Traumatic brain injury (TBI) is a major cause of death and disability. TBI is frequently followed by cerebral ischemia which is a great contributor to secondary brain damage. The main causes of cerebral ischemia are pathophysiological changes in cerebral blood flow and metabolism. Treatment of TBI patients is currently based on intracranial pressure (ICP) and cerebral perfusion pressure (CPP) targeted treatment protocols. However, ICP and CPP alone do not provide information of the oxygen availability in the brain. Monitoring of brain tissue oxygenation (BtipO2) may give additional and valuable information about the risk for development of ischemia in TBI patients. The aims of this thesis were to study BtipO2 monitoring devices in-vitro regarding accuracy and stability, to detect threshold level of cerebral ischemia in-vivo and finally to examine the cerebral oxygen levels and cerebral metabolism in TBI patients. The BtipO2 probes performed with high accuracy and stability at different clinically relevant oxygen concentrations. A pig TBI model was developed by step-wise intracranial volume/pressure increase. Volume increase resulted in a gradual increased ICP, decreased CPP, intracranial compliance and BtipO2, respectively. Brain death (BD) was confirmed by negative CPP and negligible amount of previously injected microspheres in the brain tissue. The model simulated the clinical development of BD in humans with a classical pressure-volume response and systemic cardiovascular reactions. The model should be suitable for studies of brain injury mechanisms. From the same in-vivo model it was also possible to detect the threshold level of cerebral ischemia in the pig, where BtipO2 below 10 mmHg and CPP below 30 mmHg was associated with an impaired cerebral metabolism (microdialysis lactate to pyruvate ratio >30). BtipO2 together with cerebral microdialysis were studied in 23 severe TBI patients. We observed different patterns of changes in BtipO2 and cerebral microdialysis biomarkers in focal and diffuse TBI.  Increased cerebral microdialysis levels of glutamate, glycerol or the lactate/pyruvate ratio were observed at BtipO2 < 5 mmHg, indicating increased vulnerability of the brain at this critical level of tissue oxygenation in TBI patients.

Diss. (sammanfattning) Stockholm : Karol. inst., 2002. / Härtill 5 uppsatser.

Head injuries are a major health concern for sport participants as 90% of emergency department visits for sport-related brain injuries are concussion related (Canadian Institute for Health Information, 2016). Recently, reports have shown a higher incidence of sport-related concussion in female athletes compared to males (Agel et al., 2007). Few studies have described the events by which concussions occur in women’s hockey (Delaney et al., 2014, Brainard et al., 2012; Wilcox et al., 2014), however a biomechanical analysis of the risk of concussion has not yet been conducted. Therefore, the purpose of this study was to identify the riskiest concussive events in elite women’s hockey and characterize these events through reconstructions to identify the associated levels of peak linear and angular acceleration and strain from finite element analysis. 44 head impact events were gathered from elite women’s hockey game video and analyzed for impact event, location and velocity. In total, 27 distinct events based on impact event, location and velocity were reconstructed using a hybrid III headform and various testing setups to obtain dynamic response and brain tissue response. A three-way Multivariate Analysis of Variance (MANOVA) was conducted to determine the influence of event, location and velocity. The results of this study show that head-to-ice impacts resulted in significantly higher responses compared to shoulder-to-head collisions and head-to boards impacts however, shoulder and boards impacts were more frequent. All events produced responses comparable to proposed concussion threshold values (Zhang et al., 2004). This research demonstrates the importance of considering the event, the impact characteristics, the magnitude of response, and the frequency of these impacts when attempting to capture the short and long term risks of brain trauma in women’s hockey.

The long-term consequences of repetitive mild traumatic brain injuries (mTBIs), or concussions, as well as the immediate acute dangers of head collisions in sport have become of growing concern in the field of medicine, research and athletics. An estimated 3.8 million sports-related concussions occur in the United States annually, with the highest incidence having been documented in football, hockey, soccer, basketball and rugby (Harmon et al., 2013). The incidence of concussion in the National Rugby League (NRL) corresponds to approximately 8.0-17.5 injuries per 1000 playing hours, with tackling having been identified as the most common cause (Gardner et al., 2014; King et al., 2014). The highest incidence of rugby concussive impacts is a result of shoulder-to-head collisions (35%) during tackles and game play (Gardner et al., 2014). Shoulder-to-head concussive events occur primarily on the ball carrier and secondarily on the tacklers (Hendricks et al., 2014; Quarrie & Hopkins, 2008). While some studies report that the ball carrier is at a greater risk of sustaining a concussion (Gardner et al., 2015; King et al., 2010, 2014), others have demonstrated a greater incidence of tacklers being removed from play for sideline concussion evaluation (Gardner et al., 2014). Given this discrepancy, the purpose of this study was to compare dynamic response and brain tissue deformation metrics for ball carriers and defensive tacklers in professional rugby during shoulder-to-head concussive impacts using in-laboratory reconstructions. Ten cases with an injured defensive tackler and ten cases with an injured ball carrier were reconstructed using a pneumatic linear impactor striking a 50th percentile Hybrid III headform to calculate dynamic response and maximum principal strain values. There was no significant difference between the two impact conditions for peak resultant linear and rotational accelerations, as well as brain tissue deformation. Differences between metrics in this research and past research where the impacting system was not reported were discussed. These differences reflect the importance of accounting for impact compliance when describing the risk associated with collisions in professional rugby.

