Mechanics of Glenohumeral Instability.

Last updated Friday, February 04, 2005

Figure 17

Figure 18

Figure 19

Figure 20

Figure 21

The effective glenoid arc

Effective shape of the glenoid

The glenoid concavity is formed by a combination of the shape of the underlying bone and the overlying cartilage and labrum (see figures 17 and 18). (Howell and Galinat, 1989; Soslowsky et al, 1992) The effective glenoid arc may be compromised by congenital deficiency (glenoid hypoplasia), excessive compliance, traumatic lesions (rim fractures or Bankart defects) or wear (see figure 19). (Altchek et al, 1991; Baker et al, 1990; Bankart, 1938; Cooper and Brems, 1992; Cyprien et al, 1983; Howell and Galinat, 1989; Joessel, 1880; Lazarus, Sidles, Harryman et al, 1996; Lippitt, Vanderhooft, Harris et al, 1993; Matsen, Lippitt, Sidles et al, 1994; Matsen and Thomas, 1990, Matsen et al, 1990; Neer and Foster, 1980; Pappas et al, 1983; Rowe, Patel and Southmayd, 1978; Thomas and Matsen, 1989) The effective arc may be augmented by anatomical repair of fractures or Bankart lesions (see figure 20), by rim augmentation, by congruent glenoid bone grafting and by glenoid osteotomy. (Lazarus, Sidles, Harryman et al, 1996)

The effective shape of the glenoid is revealed by the glenoidogram. As the humeral head is translated from the center of the glenoid to the rim in a given direction, the center of the humeral head traces the glenoidogram, which has a characteristic gull-wing shape. The glenoidogram is different for different directions of translation as shown in (see figure 21) which demonstrates data recorded for the superior, inferior, anterior, and posterior directions in a typical shoulder. The shape of the glenoidogram can be predicted from the humeral radius of curvature, the glenoid radius of curvature and the balance stability angle (footnote 1).

Glenoidograms

Predicted glenoidograms are qualitatively similar to glenoidograms measured experimentally (see figure 5). The glenoidogram also reveals another important aspect of shoulder stability: the slope of the glenoidogram at any point is equal to the tangent of the balance stability angle (which equals the stability ratio) at that point. For most glenoidograms it can be seen that the slope is steepest when the humeral head is centered in the glenoid. Thus the joint has the highly desirable property of being most stable when the head is centered. As the humeral head is moved away from the center, the slope of the glenoidogram and the stability ratio become less.

Thus, as the head is displaced from the glenoid center, it becomes progressively more unstable. Once enough force is applied to displace the head from the center, that same amount of force would easily displace the humeral head over the glenoid lip. Note also that when the humeral head is translated to the lip of the glenoid, the stability ratio is, as expected, zero. These observations relate to the "jerk" tests described for anterior (Lerat et al, 1994) and posterior (Matsen, Lippitt, Sidles et al, 1994) instability: in these tests there is no translation of the humeral head until the point where sudden and substantial translation occurs. (Lazarus, Sidles, Harryman et al, 1996; Lippitt, Vanderhooft, Harris et al, 1993)

Footnotes

Table 1

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