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.
In dislocation of the elbow the bones of the forearm are most commonly displaced backward. More rarely they may be partially displaced either inwardly or outwardly and with or without an accompanying backward displacement. The lateral ligaments are strong, the anterior and posterior weak. The formation of the bones permits anteroposterior movement and resists lateral movement; hence the frequency of anteroposterior and the rarity of lateral luxations. To understand and recognize these dislocations and distinguish between them and fractures requires a knowledge of the shape of the bones, the position of the articulations, and especially of the relations and significance of the various bony prominences, in other words, surface anatomy. In doubtful cases compare the normal with the injured elbow.
Fig. 303. - Posterior luxation of the elbow; surface view of the outer side.
In backward dislocation the radius and ulna are pushed backward and the lower end of the humerus comes forward. It is most commonly caused by falls on the outstretched hand and not by direct injury to the elbow.
On the cadaver hyperextension with or even without a slight twisting readily produces the displacement.
The internal and external lateral ligaments are torn loose from their respective condyles and the anterior ligament is ruptured. The posterior ligament is stretched from the olecranon process to the humerus, and with the periosteum may be lifted up but not ruptured. This is especially the case with the periosteum above the external condyle, as shown by Stimson.
The amount of tearing of the muscles depends on the amount of displacement. The flexor muscles may be partly torn from the internal condyle or the extensors from the external. The brachialis anticus probably will be somewhat torn near its insertion in front of the coronoid process. The biceps is not torn but may in some cases be caught behind the external condyle. The orbicular ligament remains intact and holds the radius in its proper relation to the ulna.
The position assumed by the bones is usually one of slight flexion, approximately 120 degrees (Hamilton).
Viewing the elbow from the side, the anterior portion of the arm above the crease is fuller than is normally the case. Posteriorly the olecranon is seen projecting, and above it is a distinct hollow. On the outer side of the joint immediately in front of the olecranon is seen a prominent projection caused by the head of the radius. It is to be recognized by placing the thumb on it and rotating the hand. Almost directly above it may be felt, - though it is not at all distinct, - the external condyle (Figs. 303 and 305). On the inner side are seen two rounded bony eminences. The posterior and upper of these is the larger; it is the internal condyle. Below and anterior to this is another; it is the inner edge of the trochlear articulating surface (Figs. 304 and 306).
Fig. 304. - Posterior luxation of the elbow; surface view of the inner side.
Measurements from the condyle to the acromion process show that they are the same on the injured and the healthy sides. Measurements from the condyle to the styloid process of the ulna show shortening on the injured side. As the lateral ligaments are torn there is abnormal lateral mobility. If the forearm is placed at right angles to the arm, it is seen that the tip of the olecranon no longer lies on a plane drawn through the long axis of the arm and the two condyles, but is considerably posterior to it.
The diagnosis as pointed out by Stimson should be based on the positive recognition of the position of the olecranon, the two condyles, and the head of the radius.