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Browsing by Author "Puttlitz, Christian M., advisor"

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    A biomechanical analysis of venous tissue in its normal, post-phlebitic, and genetically altered conditions
    (Colorado State University. Libraries, 2009) McGilvray, Kirk Cameron, author; Puttlitz, Christian M., advisor
    The incidence of vein disease is very high, affecting more than 2% of the hospitalized patients in the United States; a number that is expected to increase. Post phlebitic veins, the result of chronic deep vein thrombosis, is considered to be one of the most important venous disease pathologies. Unfortunately, little information is currently available on the biomechanical effects of thrombus resolution in the deep veins. The aim of this research was to characterize the biomechanical response of both healthy and diseased venous tissue using a murine model. It was hypothesized that biomechanical response parameters derived from healthy and diseased tissue would give insight into the resultant clinical complications observed in patients following thrombus resolution. Biomechanical analysis revealed that statistically significant deleterious changes in vein wall compliance were observed following thrombus resolution. Data also revealed that matrix metallopeptidase 9 expression has a statistically significant effect on the biomechanical response of the tissue. These results indicate that clinical complications following deep venous thrombosis manifest from significant decreases in the compliance of the vein wall. Finite element analyses were also performed. Biomechanical data served as input material parameters for modeling. Finite element modeling was used to evaluate the response of the inferior vena cava under physiologic loads. The results indicate that peak stresses are generated in the circumferential direction of loading during luminal pressurization. Decreased dilatation was observed following thrombus resolution. The data indicates that deep venous thrombosis lead to increased vein wall stress in correlation with decreased luminal distensability.
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    A Haversian bone model of fracture healing in a simulated microgravity environment
    (Colorado State University. Libraries, 2015) Gadomski, Benjamin C., author; Puttlitz, Christian M., advisor; Browning, Raymond, committee member; Donahue, Tammy, committee member; Heyliger, Paul, committee member
    Ground-based models of weightlessness and microgravity have provided valuable insights into how dynamic physiological systems adapt or react to reduced loading. Almost all of these models have used rodent hindlimb unloading as the means to simulate microgravity on isolated physiological systems. Unfortunately, results derived from rodent studies are significantly limited when one tries to translate them to the human condition due to significant anatomical and physiological differences between the two species. Therefore, it is clear that a novel animal model of ground-based weightlessness that is directly translatable to the human condition must be developed in order for substantial progress to be made in the knowledge of how microgravity affects fracture healing. In light of this, four specific aims are proposed: (1) develop a ground-based, ovine model of skeletal unloading in order to simulate weightlessness, (2) interrogate the effects of the simulated microgravity environment on bone fracture healing using this large animal model, (3) develop a computational model of weightbearing in ovine bone under different experimental conditions in order to characterize the loads experienced by the fracture site, and (4) develop countermeasures that enhance bone fracture healing in the presence of simulated microgravity. Successful completion of this project will substantially elevate the understanding of how fracture site loading affects the subsequent healing cascade in the presence of microgravity and will form the foundation for designing future rehabilitation protocols to facilitate bone healing during long-duration spaceflight.
