Material Properties of the Intervertebral Disc

The intervertebral disc is the primary articulation between adjacent vertebral bodies in the human spine. Degenerative disc disease is the leading cause of pain and disability in the adult; replacing the diseased disc with an artificial material is becoming the treatment of choice in cases that require surgery. At the same time, injuries to the spinal column are common in incidents that are associated with high accelerations, such as falls, sports injuries, road traffic accidents, and acts of violence; these injuries are often associated with long term disability. Injury induced by road-traffic accidents alone is predicted to become the third leading cause for burden of disease by 2030 according to the World Health Organization.

A comprehensive understanding of the mechanisms associated with injury to the spine is lacking, especially in high-energy trauma. Official NATO reports acknowledge the limitations of current injury criteria and lack of injury risk curves for the spine; these criteria and curves would allow us to evaluate vehicles and protection systems appropriately. Similarly, the human-like response of anthropometric test devices (or dummies) in predicting the response of the human spine under load is questionable. As a result, strategies to enhance protection and to improve protective equipment are being developed using sub-standard technologies.

Finite element (FE) modelling - a type of computer simulation of the mechanics of structures - of human injury, of implants and of protective systems are important engineering tools that allow us to understand the mechanisms involved in an injurious event and to develop new and improved evaluation criteria, techniques, materials and designs in a cost-efficient manner. As computational power becomes more abundant, FE modelling for optimal design is a clear strategic direction in industry as an alternative to expensive and labour intensive experiments. A critical parameter, however, that influences the predictive ability of FE models of human response is the quality of the input data that are associated with the material behaviour of human tissue. Such data, particularly at loading rates relevant to injury, are sparse for most human tissues; this is definitely the case for the intervertebral disc.

The aim of this project, therefore, is to quantify the material behaviour of the human intervertebral disc across physiological and injurious loading rates. The intention is to inform implant design and to increase the accuracy of FE models of human injury in order to improve their ability to simulate the response of the spine under load.

The ultimate beneficiary of this work is the patient with spinal injury, and society and economy through prevention of spinal injury.
The main, direct beneficiaries of this work are researchers and industry that develop computer (specifically, finite element) models of the spine to predict joint motion, loading, and potential for injury in order to enable the design of better surgical interventions, safety systems, personal protection equipment and other mitigation technologies. Better protection from, and better surgical reconstruction of, spinal injury have a clear, direct economic and societal impact through reduction of healthcare costs and improvement in quality of life. The impact is not only due to the fewer injured people, but also due to the use of more efficient means of design for protection. The work can contribute to impact at the long term in goverment policy and international standards in orthopaedic, automotive and military domains.

The proposed research will achieve its impact by the implementation of a material model of the intervertebral disc (IVD) into finite element (FE) models of the spine, and by informing the selection of materials for use in IVD implants. It will also achieve impact when the software program that will be developed as part of this work is taken upon by other researchers or industry for application in characterisation of human tissue or other materials and physical processes.

Dissemination of the outcomes of this work to the beneficiaries will be achieved primarily through presentations at appropriate scientific conferences, publication in peer-reviewed journals with appropriate readership, and personal communications to the industrial stakeholders. Direct communication to the immediate beneficiaries - both academic and industrial - will ensure that there is immediate impact at the end of the grant period.

Furthermore, I am part of the The Royal British Legion (TRBL) Centre for Blast Injury Studies at Imperial, a very high-visibility 'project' for both Imperial and TRBL. This partnership, among others, is ensuring avenues for technology transfer and public engagement. For example, the Centre publishes an annual report that is available to download freely from the website. This activity ensures that the work is communicated appropriately to the public.