Thomas Oxland

Spinal muscles play an important role in two inter-related clinical problems in the thoracolumbar spine: 1) age-related progressive kyphosis and 2) proximal junctional kyphosis (PJK) following correction surgery. Although these disorders occur largely in the thoracic spine and show symptoms in weight-bearing postures, studies have not investigated thoracic muscles in postures other than supine. Moreover, almost all the image-based thoracolumbar models are developed from muscle data obtained from supine imaging, which questions its credibility. Hence the objectives of this study were to i) analyze the effect of posture on thoracic spinal muscle parameters in different postures, and ii) develop a method for translation of MRI-derived spinal and muscle data into a thoracolumbar biomechanical model. Two regions (T4-T5 and T8-T9) of the thorax of six healthy volunteers were imaged (0.5T MROpen, Paramed, Genoa, Italy) in four postures (supine, standing, sitting, and flexion). Descriptive guidelines were developed to identify and quantify three muscles- trapezius (TZ), erector spine (ES), and transversospinalis (TS) from axial MR images. Intra- and Inter- segmentation repeatability was assessed using ICC(3,1). The effect of spinal level and posture on muscle parameters (cross-sectional area (CSA) and position (radius and angle)) was evaluated using 2-way repeated measures ANOVA (p
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To better understand the link between spinal cord impact and the resulting tissue damage, computational models are often used. These models typically simulate the spinal cord as a homogeneous and isotropic material. Recent research suggests that grey and white matter tissue differences and directional differences, i.e. anisotropy, are important to predict spinal cord damage. The objective of this research was to characterize the mechanical properties of spinal cord grey and white matter tissue in confined compression. Spinal cords (n=11) from the thoracic and cervical regions of pigs (Yorkshire and Yucatan) were harvested immediately following euthanasia. The spinal cords were flash frozen (60 secs at -80 oC) and prepared into four types of test samples: grey matter axial, grey matter transverse, white matter axial, white matter transverse. For each sample type, 2 mm diameter biopsy samples were collected, thawed, and subsequently tested with a custom confined compression apparatus. This was performed within 6 hours of euthanasia, minimizing time post-mortem effects. All samples were compressed to 10% strain at a quasi-static strain rate (0.001/sec) and allowed to relax for 120 secs. A quasi-linear viscoelastic model combining a first-order exponential with a 1-term Prony series was used to characterize the loading and relaxation responses respectively. The effect of tissue type (grey matter vs. white matter), direction (axial vs. transverse), and their interaction were evaluated with a two-way ANOVA (p
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There are many cellular and therapeutic treatments for traumatic spinal cord injury (tSCI) that have shown promise in animal models, however, these treatments have been unsuccessful when applied to humans. A possible reason for this discrepancy is that animal model SCIs are well-controlled, whereas human SCIs are heterogeneous in terms of population, severity and mechanism. It is known that the mechanical injury parameters play an important role in dictating the pathophysiology of SCI. However, to fully describe the mechanics of tSCI, it is necessary to understand the mechanical properties of the spinal cord. In order to investigate the mechanical properties of the rat spinal cord, I created a finite element model to simulate experimental contusion SCIs on rats based on research by Bhatnagar et al., and evaluate morphological similarity between computationally predicted and experimentally deformed spinal cords. The model was used to determine the relative stiffnesses of the grey and white matter by iteratively assigning mechanical properties to each tissue, deforming the spinal cord to match the shape of the experimentally deformed cord, and observing the morphological similarity of the predicted and experimentally deformed grey matter shapes. Using a linear elastic, homogeneous, isotropic material model for both tissues examined, I found that for six of the seven spinal cords examined, the best model agreement occurred when the white matter was modeled as 2-3 times stiffer than the grey matter, while each tissue was held at a Poisson's ratio of 0.45. Furthermore, I found that for contusion injuries inflicted upon the mediolateral center of the spinal cord, the model predicted the deformation well, while for off-center contusions, the model was unable to capture the shape of the grey matter on the side contralateral to the contusion location. I demonstrated that the spinal cord white matter appears to be stiffer than the grey matter and that current strategies of modeling spinal cord injury do not adequately capture the complexity of the deformation of the cord as a whole.My research gives further reason to investigate the mechanical properties of the spinal cord for the purpose of computationally modeling spinal cord injury.

