The accessory navicular (os tibiale externum) is the most common accessory ossicle in the foot, present in approximately 4-21% of individuals on the medial navicular. Three types are described: Type I is a small sesamoid within the tibialis posterior tendon (os tibiale); Type II is a large ossicle connected to the navicular by a fibrocartilaginous synchondrosis (the most symptomatic); and Type III is a completely fused enlarged navicular prominence (cornuate navicular).
Type II accessory navicular syndrome produces medial midfoot pain at the bony prominence in adolescents and young adults, worsened by activity and direct pressure. MRI demonstrates synchondrosis stress and bone marrow oedema at the junction. The Kidner procedure excises the accessory navicular and advances the TPT insertion to the navicular body to restore arch support. PTTD is more common in patients with accessory navicular as the TPT has less effective mechanical advantage when inserting into the more proximal os rather than the distal navicular tuberosity.
A large Type II accessory navicular connected to the navicular by a fibrocartilaginous synchondrosis accumulates repetitive shear stress from TPT pull and direct footwear pressure, producing medial midfoot pain and MRI synchondrosis oedema; Kidner procedure excises the ossicle and advances the TPT insertion restoring medial arch support.
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