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Luciano Luporini Menegaldo
11-21-1994, 03:35 AM
I'm sending the most conclusive replie i got after my request in biomch about bo
udary conditions of a spinal fixation device. Unfortunetly, i lost another an
swer, but this is the most conclusive. I lost too another mail who asked abou
t more details of my project. I'm realy sorry.


Luciano Menegaldo



Dear Luciano,

Here are some details (or reflexions) to your questions related to device loads
.

With our finite element model, we used 2 different ways to input
load conditions:

1) When we want to simulate orthotic treatments (brace), we apply
forces (imposed forces) on the patient thorax. These forces were
calculated from pressure measurements performed at the interface
between the brace and patient skin.

2) When we want to simulate bending tests, we apply displacements
(imposed displacements) on the boundary vertebrae. These
displacements were calculated from X-Rays taken in up-right and
bending positions.

Several non-linear elements (cable, contact elements) are included
in our model.
As for the solution, large displacements (non-linear) were taken
into account.

With our experience on these, I could say that the convergence
is better achieved when DISPLACEMENTS are imposed to a structure.

So, if you know the displacements (ie the way your device will
deform the spinal structure), you can apply these directly
on the boundary vertebrae.

Otherwise, If you want necesserly to impose forces (given by
the instrumentation) to the vertebrae, the way to impose forces
(punctual, distributed, ...) depend on the precision you need and
also the refinement of your model. But you need to hypothesized (or
measure) the force directions and magnitudes...

I think that instrumentation displacements are better documented
than instrumentation forces.


I hope these reflexions can help your work... Feel free to
comunicate with me if you need more informations, or to continue the
discussion.
Otherwise, I will be interested to keep in touch with your work
(ie I will appreciate to hear from you & your work, as it will
progress)

Regards,

Carl-Eric Aubin

--
Carl-Eric AUBIN
Ecole Polytechnique
Departement de genie mecanique
Groupe de Recherche en Biomecanique & Biomateriaux (GRBB)
C.P. 6079, Succ. "Centre-Ville", Montreal, Quebec, H3C3A7 (CANADA)
Email: aubin@grbb.polymtl.ca =========== ,_ o
Tel: (514) 340-4968 ========== ,_ o ========== / //\,=
Fax: (514) 340-4611 ========== / //\,= ===== \>> | ====
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