This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
Separating the greater tuberosity from the deltoid muscle, the acromion process, and coraco-acromial ligament, is the large subacromial bursa. It does not communicate with the joint, except rarely in old people. Effusions into it cause an increased prominence of the deltoid muscle, and pus seeking an outlet is likely to show itself at the anterior edge of the muscle and less often at its posterior edge. These effusions, which are liable to be present from contusions, sprains, etc., should not be mistaken for intra-articular accumulations.
The long tendon of the biceps muscle enters the joint through the bicipital groove between the two tuberosities. With the arm hanging by the side it points directly forward; it passes over the head of the humerus and under the coraco-acromial ligament about midway between the coracoid and acromion processes to insert into the upper edge of the glenoid cavity. It is covered by a synovial sheath which passes with it through the opening in the capsule and a short distance along the bicipital groove. As this sheath does not communicate with the joint the tendon is in one sense extra-articular. It is held in the groove by a fibrous expansion, extending from the pectoralis major tendon below to the capsule above, called the transverse humeral ligament. This ligament is so strong that luxation of the tendon is uncommon; even when the humerus is luxated the tendon is rarely displaced.
Beneath the tendon of the subscapularis there is a bursa which frequently communicates with the joint. This opening tends to weaken the capsule and it is at this point and just below that the head bursts through in dislocations.
The capsule of the joint and the synovial membrane may be prolonged beyond the rim of the glenoid cavity under the tendon of the infraspinatus, or a bursa at this point may communicate with the joint.
Other bursae may be present, but are unimportant. One is between the coracoid process and the capsule and another under the combined tendon of the coracobrachialis muscle and the short head of the biceps.
Liquid accumulations occur both from injury and disease. The liability of confounding them with those in the subacromial bursa has been alluded to above. As a result of disease, most often osteo-arthritis or tuberculosis, considerable liquid may accumulate in the joint. As the tension increases the arm becomes abducted about 50 degrees and the effusion tends to escape through the openings in the capsule (Fig. 268).
A distention of the joint will cause the deltoid to be more prominent. If the affection is in an old person, as is liable to be the case in osteo-arthritis, there is apt to be a communication with the subacromial bursa and this will become distended. If the liquid is purulent it has a tendency to work its way laterally under the deltoid and break through at its anterior or posterior borders and show itself at the folds of the axilla.
In osteo-arthritis (arthritis deformans) the long tendon of the biceps as it passes through the joint may be dissolved and the belly of the muscle then contracts and forms a lump on the middle of the arm anteriorly.
Pus frequently finds an exit along the bicipital groove and follows it downward and shows itself just at the edge of the anterior axillary fold near the middle of the arm.
If the pus passes out by way of the subscapular bursa it passes below the subscapular tendon and into the axilla anteriorly. If it passes backward it may emerge through the bursa beneath the infraspinatus muscle, and then either work its way downward into the posterior portion of the axilla, or if it works upward may travel either above or below the spine of the scapula and show itself on the dorsum.