Os trigonum syndrome which also called posterior ankle impingement syndrome is a rare cause of ankle pain. In this report, we discuss a 39 year old female nurse with os trigonum syndrome resulting posterior ankle pain and swelling. On physical examination posterolateral aspect of left ankle was swolen and painful on palpation. Neurologic and sensory examinations were otherwise normal. Laboratory tests revealed normal complete blood count, sedimentation, C-reactive protein, rheumatoid factor and antinuclear antibody levels. There was vitamin D deficiency with normal levels of calcium, phophorus, alkaline phosphatase, liver and renal function tests. Magnetic resonance imaging of left ankle revealed os trigonum accompanied by subtalar effusion and soft tissue edema. Non- steroidal anti-inflammatory drug, cold pack application and gentle ankle range of motion exercises were recommended. She responded well to therapy and followed clinically. We aimed to emphasize the os trigonum syndrome in differential diagnosis of posterior ankle pain in order to provide early diagnosis and treatment with a literature review.

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