Full-field strain distribution in multi-vertebra spine segments: An in vitro application of digital image correlation

The biomechanics of the spine is experimentally assessed in terms of range of motion and overall stiffness. Quantification of the surface strain distribution is currently limited either to the vertebrae or the discs, whereas a full-field approach to measure the strain distribution in a multi-vertebra segment is currently missing. The aim of this work was to explore the feasibility of using Digital Image Correlation (DIC) to measure the strain distribution simultaneously on the vertebral bodies and the intervertebral discs of spine segments in different loading configurations. Three porcine spine segments were tested. A white-on-black speckle pattern was prepared which covered the hard and soft tissues. Two different loading configurations (flexion and lateral bending) were reproduced, while two sides of the spine were analyzed with DIC. Measurements were successfully performed on the entire region of interest of all specimens, in both configurations. The DIC analysis highlighted the strain gradients present on the spine segments including tension and compression associated with bending, the direction of principal strains in the different regions, as well as bulging of the discs under compression. Strains of tens of thousands microstrain were measured in the discs, and below 2000 microstrain in the bone. This work showed the feasibility of applying DIC on spine segments including hard and soft tissues. It also highlights the need for a full-field investigation, because of the strain inhomogeneity in the vertebrae and discs.