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Graduate Student Supervision
Doctoral Student Supervision
Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.
Hip fracture has serious repercussions at both the societal and personal levels. For better fracture prevention, it is essential to understand the material changes of femoral cortical bone that contribute to hip fragility, and the deformation and fracture process during hip fractures. Therefore, the aim of this dissertation was to study the mechanisms of hip fracture from both structural and mechanical perspectives. Using quantitative backscattered electron (qBSE) imaging and polarized Raman microspectroscopy, periosteal hypermineralization in aged human proximal femur was found with significantly higher mineral content/mineral-to-matrix ratio than lamellar bone. Accompanying the increased mineralization was the “brittle” cracking behavior upon microindentation in the hypermineralized tissue. Small- and wide-angle X-ray scattering (SAXS/WAXS) measurement showed substantially thinner, shorter and more irregularly distributed mineral platelets in the hypermineralized region, indicating the material changes at the ultrastructural level. Combined second harmonic generation (SHG) and two photon excitation fluorescence (TPEF) techniques were used to study shear microcracking and its association with the organization of collagen fibrils in the femoral cortical bone. Unique arc-shaped shear microcracks, differing from either tensile or compressive microcracks, were identified at the peripheral zone of the osteons. These microcracks were further located within the “bright” lamellae where collagen fibrils are primarily oriented at the circumferential direction to the osteons’ long axes. Microcracking analysis on clinically retrieved femoral neck components identified shear, compressive and tensile microcracks associated with major fractures. The results pointed to the central role of the superior cortex in resisting a hip fracture, whereby higher density of microcracks and buckling failure were found in the superior cortical bone. BSE imaging at the fracture sites found the direct involvement of hypermineralization, which lacked crack deviation and had fewer microcracks than the tough lamellar bone. This dissertation answered fundamental questions regarding the role of femoral cortical bone in clinical hip fractures, and elucidated the underlying failure mechanisms due to microstructural changes and the complex stress states under external loading. The findings thus provided new insights into better identifying at-risk population of hip fracture.
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Understanding vertebral mechanics is of interest for identifying persons at risk of fracture, whether that is due to everyday loading such as in osteoporotic fracture or as a result of dynamic loading leading to a traumatic fracture. Vertebral fractures negatively impact the quality life of patients and represent a large financial burden on the healthcare system. A powerful but underutilized tool that can be used to study vertebral loading and fracture is digital image correlation (DIC). DIC is a non-contact optical method for measuring the displacement on the surface of materials, including bone. In this thesis, DIC was used in a laboratory setting to provide a more complete understanding of the response of vertebral bodies to compressive loading. The first investigation compared measurements from DIC with strain gages, a commonly accepted experimental method for measuring the bone surface response. For porcine vertebral bodies, the agreement was strong between the strain gages and DIC-measured strains indicating that DIC can be successfully used on bone. Based on those findings, experimental studies were performed using DIC to identify fracture of the anterior cortex and to quantify rate-dependency of the vertebral body response. For the fracture study, high DIC strains on the anterior cortex of vertebral bodies corresponded well with the locations of damage identified by observation of the video. For the rate-dependency study, the DIC displacement patterns were similar for the slow and fast rate tests, but the displacements from the slow rate tests had higher magnitudes, as expected for viscoelastic materials such as bone. Finally, specimen-specific finite element (FE) vertebral body models were created and DIC was used to validate the displacement and stiffness response. The FE models were predictive of the experimental stiffnesses measured using DIC on the surface of the vertebrae. This thesis demonstrates the utility of DIC for experimental vertebral body investigations and for validation of FE models. Through these studies and future work, DIC has advanced and will continue to advance the understanding of vertebral mechanics under everyday loads as well as in simulated osteoporotic and healthy bone trauma.
