Three-dimensional surface strains distribution on healthy and degenerated human intervertebral discs

The intervertebral discs are exposed to a wide array of pathological, genetic, and environmental factors that may give rise to degenerative and mechanical changes. Degeneration alters the structural integrity of intervertebral discs, leading to low-back pain which affects 75% of us in our lifetimes. Investigating the effects of these changes on surface IVD strains may lead to better preventative strategies and surgical techniques. In this study, I aimed to evaluate the effect of the disc degeneration on the mechanical behaviour of human intervertebral discs, measuring the three-dimensional surface strain distributions. Five-vertebrae human spine segments were prepared. Comparative biomechanical tests were performed in four different loading conditions (flexion, left and right lateral bending and pure compression) in the elastic regime. Each condition was tested with three different loading speed: slow, normal and fast. Specimens were tested twice: before and after T11-T12 intervertebral disc degeneration. Surface principal strains were measured using a contact-less Digital Image Correlation (DIC) system. Comparison of the maximum and minimum principal strains measured before and after degeneration was carried out. Results showed that, after intervertebral disc degeneration, the maximum and minimum principal strains tend to increase. In particular, the degenerated disc provided larger deformations than the adjacent control intervertebral disc. The 3D strain distribution showed how the intervertebral discs compliance varied in relation to the side where the load was applied, highlighting how the magnitude of the full-field strain measurement changed in the different loading conditions. These findings advance the state of knowledge on mechanical changes occurring during degeneration, which help reduce the risk of fracture, investigate new treatments, and improve spinal implant designs.