Hi,
I would like to know why the constraining area of the resultant force in the glenoid has a circular and not an elliptic shape. What are the consequences of this circular shape on the measurement of the resultant force at the GH joint ? Would it be more representative to place nodes directly on the border of the scapula ?
Thank you.
Pierre
Hi Pierre,
I think the ideal situation would be to place the contact nodes directly on the edge of the glenoid, but the shape of this probably varies quite a bit between individuals. I think the current circle was meant to be an approximation that was also easy to parameterize if somebody wanted to do that.
Notice also that there is more to this constraint than the shape or the curve on which the points are placed. The contact is modeled by means of pushing elements rooted in these nodes, and the pushing elements also have a direction that should be perpendicular to the glenoid surface at the point. So the contact elements represent not only the shape of the edge but also the orientation of the articular surface.
Best regards
John
Hi John,
I understand. I will consider the circle as the limit of the articular surface. However, what does happens if the force lies somewhere outside the articular surface, but inside the glenoid rim ? Can we say that this force is destabilizing the shoulder ?
Also, as long as the resultant force stays inside the constraining area (inside the articular surface), the stability is maintained. So, is the orientation of the same resultant force is arbitrary ? If, for example, the resultant force shifts superiorly really quickly but stays inside the articular surface, can we speculate about an eventual superior instability ? If not, do we have to necesserily simulate the translation of the humeral head, in order to speculate about the GH joint stability ?
Thank you.
Pierre
Hi Pierre,
Mechanically speaking, the edge of the glenoid as defined by the points is also the limit of the articular surface. Stability is obtained when the reaction force is within the convex hull of these points. This means that if the points form a non-convex polygon, then the joint can still be stable if only the reaction force it within the convex hull of this polygon.
The way the system reacts to this stability criterion is by activating the muscles in such a way that the reaction force will be stable. If for some reason the muscle configuration is unable to do this, then the solver will fail to recruit the muscles and you will get an error. Remember that the recruitment might also return a solution with overloaded muscles, and this might also be due to inability to stabilize the joint with the muscle strengths it has available.
Best regards,
John
Hi John,
Ok. However, the question remains to know if the direction of the computed resultant force is close to reality or just arbitrary, and if it can be used as a stability determinant. If not, how can we study the stability of the joint, considering that the humeral head cannot be translated in the model ? Do we have to necessarily allow the translation of the humeral head ?
Thank you very much for your quick answers.
Hi P-O,
I do not think there is any way of knowing whether the computed direction is correct or not. Instrumented prostheses can measure the direction, but a prosthetic shoulder is obviously much different from an anatomical shoulder so we do not know whether the measurements are representative for a real shoulder.
Constraining the the reaction forces inside the glenoid fossa is a sound principle because instability will certainly occur if this is not the case. However, we cannot simulate what the consequence of such an instability would be: Complete dislocation of a small migration of the humeral head?
With the new force-dependent kinematics we can perhaps do this, but it requires knowledge of the nonlinear stiffness of stabilizing passive-elastic elements around the joint.
Best regards,
John
Hi John,
This is a fair and clear answer. It will guide my interpretations of the results for my future paper submissions.
If I increase the constraining area of the reaction force in such a way that the force would be allowed to step outside the glenoid rim, would it means that the reaction force could destabilize the GH joint ? Does the resultant force would still be valid ?
Also, if I use the Quadratic recruitment criterion, the direction of the resultant force varies much, because the deltoid is much recruited compared to the rotator cuff muscles. In comparison, the MinMax criterion compute a resultant force which does not vary much and stays centered in the glenoid fossa. However, I read that in some cases, the MinMax criterion is not fully representative of the muscle recruitment, because it creates jumps in the muscle recruitment. How can I determine which criterion is the best suited for the abduction of the shoulder ? Maybe a mix of both recruitment (Auxiliary criterion) ?
With the help of Sylvain Carbes, I’m presently working on the implementation of a translational stiffness at the GH joint. I hope that this will lead to an improvement of the model and a good prediction of the translation of the humeral head.
Thank you very much.
Pierre
Hi there,
I come back to you with the constraining area of the glenoid cavity. When we simulate a massive rotator cuff tear (removing all the rotator cuff and the biceps long head) with the intact model, the model estimates that it is still possible to perform the movement up to 150 degrees without muscular saturation, which means that it is able to respect the stability constraint. However, numerous clinical studies shows that a massive rotator cuff impair shoulder function in most cases. Thus, we feel that the constraining area may not be enough restrictive and that the resulting activation of the cuff may not be sufficient.
Our idea, which has already been proposed by others, would be to reduce the constraining area in order to force the reaction force to be closer to the glenoid centre. This will force the rotator cuff muscle to be more sollicited or, in case of massive rotator tear, this will cause an early muscular saturation, indicating that the shoulder cannot be stabilized.
What do you think ? Do you feel that the constraining area is ok like it is now ?
Thank you.
Pierre
Hi P-O,
I have no first hand knowledge about glenoid shapes, but my second hand knowledge is that they vary a lot in shape. Krystyna Gielo-Perczak, I believe, investigated the matter during her time with Liberty-Mutual and often spoke with me about the need to investigate the biomechanical consequence of different glenoid shapes. Her idea, which I think is quite correct, is that if the glenoid is already shallow, then the sensitivity to small changes of shape will be high.
Best regards,
John
Hi John,
Glenoid shape is a quite interesting idea. We also work with Krystyna and plan to evaluate this aspect with her.
Thank you.
Pierre