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    Constitutive modeling of the biaxial mechanics of brain white matter
    (Colorado State University. Libraries, 2016) Labus, Kevin M., author; Puttlitz, Christian M., advisor; Donahue, Seth, committee member; Heyliger, Paul, committee member; James, Susan, committee member
    It is important to characterize the mechanical behavior of brain tissue to aid in the computational models used for simulated neurosurgery. Due to its anisotropy, it is of particular interest to develop constitutive models of white matter based on experimental data in order to define the material properties in computational models. White matter has been shown to exhibit anisotropic, hyperelastic, and viscoelastic properties. The majority of studies have focused on the shear or compressive properties, while few have tested the tensile properties of the brain. Brain tissue has not previously been tested in a multi-axial loading state, even though in vivo brain tissue is in a constant multi-axial stress state due to fluid pressure, and data from uniaxial experiments do not sufficiently describe multi-axial stresses. The main objective of this project was to characterize the biaxial tensile behavior of brain white matter via experimentation and constitutive modeling. A biaxial experiment was developed specifically for testing brain tissue. Experiments were performed at a quasi-static loading rate, and an Ogden anisotropic hyperelastic model was derived to fit the data. A structural analysis was performed on biaxially tested specimens to relate the structure to the mechanical behavior. The axonal orientation and distribution were measured via histology, and the axon area fraction was measured via transmission electron microscopy. The measured structural parameters were incorporated into the constitutive model. A probabilistic analysis was performed to compare the uncertainty in the stress predictions between models with and without structural parameters. Finally, dynamic biaxial experiments were performed to characterize the anisotropic viscoelastic properties of white matter. Biaxial stress-relaxation experiments were conducted to determine the appropriate form of a viscoelastic model. It was found that the data were accurately modeled by a quasi-linear viscoelastic formulation with an isotropic reduced relaxation tensor and an instantaneous elastic stress defined by an anisotropic Ogden model. Model fits to the stress-relaxation experiments were able to accurately predict the results of dynamic cyclic experiments. The resulting constitutive models from this project build upon previous models of brain white matter mechanics to include biaxial interactions and structural relations, thus improving computational model predictions.
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    Development of a finite element model of supracondylar fractures stabilized with variable stiffness bone plates
    (Colorado State University. Libraries, 2019) Sutherland, Conor J., author; Puttlitz, Christian M., advisor; McGilvray, Kirk, advisor; Easley, Jeremiah, committee member; James, Susan, committee member
    Approximately 10% of orthopaedic fracture fixation cases lead to non-union, requiring surgical intervention. Inadequate fixation device stiffness, which causes unwanted fracture gap motion, is believed to be one of the largest factor in poor healing as it prevents ideal tissue proliferation in the callus. By altering the thickness of orthopaedic bone plates, it was theorized that the fracture gap micro-mechanics could be controlled and driven towards conditions that accommodate good healing. The first goal of the project was to create computational FEA models of an ovine femoral supracondylar fracture stabilized with a plate of varying thickness. The models were used to investigate the mechanical behavior of the plate and the callus under different physiological loading conditions. The second goal of this study was to validate the computational model with bench-top experiments using an ex-vivo ovine femoral fracture model. To achieve these goals, novel plates were designed and manufactured with different stiffnesses (100%, 85%, and 66% relative stiffness) to be used to treat a femoral supracondylar fracture model in ovine test subjects; both in-vivo and ex-vivo. The FE models were shown to accurately predict the stress/strain mechanics on both bone and plate surfaces. Micromechanics (strain and pressure) predictions in the fracture gap were reported and used to make tissue type proliferation predictions based on previously reported mechanics envelopes corresponding to bone remodeling. The results indicated that changing plate thickness successfully altered the construct stiffness and consequently, the predicted healing tissue type at the fracture site. The FE methods described could help improve patient specific fracture care and reduce non-union rates clinically. However, further in vivo testing is required to validate the clinical significance of the methods described in this thesis.
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    Development of an artificial temporomandibular joint disc replacement
    (Colorado State University. Libraries, 2023) Kuiper, Jason Paul, author; Puttlitz, Christian M., advisor; Prawel, David, committee member; McGilvray, Kirk, committee member; Henry, Charles, committee member
    The temporomandibular joint (TMJ) is a complex bilateral ginglymoarthroidal joint containing a fibrocartilaginous disc and is essential for chewing, speaking, and swallowing. Due to the high loading frequency, small imbalances in joint homeostasis can overcome the natural capacity for adaptation and lead to a cascade of degenerative changes. For progressive TMJ disorders, resection of the TMJ disc is the leading treatment, but disc resection inherently increases stress and friction on the articular cartilage surfaces, leading to a progression to total joint replacement in 11.7% of patients. The current methods of treatment for disorders of the TMJ musculoskeletal complex are predominantly palliative and do not reliably address disorders of arthrogenous origin. Unfortunately, no synthetic TMJ disc replacements currently exist due to profound implant failures in earlier attempts. Introduction of a robust artificial TMJ disc replacement after resection will prevent further joint degradation and improve patient outcomes. Rigorous preclinical evaluation of artificial TMJ disc replacement strategies must be conducted to support future translation to humans. Therefore, the following aims are proposed: (1) Characterize the biomechanical behavior of the ovine temporomandibular joint soft tissues, (2) identify and evaluate a material candidate for a temporomandibular joint disc replacement, (3) develop in silico and in vitro methods for evaluating design candidates for artificial TMJ disc replacement, and (4) implement a temporomandibular joint disc replacement strategy in an ovine model.