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Rollover accidents are dynamic and complex events in which head contacts with the vehicle interior can cause catastrophic neck injuries through head-first impact. Ex vivo cadaver tests are valuable for studying these mechanisms of head-first axial loading neck injuries; however, they lack a biofidelic representation of neuromuscular control, postural stability, and overall spine posture. Computational modeling can be used to evaluate changes in the risk of neck injury under the influence of muscle forces, yet the exact muscles and levels of forces that are involved leading up to a head-first impact are unknown. Knowing the state of the neck prior to impact is critical to improving cadaveric and computational models of neck injury. Four human volunteer experiments were conducted to determine whether inversion, head position, muscle tensing, and dynamic motion influence the cervical spine alignment. These four studies included: (1) static inversion, (2) muscle tensing, (3) moment generation, (4) dynamic flexion/extension. For each experiment, cervical alignment was captured using fluoroscopy and muscle activity was captured using electromyography. The inverted posture and muscle activations were found to be different than the upright relaxed posture and the differences depend on the position of the head (study 1). Actively tensing the neck muscles in a free unconstrained task (study 2) and in generating flexion and extension forces with head constraint (study 3) resulted in different cervical alignment compared to the initial resting spine. Not only do these neck muscle contractions induce postural changes, they also provide a substantial stiffening effect to the neck. Finally, dynamically arriving at the neutral position did not result in the same cervical alignment as static neutral and the alignment depended on the direction that neutral is approached from (full flexion or full extension). These findings suggest that it may not be sufficient to replicate the upright resting posture in cadaveric and computational models of neck injury. Adopting in vivo postures and muscle activations, relevant to head-first impact, in the laboratory may help in replicating the spectrum of injuries observed in real life rollovers, an important step toward injury prevention.
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Cervical spine and spinal cord injuries are significant health concerns. Although lateralforces are present during real-world head-first impacts, there is a lack of information aboutcombined lateral bending moments with axial compression. The general aim of this researchwas to evaluate the effects of lateral bending in dynamic axial compression of the cervicalspine on kinetics, kinematics, canal occlusions, and injuries of the cervical spine and thisrequired the development of novel loading and measurement apparatus. We experiencedtechnical challenges in experimentally producing lateral bending moments requiring novelloading methods. Also, as acoustic emission (AE) signals could provide more objectiveestimates of the timing of injuries produced experimentally, these techniques were developedfor use in the spine.In Study 1, techniques were developed to measure the time of injury of isolated spinalcomponents using AE signals. Injuries to human cadaver vertebral bodies resulted in AEsignals with higher amplitudes and frequencies than those from ligamentum flavumspecimens.Study 2 presented a theoretical and experimental evaluation of the effects of testconfiguration on bending moments during eccentric axial compression. Designrecommendations were provided that allowed us to apply appropriate bending moments inthe subsequent studies.In Studies 3, 4, and 5 dynamic axial compression forces with lateral eccentricities wereapplied to human cadaver cervical spine segments and AE signals were used to detect thetime of injury. High lateral eccentricities resulted in lower peak axial forces, inferiordisplacements, and canal occlusions and greater peak ipsilateral bending moments, bendingrotations, displacements, and spinal flexibilities in lateral bending and axial rotationcompared to low eccentricity impacts. Also, low and high lateral eccentricities producedprimarily hard and soft tissue injuries, respectively. In this three-vertebra model, AE signalsfrom injuries to endplates and/or vertebral bodies had higher amplitudes and frequencies thanthose from injuries to the intertransverse ligament and/or facet capsule.The effects of lateral bending in dynamic axial compression on injury mechanisms of thecervical spine and the injury detection techniques demonstrated in this thesis may potentiallyassist in the development and improvement of injury prevention and treatment strategies.
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Knowledge of proximal femur failure mechanics has a pivotal role to play in predicting who might suffer a hip fracture. Previous researchers of sideways falls resulting in hip fracture have investigated the roles of several bone parameters such as bone mineral density and morphology, as well as different modelling boundary conditions. While important advances have been made, current models use constant displacement rates applied at the greater trochanter, which may not allow the bone to respond as it would in a sideways fall. Proximal femurs have been shown to be sensitive to displacement rate, but impacts like those in a sideways falls have never been examined. The goal of this thesis was to compare the results of constant displacement rate testing to more biofidelic, inertially driven, impact fall simulation testing and determine how these methods influence specific test outcomes. In study 1, sub-failure loads were applied to single bones at constant displacement rate, followed by impact loading in the fall simulator. Stiffnesses, energies and strains at a point were compared. In study 2, the same bones were compared using digital image correlation to examine bone strains on the anterior-superior femoral neck. In study 3, two cohorts of bones were loaded to failure, one at constant displacement rate and the other in the impact fall simulator. Initial failure locations, fracture patterns, stiffnesses and energies to failure were compared. The results of study 1 indicate that the behaviours of the bones were not affected by the change in loading. In study 2, I found that individual specimen strain maps were sensitive to the change in loading parameters; however, pooling the data from all the specimens yielded no statistical difference. In study 3, I discovered that final fracture patterns were different, but initial failure locations, stiffnesses and energies to fracture were not. Additionally, femurs tested in the fall simulator did not show significant viscoelasticity. These data indicate that sub-failure measures of mechanical bone behaviour are insensitive to changing between constant displacement rate and impact fall simulation testing; however, strain pattern and final fracture behaviours are influence by changingthe loading protocol.
