The accessory navicular is a supplementary piece of bone on the inside of the foot just above the mid-foot ( arch ) nearby its top part. The bone is included within the tibialis posterior tendon which attaches into the navicular bone at the top of the mid-foot. The additional bone can also be referred to as os navicularum or os tibiale externum. It is hereditary, so is present since birth. There are several different kinds of accessory navicular and the Geist grouping is most commonly used. This categorization separates the accessory navicular into three variations:

The Type 1 accessory navicular bone:
This is the typical ‘os tibiale externum’ and make up to 30% of the occurrences; it’s a 2-3mm sesamoid bone included within the distal area of the tendon without having any connection to the navicular tuberosity and may even be divided from it by up to 5mm

The Type 2 accessory navicular bone:
This variety makes up about 55% of the accessory navicular bones; it’s triangular or heart-shaped and attached to the navicular bone via cartilage. It might eventually merge to the navicular to create one bone.

The Type 3 accessory navicular bone:
Prominent navicular tuberosity. This could have been a Type 2 that’s fused to the navicular

The typical symptom associated with the accessory navicular is the prominence on the medial area of the mid-foot. Because of the additional bone there, this has an effect on how well the arch muscles work and might result in a painful foot. Inflexible type footwear, like ice skates, can also be very painful to use as a result of enlarged pronounced bone.

The treatment is geared towards the symptoms. When the flatfoot is a concern, then ice, immobilisation along with pain relief medication may be needed to start with. After that, physiotherapy and foot orthotic devices to aid the foot are used. When the pain is due to force from the style of shoes that must be worn, then donut type padding is used to get force off the sore area or the shoes may need to be modified.

If these non-surgical treatment options are not able to reduce the symptoms of the accessory navicular or maybe the problem is an ongoing one, then surgical procedures could be an appropriate option. This involves taking out the accessory bone and fixing the attachment of the posterior tendon so its function is enhanced.