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The biomechanics of the cervical spine during shear loading are not well-established as compared to other loading regimes. This deficit may be problematic as there is evidence that shear loading may contribute to fracture-dislocation injuries, which often lead to spinal cord injury. Because of this deficit, existing safety standards, such as those used in the automotive industry, may not provide sufficient protection against spinal cord injuries in the cervical region. The present work aims to address this deficit in two ways: through the characterization of the load-displacement behaviour of the cervical spine during shear loading, and through an analysis of the effect of test apparatus design on specimen artefact loading during shear testing. In the mechanical testing phase of the project, fresh-frozen human cervical functional spinal units were loaded to 100 N using a materials testing machine and custom-designed test apparatus. Three directions (anterior, posterior, lateral) were tested in each of three specimen conditions (intact, posterior ligamentectomy, disc-only). Significant decreases in stiffness were found in both the anterior (∆81 N/mm) and posterior (∆15 N/mm) directions between the intact and disc-only conditions, respectively. A computational model was then developed to investigate the effects of test apparatus design on artefact loading and coupled rotations, which had proved problematic during previous attempts to apply axial compression preloads during shear testing. Three axial compression force application methods (point load, rotationally constrained, follower load) were modeled during testing up to 10 mm anterior shear, with axial compressive loads up to 800 N for each method. A subset of the simulations were validated experimentally using porcine functional spinal units. It was found that the follower load provided the best reduction of both artefact moments and coupled flexion-extension rotations. This work provides additional scope to existing shear biomechanics data, as well as insight into how test apparatus design may influence results during shear testing of the cervical spine. These results may be used to improve the definition and validation of existing finite element models of the human neck, where such models may reduce the incidence or severity of spinal cord injury through improved automotive safety.

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The effect of timing of decompression following traumatic spinal cord injury (SCI) remains unclear, as clinical and preclinical studies have demonstrated varying results. There remains a question of whether certain sub-groups of SCI could see greater benefits than others from early decompression. Dislocation is the most commonly seen injury mechanism but has never been investigated with respect to residual compression in an animal model. The goal of this thesis was add residual compression to an existing rat cervical dislocation model and to examine the effect of time of residual compression and velocity of injury with this model.Dislocation injuries were conducted on forty-six male, Sprague-Dawley rats in four groups: two timings of decompression (24 minutes, 240 minutes) and two velocities (10mm/s, 500mm/s). All injuries involved dislocation between the C5/C6 vertebrae in an anterior-posterior direction to 1.45mm and residual compression of 0.8mm. Animals were evaluated for motor function using the Martinez open field, grip strength, and grooming tests for 6 weeks post-injury. High velocities consistently produced more severe injuries than low velocity. Correlation coefficients between 0.46 and 0.58 (p
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Introduction: The development of adjacent segment degeneration (ASD) as a common complication of spinal fusion is believed by some clinicians and researchers to have roots in kinematic changes and altered loading at the intervertebral levels beside the fusion (i.e. adjacent). Dynamic spinal implants and minimally invasive surgeries were introduced to minimize such kinematic changes and load alterations in attempts to prevent ASD. However, little is known whether the kinematic changes at the adjacent level to a fusion are common in vivo occurrences. Further, the role of iatrogenic muscle damage on loading at the adjacent levels has not been investigated previously.Objectives: (1) To assess the current clinical evidence of in vivo kinematic changes at the levels adjacent to a lumbar spinal fusion. (2) To investigate the role of iatrogenic muscle damage on loading at the adjacent levels.Methods: (1) A systematic search in the PubMed database was performed for studies that addressed kinematics of the segment adjacent to a lumbar spine fusion or any other spinal implant. (2) A musculoskeletal model of the lumbar spine with 210 muscles was developed. Muscle damage was simulated by detaching the muscles from the posterior elements of the operated vertebrae and its effect on spinal loads at the adjacent levels was assessed during upright standing. Results: (1) The search identified 39 articles, among which 29 studied fusion. None of the studies observed any increase in range of motion (ROM) of the caudal adjacent segment, while for the rostral adjacent level the ROM was reported to increase in 10-30% of the patients. (2) The axial forces at the adjacent levels increased with muscle damage, with the largest increases being at the rostral adjacent level (73%) in comparison to the caudal level (32%).Discussion: The results of both studies imply higher susceptibility of the rostral adjacent level to disc degeneration, which is in harmony with the clinical prevalence of ASD occurring in 70 to 100% of the cases at the rostral level. The findings suggest that muscle damage secondary to spine surgery may play a key role in adjacent segment changes, independent of the spinal instrumentation.

