Biomechanics of cervical spine and spinal cord injury under combined axial compression and lateral bending loading

Cervical spine and spinal cord injuries are significant health concerns. Although lateral forces are present during real-world head-first impacts, there is a lack of information about combined lateral bending moments with axial compression. The general aim of this research was to evaluate the effects of lateral bending in dynamic axial compression of the cervical spine on kinetics, kinematics, canal occlusions, and injuries of the cervical spine and this required the development of novel loading and measurement apparatus. We experienced technical challenges in experimentally producing lateral bending moments requiring novel loading methods. Also, as acoustic emission (AE) signals could provide more objective estimates of the timing of injuries produced experimentally, these techniques were developed for use in the spine. In Study 1, techniques were developed to measure the time of injury of isolated spinal components using AE signals. Injuries to human cadaver vertebral bodies resulted in AE signals with higher amplitudes and frequencies than those from ligamentum flavum specimens. Study 2 presented a theoretical and experimental evaluation of the effects of test configuration on bending moments during eccentric axial compression. Design recommendations were provided that allowed us to apply appropriate bending moments in the subsequent studies. In Studies 3, 4, and 5 dynamic axial compression forces with lateral eccentricities were applied to human cadaver cervical spine segments and AE signals were used to detect the time of injury. High lateral eccentricities resulted in lower peak axial forces, inferior displacements, and canal occlusions and greater peak ipsilateral bending moments, bending rotations, displacements, and spinal flexibilities in lateral bending and axial rotation compared to low eccentricity impacts. Also, low and high lateral eccentricities produced primarily hard and soft tissue injuries, respectively. In this three-vertebra model, AE signals from injuries to endplates and/or vertebral bodies had higher amplitudes and frequencies than those from injuries to the intertransverse ligament and/or facet capsule. The effects of lateral bending in dynamic axial compression on injury mechanisms of the cervical spine and the injury detection techniques demonstrated in this thesis may potentially assist in the development and improvement of injury prevention and treatment strategies.Applied Science, Faculty ofMechanical Engineering, Department ofGraduat

The Effect of Lateral Eccentricity on Failure Loads, Kinematics, and Canal Occlusions of the Cervical Spine in Axial Loading

Current neck injury criteria do not include limits for lateral bending combined with axial compression and this has been observed as a clinically relevant mechanism, particularly for rollover motor vehicle crashes. The primary objectives of this study were to evaluate the effects of lateral eccentricity (the perpendicular distance from the axial force to the centre of the spine) on peak loads, kinematics, and spinal canal occlusions of subaxial cervical spine specimens tested in dynamic axial compression (0.5 m/s). Twelve 3-vertebra human cadaver cervical spine specimens were tested in two groups: low and high eccentricity with initial eccentricities of 1% and 150% of the lateral diameter of the vertebral body. Six-axis loads inferior to the specimen, kinematics of the superior-most vertebra, and spinal canal occlusions were measured. High speed video was collected and acoustic emission (AE) sensors were used to define the time of injury. The effects of eccentricity on peak loads, kinematics, and canal occlusions were evaluated using unpaired Student t-tests. The high eccentricity group had lower peak axial forces (1544 ±629 vs. 4296 ±1693 N), inferior displacements (0.2 ±1.0 vs. 6.6 ±2.0 mm), and canal occlusions (27 ±5 vs. 53 ±15%) and higher peak ipsilateral bending moments (53 ±17 vs. 3 ±18 Nm), ipsilateral bending rotations (22 ±3 vs. 1 ±2o), and ipsilateral displacements (4.5 ±1.4 vs. -1.0 ±1.3 mm, p<0.05 for all comparisons). These results provide new insights to develop prevention, recognition, and treatment strategies for compressive cervical spine injuries with lateral eccentricities.Applied Science, Faculty ofMedicine, Faculty ofMechanical Engineering, Department ofOrthopaedic Surgery, Department ofUnreviewedFacult

Acoustic Emission Signals Can Discriminate Between Compressive Bone Fractures and Tensile Ligament Injuries in the Spine during Dynamic Loading

