The glenohumeral "suction cup" provides stability by virtue of the seal of the labrum and capsule to the humeral head.
Suction
A suction cup adheres to a smooth surface by expressing the interposed air or fluid and then forming a seal to the surface. A rubber suction cup is noncompliant in the center but becomes more flexible toward its periphery. Like a suction cup the glenoid surface has "feathered" edges which become increasingly flexible with increasing distance from the center. The center of the glenoid is covered with a relatively thin layer of articular cartilage. At greater distances from the center the articular cartilage becomes thicker providing greater flexibility. More peripherally the glenoid labrum and finally the capsule provide even more flexibility. This graduated flexibility permits the socket to conform and seal to the smooth humeral articular surface. Compression of the head into the glenoid fossa expels any intervening fluid so that a "suction" is produced that resists distraction.
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Figure 1 - Like a suction cup
Figure 1 - Like a suction cup
Centering the humerus in the glenoid
The glenoid suction cup stabilization mechanism is easily demonstrated in young cadaver shoulders in which the articular cartilage glenoid labrum and joint capsule are compliant. The magnitude of this stabilizing effect has not been measured.
Like stabilization from adhesion-cohesion the glenoid suction cup centers the head of the humerus in the glenoid without muscle action and is effective in midrange positions in which the capsule and ligaments are not under tension. The suction cup mechanism is disrupted in situations where the socket cannot seal to the surface of the humeral head such as an avulsion of the glenoid labrum or glenoid fracture.
Movie
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Glenoid-labrum suction cup and adhesion-cohesion forces
