A New Concept of Reverse Glenoid Track to Determine Which Reverse Hill-Sachs Lesion Should Be Treated

Background: Humeral and glenoid bone loss is commonly seen in patients with posterior shoulder dislocation, like anterior dislocation. However, which reverse Hill-Sachs lesion should be treated has not yet been clarified. Purpose: To (1) determine the contact area of the glenoid on the humeral head with the arm in various degrees of flexion while keeping maximum internal rotation and horizontal flexion and (2) clarify the surgical indication of a reverse Hill-Sachs lesion. Study Design: Descriptive laboratory study. Method: The authors examined 20 shoulders of 10 healthy volunteers. The magnetic resonance imaging scans were taken in 3 arm positions (0°, 45°, and 90° of flexion) while keeping maximum internal rotation and horizontal flexion. All the scans were reconstructed using 3-dimensional visualization analysis software. The 3-dimensional surface bone models of the proximal humerus and glenoid were reconstructed. The contact area between the humeral head and the glenoid was measured. Results: With an increase in flexion, the glenoid contact area shifted, creating a zone of contact (reverse glenoid track). The distances from the medial margin of the footprint of the rotator cuff tendon to the medial margin of the glenoid were 16.7 ± 3.2 mm (0° of flexion), 18.0 ± 3.7 mm (45° of flexion), and 19.0 ± 3.2 mm (90° of flexion), which were equivalent to 80% ± 15%, 87% ± 14%, and 92% ± 14% of the glenoid width, respectively. The distances from the medial margin of the articular cartilage edge were 11.9 ± 2.2 mm, 10.1 ± 1.4 mm, and 12.1 ± 1.9 mm, which were equivalent to 59% ± 12%, 50% ± 8%, and 59% ± 10% of the glenoid width, respectively. Conclusion: With an increase in flexion, the glenoid contact area shifted, creating a zone of contact (reverse glenoid track). The width of the reverse glenoid track was 50% to 59% of the glenoid width. A reverse Hill-Sachs lesion has a risk of engagement if it extends medially over the medial margin of the reverse glenoid track. Such reverse Hill-Sachs lesion is needed to treated.