Chapter 3 Instability

FIGURE 3-33  A: At any point in shoulder motion, there is a contact point between the glenoid and the humeral articular surface, as shown by the glenoid outline in this schematic. As the arm is abducted in maximum external rotation, these individual contact “footprints” merge into a continuous “track” of contact on the humeral articular surface. This zone of contact on the humerus is defined as the “glenoid track.” B: When there is no glenoid bone loss, the glenoid track is 83% of the inferior glenoid diameter (0.83D). The contact between the glenoid articular surface and the humeral articular surface is not 100% of the glenoid diameter because the posterior glenoid rim pushes the rotator cuff attachments 17% posteriorly. C: When there is a glenoid defect, the glenoid track is reduced by the width of the defect. In this schematic, the width of the defect is “a,” so that the glenoid track in this case would be 0.83D − a, which is the length of the line “b” in this figure.

FIGURE 3-35  The medial margin of the Hill-Sachs lesion is located lateral to the glenoid rim, so the Hill-Sachs lesion cannot engage the anterior glenoid rim. This is an “on-track” Hill-Sachs lesion. GT, glenoid track; d, glenoid defect.

FIGURE 3-34  The medial margin of the Hill-Sachs lesion extends medial to the glenoid rim, allowing the Hill-Sachs lesion to engage the anterior glenoid rim. This is an “off-track” Hill-Sachs lesion. GT, glenoid track; d, glenoid defect.