Chronic Posterior Ankle Pain in a Recreational Runner: A Rare Case of Os Trigonum Syndrome Diagnosed in an Outpatient Setting

Background: Chronic posterior ankle pain is a relatively common complaint among athletes and runners, but accessory bone–related etiologies such as Os Trigonum Syndrome are often underdiagnosed, particularly in non-professional athletes. The os trigonum is an accessory ossicle located posterior to the talus, which can cause posterior ankle impingement when symptomatic. This report presents a case of Os Trigonum Syndrome in a recreational runner, diagnosed and managed entirely in an outpatient setting. Case Presentation: A 32-year-old female recreational runner presented to the orthopedic outpatient clinic with a 6-month history of progressive posterior ankle pain, aggravated by running and activities involving forced plantarflexion. There was no significant trauma history. Physical examination revealed tenderness over the posterior ankle, pain on passive plantarflexion, and preserved ankle stability. Initial plain radiographs demonstrated the presence of an os trigonum, and magnetic resonance imaging confirmed posterior ankle impingement without associated tendon injury. Conservative management, including activity modification, non-steroidal anti-inflammatory drugs, and physiotherapy, resulted in significant symptom improvement within 8 weeks. The patient resumed running without recurrence of symptoms at 6-month follow-up. Discussion: Os Trigonum Syndrome is a rare but important differential diagnosis in patients with chronic posterior ankle pain, even among non-elite athletes. The repetitive plantarflexion motion associated with running can exacerbate impingement in individuals with an os trigonum. This case highlights the importance of maintaining a high index of suspicion in outpatient practice, utilizing appropriate imaging to confirm diagnosis, and initiating timely conservative treatment to prevent chronic disability. Conclusion: In runners presenting with persistent posterior ankle pain unresponsive to initial therapy, Os Trigonum Syndrome should be considered. Early diagnosis in the outpatient setting can facilitate effective conservative management, enabling patients to return to physical activity without long-term sequelae.