Computational Modeling of Hard Tissue Response and Fracture in the Lower Cervical Spine under Compression Including Age Effects
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Date
2018-06-21
Authors
Khor, Fiona
Journal Title
Journal ISSN
Volume Title
Publisher
University of Waterloo
Abstract
Almost half of motor vehicle accident (MVAs) victims experience traumatic spinal cord injuries
(SCI), which are often associated with rollover accidents. Specifically, rollovers have the highest
incidence rate of AIS2+ cervical spine injuries and more than half of the patients with SCIs
demonstrated spine fractures with the majority being burst fractures. Detailed finite element
human body models (HBMs) have been utilized to assess the safety of occupants and pedestrians
in crash scenarios, augmenting the results from crash test dummies in physical tests. HBMs can
predict the potential for injury and provide data such as fracture initiation and propagation that is
not possible to collect experimentally. Biofidelic HBMs capable of predicting tissue-level injury
require representative material properties and tissue level failure criteria. However, current
HBMs use simplified constitutive models and are not capable of predicting the fracture threshold
and fracture pattern for complex scenarios, such as the vertebrae in the neck. The objective of
this study was to investigate constitutive models with age effect that are representative of cortical
and trabecular hard tissues and assess the failure response of a C57 (C5-C6-C7) segment model
under compression loading. Two sets of material properties were identified that corresponded to the lower age of the
experimental test samples (younger than 50 years old (YO)) and the higher age of the test
samples (older than 70 YO). The available constitutive models in a commercial finite element
code (LS-DYNA) were reviewed and the constitutive models that best represent the cortical and
trabecular bone responses were analyzed. As there were no single constitutive model available
that included all the key properties of hard tissues, asymmetric and anisotropic elastic-plastic
(cortical) and crushable foam (trabecular) models were evaluated. Single element simulations
were performed to verify the constitutive models. A functional spinal unit (FSU) model was
extracted from a detailed 50th percentile HBM (Global Human Body Models Consortium
(GHBMC) M50-O v4.3) and a centric compression simulation was performed to identify the best
performing constitutive model compared to experimental data. Various eccentricity cases of the
compression experiments were simulated as well such as anterior, posterior and lateral. The anisotropic model predicted failure values and fracture patterns in better agreement with experimental data compared to an asymmetric or isotropic and symmetric model. This study showed the importance of including age effects that correspond to the age of experimental test
subjects. This study also showed that simulations could provide additional insight regarding
fracture initiation and progression, which is challenging to measure in dynamic experiments.
Gender, segment level, and strain rate effect were not included in this work, which are
limitations of the current study. In addition, the lack of human cervical spine experimental data
for improved model validation is another limitation of this study. In conclusion, this study
successfully utilized uncalibrated material properties of cortical and trabecular bone tissue from
literature and accurately predicted failure outcomes of compression experiments with the
implemented constitutive models.
Description
Keywords
computational modelling, cortical bone, trabecular bone, compression, age effect, constitutive model, fracture, anisotropy, asymmetry, element erosion, human body models