Distal radius fracture model: Segment coupling and AnyForceMomentMeasure validation

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1

Hi all,

I am a master student at the University of Twente and I am developing a distal radius fracture model in AnyBody. I have a question regarding my modelling approach and validation results.

To model the fracture, I split the radius into RadiusProximal (~90%) and RadiusDistal (~10%). To preserve AMMR anatomy, I kept the original segment Seg.Radius as a massless, inertia-free ghost segment:

• no muscles or joints attached
• rigidly welded to RadiusProximal
• used only as a kinematic reference for anatomical nodes (sRel/ARel)

All muscles, joints, and wrapping surfaces were reassigned to the new fracture segments. The two fracture parts are connected using a rigid kinematic coupling to preserve overall consistency with the intact model.

With this setup:
• muscle forces and joint reaction forces match the intact AMMR model
• ghost segment contributes Fout = 0

I compared the force output of the fracture driver between the two segments with a reference model using AnyForceMomentMeasure2 applied to the intact AMMR model with:
• cutsystem = On (CutSystemNormal = y: long bone axis pointing towards elbow)
• cut at the same fracture plane
• including original radius, all muscles connected to radius, and joint reaction forces on proximal side of the fracture plane

My final goal is to extend this model to force-dependent kinematics (FDK) to simulate motion of fracture fragments under static loading conditions.

Questions

  1. Is this ghost-segment + rigid coupling approach a valid way to preserve intact-model mechanics in AnyBody?

  2. Or should all dependencies on the original Seg.Radius be fully removed (including mirrored nodes and anatomical references)?

  3. Is AnyForceMomentMeasure2 with a cut system a valid way to verify equivalence between the intact and fracture models? Should both approaches theoretically yield identical force results?

2

Hi,

I think a massless ghost segment will not harm, but there is always a danger to forget/miss something. If you completely remove - it might help you find it. You only need to refer StdParams of the original segment and reuse sRel definitions from the radius segment. But let’s assume you did everything well.

AnyForceMomentMeasure2 should give you identical results only in a gravity-free and movement-free conditions. Because the inertia and gravity forces will be applied to the CoM, which will be located proximally to the cut. In case of 2 segments, both segments will have they own inertial forces.

Kind regards,

Pavel