Navicular Bone, Accessory

This trait, present in about 5% of persons, causes an undue prominence on the medial side of the foot. It is sometimes referred to as an accessory or secondary medial malleolus. Sometimes it is fused with the navicular to form an abnormally large tuberosity on the latter bone (Moseley, 1957). Geist (1925) reported that 10 to 14% of 'normal' feet have an accessory navicular bone. It can be symptomatic and even require surgical treatment in some instances. Irritation of the bony prominence and the frequently associated pes planus are responsible for the symptoms (Kiter et al., 2000). The accessory navicular bone has been classified into 3 types: type 1 is a small sesamoid bone embedded within the distal portion of the posterior tibial tendon; type 2 is an accessory bone united to the navicular by a 1- to 3-mm thick synchondrosis; and type 3 is a fused form of type 2. Types 1 and 2 comprise 70% of all cases and are usually involved when symptoms are reported (Ray and Goldberg, 1983).

Kiter et al. (2000) presented a Turkish pedigree consistent with autosomal dominant inheritance with male-to-male transmission and probably incomplete penetrance.