The order of the appearance of the osiification centres on radiographs of the elbow can be remembered using CRITOL or CRITOE. In the above examples, the first two are normal and the final radiograph demonstrates avulsion of the internal ossification centre.
The internal epicondyle always ossifies before the trochlear.
This is of importance as an avulsion may displace an ossification centre and it is important to pick this up rather than assign the appearance to a normal ossification centre. For example, if the medial (internal) ossification centre is absent, but the trochlear is thought to be present, this would indicate pathology i.e. avulsion of the internal epicondyle which now lies in the position of the trochlear.