Coracoid bursa.

1- there was a normal communication between sub coracoid and subacromion bursae.
2- Both sub coracoid and subacromion bursae did not have communication to joint space.
3-Both coracoid bursa and sub scapular recess were in close relation and were separated only by thin fibrous wall.

* : subscapularis recess.
arrow head: subcoracoid bursa.
SSc: subscapularis muscle and tendon.
CB: coracobrachialis muscle and tendon.

4- Most common causes of fluid filled subcoracoid bursa:
-Full thickness tear of supraspinatous tendon (in case of supraspinatous full thickness tear with connection between both subcoracoid and sub acromial bursae; both were be filled with fluid.
-Subcoracoid impingement has also been suggested as a cause of subcoracoid bursal distention.  A normal coracohumeral distance measures  8-11 mm with an average value of 5.5 mm in
symptomatic individuals.  

A fat-suppressed proton density-weighted axial image demonstrates a partial thickness subscapularis tendon tear (arrow), and a narrowed coracohumeral distance (dotted line, measuring 3 mm)

-Subcoracoid fluid collection was also evident with  rotator interval injury.
-Trauma resulting in medial location of the biceps tendon lead also to fluid filled subcoracoid bursa.

-Isolated coracoid bursitis is best considered a diagnosis of exclusion, after all other associated pathology has been ruled out. 

Conclusion

Radiologists often mistake a distended subscapularis recess for a distended subcoracoid bursa.  It is essential to properly distinguish these two potential spaces about the shoulder, since fluid within the subcoracoid bursa is considered pathologic, while the fluid in the subscapularis recess is due to a normal communication with the glenohumeral joint.  Isolated subacromial bursitis should be considered a diagnosis of exclusion after all other associated pathology has been ruled out. Neither the subcoracoid bursa nor the subacromial bursa should communicate with the glenohumeral  joint  when the rotator cuff is intact, but they may communicate with one another.  When this interbursal communication exists, subcoracoid bursal distention can be a sign of a full thickness rotator cuff tear.  Distention of the subcoracoid bursa has also been recognized in subcoracoid impingement and rotator interval tears, and may be associated with other pathology of the rotator interval such as adhesive capsulitis.  Identification of a fluid-filled subcoracoid bursa should thus prompt a diligent search for associated pathology of the shoulder.

18 A

(18a) A sagittal T2-weighted image demonstrates a thickened coracohumeral ligament (arrow), infiltration of the subcoracoid fat triangle (short arrow), and a distended subcoracoid bursa (arrowheads). (18b) The coronal fat suppressed T2-weighted image demonstrates thickening and edema of the inferior glenohumeral ligament typical for adhesive capsulitis. The small subacromial fluid collection (arrowheads) did not communicate with the subcoracoid bursa, and there was no full thickness rotator cuff tear.