Stress evaluation along the posterior annular circumferential tears on the L5-S1 spinal unit as an index of tear progression

Low back pain is a critical health issue related to the formation and growth of tears (or lesions), along with degenerative changes, which is commonly observed in the posterior regions of intervertebral discs (IVD) in the lower spine segments. Early and accurate prediction of growth of these tears is a challenging task with immense clinical significance. Finite element method is a promising technique in this direction, however, the combined effects of tears and degeneration in the posterior region of annulus has not been studied. The present work focuses on a numerical evaluation of the contribution of annulus material degeneration, the position of tear and regular physiological movements (extension, flexion, left and right lateral bending) on the growth of the posterior circumferential tear in the annulus of the L5-S1 spinal unit. The material models developed previously to describe degeneration effects were adopted for defining the annulus material. The tear is modelled by altering the mesh to determine its effects in the presence of degeneration in the annulus. Degeneration, which is a loss of functional ability due to age-related effects, influenced the stress response significantly across all types of movements. Flexion movement is found to have the most predominant damaging effect on the tears, evident by the higher stresses along the tear boundary. Tears near the outer periphery of the annulus were likely to experience higher magnitudes of stress for an applied load and, hence, were suggested to be more critical. Extension movement seemed to aid in the recovery of the tear, supporting the results of clinical studies. This study highlights the complex interaction between physiological movements and the progression of posterior circumferential tears under different stages of degeneration, which can enable clinicians to develop and implement suitable patient-specific treatment modalities.