O3280: The Etiology of the Os Subfibulare: A Prospective Ultrasound Study

Background: Debate remains regarding whether the os subfibulare/subfibular ossicle is congenital or post-traumatic.1,2 While studies have demonstrated its appearance after radiographic negative pediatric lateral ankle injuries, the etiology remains unclear.2 Ultrasound provides the capability to evaluate ligament, bone, and cartilage injuries that are not visible on radiographs.3 The main objective of this study was to assess for the development of an os subfibulare/subfibular ossicle at least 6 months after a first-lifetime lateral ankle injury.

Methods: Data were extracted from an IRB approved prospective lateral ankle ultrasound study at a single tertiary pediatric sports medicine and orthopedic institution. Patients ages 5-12 years old at time of enrollment with a diagnosis of first-time lateral ankle inversion injury presenting ≤ 30 days of injury were included. Demographics, patient-reported outcomes (PROs) (PROMIS pain, mobility, and anxiety scores, and Pedi-FABS), X-rays, and ultrasound imaging were obtained at injury presentation. Radiographs, ultrasound of the lateral ankle , and PROs were repeated on the previously injured side at least 6 months post enrollment. Statistical analysis was done via Fisher’s exact, T-test, Mann-Whitney Test, Chi-Square test, and Cohen’s Kappa tests.

Results: A total of 67 patients (71.6% female) with a mean age of 10.5 ± 1.92 with a first-time lateral ankle inversion injury were enrolled. 14 patients (20.9%) were found to have an acute lateral ankle avulsion fracture through ultrasound imaging alone, while 3 patients (4.5%) showed an acute avulsion fracture through both X-ray and ultrasound imaging. Four avulsions were purely chondral, while the remaining 13 were osteochondral. 27 of 47 (57.5%) eligible patients returned at least 6 months later for repeat imaging and PROs, with only six of the 12 (50.0%) eligible patients diagnosed with an avulsion at their initial visit returning. Five of the six patients (18.5%) who demonstrated an avulsion on ultrasound at their clinic visit and returned for follow-up developed an os subfibulare on repeat ultrasound imaging. However, one of these five patients did not demonstrate os subfibulare development on repeat X-Ray imaging. The remaining 21 patients that did not demonstrate an acute avulsion at the time of injury also did not develop an os subfibulare on follow-up imaging. Cohen's Kappa determined there is perfect agreement between initial US imaging and os subfibulare US detection at a follow-up visit (κ = 1.000, p < 0.001).

Conclusion: This study suggests that the os subfibulare/subfibular ossicle may develop as a result of a distal fibula avulsion fracture, which may often be non-apparent on acute plain radiographs. Ultrasound may be useful in identifying radiographically occult lateral avulsion fractures that can lead to the development of an os subfibulare.