Peter Cripton

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.

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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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