What are secondary ossification centers?
Secondary ossification centers appear and then fuse later with the primary ossification center as seen on radiographs at predictable times during skeletal maturation. For example, multiple secondary ossification centers are seen around the elbow, which appear at different ages. Their usual sequence can be remembered by the mnemonic CRITOE: C apitellum (1 year), R adial head (3 years), I nternal (medial) epicondyle (5 years), T rochlea (7 years), O lecranon (9 years), and E xternal (lateral) epicondyle (11 years).
Why are secondary ossification centers particularly important to understand in the setting of elbow trauma?
One important reason to understand this sequence is that a type I Salter-Harris fracture through the physis of the medial epicondyle may cause displacement of this ossification center into the region of the trochlea. This displacement might create the false impression that the trochlear ossification center is present, whereas the medial epicondylar ossification center has not yet appeared. Knowledge of this sequence allows one to identify this appearance appropriately as a displaced fracture.