INTRODUCTION: Degeneration of lumbar discs is related to progressive changes in disc morphology such as disc height and annular volume, changes in disc tissue composition such as water content, porosity and drained modulus and finally increased annular lesions. The current study is related to the effect of such progressive changes on disc biomechanics.
METHODS: A poroelastic non‐linear finite element model of L4‐5 that includes biological parameters was used in the current study. Models of healthy, mildly degenerated and moderately degenerated lumbar discs with and without annular tears were developed and validated with in‐vitro results. Biomechanical responses of these models were analyzed for a moment load of 6.6 Nm in all the three principal directions.
RESULTS: Presence of tears in healthy, mild and moderately degenerated discs produced less than 5% increase in motions in all loading modes. As disc degeneration progressed the principal motions increased with mild degeneration (85% in flexion/extension;105% in torsion and 75% in lateral bending) and further progression of disc degeneration decreased the motions (55% in flexion/extension;50% in torsion and 30% in lateral bending). The principal motions in a moderately degenerated disc were still higher than those in a healthy disc. As the disc degenerated, maximum pressure in the nucleus increased from 0.2 MPa to 0.5 MPa, maximum shear stress in the annulus increased from 0.2 MPa to 0.7 MPa and maximum principal stress in the endplate increased from 3 MPa to 5 MPa.
DISCUSSION: The results show that change in motion due to degeneration is mainly due to tissue composition and morphology. Mildly degenerated disc showed an increase in motion which may cause spinal instability. Stresses in all components of the degenerated discs were greater than those in a healthy disc indicating higher possibilities of further failure in the disc components.