Sub-spinous Dislocation.—Backward dislocation is usually termed sub-spinous, although in a considerable proportion of cases the head of the humerus does not pass beyond the root of the acromion process (sub-acromial) ([Fig. 17]). This dislocation is usually produced by a fall on the elbow, the arm being at the moment adducted and rotated medially, so that the head of the humerus is pressed backwards and laterally against the capsule, which ruptures posteriorly. All the muscles attached to the upper end of the humerus are liable to be torn, and the tuberosities are frequently avulsed. The long tendon of the biceps may slip from its position between the tuberosities, and prevent reduction or favour re-dislocation, necessitating an open operation.
In the milder cases the clinical features are not always well marked, and on account of the swelling this dislocation is apt to be overlooked. In addition to the ordinary symptoms, the shoulder is broadened, there is a marked hollow in front in which the coracoid projects, and the arm is held close to the side with the elbow directed forward. The head of the bone may be seen and felt in its abnormal position below the spine of the scapula.
Reduction can usually be effected by making traction on the arm with medial rotation, and pressing the head forward into position, while counter-pressure is made upon the acromion.
Prognosis.—The ultimate prognosis in dislocations of the shoulder should always be guarded. The axillary nerve may be stretched or torn, and this may lead to atrophy of the deltoid; or other branches of the brachial plexus may be injured and the muscles they supply permanently weakened. In a certain number of cases traumatic neuritis has resulted in serious disability of the limb. The movements of the shoulder-joint may be restricted by cicatricial contraction of the torn portion of the capsule and of the damaged muscles. A marked tendency to recurrent dislocation may follow if abduction movements are permitted before repair of the capsule has had time to occur.
Dislocation of the Shoulder complicated with Fracture of the Upper End of the Humerus.—In these injuries the dislocation is almost always of the sub-coracoid variety, and the most common fractures by which it is complicated are those of the surgical neck, the anatomical neck, or the greater tuberosity. The most common cause is a fall directly on the shoulder, and it seems probable that the head of the bone is first dislocated, and, the force continuing to act, the upper end of the humerus is then broken; or the two lesions may be produced synchronously.
When seen soon after the accident, the existence of the fracture of the humerus is liable to be overlooked, the condition being mistaken for dislocation alone, or for a fracture through the neck of the scapula. On careful examination under an anæsthetic, however, it is observed that not only is the head of the humerus absent from the glenoid cavity, but that it does not move with the rest of the bone, abnormal mobility and crepitus are recognised at the seat of fracture, and the upper arm is shortened. The extravasation in the axilla is usually greater than that accompanying a simple dislocation, and the pain and shock are more severe. A fracture through the neck of the scapula alone is readily recognised by the ease with which the deformity is reduced, and the way in which it at once recurs when the support is withdrawn. In many cases it is only by the aid of a radiogram that an accurate diagnosis can be made ([Fig. 25]).
Fig. 25.—Dislocation of Shoulder with Fracture of Neck of Humerus.
(Sir Robert Jones' case. Radiogram by Dr. D. Morgan.)
Treatment.—Unless the dislocation is reduced at once, the movements of the arm are certain to be seriously restricted, and painful pressure effects from excess of callus are liable to ensue. An attempt should first be made, under anæsthesia, to replace the head in its socket, by making extension on the arm in the hyper-abducted (vertical) position, and manipulating the upper fragment from the axilla.