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Cervical spine and spinal cord injuries (SCI) have catastrophic and permanent neurological consequences and are known to occur from head-first impacts in many activities where helmets are worn. A particularly dangerous posture for catastrophic cervical SCI from head-first impacts occurs when the head is flexed (nodded) approximately 30 degrees downward such that the cervical spinal column becomes aligned. In this posture, the neck reacts axially along its stiffest axis such that high forces develop over small displacements. The deceleration of the torso creates strain energy in the vertebrae beyond their tolerance. It has been shown that increasing constraint on the head at impact places the cervical spine at greater risk of injury compared to less constraining head conditions that allow the head to rotate and translate along the impact surface. At impact speeds near the tolerance for injury, this degree of head constraint can make the difference between avoiding neck injury altogether or the development of unstable neck fractures.This thesis involves the design, construction, and testing of a mechanical head, neck, and a helmet prototype that induces horizontal motion to the head as a neck injury mitigation approach in aligned column head-first impacts. In addition, a new in vitro cervical spine model of head-first impact was developed. All testing utilized a free standing drop tower to create an experimental model of the head, neck and torso system.The head and neck model exhibited an impact response that was in good agreement with the in vitro human response and was sensitive to surface compliance and platform angle. The lower-neck, head, and impact surface were instrumented to provide estimates of impact severity. The helmet prototype, of realistic size, mass, and inertia, showed that when the induced head motion acted to increase the obliqueness of the impact, a combined injury metric comprised of peak neck axial force and peak bending moment was reduced by 30% to 51% compared to testing without induced head motion. These reductions in lower-neck reaction loads were achieved without significant increases, or accompanying decreases in head accelerations. This work is being used to develop and test subsequent helmet prototypes.
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Despite concentrated research efforts there is currently no treatment for spinal cord injury (SCI). Several researchers have identified that cerebrospinal fluid (CSF) may have a role in the biomechanics of the injury event and in the secondary physiologic response, but this has not been closely examined. The aim of this thesis was to develop a large animal model and a benchtop model of human SCI, and to use these to characterise (1) the pressure response of the CSF during the SCI event, (2) the effect of CSF thickness on mechanical indicators of injury severity, and (3) the pressure differentials and cord morphology associated with thecal occlusion and decompression.Study 1 presented the large animal model and provided preliminary CSF pressure transient data that indicated further investigation was warranted. In Study 2, the CSF pressure transients from medium and high severity human-like SCIs were characterised. The peak pressures at 30 mm from the impact were within the range associated with experimental traumatic brain injury, but the wave was damped to peak pressures associated with noninjurious everyday fluctuations by 100 mm. In Study 3, results from the bench-top model demonstrated that the thickness of the CSF layer is directly proportional to the resultant peak CSF pressure, cord compression and impact load.In Study 4, the cranial-caudal CSF pressure differential increased gradually over eight hours of thecal occlusion. Decompression eliminated or reduced the differential, after which it did not change significantly. These results indicate that lumbar CSF pressure measured prior to decompression may not be representative of CSF pressure cranial to an injury. In Study 5, the change in spinal cord and thecal sac morphology after surgical decompression was assessed with ultrasound. Moderate SCI was associated with a residual cord deformation and then gradual swelling, while high severity SCIs exhibited immediate swelling which occluded the thecal sac within five hours. The different aspects of CSF response to SCI demonstrated in this thesis can potentially be used to assess and validate current and future models of SCI, and to guide future studies of clinical management strategies such as CSF drainage and early decompression.
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Master's Student Supervision
Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.
Whiplash is a common automotive injury and can cause debilitating symptoms including neck stiffness and pain, headaches, and other cognitive/psychological symptoms. The diagnosis, treatment, and prevention of whiplash is challenged by the lack of scientific consensus on the mechanism of injury and source of symptoms. One theorized whiplash injury mechanism postulates that the rapid head/neck motion during a rear-end collision causes pressure transients in the spinal cerebrospinal fluid (CSF) which strain the sensory nerve cells in the cervical dorsal root ganglia. This study aimed to improve understanding of this potential injury mechanism by 1. Developing an in-vivo whiplash injury model to produce controlled and repeatable whiplash exposures, and 2. Correlating the head kinematics in simulated whiplash exposures with the cervical CSF pressure response. A custom test apparatus consisting of programmable servo-motors was developed that could simulate specific programmed whiplash exposures in a porcine model. An in-vivo porcine model was selected due to the gross anatomical and scale similarities to humans, and to enable measuring live spinal CSF pressure. Four anaesthetized Yorkshire pigs underwent surgery to implant three fiber-optic pressure transducers in the cervical intrathecal space. Cranial surgery was also performed to rigidly attach accelerometers and angular rate sensors for measuring head kinematics. Each instrumented animal experienced multiple whiplash exposures where the severity and shape of the whiplash exposure was altered. Relevant head kinematic parameters were extracted and correlated with the cervical CSF pressure response. Qualitatively, all whiplash exposures across the different subjects produced similar patterns of local and global maxima in CSF pressure. Maximum angular rate of the head and Neck Injury Criterion (relates the relative motion of the head and torso) were positively correlated with local pressure maxima, while delay in extension onset was negatively correlated with the local pressure maxima. Additionally, maximum head extension and time to maximum extension were positively, and negatively, correlated with global pressure maxima, respectively. These findings can be used to inform the design of automotive safety systems such as active anti-whiplash seats to reduce the pressure amplitudes in the cervical spine and risk of injury during whiplash exposures.
