The spinoglenoid ligament and its relationship to the suprascapular nerve.

Entrapment of the suprascapular nerve by the inferior transverse scapular ligament or spinoglenoid ligament (SGL) has been discussed frequently in the literature, but it has not been well documented anatomically. Therefore the mechanism of entrapment is not well understood. When isolated atrophy and denervation of the infraspinatus muscle have been noted, compression of the muscle's motor branch at the spinoglenoid notch has been implicated. This anatomic and morphologic study investigates the role of the SGL in entrapment neuropathy of the infraspinatus. We used 23 shoulders from 19 cadavers, 5 women (8 shoulders) and 14 men (15 shoulders), with a mean age of 67.9 (54 to 78) years. The presence or absence of the SGL was noted. The length, width, and orientation of the SGL; size and shape of the tunnel to the infraspinatus fossa; and distance of the notch to the posterior glenoid rim were determined. The SGL was present in 14 (60.8%) shoulders, 5 (36%) women and 9 (64%) men. The SGL was wider at the superior entrance of the tunnel and fanned and twisted toward the inferior aspect. In all specimens the SGL fibers inserted into the posterior shoulder capsule. The mean length for the upper part of the SGL was 17.5 +/- 2.6 mm in men and 15.8 +/- 1.8 mm in women, and the lower part was 14.1 +/- 2.4 mm and 12.9 +/- 1.8 mm, respectively. The widths of the SGL at the origin of the scapular spine were 12.2 +/- 3.9 mm for men and 10.4 +/- 2.7 mm for women, whereas the insertion site widths were 15.8 +/- 2.2 mm for men, and 16.1 +/- 3.8 mm for women. The midportion width of the SGL was 6.8 +/- 1.9 mm in men and 5.8 +/- 2.1 mm in women. During cross-body adduction and internal rotation of the glenohumeral joint, the interaction of the SGL and the posterior capsule resulted in a tightening of the SGL. The suprascapular nerve moved laterally and stretched underneath the SGL in this position.