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Currently a treatment for spinal cord injury (SCI) remains elusive to clinicians and researchers. This is, in part, due to variation between primary injury mechanisms and diversity of mechanical impact factors such as impact velocity, depth, force, and acceleration. This research examines both the individual and combined effects of impact velocity and depth on the cervical spinal cord and also aims to understand the contribution of the energy applied, not only the impact factors. In this study, contusion spinal cord injuries were induced in 54 male, Sprague-Dawley rats at impact speeds of 8 mm/s, 80 mm/s, or 800 mm/s with displacements of 0.9 mm or 1.5 mm. Animals recovered for seven days followed by behavioural assessment and examination of the spinal cord tissue for demyelination and tissue sparing at 1 mm intervals ±3 mm rostrocaudally to the epicentre. In parallel, a finite element model of the rat spinal cord was used to examine the resulting maximum principal strains in the spinal cord during impact.Impact depth was a consistent factor in qualifying axonal damage in the spinal cord, tissue sparing, and resulting behavioural deficit. Increased impact velocity resulted in significantly different impact energies and measureable outcomes at the 1.5 mm impact depth, but not the 0.9 mm impact depth, identifying threshold interactions between the two factors. The difference of injury severity to velocity at different impact depths identifies the existence of threshold interactions between the two impact factors.Linear correlation analysis with finite element analysis (FEA) strain showed significant (p≪0.001) correlations with axonal damage in the ventral (R ²=0.86) and lateral (R²=0.74) regions of the spinal cord and with white matter (R²=0.90) and grey matter (R²=0.76) sparing. Non-parametric correlation analysis identified strong correlations between grey and white matter strain with open field behavioural scores (p=0.005,r_s=-0.94).The results shown by this work extend the research identifying significant correlation between maximum principal strain and neurologic tissue damage. Furthermore, a relationship between the impact depth and velocity of injury demonstrated a more rate sensitive response of the spinal cord at the 1.5 mm impact depth than at the 0.9 mm impact depth.