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Safety devices meant to protect the head and neck are often evaluated with the use of an anthropometric test device. Anthropometric test devices are designed for a specific loading scenario and typically are biofidelic only in that application. There is no single surrogate appropriate for the multiplane loading that often occurs in real-world scenarios. In this thesis, I present the development of a surrogate upper cervical spine model (C0-C2 vertebrae) for eventual use in an omnidirectional anthropometric test device neck and its validation under quasi-static loading. We obtained CT scans from a 31-year-old male with no cervical spine pathologies from Vancouver General Hospital. These scans were segmented, modified and 3D printed in aluminum. The transverse, alar, and nuchal ligaments were replicated in the model as they are believed to be the most deterministic to the kinematics of the region. For testing, a custom spine machine was used to apply pure moments in flexion-extension, lateral bending, and axial rotation to the specimen at quasi-static rates. Movements of the vertebrae were tracked using a motion analysis system. In this way, the applied moments and corresponding movements of the vertebrae can be recorded and evaluated. Range of motion, neutral zone and mean helical axis of motion were extracted from the resultant moment-rotation plots and compared to the cadaveric literature. Quantitative curve shape analysis was carried out to assess the shape of the prototype moment-rotation curve to those from cadaveric literature. The range of motion and neutral zones in flexion-extension, lateral bending, and axial rotation are within range of the cadaveric results presented. Quantitative curve shape comparisons resulted in biofidelity ratings from fair to excellent. The mean helical axis of motion was aligned with that reported in cadaveric studies in axial rotation and slightly anterior to what was expected in flexion-extension. Reproducing the kinetic and kinematic responses of surrogate spinal segments will aid in the construction of a biofidelic omnidirectional durable surrogate neck. Such a neck could be used to evaluate, improve, and optimize head and neck safety equipment for transportation, occupational, and sports settings.
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Globally, millions of spinal cord injuries (SCIs) and traumatic brain injuries (TBIs) occur annually [WHO, 2013], [Dewan, 2019], most commonly in vehicular crashes, falls and sports [Cripps, 2011], [Singh, 2014]. These injuries lead to paralysis, a decrease in mental health [DeVivo, 1991], [Losoi, 2016], and/or financial hardship [McGregor, 1997], [Lenehan, 2012]. SCIs and TBIs are most likely to occur in men [Singh, 2014], [Nguyen, 2016], [CDC, 2016], at the C4-C5 level [Prasad, 1999]. As SCIs and TBIs have a high occurrence and dire health consequences, it is crucial that they are prevented. Safety equipment meant to prevent head and neck injuries, such as helmets and seatbelts, are evaluated using anthropometric test devices (ATDs). Unfortunately, there is no ATD neck that is biofidelic for the multiplane loading that occurs in real-world scenarios [Nelson, 2010]. As such, the aim of this research was to take a step towards creating a biofidelic omnidirectional surrogate neck for the evaluation of these safety devices. This first step was to design and construct an anatomically accurate sub-axial functional spinal unit (FSU) that produced a repeatable biofidelic response in quasi-static flexion-extension (FE), axial rotation (AR), lateral bending (LB), and coupled motion. This research was carried out in three main phases. The first was to design, construct and test synthetic sub-axial cervical segments in AR and FE to obtain a basic understanding of their inherent complex motion. The second was to perform compression and tension testing on potential surrogate intervertebral disc and ligament materials, respectively. This was to identify the structural properties of the components in order to select biofidelic materials, when compared to published cadaveric results, for use in the final FSU. In the final stage, anatomically accurate C4 and C5 vertebrae were constructed from the CT scans of a 31-year-old male. An FSU was built using the previously selected soft tissues and when compared to published cadaveric results, showed acceptable biofidelity in AR, FE, LB and coupled motion. The process of constructing this biofidelic FSU will inform future construction of a full surrogate neck to be used in the testing of head and neck safety equipment. [An errata to this thesis/dissertation was made available on 2021-03-03.]
