Image-based biomechanical assessment of vertebral body and intervertebral disc in the human lumbar spine

Maquer, Ghislain Bernard (2013). Image-based biomechanical assessment of vertebral body and intervertebral disc in the human lumbar spine. (Dissertation, University of Bern, Faculty of Medicine, Institute for Surgical Technology & Biomechanics)

Text Image-based biomechanical assessment of vertebral body and intervertebral disc in the human lumbar spine.pdf - Published Version Restricted to registered users only Download (7MB)

Life expectancy continuously increases but our society faces age-related conditions. Among musculoskeletal diseases, osteoporosis associated with risk of vertebral fracture and degenerative intervertebral disc (IVD) are painful pathologies responsible for tremendous healthcare costs. Hence, reliable diagnostic tools are necessary to plan a treatment or follow up its efficacy. Yet, radiographic and MRI techniques, respectively clinical standards for evaluation of bone strength and IVD degeneration, are unspecific and not objective.
Increasingly used in biomedical engineering, CT-based finite element (FE) models constitute
the state-of-art for vertebral strength prediction. However, as non-invasive biomechanical
evaluation and personalised FE models of the IVD are not available, rigid boundary
conditions (BCs) are applied on the FE models to avoid uncertainties of disc degeneration
that might bias the predictions. Moreover, considering the impact of low back pain, the
biomechanical status of the IVD is needed as a criterion for early disc degeneration.
Thus, the first FE study focuses on two rigid BCs applied on the vertebral bodies during
compression test of cadaver vertebral bodies, vertebral sections and PMMA embedding.
The second FE study highlights the large influence of the intervertebral disc’s compliance on
the vertebral strength, damage distribution and its initiation. The third study introduces a
new protocol for normalisation of the IVD stiffness in compression, torsion and bending
using MRI-based data to account for its morphology. In the last study, a new criterion (Otsu
threshold) for disc degeneration based on quantitative MRI data (axial T2 map) is proposed.
The results show that vertebral strength and damage distribution computed with rigid BCs
are identical. Yet, large discrepancies in strength and damage localisation were observed
when the vertebral bodies were loaded via IVDs. The normalisation protocol attenuated the
effect of geometry on the IVD stiffnesses without complete suppression. Finally, the Otsu
threshold computed in the posterior part of annulus fibrosus was related to the disc
biomechanics and meet objectivity and simplicity required for a clinical application.
In conclusion, the stiffness normalisation protocol necessary for consistent IVD comparisons
and the relation found between degeneration, mechanical response of the IVD and Otsu
threshold lead the way for non-invasive evaluation biomechanical status of the IVD. As the
FE prediction of vertebral strength is largely influenced by the IVD conditions, this data
could also improve the future FE models of osteoporotic vertebra.

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