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

RESULTS
As a preliminary test, a 0.6 mm compressive vertical displacement was imposed on the
IVD model, during 8h. Pressure on the anterior-posterior axis of the IVD and the total
forces acting on the model over the vertical axis were measured (Fig. 3). Four nodes
were chosen for the measurements, corresponding to anterior and posterior NP and
anterior and posterior AF. The IVD was already conditioned on t=0s.
DISCUSSION
a) b)
Fig. 3 Constant displacement: a) Pressure on the anterior-posterior axis of the IVD;
b) Total Compressive Forces on the IVD model
The geometry of the full lumbar MS FE model seems to be coherent with the most upto-date
models available in the literature, so the simulations have anatomical validity
[12]. The results of the preliminary numerical simulations seem to be in accordance
with the literature, as the force decreases over time (stress-relaxation effect), influenced
by the viscoelastic behavior of the NP [14, 15]. Pressure also changes over time, as
expected. It is mostly decreasing on the NP and increasing on the AF, which means that
the fluid flowing through the IVD is occurring as expected, i.e., from inside to outside
[13]. The posterior part of the AF is more vulnerable to the compressive loading,
because of its lower thickness, in comparison with the anterior part. More detailed
discussion about the anisotropic behavior of the AF should be addressed to the work of
Lopes and Alves (2012) [11].
CONCLUSION
The Terzaghi’s test was important to validate the innovative poroelastic formulation
here described. In addition, the FE model may possibly be built with the linear 4-node
tetrahedron (with an extra bubble node) or the quadratic 10-node tetrahedron,
instead of the current 27-node hexahedron.
From the global results produced, some preliminary conclusions may be drawn, even
regarding that this is an on-going work. The full MS FE model is reproducing the
characteristics of a natural MS, due to the reasonable geometric accuracy. However, the
most important feature of this model is the implemented hyper-visco-poro-elastic
behavior, even though some important updates are currently being implemented,
namely the poroelastic swelling behavior, based on Wilson’s model (2005) [7].
Therefore, the in-development open source FE solver is becoming appropriate to the
description of IVD and to the understanding of the several pathways that lead to the
DDD.