Avulsed elbow medial epicondyle

At birth both the ends of the forearm and humerus that make up the elbow are cartilaginous. During childhood, from approximately 6 months to 12 years, the six elbow ossification centres form.

The exact age at which these ossification centres appear in not as important as the order in which they appear. The order of ossification is given by the acronym CRITOL.

C   Capittelum
R   Radial head
I   Internal (medial) epicondyle
T   Trochlea
O   Olecranon
L   Lateral (external) epicondyle

The principle of the sequence is that a latter ossification centre should not be seen without the former being seen. Unfortunately the sequence is not always followed, but the trochlea invariable ossifies after the internal epicondyle. Thus, if the trochlea is seen, the internal epicondyle must be seen elsewhere on the epicondyle.

Paradoxically, the grossly displaced internal epicondyle may be more difficult to see that a minimally displaced epicondyle. This is because the grossly displaced epicondyle tends to overlie the joint and may be misinterpreted as another centre.

The films show an elbow effusion as demonstrated by a posterior fat sign, which occurs as the medial epicondyle is intra-capsular. The trochlea is seen to be present, but the internal epicondyle is not normally located. It overlies the joint space on the lateral view, and can be seen projected over the capitellum on the frontal view.

Early identification of an avusled medial epicondyle is required as the fragment is almost always displaced inferiorly, but may also be displaced anteriorly or posteriorly as well and may enter the joint space, commonly between the trochlea and the coronoid process of the ulna, and cause entrapment.