**Conflict of interest**

pressure of the FSU C6-C7 in order to indicate the model weaknesses and optimize

In the first part of this study an *one-way* sensitivity analysis is performed in order to indicate, whether one of the given input parameter, namely stiffness or damping term, has a dominant influence on the model behavior. The experimental results show, that both parameters exhibit an identical impact on the disc pressure. However the variations of the damping term indicate a slightly stronger effect on the intradiscal pressure measurements, which is reflected in relatively higher value of the calculated sensitivity coefficient. When applying compressive loads from 100 N up to 800 N on the FSU model and varying the analyzing parameters a not foreseeable response pattern in the disc pressure is explored. Simultaneous change of the load and the corresponding parameter values results in a non-linear outcome regarding the intradiscal pressure, which is not detected in the simulations that

Further, it could be shown that the correlation between disc area and disc pressure can be approximated by a third-degree polynomial. This allows a further possibility for model validation of the simulated intervertebral disc pressure. For this purpose, the simulation result can be compared with the intervertebral disc

An essential point to be considered in the next step is the implementation of the musculature. This is not taken into account in this model. It is still unclear what influence other cervical parameters, e.g. the facet joints, ligaments or muscles have and how these affect the overall mechanic when changed. Therefore, following this inves-

In case when additional elements are integrated into the model and the number of input factors grows, another broadly used method called multivariate sensitivity analysis can be applied in order to investigate the model response affected by the simultaneous variations of the underlying parameters. This procedure can help to optimize the model structure by finding the variables, that primarily impact the model outcomes. Moreover, using the sensitivity analysis methods the values of the principal parameters can be determined so that realistic simulation of model

The experimental design of the presented sensitivity analysis follows the recommendations found in the literature. In the future work, the boundary conditions of the experiments should be extended. For instance, the range of the stiffness value might be increased up to 8*:*<sup>3</sup> <sup>10</sup><sup>6</sup> as it was used in the model proposed in [14]. Then the response of the current FSU model can be compared with the outputs of

We like to thank Prof. Dietrich Paulus, Institute of Computer Visualistics, University Koblenz-Landau for the fruitful discussion and Dr. Francis Kilian, Head of the Clinic for Spinal Surgery, Head of the Spinal Center Catholic Clinic Koblenz-Montabaur and PD Dr. Roland Jacob, specialist in ear, nose and throat medicine for

pressure calculated by the polynomial with a known disc surface area.

tigation, the effect of model parameters of others spinal structures, such as facet alignment and size, on the load on the intervertebral discs will be evaluated. Further, it must be questioned critically whether these results can be transferred to a model with a larger spinal column section. In order to discuss this question, in a further step not only an FSU should be considered, but the sensitivity of model parameters in a

model that contains an entire spinal column section should be analyzed.

the model design.

*Recent Advances in Numerical Simulations*

behavior is possible.

the referenced model.

**Acknowledgements**

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guidance on medical and anatomical questions.

consider the exerting external force of 80 N.

The authors declare no conflict of interest.
