM model can be extended by a second step for nonlinearity with a fiber-based constitutive model. A closedform solution for the latter was developed and provides rapid results for nonlinear regression. In summary, this work has built a novel exploratory tool to extract regionspecific anisotropic properties on intact tissue samples. GAIM can be applied to provide information on the mechanical function of healthy tissue subjected to complex physiologic loads, identify regions within a tissue that exhibit irregular mechanical behavior (possibly due to disease or damage), and provide structural information from the mechanical function of tissues that are not amenable to structural tests.