Patellar tendon stiffness

Main Forum
1

Hi AnyBody,

I have a model based on AMMR.v2.0.0. I am interested in the knee joint loading. The predicted femur/tibia contact forces are in a reasonable range. However, the predicted patella/femur contact forces are nearly two times of the experimental data. The reason is likely the patella tendon stiffness in the existing (AMMR.v2.0.0) model is too high. My question is how to adjust the patella tendon stiffness?

Thanks,

John

2

Hi John,

Which AMMR model are you referring to?

The normal knee model do not have a patella/femur stiffness implemented, it is made with kinematic constraints.

Are you sure you are looking at the right property in the modeltree and that the coordinate system these forces are given in corresponds to the value you are comparing with.

If on the other hand you are running an FDK model for the knee it is a different story, then the stiffness will influence results.

Best regards
Søren

3

Søren,

Thanks for the quick response. I mean the tendon pulling the patella to the tibia, as shown in red line of the attached picture. The stiffness of the patella tendon should have a big effect on the patella/femur contact force. I need adjust the stiffness of this tendon.

John

4

I have used AMMR.v2.0.0 and defined the model as:

…
#define BM_LEG_MODEL LEG_MODEL_TLEM

#define BM_LEG_MUSCLES_BOTH MUSCLES_3E_HILL

#define BM_SCALING SCALING_LENGTHMASSFAT
…

This is equivalent to “Twente Lower Extremity Model v.1.2” in AMMR.v2.1.0

John

5

Hi John,

The connection between patella and tibia is a kinematic constraint made as a AnyKinPLine, so it do not have a stiffness… but as i wrote earlier if you are running an application which replaces the knee with an FDK model then the kinematic constraint is replaced by a ligament with a stiffness.

Best regards
Søren

6

Søren,

Thanks.

Do you know a good existing model that applied FDK to knee joint, which I can adopt and modify? The only model I found in AMMR.v2.1.0 is THA-KneeBendDemo, which applied FDK to a hip joint.

John

7

Søren,

I tried to include the FDK into knee patellar/femur joint, according to “THA-KneeBendDemo” (see attachment). I got a error message: some DOFs have not been constrained. Do you have time to look at my model and give me some suggestions? When FDK is switched off, the model works.

Thanks,

John

8

Hi John,

Please have a look in the Beta folder there is a TKA_kneebenddemo model.

Replacing joints in the model with FDK driven joints requires some experience and may lead to models errors on the way until the proper stiffness in the model has been established.

Please see this section of the wiki https://github.com/AnyBody/support/wiki/All-about-Force-Dependent-Kinematics

Among other things you need to:
[ol]
[li] ensure proper stiffness applied on the DOF you ask FDK to solve for.
[/li][li] use a reasonable starting guess on the position for the FDK dof
[/li][/ol]

Best regards
Søren

9

Søren,

Thanks for your help.

The TKA-kneebenddemo model in Beta folder is not available. Is it possible that you send it to me or put it github?

John

10

Søren,

I found the Beta-TKA-kneebenddemo model, which you mentioned, in the older AnyBody version v6.1. I can start from there.

Thanks,

John