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Structural changes that occur with intervertebral disc (IVD) degeneration have been established. However, there has been a major challenge in differentiating changes that occur solely due to aging from those that might be considered pathological; uncertainty still exists regarding the exact anatomic and physiologic basis for several clinical symptoms. Diffusion-weighted imaging techniques in MRI have emerged as a potential method to further our understanding of the effects of degeneration on the mechanical behaviour of the intervertebral disc. This thesis describes three stages of work: first, a force measurement system was developed to measure forces acting on the intervertebral disc in the MRI environment using Fibre Bragg Grating sensors. Secondly, a loading rig was designed that applied loading conditions simulating postures of the spine (supine, standing, flexion, and extension) to lumbar cadaveric specimens, and an imaging protocol was developed to obtain ADC measurements within defined regions of the disc. ADC values were then compared in these regions between loading conditions. In the final stage, a device was designed to apply compressive strain to annulus fibrosus tissue samples. In this part of the study, the relationship between ADC values and both compressive strain and degenerative state were evaluated.We successfully developed a load measurement system, which was then used to record forces acting on the disc during diffusion imaging. We found ADC values to be significantly higher in the NP than the AF; no change in ADC values was observed between different regions of the AF, regardless of applied load. Further, we found no change in ADC between any of the four loading conditions within any region of the disc.In samples of AF tissue, we found no change to ADC with increasing compressive strain, regardless of degenerative level. A significant decrease in ADC values was found between severely degenerated discs and both mild and moderately degenerated discs. These results expand our understanding of the limitations of ADC measurements to detect changes to mechanical behaviour of the disc, and may help guide the direction of future studies towards other avenues of investigation of disc mechanics.
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Vehicle rollovers account for 3% of motor vehicle crashes yet cause one-third of all crash-related fatalities. Despite advanced cervical spine injury models, a discrepancy exists between clinically reported injuries and cadaver test pathologies. One possible explanation for this discrepancy is that the intervertebral posture and simulate muscle tone used in cadaver models (and computer models) typically mimic an upright and relaxed condition that may not exist during a rollover. The aim of this work was to characterize vertebral alignment and neck muscle responses in the cervical spine by studying a human subject in a simulated impending headfirst impact, in an upside-down configuration. A custom inversion device was built to expose human subjects to a 321 ms inverted free fall drop. An onboard fluoroscopic C-arm captured cervical vertebral motion while indwelling electromyography captured the response of 8 superficial and deep neck muscles. The subject shoed consistent muscular responses in 4 repetitions of the free-fall exposure. Moreover, the muscle response pattern was different from the scheme used in existing cervical spine injury models and observed in previous quasi-static tests conducted in our lab. The general trends in muscle-induced changes to vertebral alignment were consistent with our previous work. C3-C6 translated anteriorly and inferiorly in response to the inverted free fall stimulus, and the head moved into flexion. These observations suggest that, at the time of impact, the in vivo state of the neck may differ considerably from its initial alignment prior to the forewarned impact. The in vivo data set acquired from this experiment of vertebral and muscular responses could be used to improve and validate current injury models and advance injury prevention strategies in rollover crashes.
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With head injury being the leading cause of death from skiing and snowboarding in North America, a better understanding of the mechanisms at play and improved preventative measures are necessary. Safety certification standards exist for snow sport helmets in an effort to evaluate potential technologies as well as ensure helmets offer protection to the user. However, current protocols are seen to be oversimplifications of real world head impacts, particularly from skiing and snowboarding. The purpose of this work is to mechanistically characterize snow sport head injury and design a test apparatus capable of representing these real world head impact scenarios.In an effort to characterize the fall mechanisms and injuries of snow sport head impact, a clinical investigation was performed. A 6 year retrospective clinical case review yielded a database of 760+ incidents for which basic demographic information, gross mechanism detail, nature and severity of injuries sustained and helmet use data was collected. In addition to epidemiological insight, the database highlighted the need for a revised standard testing protocol through observation of several general fall scenarios, a high prevalence of concussion (considered a low-energy injury) and the majority of impacts occurring to snow or ice surfaces.This information, in conjunction with existing biomechanics literature, informed the design of a helmet testing apparatus capable of recreating snow sport head impact mechanisms. Through a formal design process involving stakeholder discovery, development of design requirements, concept generation and evaluation, and detailed design, a final apparatus was decided upon and fabricated. To investigate if the test apparatus was capable of satisfying the requirements set forth, namely impact velocity and repeatability, verification testing was performed. Recommendations are made for conditions that remained either partially met or unmet.To address the need for an improved understanding of snow sport head injury mechanisms in the context of helmet testing, clinical data and existing literature was used. As a result, a test apparatus capable of more representative impact testing protocols was developed. Aspects of this work can be adopted by the head injury research and helmet standards communities in order to improve design and evaluation of preventative equipment.
