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

surfaces on the 3D distal extension model (Fig. 4 in blue) and were assumed to be
rigidly fixed.
A 5mm distraction of the VEPTR is realized approximately every 6 months to keep
up with the child’s growth. Through an experiment in our laboratory made by an
experimented surgeon (co-author), a force of approximately 300N was measured to
achieve a 5mm distraction of the VEPTR. This result was in agreement with Teli et
al. [8]. Indeed, Teli et al. [8] measured every 1mm until a limit of 12mm, the in vivo
force generated by a surgeon distracting a growing rod system (Tab. 1). The mean load
measured for a 5mm distraction was 308N. Then finite element analysis was carried out
for a static compressive force of 700N which represents the distraction force with a
safety factor. To modulate a growing rod construct using the VEPTR we simulated
the proximal and distal spinal anchors as a set of pedicle screws rather and simulated in
the standard proximal VEPTR rib cradle, and distal laminar hook. The screws are
represented by two surfaces on the 3D proximal extension model for the application of
the force (Fig. 4 in red). Contacts within the VEPTR were modeled using the
augmented Lagrange algorithm with face to face contact element and divided into three
zones: contact between the distal and proximal extension (Fig. 5a); contact between
distal extension and the distraction lock (Fig. 5b) and contact between the proximal
extension and the distraction lock (Fig. 5c). These contacts were modeled as
frictionless.
a) b)
Fig. 3 – a) Illustration of the mesh b) Bilinear isotropic hardening curve of titanium alloy
Distraction
(mm)
Mean load
(N)
Fig. 4 – Boundary condition: fixed support (in blue) and application of force (in red)
1 2 3 4 5 6 7 8 9 10 11 12
133 203 247 282 308 354 364 354 402 430 404 485
Tab. 1 – Force (N) – distraction value (mm) (Teli et al. [8])