Experimental and finite element analysis of mechano-electrochemical effects in intervertebral disc biomechanics

Lower back pain places an enormous economic burden on society and health authorities as it affects up to 80% of the population and has been attributed to intervertebral disc injury as well as the degenerative adaptations which occur with advancing age. Much of our understanding of disc mechanics comes from mathematical and finite element models; however, there is a lack of empirical data which is required for model validation. Additionally, the influence of mechano-electrochemical phenomena on fundamental mechanical properties such as permeability and Poisson's ratio is still not fully understood. Therefore, this thesis aimed to investigate the influence of such phenomena on disc mechanics whilst providing a comparison between multiphasic FE models and experimental data. Direct permeation experiments found that fluid velocity may be augmented through the nucleus pulposus via ionic osmotic pressure gradients which consist of fixed charge, mobile ion and electrical potential gradients. A novel method to fully characterise, for the first time, the Poisson's ratio of the disc was developed. Poisson's ratio of the nucleus pulposus was found to be strain dependent and lower than previously thought, whilst solid matrix viscoelasticity may influence disc mechanics at high strain-rates. Confined compression experiments on the degenerate human nucleus pulposus found that the solid matrix bears the majority of load under axial compression due to the depletion of proteoglycans and the consequential reduction in mechano-electrochemical effects in this tissue. Poor agreement with biphasic theory may provide evidence of the degenerate nucleus pulposus exhibiting a heterogeneous structure and a dual permeability phase. Multiphasic models were developed and compared to experimental data. Differences were found between biphasic and triphasic models which resulted in varying agreement with experimental data thus the correct selection of numerical framework was found to be crucial when investigating disc mechanics. The data presented in this thesis are important for informing our overall understanding of disc mechanics, aiding the development future models as well as providing a benchmark for potential replacement materials to be critiqued against, particularly in terms of permeability and fluid pressurisation which are crucial to the load bearing capacity of the tissue.

Name dc:type.qualificationname

phd

Level dc:type.qualificationlevel

doctoral-pg

Grantor dc:publisher.institution

University of Strathclyde

Year dc:date.issued

2013

Author dc:creator

• Farrell, Mark D.

DOI

10.48730/1wem-6v48

Identifier

T13583

OAI identifier oai:identifier

oai:strathclyde:9w032305v