Development and application of methods for the biomechanical characterization of spine ligaments and intervertebral discs

The spine is one of the major organs subject to trauma or genetic problems. Today 30% of people suffer from back pain and every day a large number of surgical interventions on the spine are performed to treat those patients with severe spinal deformities (i.e. scoliosis or kyphosis). From a statistical analysis, the percentage of failures for this type of interventions is around 25-30%. The aim of my PhD thesis was the improvement of the knowledge of the strain distribution on biological tissues, in particular on ligaments and intervertebral discs of the human spine. The first part of this thesis aimed at improvement of the methodologies used to measure the strain distribution, simultaneously on hard (vertebrae) and soft tissues (ligaments and intervertebral discs), using Digital Image Correlation. The second part of my research studied the biomechanical behaviour of the intervertebral discs and of the different ligaments. The disc acts as a shock absorber for the spine, reducing shocks and impacts. The anterior longitudinal ligament (ALL), supraspinous and interspinous ligaments were studied analysing how they were deformed under different loading conditions. These ligaments limit the movement of the spine during flexion reducing the overload on the intervertebral disc. The ALL does not offer great mechanical strength during lateral bending and axial torsion. Summarizing, the study underlines the necessity of having a full-field strain analysis tool to enhance the knowledge of the biomechanics of the spine and the interaction between different types of tissue. Furthermore, the results reported in this thesis could be useful also to build better multibody spine models and to include more realistic properties in finite element models. These results could be a starting point for future works in which the effect of different surgical procedures and the use of new surgical devices could be investigated.