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.

While the non-linear behavior of spine segments has been extensively investigated in the past, the behavior of the Anterior Longitudinal Ligament (ALL) and its contribution during flexion and extension has never been studied considering the spine as a whole. The aims of the present study were to exploit Digital Image Correlation (DIC) to: (I) characterize the strain distribution on the ALL during flexion-extension, (II) compare the strain on specific regions of interest (ROI) of the ALL in front of the vertebra and of the intervertebral disc, (III) analyze the non-linear relationship between the surface strain and the imposed rotation and the resultant moment. Three specimens consisting of 6 functional spinal units (FSUs) were tested in flexion-extension. The full-field strain maps were measured on the surface of the ALL, and the most strained areas were investigated in detail. The DIC-measured strains showed different values of peak strain in correspondence with the vertebra and the disc but the average over the ROIs was of the same order of magnitude. The strain-moment curves showed a non-linear response like the moment-angle curves: in flexion the slope of the strain-moment curve was greater than in extension and with a more abrupt change of slope. To the authors’ knowledge, this is the first study addressing, by means of a full-field strain measurement, the non-linear contribution of the ALL to spine biomechanics. This study was limited to only three specimens; hence the results must be taken with caution. This information could be used in the future to build more realistic numerical models of the spine.

Contrasting results are reported when the spine is tested at different strain rates. Tissue specimens from the ligaments or the intervertebral discs (IVD, including annulus fibrosus and nucleus pulposus) exhibit higher stiffness and lower dissipation at high strain rates. Counterintuitively, when spine segments are tested at high rates, the hysteresis area and loop width increase. It is unclear how the load is shared between the different structures at different loading rates. The hypotheses of this study were: (i) As the IVD stiffens at higher loading rates, the strain distribution around the disc would be different depending on the loading rate; (ii) Preconditioning attenuates the strain-rate dependency of the IVD, thus making differences in strain distribution smaller at the different rates. Six segments of three vertebrae (L4–L6) were extracted from porcine spines and tested in presso-flexion at different loading rates (reaching full load in 0.67, 6.7, and 67 s). The full-field strain maps were measured using digital image correlation on the surface of the IVDs from lateral. The posterior-to-anterior trends of the strain were computed in detail for each IVD, and compared between loading rates. The values and the direction of principal strain on the surface of the IVDs, vertebrae, and endplates remained unchanged at different rates. In the transition zone between IVD and vertebra, only slight differences due to the loading rate appeared but with no statistical significance. These findings will allow better understanding of the rate-dependent behavior and failure of the IVD.