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.
Posterior dislocations are always beneath some portion of the spine of the scapula, hence they have been called subspinous. When the head lies anteriorly under the posterior portion of the acromion process they have been called subacromial.
Posterior luxations are rare. They occur either when the arm is abducted with strong internal rotation or by direct violence, such as a blow on the anterior portion of the shoulder, which forces the head out of its socket backward. The posterior portion of the capsule is torn and the head lies posterior to the glenoid cavity with its anatomical neck resting on the rim and the lesser tuberosity in the glenoid fossa. The arm is inverted and abduction and rotation impaired. The capsule is ruptured by internal rotation while the arm is in a position of abduction, and then a push sends the head posteriorly. We have seen it as a congenital affection resulting from injury in childbirth.
Fig. 254. - Raising the arm to a vertical line or a little more relaxes the tendon of the coracobrachialis and short head of the biceps muscle and allows the lesser tuberosity of the humerus to pass beneath it when the arm is rotated inward to place the hand on the opposite shoulder.
Fig. 255. - Posterior luxation of the shoulder. The head of the humerus makes a prominence beneath the spine of the scapula and the arm is rotated inward. (From a photograph of author's patient by Dr. A. P. C. Ashhurst).
The infraspinatus, teres minor, and sometimes the subscapularis muscles are ruptured and frequently there are accompanying fractures of the tuberosities or some part of the scapula. The head makes a prominence posteriorly and the arm hangs to the side and in a position of inward rotation. Reduction, if the injury is recent, is likely to be easily effected by pushing the head directly forward into its socket.