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

(i) ISO configuration and load prescriptions (2000 N) tend to overstress the rods if
compared to a more physiological way of loading the system (M1 model, 500 N vertical
load): nevertheless, as the experimental model is unable to simulate the response of the
biological environment, this could be a worst-case scenario deliberately chosen by ISO
in order to guarantee the safety of the device in case the tests are successfully passed.
(ii) In ISO experimental set-up, two discs are simulated by means of the presence of the
springs but such a tissue preservation is seldom maintained in the clinical practice when
fusion of the segment is searched (discs are often excised): in this light, the ISO set-up
well replicates the physiological boundary conditions (the stiffness of the springs is
correctly chosen, as stresses in the rods in S1 and M1 are similar when the same vertical
load is applied) but actually this condition is far from clinical reality.
(iii) Some differences were found as regards the positions of the most stressed areas: in
fact, both S1 and S2 tend to load only the posterior part of the rods, while the
application even of the same vertical load to the physiological environment (M1, M2
and M3) tends to move the most stresses area toward a more medial position (Figure 3),
due to the natural curvature of the spine and to the inclination of the pedicles (both not
taken into account in the experimental set-up).
L
A
P
M
S1 S2 M1
Figure 3. VM stress patterns for F1 case.
(iv) Everyday activities, different from flexion caused by the application of the vertical
load, cause stresses of lower entity compared to the ones caused by the 2000 N vertical
load: anyway, the positions of the most stressed areas in these cases are different from
the posterior part of the rods, which is the only one loaded by the standard
configuration.
(v) ASTM configuration is much more severe than reality and positions of the most
stressed areas are similar to ISO; the vertical load that induces the same state of stress as
ISO configuration is approximately 400 N and in case an anterior support is used (e.g.
titanium cage), the reduction of the stress on the rods is important (about one order of
magnitude).
6. REFERENCES
1. Galbusera, F., Bellini, C. M., Anasetti, F., Ciavarro, C., Lovi, A., Brayda-Bruno, M.,
Rigid and flexible spinal stabilization devices: a biomechanical comparison, Med.
Eng. Phys., 2011, 33(4), 490-6.