How to step outside the glenoid fossa

Main Forum
1

Hi guys,

My question is about the GHReactions edgelines of the shoulder model. Those lines ensure that the gh resultant force stays inside the glenoid fossa. What do I need to do if I want to prove that the resultant force, for one particular case (ex. rotator cuff tear), would step outside the glenoid fossa ? Would it be sufficient to just broaden the limit of the glenoid, i.e. increasing the radius of the circle at the base of the cone ?

Thank you.

Pierre

2

Hi Pierre,

If you broaden the limit of the glenoid you give the chance to the reaction force to step outside the previous limit. I think what you would need on top of that is a way to measure that the force has or has not gone out of the previous limit. I guess you can figure out a smart way to check it.

Best Regards, Sylvain.

3

Hi,

is it wrong to merely switch off the GHReactions?

Regards
Hauke

4

Hi Hauke,
this will lead to problems and is not recommended.
Best regards,
Sebastian

5

What kind of problems? As I did it and no error occured. Or will it lead to wrong result, e.g. forces. etc.?

Thanks so far :slight_smile:

6

Hi Hauke,
as written in the GH reactions files: "This file creates realistic reaction between humerus and scapula,
the ordinary reactions in the spherical GH joint has been switched off.

The ideas is to use a setup of strong pushing muscles
which ensure that the GH reactions force is alway inside the glenoid cavity.
The reactions are made by a number of strong muscles pushing
from the GH joint center towards the edge of the glenoid cavity,
here created as a circular path. So if any of these muscles are
activated the reaction force will either point ot the edge or inside
the cavity, it can not go outside. These muscles are recruited like
any other muscle in the model, they are just very strong and not
close to the activity envelope, so they do not distrub the
recruitment of the other muscles. In other words the model “feels”
that the reaction force should fall within the cavity and it will
change the recruitment of the normal muscles to ensure this, but it
do not feel any cost of having a high normal force there, even
though it is created by muscles, because these muscles are very strong,
and should not be lying on the muscle activity envelope.

This way of calculating the reaction force can be switched off by
excluding the file "GHReactions.any "from the Arm3D/jnt.any
and instead using the normal reactions of the GH joint.
"
You can switch the reactions off, but then it is not ensured that the reaction forces are pointing in the right direction - inside the glenoid. It depends on your case if this is ok or not.
Best regards,
Sebastian

7

Hi Sebastian,

I don’t quite get it…

Pothekid asked:
“What do I need to do if I want to prove that the resultant force, for one particular case (ex. rotator cuff tear), would step outside the glenoid fossa ? Would it be sufficient to just broaden the limit of the glenoid, i.e. increasing the radius of the circle at the base of the cone ?”

As far as I understand he wants the force to be able to step outside the fossa, which means there must be the possibility to step out. When I switch off the GHReaction force, this is given, isn’t it?
So, where is the difference between switching of the force and broadening the radius of the cone?

This is really important for my study, as there might occur the case that the force steps outside.

Thanks for your response,
Hauke

8

Hi,
if you just broaden the radius you still contrain the reaction force to stay inside of this new radius. By switching them off the reaction force can go everywhere. I many cases there might be no difference at all, but in some motions there might be a difference.
Best regards,
Sebastian