Talar Avulsion Fracture

Another talar avulsion fracture seen on lateral foot film

The image on the left is an avulsion fracture of the posterior process.  However, the image on the right is not.  It is a normal variant called the os trigonum, present in 2.5% to 14% of normal feet.  It is an accessory ossicle that is round, oval, or triangular and of variable size located just posterior to the lateral tubercle of posterior talar process.  Its edges appear smooth, with dense cortical bone.