The patient had presented with a sprained ankle, and a tibiotalar joint effusion is evident anteriorly.
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Lateral view of the ankle demonstrates a well-corticated rectangular bone fragment posterior to the left talus. This is consistent with an os trigonum, a common accessory bone found just posterior to the lateral tubercle of the posterior talar process. In this case, the os trigonum, appears to be partially fused with the lateral tubercle.