Plaster bandages are the most convenient means of immobilising the parts after reduction.
LUXATION OF THE SCAPULO-HUMERAL JOINT.
This luxation, like that of the femoro-tibial articulation, is exceptional. It may assume one of two forms, depending on whether the head of the humerus is displaced inwardly, or towards the back of the glenoid cavity; but as a rule luxation occurs inwardly. Forward luxation of the head of the humerus is almost impossible, in consequence of the resistance offered by the tendons of the flexor brachii and antea spinatus muscles. Similarly, luxation outwards is very difficult, the tendon of the postea spinatus being very powerful and offering enormous resistance.
Inwardly, on the other hand, the insertion of the subscapularis is much less powerful, and there is no real opposition to movement of the head of the humerus.
Causation. Violent mechanical shocks transverse to the upper third of the arm may, by sheer force, displace the head of the humerus in an inward direction, causing rupture of the internal wall of the capsular ligament and of the subscapularis muscle. Jumping from high to low ground and falling on the front limbs tend to displace the glenoid cavity in front of the head of the humerus, and often result in luxation of the bone in a backward direction, a luxation, however, which almost always assumes a postero-internal direction. The commonest causes of these luxations are the sideward falls of animals which have attempted to cover others. Whether the subject be a bull or a cow, if the stationary animal suddenly moves to one side, or if the moving animal is frightened by the appearance of a dog, one of the front limbs may be violently dragged away from the body; the resistance of the capsular ligament and internal muscles of the shoulder may be overcome and luxation produced.
Symptoms. Symptoms are immediately apparent: no weight can be placed on the injured limb, and the animal moves on three legs. All muscular action is avoided, the limb is slightly shortened as a consequence of the head of the humerus slipping behind the shoulder, which is held rigidly during movement; the points of the claws are dragged along the ground.
On local examination the point of the shoulder appears to be deformed and outwardly displaced, in consequence of the pressure exercised by the displaced head of the humerus. Below the glenoid cavity and coracoid process lies a depression, at the base of which the displaced humerus can be felt. This depression, however, is soon filled up by the sero-sanguinolent effusion consequent on luxation.
Diagnosis. Bearing in mind the conditions to which the accident is due, the diagnosis presents no great difficulty.
Prognosis. The prognosis is grave, for although it is relatively easy to reduce the displacement, it is very difficult to keep the joint fixed in position.
Treatment. To effect reduction, the animal should be cast on the sound side, and a loop of webbing passed under the arm, in order to provide for counter-extension. The limb is extended by direct traction on the cannon bone or fetlock, whilst the operator endeavours to effect reduction with one hand placed in front and one behind the joint.