Quadrilateral space syndrome involves dysfunction of the axillary nerve, perhaps by entrapment or compression, resulting in the functional denervation of the teres minor.
The quadrilateral space is a potential space formed by the long head of the triceps medially, the humerus laterally, the teres minor above, and the teres major below. The axillary nerve and posterior circumflex humeral artery travel through this space.
The Sanders article describes the MRI appearance, which is that the muscle appears streaked with white on MRI and atrophied (See illustration A) consistent with fatty atrophy. Sanders group report this finding in 3% of shoulder MRIs. The posterior circumflex humeral artery also travels with the axillary nerve as it travels through this space. Loss of capsular volume on an arthrogram study is suggestive of adhesive capsulitis.
Illustration B is a diagram which shows the borders of the quadrilateral (or quadrangular) space.
Sanders TG, Miller MD. Systematic approach to MRI interpretation of sports medicine injuries of the shoulder. Am J Sports Med. 2005 Jul;33(7):1088-105.
PMID:15983127 (Link to Abstract)
Sofka CM, Lin J, Feinberg J, Potter HG. Teres minor denervation on routine magnetic resonance imaging of the shoulder. Skeletal Radiol. 2004 Sep;33(9):514-8.
PMID:15221220 (Link to Abstract)