Histological features of the distal insertion of the middle glenohumeral ligament on the subscapularis tendon.

BACKGROUND

The middle glenohumeral ligament (MGHL) is the least well characterized glenohumeral ligament and its distal insertion remains matter of debate. The aim of this study was to describe the histological features of the distal insertion of the MGHL and its interactions with the subscapularis tendon. Our hypothesis was that the MGHL inserts distally on the articular surface of the subscapularis tendon.

METHODS

Macroscopic and histological analysis of the insertion of the MGHL was performed in 16 shoulders obtained from 10 fresh cadaver dissections from a posterior approach. Macroscopic analysis involved observation of the distal insertion of the MGHL and measurement of its distance from the humerus. The distal insertion site was then extracted without bone, fixed in paraffin and stained with hematoxylin-eosin-saffron for histological analysis of longitudinal sections of the MGHL.

RESULTS

The MGHL inserted macroscopically into the subscapularis tendon in all 16 shoulders, at a mean distance of 13 mm (range, 5-30 mm) from the humerus. Histological analysis revealed that the MGHL inserted on the deep layer (articular surface) of the subscapularis in all 16 shoulders, at a distance from the humerus. The junction angle was acute in all cases. Two different connection mechanisms were observed. While a majority of MGHL collagen fibers extended into the articular capsule, increasing its thickness, the remaining MGHL collagen fibers penetrated into the subscapularis tendon in the longitudinal direction of the ligament, mingling with the larger perpendicular collagen bundles of the subscapularis tendon. The junction was in some cases in the subscapularis tendon and in others in the subscapularis myotendinous junction.

CONCLUSION

The anatomically exceptional ligament-to-tendon distal insertion of the MGHL into the subscapularis was histologically confirmed in all. These findings support recent results indicating that the MGHL does not insert onto the humerus but into the articular surface of the subscapularis tendon, forming a blend of ligament fibers up to its terminal insertion on the lesser tuberosity. These results suggest that MGHL mechanical constraints are transferred into the subscapularis tendon rather than at the bony insertion on the lesser tuberosity.