Design of a new intervertebral disc prosthesis

In the degenerative disc disease, the traditional treatment consists in a removal of the intervertebral disc followed by the fusion of the two adjacent vertebral bodies (arthrodesis). An alternative treatment is the use of an artificial intervertebral disc. The advantage of an artificial intervertebral disc is that the mobility of the vertebral column will be restored. Aim of this paper consists in the definition of a prosthetic device for insertion in the intervertebral zone. Different types of artificial bone prostheses have been proposed, seldom based on similarity with physiological discs, then made with a central part (nucleus) consisting of hydrogel and an outer containment belt (annulus) consisting of a plastic material of greater stiffness. Upper and lower containment plates allow realizing a closed structure to be inserted in place of the original disc. Then, in our solution, the external parts (annulus and plates) were modeled by HDPE and the inner part (nucleus) by hydrogel. Both the materials are highly biocompatible. Both materials were modeled by the Ogden parameters related to elastomers. In fact, under the action of near-bending and twisting forces, the disc is subject to large deformations that the materials are able to withstand in elastic conditions. In order to assign the correct Ogden parameters to the materials, a procedure was used to fit the curves acquired by literature. The discs were subjected to compressive load as well as to forward and back bending (flexion and extension), lateral bending, torsion and combined loading.