DILOCATIONS.—The head of the humerus may be dislocated from the glenoid cavity of the scapula as the result of accident, in three different directions ; namely, downwards and inwards, into the axilla.
Secondly, forwards and inwards.
Thirdly, backwards on the infra-spinatus fossa, or on the dorsum of the scapula.
Partial dislocations, or subluxations of the head of the humerus, as the result of acci dent, have been much spoken of, and accounts of such supposed accidents are to be found in the works of practical surgeons. While we would not deny that cases deserving the name of partial luxations of the head of the humerus do occasionally present themselves to the surgeon, in our experience all such cases have been found, on strict inquiry, not to have been the direct effect of accident, but the result of chronic disease, or of congenital malformation of the shoulder joint. And we here formally deny that the case of partial luxation of the head of the humerus, as the result of accident, has ever been satisfactorily proved, either in the living or the dead sub ject.
1. — Dislocation downwards and inwards into the axilla. —The dislocation of the hu merus downwards is unquestionably the most common, and is generally produced by a fall on the elbow, or palm of the hand, the arm being at the time extended from the body. The humerus, therefore, immediately prior to the accident, would be so related to the glenoid cavity as to form with it an acute angle inverted ; and the head of the bone, thus gliding from above downwards, is forced violently against the lower part of the cap sule, which is stretched and lacerated so as to allow the head of the humerus to escape ; this result is further aided by the weight of the body, and by the contraction of the great pectoral, latissimus dorsi, and teres major muscles. The new position assumed by the head of the dislocated bone is on the inner side of the anterior margin of the scapula, between the subscapular muscle anteriorly, and the long head of the triceps, posteriorly. The pectoralis major, latissimus dorsi, and teres major muscles act upon the arm as on a lever, of which the elbow is the fulcrum, and the point of resistance is at the articulation ; while the elbow rests on the ground, and the weight of the body presses on the lower part of the capsular ligament of the shoulder joint, the muscular folds of the axiila being in stinctively thrown into violent action, make an effort to approximate the arm to the side; but as these muscles cannot move the lower extremity of the humerus, on account of the elbow resting on the ground, the head of the bone becomes the moving point, and bursts through the lower part of the capsular liga ment, and is dislocated into the axilla. Dis location downwards may, according to some authors, be produced by a violent blow on the outer part of the shoulder, below the acromion ; but in that case it is often compli cated with fracture of' the scapula or humerus. It is furth.er possible that it may result from simple muscular action, as in the act of lift ing a heavy weight, or during an attack of epilepsy ; in either case a violent effort is re quired, whether the effect be attributed to the agency of the deltoid, in depressing the head of the bone, or, as Boyer supposes, to the action of the great pectoral, latissimus dorsi, and teres major muscles, simultaneously co operating with the elevators of the arm.
Symptoms.— The usual signs of this disloca tion into the axilla, are the following : —A hol low is formed below the acromion, in conse quence of the displacement of the head of the humerus from the glenoid cavity. The deltoid muscle is flattened and dragged down with the depressed head of the bone, so that the na tural roundness of the region of the shoulder is lost. The arm is somewhat longer, and the anterior fold of the axilla is deeper than na tural, because the new situation occupied by the head of the bone on the subscapular fossa of the scapula, is below the level oC its na tural position in the glenoid cavity Cflg. 434.). The elbow is with difficulty made to touch the patient's side; this movement is the source of much pain, as it causes the head of the dislocated bone to compress the nerves in the axilla; and upon this account the patient himself supports his arm at the wrist with the other hand. The head of the os humeri can be felt in the axilla, but not except the elbow be considerably removed from the side. " I have," says Sir Astley Cooper, " several times seen surgeons de ceived in these accidents, by thrusting the fingers into the axilla, when the arm is close to the side, when they have directly said. This is not a dislocation ;' but upon raising the elbow from the side, the head of the bone could be distinctly felt ; for that movement throws the head of the bone downwards, and more into the axilla." The surgeon finds some difficulty in overcoming the fixedness of posi tion of the humerus in its new situation. The patient's voluntary power of abduction of the arm, and of rotation, are lost; the motion of the limb forwards and backwards is preserved. There is great difference in respect to the movements which can be communicated to the limb, depending on the tone of the mus cles ; because, if the muscles are relaxed and feeble, from age or any other cause, the sur geon may be able to move the patient's arm freely, and to raise it up to the head, and even press the elbow close to the side. On moving the limb, a slight crepitus will sometimes be felt, but by a continuance of the motion, this soon ceases ; the crepitus, however, in these cases is never like the rough grating which is felt when a fracture is examined. The direc tion of the longitudinal axis of the arm is changed; for the lower extremity of the hu merus being placed outwards from the side, its longitudinal axis, if prolonged upwardss instead of passing towards the glenoid cavity, may be observed to be directed inwards to wards the axilla. In this accident, numbness of the fingers is somethnes complained of, arising from the pressure of the head of the bone upon some of the nerves of the brachial plexus.