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The spinal canal occlusion transducer (SCOT) is a sensor used to detect changes in geometry of the cervical spinal canal. A constant current field is created in saline filling the SCOT casing. Radial compression of the SCOT casing results in a change in cross sectional area and an increased resistance to current flow. Sensing elements along the SCOT are used to detect the change in potential difference along the length of the probe and quantify the change in area of the SCOT casing. Shortfalls of the existing SCOT iteration were identified for improvement. These included long term durability and stability of the signal. A new design for the SCOT probe was sought with the aim of addressing these shortfalls. Six prototype SCOT probes were designed, manufactured and tested. Prototypes considered sensing elements, the casing materials used, input signal conditions and the method used to construct the SCOT probe. Signal to noise ratio and stability of signal were used to compare the prototypes against the original SCOT. The design chosen consisted of a heat shrink casing, 3.97 mm diameter SS ball bearings for ground and excitation elements, and 3.18 mm diameter SS ball bearings for sensing elements. Electrical connections were soldered. A sinusoidal input with frequency of 3 kHz and peak to peak amplitude of 1.5 V was used. The minimum measurable total canal area was 107.6 ± 1.2 mm². The signal to noise ratio of the new SCOT was 74.3 dB and the variation of unoccluded SCOT signal was found to be 1.4 ± 2.5 mVRMS compared to 74.4 dB and 2.3 mVRMS for the previous iteration. The new SCOT was used in a flexibility study of cervical spine segments. Specimens were loaded in flexion-extension, left-right lateral bending and axial rotation up to 4 Nm at 2 ˚/s. Mean maximum percent decreases in total canal area were 5.8%, 15.9%, 5.1%, 4.6%, 4.6% and 4.2% for flexion, extension, left axial rotation, right axial rotation, left lateral bending, and right lateral bending, respectively. Only extension consistently demonstrated mean maximum decreases that exceeded the mean error of the SCOT (5.3 ± 1.3 mm²).
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Post traumatic spinal cord swelling can occur as a result of a spinal cord injury and may have negative effects on a patient’s neurological outcome. Spinal cord swelling is hypothesized to be associated with an increased pressure in the cord tissue. Experimental measurement of pressure in the in vivo cord would facilitate the study of spinal cord swelling and its effects. The spinal cord is a soft biological tissue consisting of fluid and solid components in which measuring and interpreting pressure is challenging.The objective of this research was to evaluate the feasibility of using fiber optic pressure sensors to directly measure intraparenchymal cord pressure. Fiber optic pressure sensors were used to measure intraparenchymal cord pressure in ex vivo pig cords under two conditions. A focal stress was incrementally applied to the cord to simulate sustained compression and decompression (i). Hydrostatic pressure was applied to the cord to simulate swelling (ii). The hydrostatic pressure was applied in three phases: a ramp to increase the pressure, a one hour hold at constant pressure and a ramp to decrease pressure using a fluid filled tank.During applied focal stress (i), results showed distinct intraparenchymal cord pressure increases and similar trends across trials. Most trials had a linear trend or region with strong correlations (r² > 0.9) between applied force and intraparenchymal cord pressure. However, when combining all trials, this association weakened (r² = 0.648). During ramping applied hydrostatic pressure (ii), the intraparenchymal cord pressure increase followed closely to the pressure in the surrounding fluid. In contrast, during the hold, the intraparenchymal cord pressure gradually increased while the pressure in the surrounding fluid remained unchanged. This resulted in a significant difference between the pressure changes seen in the cord and in the surrounding fluid. We conclude that the fiber optic pressure sensors are capable of measuring fluid pressure in spinal cord tissue. Based on the content of this thesis, we recommend the use of these sensors to examine relative intraparenchymal cord pressure in events occurring at a rate of 5 N/s up to 300 N in our in vivo porcine model for SCI.
