It's an amazing joint, with a great range of movements. Despite this, shoulder instability is uncommon and mostly follows an obvious injury. Our shoulder muscles are continually working to keep the arm bone nicely against the shoulder blade - very effectively.
Our hip joint is also a ball in a socket, but the socket covers most of the ball, making it very stable. The socket of our shoulder is just a shallow saucer, to allow for a much greater range of movement.
This is particularly so for our right shoulder. The left one has a little less movement.
It is usually anterior dislocation of the shoulder, from injuries involving overhead action.
This initial injury can involve damage to the rim of the socket (Bankart lesion) and/or the rounded head of the arm bone (Hill Sachs lesion) as well as to the muscles and ligaments of the shoulder.
Shoulder dislocation injury in adolescence, has a 100% recurrence rate. Between 18 and 30, it is still 90%.
Surgery is the usual advice for such sports injuries.
Prolotherapy is a good alternative.
This involves injecting a sugar solution to produce temporary inflammation and subsequent ligamentous tissue strengthening.
You apparently need to treat the tender anterior glenoid ligaments, medial to the biceps tendon with the person sitting up. Then lie them down and with the arm at 90 degrees to the body, elbow bent and hand up, so you can get at the bottom ligaments. You can also inject into the shoulder joint.
This is also uncommon. The person has some simple shoulder action suddenly come to an abrupt and painful stop. Movements are all limited.
There is no sign on examination, to suggest dislocation nor fracture.
I've used the same technique as for anterior dislocation, but without a pain relieving injection. Subscapularis is stretched and then the arm is flung suddenly around so that the forearm hits the body.
I have now seen "dead arm syndrome" in an overhead throwing athlete, which is also attributed to subluxation ("transient anterior subluxations with sudden paralysing pain when the arm is out, back and palm up.".)
James Cyriax¹ describes his experience as...
"After a dislocation or sprain, residual capsular laxity at the gleno-humeral joint may give rise to momentary subluxation of the head of the humerus as the arm moves upwards towards the horizontal position. At about 80 degrees, it clicks back into place, perhaps with some discomfort."
Strengthening exercises for all the 3 major muscles passing between scapula and humerus and also for biceps, are in order to protect the shoulder joint here.
Here the person is "double jointed" and both shoulders are loose, even if the other one is painless.
1. James Cyriax Textbook of Orthopaedic Medicine vol one 5th ed 1969 Bailliere Tindall & Cassell
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