The accessory navicular syndrome, also known as os naviculare syndrome occurs when a type II accessory navicular becomes painful due to movement across the pseudo-joint between the ossicle and the navicular bone.
Radiographic features
Ultrasound
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.
MRI
The syndrome presents on MRI with bone marrow edema signal i.e.
T1: hypointense
T2/STIR: hyperintense
This can involve both the accessory ossicle and the navicular bone.
Treatment and prognosis
Once avulsion fracture is ruled out, acute pain can be managed by corticosteroid injection and immobilization of the foot for 2-3 weeks. For refractory cases, surgical management can be considered.