Coordinate System in ABAQUS

[Yidan Xu]

Hi,
I am a new graduate student and try to learn from this project to do my thesis. However, I faced a problem which I can't figure out. I imported the tf_joint.inp (the meshed model) into the ABAQUS, which successfully showed up. But the coordinate system is not the same as the model in FEBio. Since I want to assign the origin point as the reference point to femur(rigid body) and assign this point as the center of flexion-extention, it's really important to know the coordinate of the origin at the mid-point of medial and femoral condyles. Or if you can tell me how to transfer the coordinate system?

Any helps, I will appreciate a lot!

Sincerely,
Yidan

[Ariel Schwartz]

Hi Yidan,

Most of the knee modelling we do in our lab is done in FEBio, so I may not be able to give you specifics on how to do what you want in ABAQUS. However I can explain how we define our tibio-femoral joint in our FEBio models, which may give you some ideas.

All of our knee models are defined in the MRI image coordinate system, I do believe that regardless of the FEA platform you are using they should all be in the same coordinate system. This does mean that the origin of the coordinate system is not located at the femur origin. In order to flex the knee, we create a system of 3 cylindrical joints (flexion/extension, abduction/adduction, internal/external). You can find a description here: https://simtk.org/plugins/moinmoin/open ... odyMotions. We define the axes and origins of these joints using user-specified bone landmarks, and then we have a script to calculate the axes (https://simtk.org/plugins/moinmoin/open ... tomization). Once these joints are modelled, flexion of the knee can be accomplished by simply fixing the Tibia bone, and prescribing a rotation to the flexion/extension joint, while leaving all other degrees of freedom free.

I recommend looking at our FEBio models to help you understand how we do this, and then you can find a similar approach in ABAQUS.

I hope this helps.
Best,
Ariel

[Yidan Xu]

Hi Ariel,

Thanks for your kind reply!

I read the website you sent and the guide file for users. The guide said that the geometries (IGS model) are provided in a coordinate system aligned with the first sagittal MR image, while the meshed model used the same coordinate system as the geometry model. Moreover, the coordinates of the landmarks on the website also used the MRI coordinate system. However, when I tried to input the coordinates of DFP into the IGS model, the DFP is not the point at the midway between the medial and lateral condyles. So I was wondering if I misunderstood something. I was trying to find the landmark's coordinates under the geometry model's coordinate system.

Thanks for helping me! I appreciate it a lot!

Sincerely,
Yidan

[Yidan Xu]

Hi guys,

As an update to you, I have solved this problem. By importing the STL file into the ParaView, and export the points on the condyles, and then calculate the center of the condyles by using MATLAB. If anyone face the same problem, you can try this method.

Sincerely,
Yidan

[BABATUNDE AFOLABI]

Hello Yidan,

Please can you explain this in a more simpler step by step format. I ran into the same issue but i don't seem to understand how you said you went about resolving it.

Thank you.

Sincerely,
Peter

[nishendra singh]

For finding the center-point of the femoral condyles in Abaqus, you can use the "Create reference point" -> "Mid way bw two points" , option in the Abaqus part manager. You can pick the nodes on either side of the condyle and generate a center point. Then use the "Translation tool" in the Assembly menu to shift the entire assembly to the coordinate systems shown by the yellow lines.