Accessory Ossicles and Sesamoid Bones: Spectrum of Pathology and Imaging Evaluation

Accessory Navicular (Os Tibiale Externum, Os Naviculare Secundarium)

There are 3 types of accessory navicular.^[12,13] Type I, or os tibiale externum, occurs when an ossification center forms a sesamoid bone within the tibialis posterior tendon, near the navicular insertion. Type I ossicles are generally well-defined round or oval bones, measuring approximately 2 to 3 mm in diameter, and are located up to 5 mm medial and posterior to the medial aspect of the navicular. In type II accessory navicular, the ossification center measures approximately 9 to 12 mm in size, and resides adjacent to the tubercle of the navicular bone. A residual cartilaginous synchondrosis joins the triangular type II ossicle approximately 1 to 2 mm medial and posterior to the navicular. The majority or entire tibialis posterior tendon inserts on the type II accessory ossicle. The type III accessory navicular, or cornuate navicular, is a prominent navicular tuberosity, which is essentially a type II ossicle connected to the medial aspect of the parent navicular by an osseous bridge.

Of the 3 patterns, the type II accessory navicular is most commonly associated with medial foot pain.^[14,15] A valgus stress injury may fracture the synchondrosis, resulting in abnormal motion. MRI is the most specific imaging modality for detecting the symptomatic accessory navicular, showing edema in the bone and soft tissues (Figures 10 and 11).

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A 24-year-old woman with medial foot pain following injury. (A) An anteroposterior radiograph shows a type II accessory navicular (arrow) exhibiting an articulation with the navicular bone that simulates a fracture. (B) Sagittal inversion recovery image of the mid-foot shows a type II accessory navicular that articulates with the navicular bone, with associated bone marrow edema and cystic changes around both sites of the articulation (arrow).

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A 39-year-old man with persistent medial foot pain and a history of prior trauma to the foot and ankle. An axial CT image shows a type II accessory navicular (arrow) articulating with the medial aspect of the navicular bone, with irregular articulating surfaces and osteophytes.

Appl Radiol. 2007;36(10):28-37. © 2007 Anderson Publishing, Ltd.

Presented in part at the 106th Annual Meeting of the American Roentgen Ray Society, Vancouver, Canada, May 2006.

Cite this: Accessory Ossicles and Sesamoid Bones: Spectrum of Pathology and Imaging Evaluation - Medscape - Oct 01, 2007.