Browsing by Author "Puttlitz, Christian, advisor"
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Item Open Access Additive manufacturing of an intervertebral disc repair patch to treat spinal herniation(Colorado State University. Libraries, 2021) Page, Mitchell Ian, author; Puttlitz, Christian, advisor; Heyliger, Paul, committee member; Susan, James, committee member; Kirk, McGilvray, committee memberChronic low back pain is ubiquitous throughout society. The consequences of this disease are extensive and lead to physical, mental, and financial suffering in the affected population. Herniation of the intervertebral disc (IVD) is the primary cause of chronic low back pain due to the essential mechanical role of the IVD in the spinal column. Degenerative changes to the IVD tissues, in particular the annulus fibrosus (AF), lead to a pronounced vulnerability to herniation. Although numerous treatments for intervertebral disc herniation currently exist, these treatments are typically palliative and prone to hernia recurrence. Accordingly, there is a distinct need for an IVD hernia therapy that can provide long-term pain relief and recovery of spinal function. One novel strategy to repair the intervertebral disc is to tissue-engineer a construct that facilitates regeneration of the healthy and functional IVD tissue. Advances in additive manufacturing technology offer the fabrication of complex tissue-engineered structures that augment biological content and biocompatible materials. Therefore, this work sought to engineer an additive manufactured repair patch for IVD herniation towards an improved treatment for chronic low back pain. Specifically, the aims of this work were to leverage experimental and computational methods to: (1) to characterize the mechanics of additive manufactured angle-ply scaffolds, (2) evaluate the tissue response of cell-laden scaffolds cultured with dynamic biaxial mechanical stimuli, and (3) to design and implement an annulus fibrosus repair patch. The mechanics of additive manufactured scaffolds for AF repair were experimentally characterized in a physiologically-relevant, biaxial loading modality. To assess sensitivity of the scaffold mechanics to additive manufacturing parameters, a broad scope of scaffold designs were evaluated with a parameterized finite element model. A custom incubator was developed, cell-laden scaffolds were cultured with a prescribed, multi-axial mechanical loading protocol, and ECM production within the scaffold was evaluated. A finite element model was developed to aid in understanding the relationship between global scaffold loading and the local, inhomogeneous cellular micromechanical environment within the scaffold. The developed TE material was translated into an implant and was implemented in a large animal model. The efficacy of the AF repair strategy was also evaluated in finite element model of the human lumbar spine. This work formed a multi-scale approach to consolidate biological and mechanical efficacy of a novel AF repair strategy. Ultimately, this approach may facilitate regeneration of the AF and represent a revolutionary treatment for chronic low back pain.Item Open Access Assessment of the effects of ligamentous injury in the human cervical spine(Colorado State University. Libraries, 2012) Leahy, Patrick Devin, author; Puttlitz, Christian, advisor; Heyliger, Paul, committee member; Sakurai, Hiroshi, committee member; Santoni, Brandon, committee memberLigamentous support is critical to constraining motion of the cervical spine. Injuries to the ligamentous structure can allow hypermobility of the spine, which may cause deleterious pressures to be applied to the enveloped neural tissues. These injuries are a common result of head trauma and automobile accidents, particularly those involving whiplash-provoking impacts. The injuries are typically relegated to the facet capsule (FC) and anterior longitudinal (ALL) ligaments following cervical hyperextension trauma, or the flaval (LF) and interspinous (ISL) ligaments following hyperflexion. Impacts sustained with the head turned typically injure the alar ligament. The biomechanical sequelae resulting from each of these specific injuries are currently ill-defined, confounding the treatment process. Furthermore, clinical diagnosis of ligamentous injuries is often accomplished by measuring the range of motion (ROM) of the vertebrae, where current methods have difficulty differentiating between each type of ligamentous injury. Pursuant to enhancing treatment and diagnosis of ligamentous injuries, a finite element (FE) model of the intact human full-cervical (C0-C7) spine was generated from computed tomography (CT) scans of cadaveric human spines. The model enables the quantification of ROM, stresses, and strains, and can be modified to reflect ligamentous injury. In order to validate the model, six human, cadaveric, full-cervical spines were tested under pure ±1.5 Nm moment loadings in the axial rotation, lateral bending, flexion, and extension directions. ROM for each vertebra, facet contact pressures, and cortical strains were experimentally measured. To evaluate injured ligament mechanical properties, a novel methodology was developed where seven alar, fourteen ALL, and twelve LF cadaveric bone-ligament-bone preparations were subjected to a partial-injury inducing, high-speed (50 mm/s) tensile loading. Post-injury stiffnesses and toe region lengths were compared to the pre-injury state for these specimens. These experimental data were incorporated into the FE model to analyze the kinematic and kinetic effects of partial ligamentous injury. For comparison, the model was also adapted to reflect fully injured (transected) ligaments. Injuries simulated at the C5-C6 level included: 1) partial FC injury, 2) full FC injury, 3) partial FC and ALL injury, 4) full FC and ALL injury, 5) partial LF and full ISL jury, 6) full LF and ISL injury, 7) partial FC, ALL, LF, and full ISL injury, and 8) full FC, ALL, LF, and ISL injury. The model was also modified to replicate injury to the right alar ligament. Five cadaveric cervical spines were tested under pure moment conditions with scalpel-sectioning of these ligaments