Illustration showing the secondary centres of ossification in the elbow. Ossification is the formation of new bone, here from cartilage. Secondary ossification occurs throughout childhood as the bones continue to grow and take form. The elbow joint has six secondary ossification centres that appear in a consistent order as the person ages. In order they are the; capitulum, radial head, internal (medial) epicondyle, trochlea, olecranon and external (lateral) epicondyle. The ages the centres first appear is variable, and is often earlier in girls, but on average, in order, they appear at the ages of 1, 3, 5, 7, 9 and 11 years.