Posterior Ankle Pain in a Ballet Dancer: Dynamic Ultrasonography of Simultaneous Onset of Subtalar Joint Arthritis and Os Trigonum Syndrome.

Posterior ankle pain during plantar flexion is frequently observed in ballet dancers, with os trigonum syndrome being one potential cause. However, concomitant subtalar joint pathology has been rarely reported. We herein present the case of a teenage female ballet dancer who presented with persistent posterior ankle pain during relevé movements, initially diagnosed with os trigonum syndrome based on computed tomography (CT) and ultrasonography, which also revealed abnormalities around the flexor hallucis longus (FHL) tendon. Despite a 15-day rest period and targeted physiotherapy focused on improving the FHL tendon gliding, her symptoms persisted. Re-evaluation revealed localized tenderness over the posterior talocalcaneal facet of the subtalar joint and pain reproduction during maximal ankle plantar flexion combined with forced subtalar pronation. Dynamic ultrasonography revealed a hypoechoic intra-articular lesion migrating during joint space narrowing, thereby reproducing the symptoms. Magnetic resonance imaging demonstrated joint effusion, and CT revealed subchondral sclerosis and bony irregularity of the posterior talocalcaneal facet of the subtalar joint, leading to the diagnosis of subtalar joint arthritis. An ultrasound-guided intra-articular corticosteroid injection led to rapid pain resolution and a full return to sport within 60 days, with no recurrence at the 90-day follow-up. This case highlights the importance of considering subtalar joint arthritis, particularly involving the posterior talocalcaneal joint, as a differential diagnosis when posterior ankle pain persists despite adequate treatment for os trigonum syndrome. Dynamic ultrasonography is valuable for identifying subtle concomitant pathologies in complex posterior ankle pain.