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Researchers and clinicians do not currently use the heterogeneity of the primary mechanism of spinal cord injury (SCI) to tailor treatment strategies because the effects of these distinct patterns of acute mechanical damage on long-term neuropathology have not been fully investigated. Computational modelling of SCI enables the analysis of mechanical forces and deformations within the spinal cord tissue that are not visible experimentally. I created a dynamic, hyperviscoelastic three-dimensional finite element (FE) model of the rat cervical spine and simulated contusion and dislocation SCI mechanisms. I investigated the relationship between maximum principal strain and previously published tissue damage patterns, and compared primary injury patterns between mechanisms.My model incorporates the spinal cord white and gray matter, dura mater, cerebrospinal fluid, spinal ligaments, intervertebral discs, a rigid indenter and vertebrae, and failure criteria for ligaments and vertebral endplates. High-speed (1 m/s) contusion and dislocation injuries were simulated between vertebral levels C3 and C6 to match previous animal experiments, and average peak maximum principal strains were calculated for several regions at the injury epicentre and at 1 mm intervals from +5 mm rostral to -5 mm caudal to the lesion. I compared average peak principal strains to tissue damage measured previously via axonal permeability to 10 kD fluorescein-dextran (Choo, 2007). Linear regression of tissue damage against peak maximum principal strain for pooled data within white matter regions yields significant (p
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Introduction: Arthritis is a degenerative disease that causes irreversible damage to a joint’s articular cartilage. Despite having high failure rates in early total ankle arthroplasty (TAA) models, recent improvements have increased the success of this procedure, providing end-stage ankle arthritis patients a viable alternative to fusion with better functional outcomes. Currently, the most prevalent cause of failure is aseptic loosening, which is believed to be affected by motion at the bone-implant interface. The objective of this study was to compare micromotion and kinematic patterns of two TAA designs. Methods: A mechanical simulator was designed to apply compressive loads and bending moments to human cadaveric ankles, intact and replaced. It induced a maximal range of motion in the ankle about 3 orthogonal axes: plantarflexion-dorsiflexion (PF-DF), inversion-eversion (INV-EV), and internal-external rotation (IR-ER). Six ankle pairs were tested and compared. The implants analyzed were the Agility™ and the Scandinavian Total Ankle Replacement (S.T.A.R.®). Using an optical motion capture system, tibiocalcaneal kinematics and the relative bone-implant motion for each implant were recorded and analyzed. Results: The Agility exhibited a greater amount of micromotion between the bone and prosthesis than the STAR for the tibial component in INV-EV (p=0.037), and for the talar component in PF-DF (p=0.002) and IR-ER (p=0.038). Micromotion magnitudes were affected by loading direction and compression. Kinematic changes were observed following replacement of the ankle joint. There were decreases in the amount of motion coupling for both implants when loaded in INV-EV and IR-ER. There were increases in joint translation for both implants in the medial/lateral direction under INV-EV loading, and for the STAR in the anterior/posterior and compression/distraction directions under PF-DF loading. No significant ROM differences were found. Discussion: Increased micromotion in the Agility supports the hypothesis that higher aseptic loosening rates are correlated with reduced initial post-op fixation. The effect of loading direction on micromotion magnitude confirms the need to apply a variety of loading conditions to obtain a comprehensive micromotion analysis. Kinematic differences between implanted and intact ankles show that there is still room for improvement towards an ankle replacement design that replicates the performance of a healthy ankle.

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Introduction: Sagittal realignment using posterior spinal fixation and fusion, with or without corrective osteotomy, is the current treatment for global sagittal imbalance. Patients may develop proximal junctional kyphosis (PJK) through failure of the uppermost instrumented or adjacent vertebra. The effects of surgical and patient variables on the development of PJK have not been studied biomechanically. Objectives: (1) To analyze pre- and post-operative intervertebral loading and the effect of osteotomy location and extensor muscle function on intervertebral spine loading using a 2D equilibrium model of sagittally imbalanced adult spine, and (2) to characterize pure moment loading pathways of multi-segment human cadaveric spines following posterior spinal fixation and in a number of surgical conditions.Methods: (1) Pre- and post-operative lateral radiographic measurements were taken of patients (N=7) and used to predict intervertebral compressive loading patterns. From pre-operative curves, the changes in loading behaviour due to simulated osteotomies and decreasing levels of extensor muscle function were assessed. (2) Six human cadaver five-segment spines in six surgical states were tested in pure flexion-extension bending to represent the post-operative loading of patients without extensor muscle function. Vertebral strain, rod strain, and specimen kinematics were measured and rod loading was used to predict load-sharing between the implant and the spine.Results: (1) Predicted intervertebral compressive loads increased up to 29% after development of PJK. Predicted compressive loads were not notably affected by the chosen level of the osteotomy but increased up to 42% after intra-operative extensor muscle loss. (2) A force couple existed between the vertebral column and the implant, supporting the majority of the applied moment. The additional compressive force on the spine due to the applied moment was predicted based on rod load measurements, found to agree with model predictions. Specimen condition had minimal or no significance on measurements.Discussion: The developed equilibrium model introduced a predictive tool for surgical planning and deformity progression. Predicted intervertebral compressive loads were higher in sagitally-imbalanced spines than asymptomatic spines (96), worsened by loss of extensor muscle function. Simulating muscle loss by applying a pure moment in vitro may provide insight to the resulting additional spine loading.

