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

EVALUATION OF DIFFERENT LOADING CONDITIONS
PRESCRIBED BY STANDARDS FOR THE FATIGUE TESTING OF
SPINAL FIXATORS
1. ABSTRACT
T. Villa 1 , L. La Barbera 2 and F. Galbusera 3
The pre-clinical assessment of the long time performances of devices for spinal surgery
has peculiar difficulties due to the very complex site of implant: experimental set-ups
and procedures prescribed by international standards necessarily represent simplified
models of the lumbar tract, thus leading to results that must be carefully evaluated. The
purpose of the work is to shed light on the real state of stress arising in the components
of a spinal fixator when tested according to different experimental procedures
comparing it to the one acting after a virtual implant in a physiological FE model of the
lumbar spine.
2. INTRODUCTION
The preclinical assessment of the mechanical reliability of implantable devices has
become a primary activity in the design of a new construct: in particular, the long term
reliability is usually assessed through experimental tests that follow the prescriptions of
standards periodically published by different international organizations (ISO and
ASTM).
The usefulness of such standards is real only if the choice of the experimental setup,
loads and number of cycles to be applied during the tests is such that the mechanical
stresses are similar to the ones the device will be subjected to after the implantation.
In the case of experimental procedures for the fatigue testing of spinal fixators, the first
standard that has been published was ASTM F 1717 (latest release in 2011), that
prescribes to test the specimen in an experimental model of vertebrectomy, in which the
lack of an anterior support is not a realistic clinical procedure: moreover, the standard
indicates the run-out number of cycles (5Mcycles) but not the load to be applied,
leaving the individuation of the load to the highest that permits to reach the run-out.
In 2008 ISO published a new standard (ISO 12189) which differs from ASTM standard
basically in the design of the set-up (that includes an anterior support obtained by
interposing springs between the vertebras) and in the prescription of the load to be
applied (2000N for lumbar application) while the run-out is the same of ASTM
standard.
The target of the work is comparing the two different experimental procedures, in terms
1
Assistant Professor, LaBS - Laboratory of Biological Structure Mechanics, Department of Structural
Engineering, Politecnico di Milano, Milan, Italy and IRCCS Istituto Ortopedico Galeazzi, Milan, Italy.
2
PhD student, Bioengineering Department and LaBS Dept. of Structural Engineering, Politecnico di
Milano, Milan, Italy.
3
PhD student, Institute of Orthopaedic Research and Biomechanics, University of Ulm, Germany.