Modeling L4-L5 and intervertebral disc

Hello all,

I want to do a finite element analysis on the intervertebral disc between L4 and L5 vertebrae for a given motion-capture data. Now, I have the geometry and the material properties of the disc and the vertebrae. I can also individualize the L4 and L5 vertebrae in Anybody and run the MOCAP model, however, the Anybody model doesn’t include the intervertebral disc. So, my question is how can I use the loading and the boundary condition from the Anybody output for a finite element analysis of the complete L4-L5 segment. Can I export the forces for the L4 and L5 segment and use that as a boundary condition and insert a disc (with geometry and material property) between the L4-L5 vertebrates and run the FE analysis? Any advice would be helpful.

Thanks,
Anup

Hi @ap21

Yes, you should be able to dump the boundary conditions and use in your FE model.
Depending on you scenario you could implement new nodes corresponding to your FE model.
You can find a few details in this older thread: AnyBody-Ansys Interface - #13 by victorwambugu

Best regards,
Bjørn

Just to confirm, should I use the AnyFe2Abq converter for L4 and L5 separately using their respective reference frame and get 2 loading files? Then, merge those two loading files with the new mesh file that contains the L4 vertebrae, L5 vertebrae, and disc geometry? I am using Abaqus for the FE analysis.

I am working on similar work, but I think discs should be modeled as body segments in anybody..

Hi Bjorn,

Or Should I use one of the local ref frame frame of L4 or L5 (AnyRefFrame) and use both the segments (AnySeg: L4 and L5) to output the AnyMechOutputFileForceExport (.xml file)? Also generate the mesh file that contains all the geometry (L4,L5, and disc). Then combine those two files (.xml and mesh file) to generate the final input file using AnyFE2Abq converter. I have some confusion regarding understanding using one ref frame to export multiple segments in AnyMechOutputFileForceExport. Any assistance would be helpful.

Thanks,
Anup

Hi all,

There are a few points that I would like to mention, that need to be taken into consideration:

  1. AnyBody model by default uses an idealized spherical joint to represent the disc behavior and assuming instantaneous centres of rotation. So only rotational deformation of the disc is present to some extent, but no compression.

  2. Personalization of just a single FSU changes very little in the results. It probably complicates the process more than it helps. You would need to introduce a complex calibration procedure for the disc and ligaments.

  3. AnyBody model also uses a kinematic rhythm that links several degrees of freedom in the lumbar spine to work as a single unit. Properly replacing a single functional spinal unit (FSU) will be very tricky implementation-wise. You would also need to use a Force-Dependent kinematics to harness the power of personalization. But it will actually make it very hard to work in a MoCap model.

My recommendation would be to go simple - export reaction force acting on the superior segment and apply an inverted force to the endplate (modelled as rigid) to simulate compression (since you are interested in the disc, but not bones themselves). Or create an AnyForceMomentMeasure2 object, which computes an equivalent load counting contribution of specified forces. As an input you could use all muscle acting on the superior adjacent segment, leaving ligament contributions out. Whatever this measure computes, can be then applied to the superior endplate as boundary conditions in the FEA.

If you still prefer to go the path with AnyFE2Abq converter, then you need to export forces in the inferior(caudal) segment ref. frame.

Kind regards,
Pavel

Hi Pavel,

Thank you for your reply. So, I want to go through the route of AnyFE2Abq converter. If I understood correctly and I want to have both the L4 and L5 seg in my FE model along with the manually created disc (for FE mesh and analysis not in Anybody), would the following pieces of code be appropriate?

I first export all the forces in segment L4 and L5 using the reference frame of SacrumSeg (is this appropriate, or do I need to use another reference frame?)

 AnyMechOutputFileForceExport testL4L5 = 
 {
   FileName = "test.xml";
   UseRefFrameOnOff = On;
   AllSegmentsInStudyOnOff = Off;
   XMLformatOnOff = On;
   RefFrame = &Main.HumanModel.BodyModel.Trunk.SegmentsLumbar.SacrumSeg.localrefframe;
   Segments = {&Main.HumanModel.BodyModel.Trunk.SegmentsLumbar.L4Seg, &Main.HumanModel.BodyModel.Trunk.SegmentsLumbar.L5Seg};
 };

Then I use that .xml file to convert to an Abaqus input file using the following command:

 AnyOperationShellExec ConvertToAbq={
   Show=On;
   FileName = ".AnyFE2Abq.exe";
   Arguments = "-i .test.xml -o .output_AB.inp -m .L4-L5-disc_mesh.inp";
   WorkDir=".\ ";
 };

The L4-L5-disc_mesh.inp is my mesh file that contains the L4 and L5 segment exported from Anybody in their local coordinate system and I have manually added intervertebral disc in between them.

Any feedback would be greatly appreciated.

Regards,
Anup

 AnyMechOutputFileForceExport testL4L5 = 
 {
   FileName = "test.xml";
   UseRefFrameOnOff = On;
   AllSegmentsInStudyOnOff = Off;
   XMLformatOnOff = On;
   RefFrame = &Main.HumanModel.BodyModel.Trunk.SegmentsLumbar.L5Seg.localrefframe;
   Segments = {&Main.HumanModel.BodyModel.Trunk.SegmentsLumbar.L4Seg};
 };
  1. Exclude joint load from the export using ForceObjectExclude
  2. Constrain L5 in FEA.

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