Residual strains estimation in the annulus fibrosus through digital image correlation

Up-to-date, back pain is among the most prevalent health issues and generally takes its origins from lesions of the annulus fibrosus (AF). While the AF ex vivo mechanical properties are increasingly well understood, in vivo data are still missing. In particular, very few studies have precisely measured the residual strains within the AF and thus the in vivo deformation state of the AF is still miss-interpreted and miss-evaluated. In this work, we propose an original and robust method for the AF residual strains quantification via digital image correlation technics. Ten pig annulus fibrosus were extracted from adjacent vertebrae followed by a radial incision to release the residual strains. The operations were filmed and then analyzed by a custom digital image correlation software in order to quantify the circumferential, radial and shear residual deformations. Our results show that residual strains are of the same order of magnitude than the in vivo one. The average circumferential strains are in tension on the outer periphery ([3.32; 5.94]%) and in compression on the inner periphery ([−6.4; −1.69]%). The mean radial residual strains are essentially in compression ([−10.4; 2.29]%). Locally, radial and circumferential residual strains can reach really large values up to 40% of compression. The mean shear strains remain very small (−0.04% ± 2.88%). This study also shows that circumferential and radial residual strains evolve linearly along the radius and non-linearly along the angle. We propose a simple model to predict their spatial variations. Our results and methods will allow the quantification of more realistic in vivo strains and stresses within the human intervertebral disc.