To determine how often os trigonum is accompanied by accessory navicular and os peroneum. A secondary aim of the study was to investigate the bone and related tendon pathologies that may develop in these three accessory bones.
A total of 110 patients who were evaluated by ankle MRI and were determined with os trigonum between 2009-2015 were included in the study. The frequency was determined of os trigonum together with accessory navicular bone and os peroneum. MR images were also evaluated in respect of bone pathologies and related tendon pathologies of these 3 accessory bones.
Of 110 patients determined with os trigonum on MRI, 21 (19%) were also determined with accessory navicular bone and 5 (4.5%) with os peroneum.When bone pathologies were examined, the most frequently seen pathology was bone marrow edema.The most common pathology in the tendons related to the accessory bones was increased amount of synovial fluid within the tendon sheath. In cases with os trigonum, bone changes were observed more frequently than pathologies of the adjacent tendons, in the accessory navicular bone cases, tendon pathologies were observed more than bone changes and in the cases with os peroneum, bone and tendon pathologies were determined at equal rates.There was no significant difference in terms of FHL tendon pathology between patients with and without edema in os trigonum. However, TP tendon was significantly more pathological in patients with edema in accessory navicular bone.
More than one accessory bone was determined in approximately one in four cases.The most common pathologies determined in these accessory bones was bone marrow edema and increased amount of synovial fluid within the tendon sheath.It is necessary to investigate and report all these findings on MRI as they play an important role in the explanation of clinical findings and treatment planning.

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