for validation of the full-injury models. Comparisons between the intact and various injury cases were made to determine the biomechanical alterations experienced by the cervical spine due to the specific ligamentous injuries. Variances in ROM and potential impingement on the neural tissues were focused upon. The overarching goals of the study were to identify a unique kinematic response for each specific ligamentous injury to allow for more accurate clinical diagnosis, and to enhance the understanding of the post-injury biomechanical sequelae.Item Open Access Computational modeling of the lower cervical spine: facet cartilage distribution and disc replacement(Colorado State University. Libraries, 2009) Womack, Wesley J., author; Puttlitz, Christian, advisorAnterior cervical fusion has been the standard treatment following anterior cervical discectomy and provides sufficient short-term symptomatic relief, but growing evidence suggests that fusion contributes to adjacent-segment degeneration. Motion-sparing disc replacement implants are believed to reduce adjacent-segment degeneration by preserving motion at the treated level. Such implants have been shown to maintain the mobility of the intact spine, but the effects on load transfer between the anterior and posterior elements remain poorly understood. In order to investigate the effects of disc replacement on load transfer in the lower cervical spine, a finite element model was generated using cadaver-based Computed Tomography (CT) imagery. The thickness distribution of the cartilage on the articular facets was measured experimentally, and material properties were taken from the literature. Mesh resolution was varied in order to establish model convergence, and cadaveric testing was undertaken to validate model predictions. The validated model was altered to include a disc replacement prosthesis at the C4/C5 level. The effect of disc-replacement on range of motion, antero-posterior load distribution, total contact forces in the facets, as well as the distribution of contact pressure on the facets were examined, and the effect of different facet cartilage thickness models on load sharing and contact pressure distribution predictions were examined. Model predictions indicate that the properly-sized implant retains the mobility, load sharing, and contact force magnitude and distribution of the intact case. Mobility, load sharing, nuclear pressures, and contact pressures at the adjacent motion segments were not strongly affected by the presence of the implant, indicating that disc replacement may not be a significant cause of post-operative adjacent-level degeneration. Variation in articular cartilage distribution did not substantially affect mobility, contact forces, or load sharing. However, mean and peak contact pressure, contact area, and center of pressure predictions were strongly affected by the cartilage distribution used in the model. These results indicate that oversimplification of the cartilage thickness distribution will negatively affect the ability of the model to predict facet contact pressures, and thus subsequent cartilage degeneration.Item Open Access Development of novel mechanical diagnostic techniques for early prediction of bone fracture healing outcome(Colorado State University. Libraries, 2021) Wolynski, Jakob G., author; McGilvray, Kirk, advisor; Puttlitz, Christian, advisor; Heyliger, Paul, committee member; James, Susan, committee member; Wang, Zhijie, committee memberTo view the abstract, please see the full text of the document.Item Open Access Multi-scale & multi-resolution experimental and analytical methods for mitigating blast risk with barrier walls(Colorado State University. Libraries, 2024) Sullivan, Kellan M., author; Mahmoud, Hussam, advisor; Puttlitz, Christian, advisor; Gadomski, Benjamin, committee member; Jia, Gaofeng, committee member; Stephens, Catherine, committee member; Pezzola, Genevieve, committee memberOver the last decade, interest in blast resistance and protection has increased as a result of the perpetual threat of terrorist groups around the world. In evaluating the Department of State (DOS) reports on terrorism since 2007, an estimated 330,000 fatalities and 430,000 injuries have been caused by terrorist attacks worldwide (2022). In the United States, various large scale explosive attacks have occurred over the years including the World Trade Center bombings in 1993, the Alfred Murrah Federal Building bombing in 1995, and the coordinated September 11th attacks in 2001. More recently, there has been a shift in the tactics of terrorist groups to use improvised explosive devices (IEDs) to target civilians due to regulations put in place after the September 11th, 2001, attacks that made it difficult for them to obtain a large amount of explosive material among other factors contributing the rise of terrorist activity. Attacks such as the Boston Marathon bombing in 2013 and the Madrid train bombing in 2004 demonstrate this shift in tactics. The upward trend of the use of IEDs around the globe since the September 11th, 2001, attacks presents a catalyst for a shift in research methods for blast mitigation techniques to provide protection to people rather than just structures. Therefore, developing methods to provide protection for people from blast effects is necessary to minimize the impact these terrorist groups have on our communities. Of the existing blast mitigation strategies, perimeter walls or barriers are specifically advantageous in that they increase standoff distances and provide an obstacle to the propagation path of the blast wave as well as primary fragmentation. The use of perimeter walls or barriers to protect structures has been well established in literature, however the use of barriers to protect people has not. The ability to predict airblast effects accurately and efficiently over a large variation in scaled ranges, within a complex environment, is important to characterize the potential severity of damage to structures and casualties among personnel in both military and civilian settings. Many different techniques have been used over the years to perform blast prediction of various airblast parameters