The os subfibulare is considered a fusion failure of the secondary ossification center or avulsion fracture at the lateral ankle ligament involving the anterior talofibular ligament alone or in combination with the calcaneofibular ligament. The os subfibulare causes symptomatic repetitive ankle sprains; however, asymptomatic forms exist. We aimed to identify the symptomatic factors of os subfibulare using ultrasonography (US).

Twenty-one feet of patients who underwent operative treatment for symptomatic os subfibulare and 24 feet of those with asymptomatic os subfibulare were examined. US images were retrospectively evaluated, and the size of the ossicle, distance from the lateral malleolus to the ossicle, and instability of the ossicle during the anterior drawer stress ultrasonography test were assessed.

The size of the ossicle was mean 6.0 mm and 5.6 mm for the patient and control groups, respectively (p = 0.61). The distance in the rest position was mean 3.9 mm and 2.5 mm (p < 0.05) and median 1.4 mm and 0.0 mm during the stress test (p < 0.05) for the patient and control groups, respectively.

US is a useful examination tool for predicting os subfibulare prognosis. Additionally, the distance from the lateral malleolus in the rest position and dynamic instability were predictive factors for symptomatic os subfibulare.

Level Ⅲ, Case control study.

Os subfibulare

Ultrasound

Anterior talofibular ligament

Predictive factor

Chronic lateral ankle instability

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