Concavity compression is the main mechanism through which the glenohumeral Joint is stabilized in its most functional positions.
Let’s illustrate this with diagrams by Steven B. Lippitt. The anatomic shoulder is characterized by the compression of the ball into the concavity of the glenoid.
The joint is stable as long as it follows the direction of all forces within the concavity.
But, the joint won’t be stable if it doesn’t contain the net force within the concavity.
You can reduce the stability of the joint by either compromising the concavity (left), or cold flow which flattens the rim of a Polyethylene glenoid.
The reverse total shoulder follows the same principles. Stability is achieved when the direction and magnitude of the net-glenohumeral force are contained within the concavity. The stronger the compressive force between the humeral cup & glenosphere, the more stable the joint against displacement forces. An insufficient compressive force can cause instability.
The joint can become unstable if the net force cannot be contained.
Unwanted contact between the humerus, scapula, or interposed soft tissue can cause the net to misalign.