simtk.org

Hi,

I am currently working with the Lumbar Spine model by Christophy. The vertebral bodies are connected by custom joints that only allow for flex-extension, lateral bending and axial rotation. My question is: is it possible to define the joints so that they represent the intervertebral discs, taking into account the compression of the discs under the weight, and their axial and rotational stiffnesses?
I read in the forum Pathspring or Bushing Force might be useful, is that correct? Which one should I use, and can I implement them modifying the OSIM file through an XML editor?

Many thanks,

Michele

Hi Michele,

This somewhat depends on what you are planning to do with the model. Can you tell us what your research question is and what types of simulations you are looking to perform?

Accounting for the axial and rotational stiffness of the discs should be relatively straight forward using the bushing force. You can use the Help->XML Browser for details of the implementation in the .osim model file. The standard BushingForce is a linear function of the joint coordinate. If you would like to use a nonlinear expression, I suggest looking at the ExpressionBasedBushingForce.

Accounting for the compression of the discs would require the addition of a translational degree of freedom (can be done in the .osim file) to the joint. Then you could use the BushingForce to model the compressive stiffness of the disc. Addition of this degree of freedom will likely make simulation more difficult, thus it should be avoided if possible.

Colin

Hello Colin,

thank you for your answer. Basically my purpose is to compute the loads acting on intervertebral discs during various movements. The lumbar spine model represents the discs as rigid, incompressible joints, while I'd like to have a more realistic representation, taking into account the deformation of the discs under the body weight and muscle forces.

As you pointed out, with the Bushing Force I can only account for the rotational stiffness; I'd like to try to add the additional DOF, to consider also the compression, but I have this doubt: if I add the DOF (with the Bushing force for the stiffness), which I understand means adding also a coordinate, is it possible to have a model that automatically calculates the deformation of the disc depending on body weight and muscle forces during the motion, or would I have to input manually in the motion files the values of the new translational coordinate?
Because in the second case, if I had to define the compression a priori, that wouldn't be useful...

Thanks again!

Best,

Michele