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Introduction: Clinical studies have demonstrated beneficial results of posterior arthrodesis for the treatment of degenerative spondylolisthesis (DS). The optimal stiffness of these fusion systems to enhance load-sharing and fusion rate while minimizing adjacent segment stresses is unknown. To our knowledge, posterior instrumentation for DS has not been tested under anterior shear loads, a highly relevant loading direction for DS. Objectives: To determine the amount of shear load supported by posterior lumbar fusion devices of varying stiffness under shear loading.Methods: The effect of implant stiffness and specimen condition on implant load was assessed in a biomechanical study. Fifteen human cadaveric lumbar functional spinal units were tested under a static 300 N axial compression load and a cyclic anterior shear load (5-250 N). Implants (High-Stiffness (HS): ∅ 5.5 mm Titanium, Medium-Stiffness (MS): ∅ 6.35 x 7.2 mm Oblong PEEK, Low-Stiffness (LS): ∅ 5.5 mm Round PEEK, and Ultra-Low-Stiffness (ULS): ∅ 5.5 mm Rod X), instrumented with strain gauges to measure loads, were tested in each of three specimen conditions simulating degenerative changes: intact, facet destabilization and disc destabilization.Results: Transducers measured implant shear loads to within ±5 N. All implants supported significantly greater shear loads as the specimen was destabilized. The LS and ULS implants supported significantly less load than the HS and MS implants for all specimen conditions. Mean implant loads as a percent of the applied shear load in order of increasing specimen destabilization for the HS implant were: 43, 67 and 76%, for the MS implant were: 32, 56 and 77%, for the LS implant were: 18, 35 and 50%, and for the ULS implant were: 16, 39 and 42%. Standard errors were below 8%.Discussion: An accurate shear load transducer was developed; the methodology is adaptable to many implant designs and materials. Implant shear stiffness significantly affected the shear load-sharing characteristics of the fusion devices. Low-stiffness implants transferred significantly greater loads to the spine, and may possibly enhance the transition to the adjacent, uninstrumented spine in vivo.

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Previous experimental studies of spinal cord injury (SCI) in rodents established the importance of fixation of the spine in survival models following a mechanical injury. However, no fixation device has been designed to provide spinal stabilization, prevent additional damage to the cord, and promote fusion at the site of injury. The present study aims to design a novel rat spinal fixation device, which will be used in future survival studies and investigates its biomechanical effectiveness in stabilizing the spine up to eight weeks post injury.A custom-made magnetic resonance imaging (MRI) compatible fixation device was designed to stabilize the C5/C6 joint. This was achieved in an animal model by creating a 1.5 mm fracture-dislocation injury between C5 and C6 spinal segments of Sprague-Dawley rats using a multimechanism SCI test system. A biomechanical evaluation of the device-spine system was conducted at these segments. Cycles of stepwise directed shear forces with a maximum of 0.98 N were applied at a known distance from the injured site producing flexion and extension bending moments, while the resulting two-dimensional motions between C5 and C6 were measured and presented in the form of load-displacement curves. This was implemented at two time points: immediately (n = 6), and eight weeks post-injury (n = 9) and the results were compared to an intact group (n = 6).Average ± S.D. flexion/extension ranges of motion (ROM) were 18.1 ± 3.3º, 19.9 ± 7.5º, and 1.5 ± 0.7º, and neutral zones (NZ) were 3.4 ± 2.8º, 5.0 ± 2.4º, and 0.7 ± 0.5º, respectively for the intact, injured/fixed, and injured/8-week groups. The results show that there is a significant difference in ROM and NZ between the injured/fixed and injured/8-week groups (p-values = 0.0002, and 0.006, respectively). The device acutely stabilizes the spine by restoring its stiffness to the initial stiffness of the intact specimen. It also proves that along with the biological factors over time, fusion is promoted at the site of injury. This study presents the design and evaluation of a novel well-characterized spinal fixation device for rats, which will be used in future experimental SCI survival models.

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