The os subfibulare: the not so normal variant

Objective: The os subfibulare is an accessory ossification center and it is controversial whether it is developmental or stigmata from prior trauma. The purpose of this descriptive study is to demonstrate that prior ankle trauma is the underlying pathophysiologic basis for development of the os subfibulare. Materials and Methods: This is a descriptive retrospective study in children who had at least two sets of ankle radiographs between 1/2014 and 10/2024 performed for trauma or ankle pain, with an os subfibulare documented on the most recent radiograph. The initial radiograph was reviewed and categorized as: normal, soft tissue swelling, or fracture. The subsequent radiograph with presence of an os subfibulare was documented: longitudinal versus transversely oriented ossicle. Results: In our cohort of 39 patients (49% male; average age 7.3 years at presentation (range 4-15 years old), 26 (67%) had an osseous avulsion fracture of the fibula and 13 (33%) had only soft tissue swelling on initial ankle radiographs. The average time between initial injury x-ray and follow-up radiograph with an os subfibulare was 3.4 years (range: 2 months-9 years). There was no significant difference in age at initial radiograph for the osseous avulsion (7.2 years) or soft tissue swelling groups (7.3 years), p=0.92. The os subfibulare was longitudinally oriented in 8, and transversely oriented in 31, and this was not statistically related to orientation of the initial fracture orientation or soft tissue swelling (P=0.428) Conclusions: The os subfibulare is not a normal accessory ossicle, but represents stigmata from prior lateral malleolar trauma, presumably related to a lateral collateral ligament avulsion fracture.