Ligamentforces Aalborg Mandible FDK


i have a question about the defintion of the ligament stiffness at the aalborg mandible file in the AMMR. In the file "LigamentModel.any" the stiffness for the ligaments is set to "Stiffness = k*lr". So I am confused because k is actual the stiffness of the ligament and lr ist the reference length. So why is the stiffness for computing the ligament forces is scaled down with the reference length?

For me it doesn't make any sense, because if you scale down the stiffness by multiply it with the reference length, the ligament forces are really really small, so they don't have a real significance to the hole simulation.

So i would be lucky, if there is anybody, who could explain me the scale down of the stiffness.
Thank you for your help

Hi Clara,

In the file "LigamentProperties.any" the same stiffness number is used for all ligaments both long and short ligaments, therefore it makes sense to scale down the stiffness using the length, so a longer ligament has less stiffness than a short ligament.
I ran two of the models and you are right that the ligament forces are small, but i am not sure this is incorrect these motions are not extreme, thus i would not expect high ligament forces.

Best regards

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Hi Søren,

ah ok, that makes sense to me. Thank you for your help.

Best regards
Clara :slight_smile:

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Hi Clara,

The reason is that in the model, I wanted to be able to provide the ligament stiffness in N per m but the ligament model we used (the one from Blankevoort) require the stiffness in N per unit strain.

Regarding the small ligament forces, then I think they make sense for these movements as the loads are small. I also did a sensitivity study on the outputs of the ligament parameters and you can see the results in this paper:

Andersen, MS, de Zee, M, Damsgaard, M, Nolte, D & Rasmussen, J 2017, 'Introduction to Force-dependent Kinematics: Theory and Application to Mandible Modeling' Journal of Biomechanical Engineering, vol. 139, no. 9, 091001.

The outputs remained almost identical within the range the ligament parameters were varied.

Best regards
Michael Skipper Andersen
Associate Professor
Aalborg University

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