Secondary ossification centers in the development of the medial malleolus.

BACKGROUND: Accessory ossicles of the medial malleolus have been reported, however, these have not been linked to a pattern of development and are considered anomalies. Here, we describe a pattern of ossification of the medial malleolus in children including a secondary ossification center.

METHODS: Twenty anteroposterior (AP) and mortise x-rays of each sex and age from 4 to 12 were randomly selected from skeletally immature patients identified at our institution. X-rays were excluded if there was a cast or splint, fracture, hardware, or obvious tibial deformity. Each x-ray was evaluated and categorized to a 4-part stage of development. These stages were then applied to randomly selected AP hip to ankle films from the same age groups.

RESULTS: Four distinct stages of medial malleolus ossification were identified on ankle x-rays. Stage 1 consists of the widening of the epiphysis that did not reach the medial border of the metaphysis. In stage 2, the epiphysis had widened medially to the level of the metaphysis, however, had not extended distally to the level of the dome of the talus. In stage 3, the proximal portion of the medial malleolus has ossified distal to the dome of the talus with ossification centers at this level identified. Stage 4 consisted of a completely fused ossification center extending distally to a mature medial malleolus. The stages were reconfirmed on AP standing hip to ankle to have a similar distribution, secondary ossification centers were more common in females aged 6 to -9 and males aged 8 to 11 years.

CONCLUSIONS: The medial malleolus develops in predictable stages which may involve a secondary ossification center in the final stages of development. These findings were initially described on AP and mortise views, then confirmed on AP hip to ankle radiographs were evaluated to exclude potentially confounding ankle pain. These secondary ossification centers were seen at similar ages on both ankle and hip to ankle x-rays.

LEVEL OF EVIDENCE: Level III.