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The Pro-Neck-Tor (PNT) helmet was developed to reduce the incidence of cervical spine injuries in head-first impacts. By guiding the head out of the path of the following torso, the helmet reduces the compressive loads on the spine. It consists of an inner- and outer-shell that are connected by an internal guide mechanism. In an impact the mechanism deploys and guides the inner shell and head relative to the outer shell. The helmet is intended to induce extension in slightly anterior-to-vertex impacts, and flexion in slightly posterior-to-vertex and vertex impacts. A passive deployment mechanism was previously designed, but was not thoroughly tested. The objective of this work was to assess the functionality of the passive PNT deployment mechanism, and suggest design improvements accordingly. The selector mechanism was prototyped and tested in a drop tower. The impact platform was positioned at three different angles (-15°, 0° and 15°) to generate posterior, vertex, and anterior impacts. The mechanism deployed in the correct direction in the -15° and 15° impact conditions, but deployed incorrectly in the 0° impact condition. Additionally, the mechanism did not induce sufficient head rotation prior to head rebound to reduce the loads on the cervical spine. A multibody model of the experimental apparatus was constructed to determine how the impact forces were transmitted to the mechanism to initiate flexion or extension deployment. A dynamic analysis revealed that the net moment on the head determined the deployment mode. This moment was affected by loading from the torso. Ultimately, the deployment mode should depend only on the location of an impact on the head. A new selector mechanism was designed, which is capable of sensing anterior impacts according to the direction of the loading vector on the head. By default, the mechanism deploys in flexion. In anterior impacts a switch is triggered, which changes the deployment mode to extension. A computational model of this mechanism showed the mechanism deployed appropriately in anterior, posterior and vertex impacts. The mechanism also produced greater head rotation prior to head rebound than the passive mechanism. Physical prototyping of this mechanism is recommended in the future.
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There is currently a disagreement between the hips that today’s screening techniques identify as likely to fracture and those that actually fracture. Specimen-specific finite element (FE) analysis based on computed tomography (CT) data has been presented as a more sensitive technique than standard screening based on bone mineral density (BMD) to screen for and identify hips predisposed to fracture. However, published studies using this technique have applied loading scenarios that do not sufficiently resemble the falls that typically lead to fracture. In particular, the effects of dynamics and strain rate stiffening have been neglected in all relevant FE studies.It is my objective to explore here these previously neglected aspects. This thesis is divided into three sections focusing respectively on: 1) the effect of material stiffness and mapping techniques on a simplified quasi-static model; 2) the feasibility, accuracy, and model sensitivity of explicit dynamic FE analysis to simulate a fall impact on the femur; and 3) the feasibility of a novel multi-scale FE technique to analyse femoral bone behaviour at specific sites with a level of detail that resolves the trabecular structure. All models are compared against experimental data obtained from mechanical tests of femurs at appropriate loading rates.Results demonstrate that both dynamic and multi-scale modelling are feasible techniques that can be developed into powerful tools. Both quasi-static and dynamic FE models demonstrated sensitivity to material modulus-density relationship, with further work required to estimate the increased stiffness of bone during impact loading. For dynamic models, surface strain patterns matched fracture locations observed from high-speed video. The multi-scale technique demonstrated its value by identifying a potential stress concentration around a vascular hole that was otherwise hidden.This study demonstrates that explicit dynamic FE analysis is a feasible technique for modelling the altered bone behaviour observed during femoral impacts, yet further research is required to design material models appropriate for impact rates. It is also demonstrated that nested multi-scale FE modelling is not only feasible but also potentially useful to identify microstructural features of bone that might make it prone to fracture even in the setting of relatively high BMD.
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The average age of people suffering spinal cord injuries (SCIs) is shifting toward an older population, frequently occurring in the spondylotic (degenerated) cervical spine, due to low energy impacts. Since canal stenosis (narrowing) is a common feature of a spondylotic cervical spine, flexion or extension of such a spine can compress the spinal cord. This thesis involves two studies investigating the effects of spondylosis on the kinematics of the cervical spine and on compression of the spinal cord during spine motion.The first study developed and evaluated an image analysis technique that measures a new combination of degenerative and kinematic continuous, quantitative variables in cervical spine sagittal plane flexion-extension image pairs. This technique, evaluated using plane X-ray, effectively quantified angular range of motion, anterior-posterior (AP) translation, intervertebral disc height, pincer spinal canal diameter, and osteophyte length. The angular accuracy and linear precision were found to be ±1.3° and approximately ±0.6mm, respectively. This compared well to previous studies and is adequate for potential clinical applications.The second study quantified the effect of increasing anterior canal stenosis on spinal cord compression during spine motion. This study used a whole porcine cadaveric cervical spine, a radio-opaque surrogate spinal cord, and an artificial osteophyte. The spine was imaged by sagittal plane X-ray during quasistatic pure moment flexion-extension bending. This study demonstrated that the cadaveric model could simulate the typical spondylotic SCI mechanisms in both flexion (bowstring stretching) and extension (pincer). Spinal cord AP diameter could be measured accurately within ±0.25mm and cord diameter differences could be measured within ±0.5mm. Cord compression due to the artificial osteophyte increased with increased canal stenosis, but never exceeded 1mm.The image analysis techniques developed in the first study and results of future studies based on these techniques may be used to improve cadaveric modelling of SCI due to low energy impacts in the presence of age-related spine degeneration. Improved understanding of injury mechanisms may aid clinical intervention to both prevent and treat SCI in the presence of age-related spine degeneration.
