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

4. RESULTS
The axial compressive load vs. displacement according to the FE simulations was found to be
in good qualitative and quantitative agreement with the in-vitro test results reported by
Eberlein et al. [13] for a load of up to 2500 N. Beyond that load, the FE model showed a
tendency to predict a higher compressive stiffness than was reported by these authors. The
angular displacement vs. bending moment response according to the FE simulation was also
in good quantitative and qualitative agreement with the in-vitro test results reported by
Bowden et al. [14], with the predicted response falling within the range of reported responses.
FE simulations were successfully carried out for all nine disc models and all load cases except
for the smallest vertebra models for thick disc and average disc height. Due to convergence
difficulties, the FE simulations were only carried out with a 800 N follower load for these
models. The principal stresses vs. principal strains for all annulus elements, for all load steps
and all load cases, for the nine different vertebra models studied, is illustrated in fig. 2. The
principal stresses vs. principal strains for all results combined are illustrated in fig. 3.
Fig.3. Principal stresses vs. principal strains output according to FE simulations for all
annulus elements in the nine different vertebra models, for all load steps in the four different
load cases. The red hatch indicates a suggested uniaxial stress strain window for candidate
implant materials
5. DISCUSSION
The goals of this study were; to create a realistic parameterized FE model of the intervertebral
disc that could be used to investigate the influence of IVD geometry on the mechanical
response of the discs and; to use this modeling technology to define a window of mechanical
responses into which a candidate implant or annulus repair material would have to fit to
restore a range of motion comparable to that of a healthy disc and / or to maintain mechanical
compatibility with the surrounding native tissue. Regarding the first aim, we found that our
FE model was in good qualitative and quantitative agreement with in-vitro studies in
literature. As could be expected, we found that the disc anatomy had large influence on the
mechanical response of the disc. The maximum absolute principal tensile (7.85 MPa) and
compressive (6.07 MPa) stress values were found in a model simulating a small vertebra-thick
disc IVD geometry. This was the case even though this model was only investigated with the
reduced follower load of 800 N. Corresponding values for a FE model simulating a large
vertebra-thin disc IVD geometry were 0.66 MPa and 1.60 MPa respectively (1000 N follower
load). Related to the second aim, we suggest a window of stress strain response (fig. 3) for a
candidate implant or annulus repair material for IVDs by combining all results from all
models and load cases. This window is relatively broad but we believe it still provides a
domain for narrowing the search for such materials.