Kager's triangle is a sharply marginated radiolucent triangle seen posteriorly on lateral radiographs of the ankle. It represents the Kager fat pad.
It is bordered anteriorly by the flexor hallucis longus (FHL) muscle and tendon, posteriorly by the Achilles tendon, and inferiorly by the calcaneus. The anteroinferior corner of the triangle is related to the posterior ankle joint, while posterolaterally it is related to the retrocalcaneal bursa.
The alteration of it's radiographic appearance in the form of obliteration or distortion of its borders are subtle indicators of pathology involving the posterior ankle.
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