Mechanics of Glenohumeral Instability.

Figure 3

Figure 4

Figure 5

Figure 6

Figure 8

Figure 9

Figure 10

Figure 11

Laws of glenohumeral stability

Basic laws

The basic laws of glenohumeral stability can be stated as follows:

1. the glenohumeral joint will not dislocate as long as the net humeral joint reaction force (footnote 1) (see figure 3) is directed within the effective glenoid arc (footnote 2) (see figures 4 and 5).
2. the humeral head will remained centered in the glenoid fossa if the glenoid and humeral joint surfaces are congruent and if the net humeral joint reaction force is directed within the effective glenoid arc.

The effective shape of the glenoid is revealed by the glenoidogram, which, rather than showing how the glenoid looks, shows how it works (see figures 6 and 7). (Lazarus et al, 1996; Matsen et al, 1994) The glenoidogram is the path taken by the center of the humeral head as it is translated away from the center of the glenoid fossa in a specified direction under defined loads. The shape of the glenoidogram indicates the extent of the effective glenoid arc in that direction. If the net humeral joint reaction force passes outside the effective glenoid arc, the joint becomes unstable. The glenoidogram is oriented with respect to the glenoid center line, a reference line perpendicular to the center of the glenoid fossa (see figures 8 and 9). The maximal angle that the net humeral joint reaction force can make with the glenoid center line in a given direction is the balance stability angle (see figures 10 and 11). The balance stability angles vary for different directions around the glenoid. The requisite for a stable glenohumeral joint is that the net humeral joint reaction force is maintained within the balance stability angles.

Footnotes

Footnote 1: The "net humeral joint reaction force" is the resultant of all muscular, ligamentous, inertial, gravitational, and other external forces applied to the head of the humeral head (other than the force applied by the glenoid).

Footnote 2: Recognizing that the rim of the glenoid is deformable under load, the "effective glenoid arc" is the arc of the glenoid available to support the humeral head under the specified loading conditions.

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