| Discussion: |
See also Case 30c
There are two types of AN. Type I AN (30%) represents a sesamoid bone in the posterior tibialis tendon at the insertion into the navicular. They tend to be round or oval and may be seen up to 5mm medial and posterior to the medial aspect of the navicular. Some feel that the Type I AN leads to flat foot deformity secondary to altered biomechanics and failure of the posterior tibial tendon.
Type II AN (70%) represents an accessory ossification in the tubercle of the navicular. These are often triangular or heart shaped, usually within 1-2 mm of the medial and posterior aspect of the navicular bone. These are usually the ones that are symptomatic with local pain and tenderness.
In this case, the flattened distal margin of the AN and its proximity to the navicular suggest a Type II lesion, with some mild increased signal intensity on the FSE images.