Accessory ossicles around the foot and ankle are common skeletal variations. They are usually derived from the failure of union of secondary ossification centers adjacent to the main bone. They might be either adjacent to the main bone or separated.
They are of significance because they are frequently confused with fractures. Most are asymptomatic but occasionally following an injury the accessory ossicle may become symptomatic.
They often have interesting Latin names and identification and naming of them really impresses the patients most of whom have been told they have a fracture!
In the last few months I have taken three examples from my patients to illustrate the point.
- She presented with pain and tenderness at the base of the fifth metatarsal and was diagnosed as a fracture.
- Her x-rays however show a rounded ossicle at the base of the fifth metatarsal.
- This is an accessory bone known as the “os vesalainum” named after/by Andreas Vesalius the 16th century anatomist (recall that xray was not invented till 1895.)
- The ossicle is a failure of ossification of the apophysis (growth plate in traction) and is in the fibres of peroneus brevis. It will usually ossify by age 14 in females.
Case 2: A 40-year-old lady injured her ankle and has ongoing pain laterally.
- Plain x-rays show a? Fracture of the anterior process of the calcaneus.
- On MRI scan she has an accessory ossicle known as the “os calcaneus secundarius”.
- Plain x-rays and an MRI scan clearly show an accessory navicular bone which is the site of her tenderness.
- Interestingly the accessory ossicle is better imaged on plain x-ray than MRI scan.
- Initial management for all these patients consisted of resting in a walking boot.
- When not settling, excision of the accessory bone and usually repair of the adjacent tendon into the bone is the treatment of choice.