While the hip is stabilized by a deep socket, the glenohumeral joint has a shallow socket that allows a wide range of motion without the ball abutting against the rim of the socket.
While the knee is stabilized by isometric ligaments, the glenohumeral joint's ligaments and capsule are lax in its functionally important mid range positions 
As pointed out in In vivo quantification of the laxity of normal and unstable glenohumeral joints laxity is not the same as instability; healthy subjects without symptoms may have as much laxity as patients needing surgical repair for symptomatic shoulder instability.
We must, therefore, ask how can the relatively large humeral head be stabilized in the small glenoid socket while still allowing a greater range of motion than any other joint and amazing feats of strength as shown in these classic videos by our late partner Douglas Harryman (see Shoulder Stability 1 and Shoulder Stability 2)
One of the special mechanisms of glenohumeral stability is concavity compression as first described in Mechanisms of Glenohumeral Stability. In concavity compression, the direction of the sum of the forces applied to the humeral head is contained within the glenoid socket.
The compliant labrum allows the socket to seal to the humeral head much as the compliant edges of a suction cup allow it adhere to a man's forehead
See Doug Harryman's video on suction cup effect; while the quality of the video is not great, the message is clear.
The suction cup is not only important in the normal shoulder: in performing a ream and run procedure for arthritis, stability is enhanced by preserving the glenoid labrum. When a healthy labrum can be preserved, the stabilizing effect of the suction cup can be observed at the time of surgery. Turn your volume up, so you can hear the 'kiss' sound as the suction is broken by applying a strong posteriorly directed force on the humerus.
Recently, our colleagues at the University of Utah and Japan revisited A stabilizing role of the glenoid labrum: the suction cup effect
Using a cadaver model, they sought to quantify the effect of the anteroinferior and posterosuperior labrum to glenohumeral stability.
They measured the peak force required to translate the humeral head in the anterior, anteroinferior, posterior, and posteroinferior directions was measured under 5 conditions:
intact labrum,
an anteroinferior labral tear,
a posterosuperior labral tear,
combined labral tear, and
no labrum.
The stability ratio was defined as the peak translational force divided by the compressive force. Within force-translation curves, they defined the suction cup effect as the force required to release the negative pressure created by an intact labrum.
They found that the suction cup effect was usually present with the intact labrum and disappeared after removal of the labrum for anterior and posterior translations. After creation of an anteroinferior labral tear, the stability ratio for posterior direction decreased and the suction cup effect disappeared. After creation of a posterosuperior labral tear, stability ratios in the anterior and anteroinferior directions decreased and the suction cup effect disappeared. The stability ratio for anterior and anteroinferior testing was more diminished by posterosuperior labral tears than anteroinferior labral tears, and the stability ratio for posterior testing was more diminished by anteroinferior labral tears than posterosuperior labral tears.
They concluded that anteroinferior labral tears decreased posterior stability and posterosuperior labral tears decreased anterior and anteroinferior stability, largely because of loss of the suction cup effect.
Comment: In order to achieve its amazing mobility and stability, the shoulder requires some specialized stabilizing mechanisms - such as concavity compression and the suction cup effect - that are not as important in other articulations. Understanding these special mechanisms provides insight into the functioning of the normal shoulder and the management of shoulder instability.
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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).
Concavity compression is enhanced by increasing the force compressing the head into the glenoid and by the deepening of the glenoid concavity by the concave cartilage and the labrum.
One of the other contributions of the glenoid labrum is the creation of the glenohumeral suction cup effect.
The compliant labrum allows the socket to seal to the humeral head much as the compliant edges of a suction cup allow it adhere to a man's forehead
The suction cup is not only important in the normal shoulder: in performing a ream and run procedure for arthritis, stability is enhanced by preserving the glenoid labrum. When a healthy labrum can be preserved, the stabilizing effect of the suction cup can be observed at the time of surgery. Turn your volume up, so you can hear the 'kiss' sound as the suction is broken by applying a strong posteriorly directed force on the humerus.
Using a cadaver model, they sought to quantify the effect of the anteroinferior and posterosuperior labrum to glenohumeral stability.
They measured the peak force required to translate the humeral head in the anterior, anteroinferior, posterior, and posteroinferior directions was measured under 5 conditions:
intact labrum,
an anteroinferior labral tear,
a posterosuperior labral tear,
combined labral tear, and
no labrum.
The stability ratio was defined as the peak translational force divided by the compressive force. Within force-translation curves, they defined the suction cup effect as the force required to release the negative pressure created by an intact labrum.
They found that the suction cup effect was usually present with the intact labrum and disappeared after removal of the labrum for anterior and posterior translations. After creation of an anteroinferior labral tear, the stability ratio for posterior direction decreased and the suction cup effect disappeared. After creation of a posterosuperior labral tear, stability ratios in the anterior and anteroinferior directions decreased and the suction cup effect disappeared. The stability ratio for anterior and anteroinferior testing was more diminished by posterosuperior labral tears than anteroinferior labral tears, and the stability ratio for posterior testing was more diminished by anteroinferior labral tears than posterosuperior labral tears.
They concluded that anteroinferior labral tears decreased posterior stability and posterosuperior labral tears decreased anterior and anteroinferior stability, largely because of loss of the suction cup effect.
Comment: In order to achieve its amazing mobility and stability, the shoulder requires some specialized stabilizing mechanisms - such as concavity compression and the suction cup effect - that are not as important in other articulations. Understanding these special mechanisms provides insight into the functioning of the normal shoulder and the management of shoulder instability.
Follow on twitter: https://twitter.com/shoulderarth
Follow on facebook: click on this link
Follow on facebook: https://www.facebook.com/frederick.matsen
Follow on LinkedIn: https://www.linkedin.com/in/rick-matsen-88b1a8133/
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).
