Accessory navicular syndrome occurs when a type II accessory navicular (or "os tibiale externum") becomes painful due to movement across the pseudojoint between the ossicle and the navicular bone.
It can be inferred on musculoskeletal ultrasound if a patient's pain is located at a type II accessory navicular and the patient is tender to transducer pressure. Ultrasound can also be useful to compare with the contralateral side.
The syndrome presents on MRI with bone marrow oedema signal (hypointense T1, hyperintense T2/STIR) in both the accessory ossicle and navicular.
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