Os Subfibularis

17 yr old girl has pain ankle ater trivial trauma. Radiograph shows radiopacity with normal soft tissues and no donor site  and normal articulation-possibly ossicle - suggest followup and other side comparison though it is not  necessary to find the ossicle on both sides. In general, accessory ossicles commonly observed in order of frequency of the lower extremity include: tibiale externum, os trigonum and os peroneum. Accessory bones that are rare in the foot include accessory interphalangeus,  anamolous os calcaneum and talus. 

Case submitted by Dr MGK Murthy, MD, DNB

An accessory, distal focus of epiphyseal ossification may develop in either malleolus. These foci are not anatomically separate entities, even though they can appear to be radiographically. They usually are asymptomatic. However, they may be injured, either acutely or chronically. If fractured, the injury can extend through a segment of the malleolus. An ossicle may also be avulsed as a ligament failure analogue, similar to a sleeve fracture of the patella. This is more common in the lateral than in the medial malleolus.

These avulsions, if not adequately diagnosed and treated, may progress to delayed union, nonunion, or a chronically painful ankle. Normally, the secondary center of ossification of the lateral malleolus appears during the first year of life, and fuses with the shaft at 15 years. 22% of normal children under the age of 16 have one or more accessory ossicles in the foot and ankle.

Majority  are asymptomatic and  few that cause pain and swelling can be managed conservatively. Surgery is the option if symptoms are recalcitrant ,  with excision and resuturing of ligament  being necessary . Cause of symptoms is not  known.    The most likely explanation is that anomalous ossification centers, not yet fused to the body of the epiphysis, have been subjected to trauma, causing disruption to the fibrous or cartilaginous attachment and results in a fibrous union or pseudo-arthrosis. Mechanical irritation or joint instability may produce local pain and tenderness and contribute to recurrent ankle sprains.