Unstable os subfibulare in the setting of tibiotalar instability.

An os subfibulare is an ossicle that is located distal to the lateral malleolus near the distal fibular tip and typically at the fibular attachment of the anterior talofibular ligament (ATFL). Although there is debate concerning whether these ossicles are unfused accessory ossification centers versus non-united ATFL avulsion fractures, they are seen frequently in the setting of ankle instability. We present a case of an active patient with an unstable os subfibulare contributing to painless "catching" of the ankle due to talar subluxation and locking. This report details the clinical presentation, radiological findings, including MRI and dynamic ultrasound in the "locked" position, and surgical treatment for this unusual presentation of this unstable ossicle. The case underscores the potential importance of the os subfibulare in a patient with ankle instability and highlights the role of advanced imaging in guiding effective treatment.