such as pressure and impulse and blast resistant design research. While experimentation remains a valuable and powerful tool, in recent years, computational and numerical models have grown in popularity for their accurate evaluation capabilities. Advanced numerical software such as hydrocodes and computational fluid dynamic programs are often used to model airblast propagation and its impact on structures. However, in more complex environments, where blast loading in large areas of interest may occur, using high-fidelity computational modeling software could be inefficient due to the computing power required. The goal of this dissertation was to develop a performance-based design framework for predicting the probability of survivability of a double-barrier system under blast loading, and the probability of different bodily injuries for personnel from the blast wave itself. In this dissertation, the gaps in research for protecting civilians from IED attacks in large open areas, understanding the impact of multiple barriers on the blast shockwave and pressures around the barriers, and investigating an absorption focused barrier were addressed. A combination of analytical, numerical, and experimental methods at multiple scales was used to develop and validate the various elements needed to conduct the performance-based design. This dissertation developed rapid computational models to predict the pressure field around a double-barrier system, analyzed a new barrier design that focuses on reducing the energy of the shockwave in order to protect people, and accounted for the uncertainty and variability in multiple parameters to establish potential risk for various scenarios for both the barrier and for people. The analyses combined numerical, analytical, and experimental methods at multiple scales, to create models to predict and assess the pressures associated with person-borne-improvised-explosive-devices (PBIEDS). The developed models used to predict and quantify the pressures around a rigid double-barrier system and the response of the wood barrier to blast loading were coupled with small- and full-scale experimental testing to validate and assess the accuracy and efficiency of the models. From the results of dissertation, it can be observed how the implementation of a double-barrier system can significantly reduce the pressures experienced around the barriers, which can lead to less potential for serious injury or damage from blast events. Additionally, it showed that the distance between the barriers plays a critical role in the pressures and therefore the potential for injury between the barriers. In addition, adopting an innovative approach to blast barrier design to consider the use of more lightweight, commonly available, non-rigid materials to increase the energy absorption to attenuate the blast shockwave rather than just reflect was proven to be beneficial.Item Open Access Viscoelastic characterization and modeling of musculoskeletal soft tissues(Colorado State University. Libraries, 2012) Troyer, Kevin Levi, author; Puttlitz, Christian, advisor; James, Susan, committee member; Heyliger, Paul, committee member; Dasi, Lakshmi, committee memberOver the last decade there has been a dramatic rise in musculoskeletal soft tissue injuries in the general, athletic, and military populations. The etiology of this increase has been largely ascribed to dynamic loading events, including strenuous physical overuse and trauma. Additionally, instability arising from soft tissue pathology or trauma can induce and/or accelerate joint degeneration. Degenerative sequelae, such as post-traumatic osteoarthritis, can cause significant debility and an associated reduction in one's quality of life. Development of successful treatment modalities for joint instability and soft tissue compromise is highly dependent upon a thorough understanding of the affected tissue's mechanical (viscoelastic) behavior. However, current soft tissue viscoelastic characterization paradigms predominantly utilize quasi-linear viscoelastic (QLV) formulae despite substantial empirical evidence which has conclusively demonstrated that these tissues violate its fundamental assumption of elastic and viscous behavior separability. Furthermore, development of more applicable nonlinear viscoelastic formulations has been hindered by the inability of currently-available constitutive models and characterization methodologies to include relaxation manifested during dynamic loading events. As a result, implementation of nonlinear viscoelastic formulae in soft tissue computational models has not been widespread. To surmount these shortcomings, this work develops a novel, nonlinear viscoelastic constitutive formulation and a corresponding experimental characterization technique which can be included in current state-of-the-art computational algorithms. Specifically, the aims of this dissertation were: (1) Develop and validate a nonlinear viscoelastic characterization technique for musculoskeletal soft tissues that incorporates relaxation manifested during loading; (2) Characterize the nonlinear viscoelastic behavior of various types of ligamentous tissues and tendon; (3) Integrate a fully nonlinear viscoelastic constitutive formulation into a finite element algorithm. Aims 1 and 2 were accomplished via development and application of a novel comprehensive viscoelastic characterization (CVC) technique and constitutive formulation to describe the nonlinear viscoelastic behavior of various human cervical spine ligaments (anterior and posterior longitudinal ligament and ligamentum flavum) and ovine Achilles tendon. Additionally, improvements in the predictive accuracy of the CVC fitted coefficients over previously accepted viscoelastic characterization techniques were quantified. Furthermore, a computationally tractable fully nonlinear viscoelastic formulation was developed and validated against an analytical solution (Aim 3). Implementation of the important nonlinear viscoelastic behavior into computational models will greatly accelerate our ability to understand the functional role of soft connective tissues in whole joint mechanics and facilitate future treatment options.