In this dissertation, theoretical, computational, and experimental methodologies are introduced to determine the rate-dependent material properties of the brain tissue. Experiments have shown that the brain tissue is significantly rate-dependent. To examine the range of strain rates at which trauma might happen, a validated finite element (FE) human head model was initially employed to examine the biomechanics and dynamic behavior of the head and brain under impact and blast loads. The strain rates to cause traumatic brain injury (TBI) were found to be in the range of 36 to 241 1/s, under these types of loadings. These findings provided a good estimation prior to exploring the required experiments for characterizing the brain tissue. The brain samples were tested by employing unconfined compression tests at three different deformation rates of 10 (n= 10 brain samples), 100 (n=8), and 1000 mm/sec (n=12). It was found that the tissue exhibited a significant rate-dependent behavior with various compression rates. Two different material characterization approaches were proposed to evaluate the rate-dependent mechanical responses of the brain. In the first approach, based on the parallel rheological framework, a single-phase viscoelastic model which captures the key aspects of the rate-dependency in large strain behavior was introduced. The extracted material parameters showed an excellent constitutive representation of tissue response in comparison with the experimental test results (R^2=0.999). The obtained material parameters were employed in the FE simulations of the brain tissue and successfully verified by the experimental results. In the second approach, the brain tissue is modeled as a biphasic continuum, consisting of a compressible solid matrix fully saturated with an incompressible interstitial fluid. The governing equations based on conservation of mass and momentum are used to describe the solid-fluid interactions. This viscoelastic biphasic model can effectively estimate the rate-dependent tissue deformations, the hydrostatic pressure as well as fluid diffusion through the tissue. Although both single-phasic, as well as bi-phasic models, can successfully capture the key aspects of the rate-dependency in large strain deformation, it was shown the biphasic model can demystify more phenomenological behavior of this tissue that could not be perceived with yet established, single-phasic approaches.

<p>Traumatic brain injury (TBI) is characterized by a violent or sudden blow to the head that causes tearing or bruising of the brain tissue and its supporting blood vessels. Determination of the mechanical properties of gray and white matter is critical for the creation of computational models of healthy and TBI-damaged brain tissues. Current in vivo methods to characterize brain tissue, such as 3D amplified MRI (aMRI) and magnetic resonance elastography (MRE), are highly vulnerable to motion artifacts and have limited techniques to exert mechanical loads on the brain. Therefore, in vitro testing was employed to estimate the chemical composition of gray and white matter using Fourier Transform Infrared (FTIR) spectroscopy and the stress responses of the brain tissues to high compressive deformations via unconfined compression. Attenuated total reflectance (ATR) was run in conjunction with FTIR spectroscopy to eliminate the need for sample preparation. Unconfined compression of gray and white matter samples was performed to 70% of the total sample height at a constant strain rate of 0.35/s. Results showed significant increases in the absorbances of white matter (<em>p</em> < 0.05) in the characteristic lipid and carbohydrate regions of the FTIR spectra when compared to gray matter. Within the initial 10% toe-region of the stress-strain curve, white matter is observed to absorb significantly greater compressive loads (<em>p </em>< 0.05) than gray matter. These results indicate an incomplete characterization of brain tissue; therefore, additional in vitro and in vivo methods are still necessary, separately or in combination, to accurately characterize brain tissue mechanics in TBI and non-TBI patients.</p>

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The results of brain tissue finite element (FE) models under high rate shear deformation are affected by several factors. This thesis evaluated the effects of hourglass control, Poisson's ratio and element type in such simulations. Moreover, a comparison of FE and analytical models were performed related to boundary conditions. The simulations and optimizations were executed in ANSYS, LS-DYNA and LS-OPT. A Rivlin hyperelastic material model with linear viscoelasticity was used to describe the mechanical response of brain tissue. Examples of inverse FE material characterization of representative brain shear experiments at strain rates of 800, 500, 120 and 90 S-1 were studied and the results were validated by the ability to predict wave traveling times and deformed configurations. The difference between experimental and idealized shear strain increased with aspect ratio. One-point-integrated brick element combined with stiffness hourglass control gave the best result. A smaller Poisson's ratio that is still physically meaningful, e.g. 0.495, is preferable. / Mechanical Engineering