Glenoid Shape and Recurrent (Atraumatic) Posterior Shoulder Instability

Atraumatic posterior instability refers to posterior subluxation of the humerus due to to laxity of supporting structures and/or the shape of the bony glenoid and labrum. In contrast to traumatic posterior subluxation, a reverse Hill-Sachs lesion is not common in patients with recurrent posterior instability.

Weishaupt et al (2000), described three shapes of the posteroinferior glenoid on CT in an attempt to diagnose recurrent posterior instability:

• Normal: No posterior bony deficiency is present.
• Lazy J: Rounding of the posteroinferior glenoid rim. Glenoid deficiency.
• Delta: Triangular deficiency of the posterior glenoid. Glenoid deficiency.

A deficient posteroinferior glenoid rim, however, was not significantly more common in unstable shoulders compared to normal shoulders. They did find that the craniocaudad length of the posterior glenoid deficiency could differentiate patients with recurrent posterior instability from those with stable shoulders.

The average craniocaudad length of the posterior glenoid deficiency in patients with recurrent posterior instability was 14.3 mm (±9.9 mm), while in patients with stable shoulders, the average length was 5.0 mm (±4.6 mm). The authors suggest a value of 12 mm as a cut-off for a sensitivity and specificity for diagnosing recurrent posterior instability of 87% and 83%, respectively.

It must be noted, however, that the CT findings could not be extended to MRI due to poor differentiation of the labrum from adjacent cortical bone

References

• Mulligan ME, Pontius CS. Posterior-inferior glenoid rim shapes by MR imaging. Surg Radiol Anat. 2005 Nov;27(4):336-9.
• Weishaupt D, Zanetti M, Nyffeler RW, Gerber C, Hodler J. Posterior glenoid rim deficiency in recurrent (atraumatic) posterior shoulder instability. Skeletal Radiol. 2000 Apr;29(4):204-10.