Mechanics of Glenohumeral Instability.

Figure 37

The glenohumeral suction cup

Suction

This mechanism provides stability by virtue of the seal of the labrum and capsule to the humeral head (see figure 37). 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. In a similar manner, 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.

The glenoid suction cup stabilization mechanism was demonstrated by Harryman et al. (Harryman, Lazarus, Sidles et al, 1996) In elderly cadaver shoulders without degenerative changes, the suction cup resisted an average of 20 ± 3 Newtons of lateral traction (about four pounds). Creating a defect in the labrum completely eliminated the suction cup effect. No suction cup effect could be demonstrated in the two shoulders with mild degenerative change of the joint surface. It is likely that this effect would be even stronger in younger living shoulders in which the articular cartilage, glenoid labrum, and joint capsule are larger, more hydrated and more compliant. 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.

How useful was this page or article? This article is rated out of 5 stars (345 ratings). Not useful at all Not very useful Useful Very useful Extremely useful