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Os Trigonum is an normal variant of an accessory bone found posterior to the talus. The os trigonum is present in around 5-15% of normal feet. Usually it is asymptomatic, until there is a minor to blunt injury on the anterior aspect of the foot. The injury becomes painful on the hyperplantarflexion test. The pain is located at posterolateral aspect of the ankle associated with swelling known as the ‘os trigonum syndrome’1,2 or posterior ankle impingement (PAI).
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3. Kose O, Okan AN, Durakbasa MO, et al. Fracture of the os trigonum: a case report. J Orthop Surg (Hong Kong) 2006;14:354-6.
4. American College of Foot and Ankle Surgeons (ACFAS). Os Trigonum Syndrome (Accessed December 15, 2013, at https:// www.foothealthfacts.org/footankleinfo/os_trigonum_syndrome.htm?terms=os%20trigonum).
5. American Academy of Orthopaedic Surgeons (AAOS). Posterior Tibial Tendon Dysfunction. (Accessed December 15, 2013, at https://orthoinfo.aaos.org/topic.cfm?topic=A00166).