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

of stress arising in the rods of the fixator, calculated by means of FE analyses and
validated in an experimental model by means of strain gauges technique.
Moreover, in order to find out if the standard prescriptions are representative of the in
vivo mechanical loading condition, the fixator has been virtually mounted in a FE model
of the lumbar tract of the spine (L2-L4 tract) that has been loaded with force and
moments that arise during everyday activities: also in this case, differences between
standards and in vivo situations have been pointed out by comparing the state of stress
calculated in the rods.
3. MATERIAL AND METHODS
3.1 FE simulation of the ISO and ASTM procedure
Two FE models of the two different experimental setups suggested by ISO and ASTM
standards have been constructed (S1= ISO model, S2=ASTM model, see Figure 1). In
order to do so, firstly a model of a prototype of STL spinal fixator (2B1 srl, Milan, Italy)
has been built as a STP file from the manufacturer’s design using Rhinoceros 4.0
Evaluation CAD program (McNeel and Associates, Indianapolis, IN, USA). The fixator
is composed by six poliaxial pedicle screws and two cylindrical bars: both the screws
and the bars are made of Ti6Al4V alloy, modeled as a linear elastic isotropic material
(Young modulus=110GPa, Poisson ratio=0.3). The screws and the bars have been
considered as perfectly tied, neglecting any relative motion and meshed using 4-nodes
tetrahedral element.
S1
S2
Figure 1: standards set-ups (left), FE models (centre), experimental validation (right).
The model of the fixator was then inserted into the Polyethylene (PE) blocks that
replicate vertebras in the configuration according to standards: blocks of the same
geometry of those reported in the standards were built in the FE model and the PE has
been modeled as a linear elastic isotropic material (Young modulus=1.05GPa, Poisson
ratio=0.4, meshed with 4-nodes tetrahedral element).
ISO standard prescribes to use an anterior support by interposing three steel springs of
controlled stiffness (375 N/mm each) between each couple of vertebras: such springs