What is the diagnosis?
Medial epicondylar avulsion with the apophysis displaced into the joint.
How does knowledge of the order in which the centers of ossification of the elbow appear help in this case?
In this case, the trochlea and external (lateral) epicondyle ossification centres are clearly seen (appear at ~10-11 years). As such the internal (medial) epicondyle ossification centre should be present (appears at ~7 years). As it is not visible, it must be sought, and not mistaken for another centre.
What mnemonic is frequently used to help in this endeavour? And at what ages do each center appear at?
CRITOE (C = capitellum (1 year); R = radial head (5 years); I = internal (medial) epicondyle (7 years); T = trochlea (10 years); O = olecranon (10 years); E = external (lateral) epicondyle (11 years).
What other elbow injury is associated with medial epicondylar avulsion?
Posterolateral elbow dislocation, which may spontaneously reduce prior to clinical assessment or imaging.
In addition to elbow dislocation (in which the ulnar collateral ligament is responsible for avulsing the apophysis) what other mechanisms is frequently responsible?
Fall on an outstretched arm, with the elbow and wrist extended. The sudden traction / contraction of the flexor pronator muscle group which attaches to the apophysis, rips it off. A direct blow is a rare cause.
There is fracture / avulsion of the internal (medial) epicondyle ossification centre which is absent from it's normal position. It is displaced inferiorly and located just medial to the coronoid.