The Shoulder

instability

Factors in glenohumeral stability:

The gleno-humeral joint is descibed classically as an incongruous ball-and-socket joint. To some authors the description of the joint as analogous to a 'golf-ball-sitting-in-a-tee' is more appropriate:

The large spherical head of the humerus articulates with the small shallow glenoid fossa of the scapula (only 25% to 30% of the humeral head is covered by the glenoid surface).
There is a relative shallowness of curvature of the glenoid compared to the humeral head.

The incongruence and the surface area mismatch lead to a considerable mobility of the joint but are also important predisposing factors to glenohumeral instability.

A hierarchy of mechanisms help to conserve stability in the glenohumeral joint. Stabilising mechanisms include:

Static factors (those that do not require the expenditure of energy by muscles)
Dynamic factors (requiring muscle energy)