The accessory navicular is a supplementary bit of bone on the inside of the foot pretty much on top of the mid-foot ( arch ) near its highest part. The bone is integrated inside the tibialis posterior tendon that attaches to the navicular bone at the top of the mid-foot. The extra bone can also be known as the os navicularum or os tibiale externum. This is congenital, so is present from birth. There are various kinds of accessory navicular and the Geist typology is most typically used. This categorization separates the accessory navicular into three variations:

Type 1 accessory navicular bone:
This is the classical ‘os tibiale externum’ and make up to 30% of the cases; it is a 2-3mm sesamoid bone inserted within the distal portion of the tendon without having any connection to the navicular tuberosity and may even be separated from it by up to 5mm

Type 2 accessory navicular bone:
This variety makes up about 55% of the extra navicular bones; it’s triangular or heart-shaped and attached to the navicular bone via cartilage. It might at some point fuse to the navicular to form one bone.

Type 3 accessory navicular bone:
Visible navicular tuberosity. It might have been a Type 2 that has fused to the navicular

The typical clinical feature associated with the accessory navicular is the enlargement on the medial area of the mid-foot ( arch ). Because of the additional bone there, this has an effect on how well the mid-foot ( arch ) muscles do the job and might cause a painful foot. Inflexible type shoes, like ski boots, can also be very uncomfortable to use as a result of enlarged pronounced bone.

The treatment will be aimed towards the signs and symptoms. If the flatfoot is an issue, then ice, immobilisation and also pain relief medicine may be required initially. Following that, physiotherapy and foot orthotic inserts to support the feet are used. If the pain is due to pressure from the type of footwear which needs to be used, then donut type padding is required to get load off the painful region or the footwear might need to be modified.

If these non-surgical treatment options fail to eliminate the symptoms of the accessory navicular or the issue is a continuing one, then surgical treatment could be a suitable alternative. This requires taking out the accessory bone and restoring the insertion of the tibialis posterior tendon so its function is improved.

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