The Influence Of Ageing On Mechanical Behaviour Of Intervertebral Segment

The loss of load carrying capacity of intervertebral segment with age depends both on disc degeneration process and on mechanics of cancellous and cortical bone of vertebral body. The aim of the present work is to compare the intervertebral segment response under axial loading in the healthy and degenerated conditions of hard and soft tissue in time. The problem is approached by using a hyperelastic formulation for the constitutive material of the intervertebral disc. Collagen fibres which reinforce the annulus fibrosus are represented by truss elements working only in traction. Disc degeneration, related to the loss in liquid content in disc tissue, is simulated by varying disc compressibility. A transversely isotropic law is assumed for cortical and cancellous bone in the vertebral body. Ageing and the correlated reduction in bone amount has a double effect on cortical shell, leading both to a decrease in thickness and material characteristics. Mechanical properties change with age even more in cancellous than in cortical bone and the load distribution pattern is consequently affected, especially if ageing in vertebral body is coupled with disc degeneration effects. A three-dimensional finite element model of the intervertebral segment is developed adopting a non-linear formulation. The normal and reduced load-carrying capacities of the intervertebral segment are analysed.