GHReactions concept recreated at the acromion

Hi guys,

I’m working with the shoulder model in the 2.0 version of AnyBody. I created a permanent contact between the humeral head and the acromion to simulate a massive rotator cuff tear.

Now I want to know the force created by this contact. Because that permanent contact can creates a rounding of the surfaces, I decided to use the same concept as with the GH Reactions, but with a smaller area of contact. The result is shown in the first attached figure.

Is this the right way to make this contact ? What are the differences of using a “GHReactions” concept, compared to switching the joint ReactionsType to ON.

Now my kinematics is an abduction from 15 to 120 thoracohumeral degrees with the 3 bones in motion (clavicle, scapula and thorax).

I used the same force summation as in GHReactions to get the resultant acromial contact force. Please have a look at this force (attached file). Can you tell me why I get a non-smooth curve ? Does this sound normal to you ?

Thank you

Pierre.

Hi Pierre

If i understand you correctly you have a force setup which correspond to the normal glenoid cavity setup plus a similar setup for the acromion, and both set of forces can be active simultaneously?

This should in principle be correct in the case that you have acromion contact.
The resultant force of the two contact areas can take directions which are somewhere in between the glenoid cavity and the acromion is this as intended?

I do not recall to have seen a non smooth curve for the reaction force, but it may be due to a shifting between the two type of contacts you have.

Best regards
Søren

Hi Soren,

You are right. The resultant force is somewhere between the two contact forces, being the sum of them.

Like i said in my other thread, I changed for AnyBody 4.0 and I now use the BergmannGH model. I found that my non-smooth curve was related to a driver which was ON but was supposed to be OFF.

Also, I used some values of the litterature to drive the clavicula, in order to have a large range of abduction from 0 to 120 degrees. Now I have a recruitment problem when simulating a massive rotator cuff tear (no cuff except the teres-minor and subscapularis). At some point during the abduction, the deltoid abruptely stops to be activated, and the biceps brachiis will eventually be activated too much. This behavior doesn’t seems normal and I suspect the acromial contact to produce this result.

I’ve made some GIF animations in a powerpoint that you can check to better see the muscle recruitment behavior. In the animation, the two red vectors represent the scapular reactions and the blue vector represents the resultant force.

Note that you have to put the powerpoint in SlideShow mode to see the animated GIF.

Thank you

Pierre

Hi Pierre

Which muscle model are you using? if it is the three element model i would advice you to try shifting to the constant strength model, this makes it simple to interpretate the results.

The problem could be that the deltoid muscles are loosing their strength and the biceps muscles take over, so it could be a muscle calibration issue.

Best regards
Søren

Hi Soren,

That’s an interesting point. Can we say that "when an humeral migration occurs, the deltoid length decreases and this has an influence on the force-length relationship of the deltoid. Beyond a certain degree, the muscle is not strong enough to raise the arm and muscle recruitment changes to use other available muscles. If this is true, then we might partially explain why some patients have difficulty raising their limb beyond a certain degree.

For the calilbration issue, I use the tendon length of each muscles after a calibration with the asymptomatic shoulder (no migration) and I don’t re-calibrate the muscles in the symptomatic shoulder. I do this because I think that the tendon length (Lt0) stay the same after the massive tear and we have to consider the same original length, in order to highlight the influence on the muscle. Does it sounds correct to you ?

Thank you

Pierre

Hi Pierre

The procedure sounds correct and we have done something similar previously in other models.

Calibration of the tendon length is a difficult issue it is hard to find data on which positions to calibrate the muscles in.

Have you looked at the strength of the Deltiod muscles for the motion in your model?

Best regards
Søren