The accessory navicular (os naviculare accessorium) is an accessory ossicle adjacent to the navicular tuberosity, present in approximately 12 to 21 percent of the population in three types: Type I (os tibiale externum, sesamoid in tibialis posterior tendon), Type II (synchondrosis with the navicular, the most clinically significant), and Type III (fully incorporated cornuate navicular). Type II produces a medial foot prominence and is associated with flatfoot because the tibialis posterior tendon inserts abnormally on the accessory bone rather than the navicular tuberosity.
| Articulations | |
|---|---|
| Landmarks |
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| Ossification | Develops as a separate ossification centre that appears between ages 9 and 11 and normally fuses to the navicular, persisting separately in approximately 15 percent of individuals. |
Symptomatic accessory navicular syndrome produces medial midfoot pain aggravated by footwear pressure over the prominence and by walking, most often in adolescents and young adults. The synchondrosis can be stressed during walking and running, producing a stress reaction visible on MRI as bone oedema adjacent to the synchondrosis. An accessory navicular is associated with tibialis posterior tendon dysfunction and flatfoot, as part of the tendon pull is diverted into the ossicle. Conservative management with orthotic arch support and footwear modification resolves most cases; surgical excision of the accessory bone with tibialis posterior tendon advancement (Kidner procedure) is performed for refractory cases.
Irritation of the fibrous junction between the accessory navicular and the navicular, under the pull of tibialis posterior, causes medial midfoot pain and a tender bump.
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