The impact of flexion mechanical loading on the intervertebral disc metabolism

International audienceIntervertebral discs (IVD) are fibrocartilaginous organs that provide the linkage and mobility of the vertebrae of the spine. Healthy, they have a high-water content, nearly 80% by volume, which diminishes with age thus decreasing intervertebral mobility. IVDs allow the vertebrae to accommodate the mechanical loads supported by the spine. Naturally through aging IVDs undergo a process of degeneration that gradually alters their properties. Degeneration is a process in principle natural that continues throughout life but can, under the influence of various factors, such as mechanical factors, accelerate suddenly and leads to a pathological Disc Degeneration (DD). The DD, asymptomatic in its early stages, is hard to diagnose with conventional means of imaging such as CT scans or projection images (X-Ray and Discography). These images provide information on the general structure of the IVD such as disc height, surface area, and the volume of the disc in three-dimension. Nevertheless, they do not generally allow for the evaluation of quantitative parameters related to physiological quantities that are useful for making an early diagnosis.The detection of this pathology, at the first signs of back pain, is therefore of great interest to quickly establish effective treatments. The regulation of the water content of the IVD plays a central role in this pathology. It proceeds from a delicate balance between mechanical stresses exerted by the adjacent vertebrae, electrochemical phenomena within the extracellular matrix (ECM), and the performance of the nutritional transport processes throughout the ECM.Previous work conducted by Ghiss et al. (2016), and Chetoui et al. (2019) made use of quantitative Magnetic Resonance Imaging (MRI) to assess the lumbar disc viability. Available Nuclear Magnetic Resonance (NMR) techniques in clinical practice allow access to intrinsic disc information such as the water content of the disc and its mechanical rigidity. Our research focuses on the biomechanical behavior model of lumbar IVDs when subjected to mechanical loads, notably the bending in the coronal plane (flexion-extension). These solicitations generate high shear strain inside the disc and therefore present a high risk of injury Costi et al. (2007). One of the main aspects of this research work is the use of the real entire geometry of the lumbar IVD.This research study aims to implement a biphasic numerical model considering the biomechanical behavior of the IVD and then evaluate the impact of flexion-extension loads on disc metabolism