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

8. CONCLUSIONS
Unsurprisingly, the disc and the facet joints have the greatest impact on the model response
and need to be modeled as accurately as possible. An insufficiently modeled disc
can therefore severely harm the predictive power of a spine model.
Concerning the question about the meaningfulness of deterministic models, we can conclude
that a model with a deterministic set of parameters is just that: A model of one
specific spine. If calibrated and validated carefully, such models might be able to represent
either a patient-specific or an “average spine”—at least, if one had enough measured
data to validate against.
Nevertheless, averages are purely statistical constructs, non-existent in reality. FE models
will find differences between any slight modification of the model (well, up to machine
precision). But does the difference matter? Likewise, an implant might perform
better than implant in the model by some arbitrary factor. But is this factor—this effect—really
relevant in relation to the wide scatter in the biomechanics of true human
spines?
Indeed, even for patient-specific models, were inter-individual variability can be neglected
for obvious reasons, model parameters are still afflicted with some amount of
measuring error and hence uncertainty, which must not be ignored, if a model's prediction
shall meet certain reliability constraints.
ACKNOWLEDGEMENTS
This work was supported by the German Research Foundation (DFG Wi-1352/12-1),
Bonn, Germany.
9. REFERENCES
1. Schmidt H, Heuer F, Drumm J, et al. Application of a calibration method provides
more realistic results for a finite element model of a lumbar spinal segment. Clinical
Biomechanics. 2007;22(4):377–384.
2. Schmidt H, Heuer F, Simon U, et al. Application of a new calibration method for a
three-dimensional finite element model of a human lumbar annulus fibrosus. Clinical
Biomechanics. 2006;21(4):337–344.
3. Wilke H-J, Wenger K, Claes L. Testing criteria for spinal implants: recommendations
for the standardization of in vitro stability testing of spinal implants. European
Spine Journal. 1998;7(2):148–154.
4. Patwardhan AG, Havey RM, Meade KP, Lee B, Dunlap B. A follower load increases
the load-carrying capacity of the lumbar spine in compression. Spine.
1999;24(10):1003–1009.
5. Shirazi-Adl A, Parnianpour M. Load-bearing and stress analysis of the human spine
under a novel wrapping compression loading. Clinical Biomechanics.
2000;15(10):718–725.
6. Masharawi Y, Rothschild B, Dar G, et al. Facet orientation in the thoracolumbar
spine: three-dimensional anatomic and biomechanical analysis. Spine.