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Elbow ossification occurs at the six elbow ossification centers in a reproducible order. Being familiar with the order of ossification of the elbow is important in not mistaking an epicondylar fracture for a normal ossification center.
The order of appearances of the elbow ossification centers is highly reliable and in most individuals, is consistent: capitellum, radial head, internal (medial) epicondyle, trochlea, olecranon and external (lateral) epicondyle.
The order of "I" and "T" are most important to remember; the trochlea ossification center should not appear before the internal (medial) epicondyle ossification center. If you can see a trochlea but no internal epicondyle, then you need to look very hard for the avulsed ossification center.
Two counting methods are taught to help remember the ages at which the ossification centers appear: 1-3-5-7-9-11 (simple) and 1-5-7-10-10-11 (more accurate).
capitellum: 1 year
radial head: 3 years
internal epicondyle: 5 years
trochlea: 7 years
olecranon: 9 years
external epicondyle: 11 years
Therefore, if the trochlear center is present, but there is no medial epicondyle then you are most likely looking at a medial epicondylar fracture where the ossification center has been avulsed and displaced. This is extremely important since these injuries should be seen by an orthopedic surgeon and internally fixed.
Exceptions have been described in the literature to the usual sequence of appearance of elbow ossification centers and recognized as normal variants 2. This can occur in up to a quarter of children with incidence much more common in girls.