Excision of accessory navicular(os naviculare) with release of the medial head of gastrocnemius
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Under normal circumstances the navicular ossifies from a single ossification centre early in childhood. This usually occurs around 3-5 years in girls and boys with fusion occurring by the age of about 13. An accessory navicular, also variously called the Os Tibiale Externum or Os Navicularum, is an extra bone that lies adjacent to the parent navicular and is a variation of the embryological origin of the navicular bone. It occurs because the medial navicular tuberosity can have its own ossification centre and it is this that develops as a separate fragment of bone, with a variable pattern of what happens to it by the time of skeletal maturity.
An Os Naviculare develops as either a sesamoid in the substance of the tibialis posterior (which attaches to the medial tuberosity of the navicular), a separate fragment of bone and cartilage which forms a synchondrosis with the parent navicular, or it can fuse completely with the parent bone as a synostosis leading to a plantar medial elongation of the navicular bone. These variations form the basis for a radiological classification of the anomaly(Geist, 1914).
The condition is asymptomatic in most, occurring in about 5-14 % of the population. The commoner causes of the onset of pain are overuse, ill fitting or nonconforming shoes, stress fractures, tibialis posterior tendinopathy or synovitis at its insertion to the accessory navicular. Rarely fracture or arthropathy effecting the synchondrosis can occur. Many patients have significant pes planus which also makes the medial side of the navicular more prominent and prone to injury.
Author:Kartik Hariharan FRCS.
Institution: Aneuran Bevan University Health Board, Wales.