Concavity compression is the primary mechanism by which the glenohumeral joint is stabilized in most of its functional positions.
Let's use some diagrams by Steven B. Lippitt to illustrate. In the anatomic shoulder, the action of the deltoid, cuff and other muscles compress the ball into the glenoid concavity
As long as the direction of the sum of all the forces is contained within the concavity, the joint is stable.
However, when the net force is not contained within the concavity, the joint is not stable
The stability of the joint can also be reduced by compromise of the concavity as in a bony Bankart lesion (left) or with cold flow leading to flattening the rim of a polyethylene glenoid (right).
These same concepts apply in the reverse total shoulder. Stability is provided when the direction of the net glenohumeral force is contained within the humeral concavity. The greater the force compressing the humeral cup and glenosphere together, the greater the stability against displacing forces - inadequate compressive force leaves the joint at risk for instability.
In summary, stability of the reverse total shoulder requires a strong compressive force aligned with a competent humeral concavity.
When the net force is uncontained, the joint becomes unstable.
Any unwanted contact between the humerus and scapula or interposed soft tissues can misalign the net force, leading to instability.
Instability of the reverse total shoulder can also be caused by compromise of the concavity of the humeral cup from polyethlene wear as shown below
or from displacement if the polyethylene liner from the humeral tray
Instability is the most common mode of failure of the reverse total shoulder. The importance of the concavity-compression mechanism in stabilizing the reverse total shoulder (RSA) is emphasized in a recent article Revision for instability following reverse total shoulder arthroplasty: outcomes and risk factors for failure in which the authors sought to identify factors associated with recurrent instability by studying patients undergoing RSA for instability, including 43 surgeries in 36 patients. Overall, 32/36 patients (89%) required 38 revisions to achieve stability at most recent followup. Loss of compression was the most common mechanism for persistent instability. "Loss of containment" means compromise of the humeral concavity. "Impingement" refers to unwanted contact altering the direction of the net force across the articulation.
Comment: Understanding concavity compression can guide the evaluation and management of the unstable biologic and prosthetic glenohumeral joint.
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Here are some videos that are of shoulder interest
Shoulder arthritis - what you need to know (see this link).
How to x-ray the shoulder (see this link).
The ream and run procedure (see this link).
The total shoulder arthroplasty (see this link).
The cuff tear arthropathy arthroplasty (see this link).
The reverse total shoulder arthroplasty (see this link).
The smooth and move procedure for irreparable rotator cuff tears (see this link).
Shoulder rehabilitation exercises (see this link).