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    Spinal cord and meningeal mechanics: viscoelastic characterization and computational modeling
    (Colorado State University. Libraries, 2018) Ramo, Nicole Lauren, author; Puttlitz, Christian M., advisor; Troyer, Kevin L., advisor; Heyliger, Paul, committee member; James, Susan, committee member
    Suffering a spinal cord injury (SCI) can be physically, emotionally, and financially devastating. With the complex loading environment typically seen in SCI events, finite element (FE) computational models provide an important economical and ethical option for investigating the mechanical etiology of SCI, evaluating prevention techniques, and assessing clinical treatments. To this end, numerous research groups have developed FE models of the spinal cord using various degrees of material and structural sophistication. However, the level of model complexity that is necessary to achieve accurate predictions of SCI has not been explicitly investigated as few studies have reported applicable tissue behavior. What are reported in the literature as "spinal cord mechanical properties" are most commonly based on ex-vivo tests of the spinal-cord-pia-arachnoid construct (SCPC). The pia and arachnoid maters are fibrous meningeal tissues that closely envelope the spinal cord, and together are referred to as the pia-arachnoid-complex (PAC). Currently available data demonstrate the PAC's importance in the overall SCPC stiffness and shape restoration following compression. However, only one previous study has reported mechanical properties of isolated spinal PAC, and therefore, conclusions about its contribution to SCPC mechanics are largely unknown. Additionally, it has been shown that SCPC material properties begin to degrade within 90 minutes of death. Considering the experimental difficulties and ethical concerns associated with in-vivo mechanical testing of the SCPC, determining the relationship between in-vivo and ex-vivo viscoelastic properties would allow researchers to more accurately analyze existing ex-vivo data. Therefore, the overarching goal of this work is to address the current gaps in knowledge regarding spinal cord and meningeal tissue mechanics and incorporate the developed material models into a FE model. Comparisons of ex-vivo and in-vivo porcine SCPC non-linear viscoelastic behavior revealed significantly different acute behaviors where the ex-vivo condition exhibited a higher stress response but also relaxed quicker and to a greater extent than the in-vivo condition. Although it only made up less than 6% of the ovine SCPC volume, the PAC was found to significantly affect the non-linear viscoelastic behavior of the SCPC which supports the conclusion that it plays an important protective mechanical role. Examining the fitting and predictive accuracy of linear, quasi-linear, and non-linear viscoelastic formulations to SCPC, cord, and PAC stress-strain data, non-linear formulations are recommended to model the SCPC and cord response to arbitrary loading conditions while the QLV is recommended for the PAC. This work provides researchers with novel insights into the complex mechanical behavior of the spinal cord and PAC. The experimental results represent an important addition to the limited literature on in-vivo versus ex-vivo neural tissue viscoelastic properties; they are also the first to quantify the non-linear elastic behavior of spinal PAC and the non-linear viscoelastic properties of the isolated spinal cord. Finally, the computational portion of this work provides a detailed report of the effects of viscoelastic formulation complexity on FE model prediction accuracy and computational time allowing researchers interested in modeling SCI to make informed decisions about the balance of accuracy and efficiency necessary for their specific modeling efforts.