ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

mobility and loading bearing capacity. It is in fact the coupling of these two features,
with the imposition of high mechanical loads, the main source of wear, degeneration
and dysfunctioning of the IVDs (Urban & Roberts, 2003).
The present work, framed on the above mentioned description, aims to contribute to the
development of a new finite element (FE) model of the Human IVD, in order to allow
the study of its biomechanics, and thus contribute to understand the main sources of
degeneration and contribute to define the specifications of a NP’s biomimetic material.
The developed FE model shall allow to simulate and to quantify the influence of the
most relevant IVD anatomical features and tissues’ mechanical properties. Among
others, one aims to be able to study the role of: the orientation, density, distribution and
circumferential and radial evolution of the fibers’ mechanical properties within the AF;
the number of layers of fibers; the initial osmotic pressure of the NP on the loading
bearing capacity; and the IVD’s flexibility, in order to improve our understanding on the
main pathological mechanism of the IVD’s degenerative diseases and contribute to the
development of new mimetic materials, treatment chirurgical strategies and medical
devices. In order to build a FE model one needs to fulfill, at least, three basic
conditions: to define the geometry and the anatomical structure of each tissue, to
characterize the materials and to define boundary and loading conditions.
3. MATERIALS AND METHODS
The L4-L5 IVD is the one more often referred as the most critical in terms of injuries,
either by Degenerative Disc Disease (DDD) or by herniation. For this reason, one has
decided to study this spinal level, and thus their dimensions were selected from the
literature, i.e., approximately 10mm of height and 40 mm of diameter (Raj, 2008). A
27-node hexahedral FE mesh was built, as shown in Fig.2.
Figure 2. IVD with simplified vertebrae
representation using a 27-node hexahedral FE
mesh.
The FE solver used for the numerical simulation is a FORTRAN home-developed open
source software, which comprises the most relevant features of the biomechanics of
soft-tissues, such as their almost incompressibility, the most general isotropic and
anisotropic hyperelastic laws and viscous effects, with a fully implicit time integration
scheme and different types of mixed u/p finite elements (Alves et al., 2010).
As aforesaid, the angles of the fibers within the AF evolve circumferentially from
ventral to dorsal regions. Being the absolute fiber angle (angle with the axial plane),
the following evolution law proposed by Holzapfel et al. (2005) was used,
| |=23.2+0.130.α (r 2 =0.55, p