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Spinal cord injuries (SCIs) are commonly studied experimentally by causing injury to rodent spinal cords in vivo and analyzing behavioral and histological results post injury. Few researchers have directly investigated the deformation of the in vivo spinal cord during impact, which is thought to be a predictor of injury. This knowledge would help to establish correlations among impact parameters, internal structure deformation, and histological and functional outcomes. The objective of this thesis was to develop a radiographic method of tracking the real-time internal deformations of an anesthetized rat‘s spinal cord during a typical experimental SCI. A technique was developed for injecting fiducial markers into the dorsal and ventral white and grey matter of in vivo rat spinal cords. Two radio-opaque beads were injected into C5/6 in the approximate location of the dorsal and ventral white matter. Four additional beads were glued to the surface of the cord caudal and cranial to the injection site (one dorsal, one ventral). Overall bead displacement was measured during quasi-static compression using standard medical x-ray equipment. Dynamic bead displacement was tracked during a dorsal impact (130mm/s, 1mm depth) by imaging laterally at 3,000 fps using a custom high-speed x-ray system. The internal spinal cord beads displaced 1.02-1.7 times more than the surface beads in the cranial direction and 2.5-11 times more in the ventral direction for the dynamic impact and maximum quasi-static compressions. The dorsal spinal cord beads (internal and surface) displaced more than the ventral spinal cord beads during all compressions. Finite element modeling and experimental measurements suggested that bead migration with respect to the spinal cord tissue was small and mostly insignificant. These results support the merit of this technique for measuring in vivo spinal cord deformation. The differences in bead displacements imply that the spinal cord undergoes complex internal and surface deformations during impact. Many applications of this technique are conceivable including validating finite element and surrogate models of the spinal cord, comparing localized grey and white matter motion during impact to histological findings, and improving SCI preventative and treatment measures.
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INTRODUCTION: The majority of individuals with chronic spinal cord injury (SCI) experience spasticity, which can often impair function and degrade quality of life. Reports by individuals with SCI suggest that whole body vibration (WBV), as can occur while riding wheelchairs, may trigger spasticity.OBJECTIVES: 1) Examine the influence of wheel design on wheelchair vibration. 2) Develop a system allowing exposure of individuals with SCI to WBV and analysis of muscle activity to identify spasticity.METHODS: 1) A wheelchair wheel comparison study: Vibration acceleration and frequency content produced by wheelchairs equipped with 2 different wheel designs (steel spoked and composite material spoked) were compared as: 1a) 13 subjects with SCI wheeled through an obstacle course simulating a wheelchair user's daily activities 1b) 22 non-SCI subjects wheeled down a ramp and over a vibration inducing obstacle. Vibration acceleration was recorded using 2 accelerometers mounted on the wheelchairs' main axle and footrest. The influence of wheelchair vibration on spasticity was assessed using questionnaires, completed by the SCI subjects.2) A controlled whole body vibration (CWBV) pilot study: 2 SCI subjects were exposed to 7-12 WBV sessions. Each exposure consisted of a single frequency, lasted 20 seconds, and was repeated on 2 separate days. The WBV was applied using an electrodynamic shaker and the subjects' leg muscles' activity was recorded using an electromyography (EMG) system. Muscle spasms were identified by calculating the ratio between periods of increased muscle activity and the period before exposure to vibration.RESULTS: No statistically significant differences (p=0.05) were found in wheelchair vibration acceleration or frequency content between the 2 tested wheel designs and no clear correlation between wheelchair vibration and spasticity was apparent. The CWBV system was able to apply vibration (-+0.5 Hz, -+0.001 g) and record muscle activity (-+7 mV). The CWBV exposures produced several muscle responses that were considered to be spasms.CONCLUSIONS: The tested composite material spoked wheels do not differ, in vibration performance, from steel spoked wheels. The developed CWBV apparatus appears suitable for studying muscle activity in response to WBV.
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