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.
There is (1) at first elevation then lowering of the shoulder, (2) flattening of the deltoid muscle, (3) projection of the elbow away from the side. (4) The normal hollow below the outer third of the clavicle is filled up; the head, covered by the deltoid, may sometimes even make a rounded prominence at this point which can frequently be felt. (5) If the elbow is raised and the hand placed on the opposite shoulder and held there the elbow cannot be brought flat on the chest (Dugas's sign), (6) with the arm to the side the distance from the acromion process to the external condyle is increased, with the arm abducted to a right-angle, the same distance is decreased as compared with the previous position as well as when compared with the arm of the opposite side (see Fig. 248).
1. Lowering the shoulder obviates pain by relaxing the deltoid and preventing it from forcing the head upward in its displaced position.
2. Flattening of the shoulder is due to the head and tuberosities being displaced inward, thus leaving the socket empty. A marked depression can be felt with the fingers below the prominent acromion process.
3. Projection of the elbow from the side is due to tension of the deltoid muscle because the head is lower than normal. In its natural position the top of the head is about level with the coracoid process; when luxated it is below it.
4. The normal hollow below the outer third of the clavicle is lost because here is where the head lies. It may form a distinct prominence and when the arm is rotated if the surgeon lays his hand at this point the tuberosities can be felt to rotate beneath. If the arm is abducted the head can usually be felt in the axilla, where it may even form a prominence.
Fig. 248. - Subcoracoid dislocation of the shoulder. The head of the humerus has slipped off its pedestal or shoulder-girdle onto the side of the thorax. This shows how the arm is shortened and why it is necessary to make traction in order to replace the humerus up again on the shoulder-girdle.
5. In Dugas's test the elbow cannot be brought to the chest because the outer end of the humerus is held close to the chest-wall. On account of the thorax being rounded like a barrel it is necessary for the outer end of the bone to rise as the inner end falls.
6. The reason for the difference in measurements when the shoulder is luxated is readily seen by the fact that the head is displaced downward and inward as shown in the accompanying figure.