Browsing by Subject "Spine"

Now showing 1 - 4 of 4
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    Experimental discovery of surgical guidelines for cervical disc augmentation
    (2012-05) Mehta, Hitesh Prathiviraj
    Intervertebral disc degeneration of the cervical spine affects over one half of all individuals over the age of 40 years and the last decade has seen an alarming increase in cervical disc degenerative disease related surgeries. In spite of newer technological advancements in devices for disc degeneration disease, spinal disc replacement and fusion, revision surgery rates have remained unchanged. 90% of the disc replacement revisions and 50% of fusion related revision can be attributed to improper device selection. Therefore, the objective of this research is to evaluate disc arthroplasty (replacement) and arthrodesis (fusion) devices and identify optimal implant size (height) selection criteria for biomechanical competence in force transmission, motion, and neurologic tissue protection. Eleven osteo-ligamentous human cadaver cervical spines were biomechanically evaluated after surgical augmentation with different sized implants for both arthroplasty and arthrodesis. The biomechanical outcomes measured were range of motion, neutral zone, stiffness, articular pillar strains, facet forces and intervertebral foramen area. Increased disc distraction was found to increase lordosis of the spine, increase compressive strains in articular pillars and increase in intervertebral foramen area. The kinematics outcomes were surgery type and implant size dependent where fusion lead to decreased range of motion, while arthroplasty maintained the range of motion with differential outcomes based upon the size of the implant. The integration of these biomechanical data demonstrate an implant size /spacer height relationship with direct clinical importance and the ability to guide clinical decision making so as to reduce revision surgery due to deviant biomechanical function.
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    Intervertebral disc degeneration, quantified by T2* MRI, biochemistry, and compressive mechanics, correlated to global functional mechanics of the lumbar spine
    (2013-05) Ellingson, Arin Michael
    Low back pain is one of the most prevalent health complaints in the US, with an estimated 70-85% of the population developing back pain at some point in their life, creating a significant financial burden. Although the causes of low back pain are poorly defined and indistinct, most often implicated as the origin of pain, is the intervertebral disc. The disc affords the spine its extensive multidirectional motion due to the complex interaction between two morphologically, biomechanically, and biochemically distinct tissues: the annulus fibrosus and the nucleus pulposus. With advancing age, injury, pathology or a combination of these, a degenerative cascade of biomechanical, biochemical, and nutritional alterations diminish the discs' ability to maintain its structure and function. Unfortunately, measurement of these properties in vivo is currently not a viable option due to the invasiveness of the procedures. Therefore, an indirect method is needed to evaluate the multifarious characteristics of a patient's disc health. Of critical interest is the relationship that functional spinal mechanics has with the morphologic, biochemical, and biomechanical properties of the intervertebral disc as they change with degeneration. Eighteen osteoligamentous cadaveric lumbar spines that spanned the degenerative spectrum were utilized in a correlation study design to evaluate the relationships between each factors of disc health: imaging, biochemical content, biomechanical competency, and functional mechanics. Each specimen was first imaged using quantitative T2* MRI techniques, where the site-specific relaxation times and features of the Pfirrmann grading system, including signal intensity and distinction between the nucleus pulposus and surrounding annulus fibrosus, were measured. Then their functional spinal mechanics were evaluated and range of motion, neutral zone ratio, bending stiffness and helical axis patterns were computed. Local biochemical content and compressive biomechanical properties were subsequently analyzed. Each outcome measure was then assessed with respect to the others using correlation statistical methods in an effort to understand the multifactorial relationships surrounding disc degeneration. The T2* relaxation times and newly defined variables, T2* Intensity Area and Transition Zone Slope, were significantly correlated to the standard Pfirrmann grading, showing the T2* MR imaging parameters are sensitive to the morphological changes associated with disc degeneration. Also, these features enable the quantitative grading of disc degeneration without subjectivity or bias but with clinically recognized features of distinction. Furthermore, T2* relaxation times were found to have a high sensitivity for detecting the proteoglycan content of the intervertebral disc, which may potentially have a profound impact on the early diagnosis of degeneration. The T2* relaxation times were also significantly correlated to the residual stress and excised strain of the disc. These multi-faceted changes that occur with degeneration impact the global mechanics of the spinal unit by increasing the neutral zone to range of motion ratio, or joint instability, and altering the bending stiffness and range of motion. Even stronger correlations were measured with alterations in the helical axis patterns of lateral bending. There was a marked increase of out-of-plane rotations and a larger migration of the instantaneous axis of rotation with worsening degeneration evaluated by MRI, local biochemistry, and local residual mechanics. Quantitative T2* MRI has the sensitivity to predict the local biochemical and biomechanical properties of the intervertebral disc. Complementary to MRI analysis, the measurement of the pathway of motion throughout the degenerative progress, using the helical axis approach, can enhance the disc assessment. Altogether, these clinically viable methods may immediately improve the characterization of the intervertebral disc for enhanced treatment and care.
