In-silico study on cumulative effects of degeneration and anterior circumferential annular tear on the L5-S1 spinal unit

Low back pain is a serious health concern prevalent in majority of the people around the world, especially in case of the elderly. The root cause for this is mostly observed to be the development of lesions/ tears complemented by degenerative effects in the intervertebral disc of L4-L5 and L5-S1 segments. This study aims to analyse the effects of disc degeneration and tears on the mechanical responses of the L5-S1 spinal unit, which has not been investigated. The annulus is represented by an anisotropic hyperelastic Gasser-Ogden-Holzapfel (GOH) model wherein the effect of degeneration is defined by varying the constants responsible for the behaviour of the material in different strain-ranges. A systematic approach is proposed for modelling the effects of disc degeneration in the annulus. Further, the commonly found anterior circumferential tear is modelled to understand its combined effects with degeneration of the annulus. The damaging effect of the tear was limited only to extension movement, causing critical stress variations in its vicinity. However, degeneration had a significant influence on both stress and range of motion of the spinal unit across all types of movements. This study highlights the complex relationship of the physiological movements with pathogenesis of tear and degeneration leading to discogenic pain thus enabling clinicians to develop conservative treatment strategies for specific age groups.