Acoustic emission (AE) sensors are a reliable tool in detecting fracture, however they have not been used to differentiate between compressive osseous and tensile ligamentous failures in the spine. This study evaluated the effectiveness of AE data in detecting the time of injury of ligamentum flavum (LF) and vertebral body (VB) specimens tested in tension and compression, respectively, and in differentiating between these failures. AE signals were collected while LF (n=7) and VB (n=7) specimens from human cadavers were tested in tension and compression (0.4 m/s), respectively. Times of injury (time of peak AE amplitude) were compared to those using traditional methods (VB: time of peak force, LF: visual evidence in high speed video). Peak AE signal amplitudes and frequencies (using Fourier and wavelet transformations) for the LF and VB specimens were compared. In each group, six specimens failed (VB, fracture; LF, periosteal stripping or attenuation) and one did not. Time of injury using AE signals for VB and LF specimens produced average absolute differences to traditional methods of 0.7 (SD 0.2) ms and 2.4 (SD 1.5) ms (representing 14% and 20% of the average loading time), respectively. AE signals from VB fractures had higher amplitudes and frequencies than those from LF failures (average peak amplitude 87.7 (SD 6.9) dB vs. 71.8 (SD 9.8) dB for the inferior sensor, p<0.05; median characteristic frequency from the inferior sensor 97 (interquartile range, IQR, 41) kHz vs. 31 (IQR 2) kHz, p<0.05). These findings demonstrate that AE signals could be used to delineate complex failures of the spine.Applied Science, Faculty ofMedicine, Faculty ofMechanical Engineering, Department ofOrthopaedic Surgery, Department ofReviewedFacult

Moment Measurements in Spine Segment Dynamic Tolerance Testing using Eccentric Compression are Susceptible to Artifacts Based on Loading Configuration

The tolerance of the spine to bending moments, used for evaluation of injury prevention devices, is often determined through eccentric axial compression experiments using segments of the cadaver spine. Preliminary experiments in our laboratory demonstrated that eccentric axial compression resulted in ‘unexpected’ (artifact) moments. The aim of this study was to evaluate the static and dynamic effects of test configuration on bending moments during eccentric axial compression typical in injurious cadaver spine segment testing. Specific objectives were to create dynamic equilibrium equations for the loads measured inferior to the specimen, experimentally verify these equations, and compare moments from various test configurations using synthetic (rubber) and human cadaver specimens. Dynamic equilibrium equations were developed based on a generic spine testing apparatus. The equations were verified by performing quasistatic and dynamic experiments on a rubber specimen and comparing calculated shear forces and bending moments to those measured using a six-axis load cell. Additional quasistatic and dynamic experiments with various test configurations were performed on rubber and human cadaver cervical spine specimens (consisting of three vertebrae and the interconnecting ligaments and intervertebral discs). Calculated shear force and bending moment curves had similar shapes to those measured and the values in the first local minima differed from those measured by 3% and 15%, respectively, in the dynamic test, and these occurred within 1.5 ms of those measured. In the rubber specimen experiments, for the hinge joint (translation constrained), quasistatic and dynamic posterior eccentric compression resulted in flexion (‘unexpected’) moments. For the slider and hinge joints and the roller joints (translation unconstrained), extension (‘expected’) moments were measured quasistatically and initial flexion (‘unexpected’) moments were measured dynamically. In the human cadaver experiments with roller joints, anterior and posterior eccentric compression resulted in extension moments, which were ‘unexpected’ and ‘expected’, for those configurations respectively. The ‘unexpected’ moments were due to the inertia of the superior mounting structures. This study has shown that eccentric axial compression produces ‘unexpected’ moments due to translation constraints at all loading rates and due to the inertia of the superior mounting structures in dynamic experiments. It may be incorrect to assume that bending moments are equal to the product of compression force and eccentricity, particularly where the test configuration involves translational constraints and where the experiments are dynamic. In order to reduce inertial moment artifacts, the mass, and moment of inertia, of any loading jig structures that rotate with the specimen should be minimized to the extent possible. Also, the distance between these structures and the load cell should be reduced.Applied Science, Faculty ofMedicine, Faculty ofNon UBCMechanical Engineering, Department ofOrthopaedic Surgery, Department ofReviewedFacultyResearche

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The Effects of Lateral Eccentricity on Failure Loads and Injuries of the Cervical Spine in Head-First Impacts

ABSTRACT Current neck injury criteria do not include limits for lateral bending combined with axial compression and this has been observed as a clinically relevant mechanis