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    Mechanics of the annulus fibrosus lamellae under physiologic loading conditions: do interlamellar connections matter?
    (2014-09) Nagel, Tina Marie
    During healthy function, the spine provides the body with stability, strength, and flexibility. Unfortunately, spinal injuries such as annular tears are prevalent in human spines after age 10 (Boos et al. 2002), and at some point in their lives, about 75% of individuals experience low back pain (Andersson 1999). There are many hypotheses related to the origin of pain, but it is often attributed to injury and/or degeneration of the intervertebral disc (IVD) in the lower, lumbar spine (Andersson 1999). While the vertebral bodies are rigid structures, the IVD is a flexible, composite structure of two main components; the nucleus pulposus (NP) and the annulus fibrosus (AF), which is a fibrous structure that surrounds the NP with largely concentric layers containing highly aligned collagen fibers. There are connections that traverse between layers (C. A. Pezowicz, Robertson, and Broom 2006). The organization and composition of the lamellae allow the IVD and thus the spine to exhibit multi-axial motion including flexion, extension, and lateral bending, common to many activities of daily living.The purpose of this dissertation was to assess the influence of the interlamellar connection through pre-failure and failure mechanics of discrete AF lamellae by creating a physiologically relevant test method to deform single and multiple AF lamellae and evaluate the kinetic response using a validated structural model. Vertebral kinematics were quantified from human in vivo flexion. Average intervertebral strains were found to be symmetric during the flexion sequence but intervertebral angles were not, suggesting a physiologic decoupling of the two.A structural model was validated for use to characterize AF lamellae. Through parameter sensitivity analysis and calculating confidence intervals of the fitted parameters, it was found that the fitted parameters were more robust when using both surface displacements and grip forces.Single and multiple AF lamellae were characterized using the biaxial protocol generated from the analysis of vertebral kinematics. Single lamella samples produced significant in-plane shear force and moments, while multiple lamellae samples did not, after accounting for the number of lamellae. This suggests isolated single lamellae experience complex loading in biaxial tension but the AF as a whole reduces this response. Parameters fitted from the structural model were not statistically different between single and multiple lamellae samples. This work suggests the interlamellar connection is mechanically significant in shear rather than a planar biaxial context. AF lamellae in shear were found to withstand significant displacement prior to failure as well as carry a non-zero load during the sliding phase. This response suggests a preventative feature within the AF region to resist and mitigate damage due to axial rotation. Although the model used was unable to characterize the shear stress of the experimental data in its present form, further improvements to the model such as more anatomically accurate interlamellar layer may improve the capabilities of the model.The work accomplished in this dissertation forms a base for further assessment of discrete AF lamella(e) and interlamellar connections. Using porcine tissue, experiments performed within Chapters 4 and 5 should be continued to increase the sample size and strengthen possible trends seen within this work. With these tools, these experiments should also be performed with a larger sample size using healthy human cadaver tissue. It would also be interesting to use these tools to assess human cadaveric tissue from the degenerative spectrum. The shear testing showed the interlamellar connection to be mechanically significant in that context, but the test configuration as well as the simplistic modeling did not elucidate whether this mechanical significance originates from a fibrous connection or a matrix material. Further testing and modeling should work towards determining the connection to attribute the mechanical significance.
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    Method to Determine Compressive Bending Performance in Static and Fatigue Loading for Pediatric Non-Fusion Devices
    (2016-01) Foltz, Mary
    Background: Early onset scoliosis is a three-dimensional deformity of the spine and trunk – diagnosed before the age of 10 – and occurs in 3 to 5 out of 100,000 infant and juvenile populations. Surgical intervention is often necessary for pediatric patients with severe curvatures. During thesis procedures, spinal rods – known as pediatric non-fusion devices (i.e. growing rods) – are placed in the patients to allow for spinal growth. ASTM standards exist for fusion constructs. Growing-rod constructs place greater demands on the rods as no fusion is performed and the rods are continually loaded. Also, the constructs are serially lengthened at 6-month intervals placing additional demands on the construct. Currently, there are no standards on how to develop and utilize a finite element model – to predict durability of commonly used constructs within the patient. Methods: Finite element models representing static compression bending were implemented using Abaqus CAE (Dassault Systemés Simulia Corporation, Providence, Rhode Island). The model geometry and loading was based on ASTM standard 1537 and 136. The models were created with C3D8R type elements. The model applied a controlled displacement until failure or contact occurred between the superior and inferior UHMWPE blocks. Failure was defined as the point at which permanent deformation of the construct occurs, due to fracture, plastic deformation, or slip. Eight constructs were analyzed based on the rod material (titanium and cobalt chrome), active length (76-mm and 376-mm), and construct type (F1717 and growing-rod). Results: The force-displacement curves and maximum principle stress for each whole model were evaluated. The longer active lengths required a smaller force for failure. Failure at the pedicle screw head was found on each model. Conclusions: A new ASTM standard for growing-rod constructs should be created based off of a longer active length, the overlap of the rod connectors, and location of the rod connectors.