SYMPTOMATIC ACCESSORY NAVICULAR BONE

Introduction

Accessory navicular bone (ANB) is located postermedial to navicular bone. It is reported that % 10-14 of the population have ANB. Sometimes it may be misinterpreted as a fracture in x-rays. There are 3 types of ANB, regarding radiographic appearances:
A) Type I ANB is a sesamoid bone (%30). It is in the distal portion of the posterior tibial tendon. It has no cartilaginous connection to the tuberosity. Its shape is oval.

B) Type II ANB is secondary ossification center of the navicular bone (50–60%). It is triangular or heart shape. Type II is connected to the tuberosity through a cartilaginous synchondrosis or fibrous syndesmosis. This is the most symptomatic type.

C) Type III has a very prominent navicular tuberosity resulting from bony fusion of the accessory ossification center with the tuberosity. It is also called "cornuate" or "gorilliform navicular".

Case Report

25 year-old male patient was presented with chronic pain in medial side of the left foot. There was no trauma history. MRI examination revealed an accessory navicular bone (ANB), located proximal to navicular bone (Figure-1). The marrrow intensity of the accessory bone is somewhat increased on proton-weighted images (Figure-2). There is also increased signal intensity surrounding the accessory navicular bone (Figure-3). The posterior tibial tendon is attached to accessory navicular bone.

In the setting of chronic medial foot pain, especially occuring after stress or physical exercise, ANB should be suspected. Plain radiographs (anteroposterior and oblique lateral views) should be obtained for the diagnosis of ANB. However, MRI has high sensitivity and specificity for the diagnosis of ANB. MRI can easily show the bone and soft tissue edema.

References

1) Issever AS, et al (2007). Accessory navicular bone: when ankle pain does not originate from the ankle. Clin Rheumatol (2007) 26: 2143–2144.
2) Mellado JM, et al (2003). Accessory ossicles and sesamoid bones of the ankle and foot: imaging findings, clinical significance and differential diagnosis. Eur Radiol 13 (Suppl 6): L164–L177.

Figure-1: Sagittal T1-weighted image shows ANB (arrow)
Figure-2: Sagittal proton-weighted shows ANB, with minimally increased marrow signal intesity (arrow).
Figure-3: Sagittal T2-weighted image revealed soft tissue edema surrounding the ANB (arrows).