Pes anserine bursitis is not an uncommon pathology especially in the context of knee osteoarthritis; however, ruptured pes anserine bursitis is a rare event. Herein, the authors present a case of diving pes anserine bursal rupture in a patient with knee osteoarthritis who developed postero-medial knee tender swelling. In our case, the bursal rupture was contained, and dived posteriorly, resulting in a cystic calf swelling; the original bursa itself was collapsed beneath the pes anserine tendon, giving a taillike appearance, such tail-like appearance could help in accurate localization of swelling origin and make the diagnosis of bursal rupture much easier. The tail-like appearance deep to the pes anserine tendon was found to be a helpful imaging feature when assessing cystic calf swelling; it should be kept in mind when pes anserine bursal rupture is a differential diagnostic consideration, as it could indicate the origin of resultant swelling. Keywords Pes anserine bursitis . Diving pes anserine bursal rupture . Knee osteoarthritis . Cystic calf swelling This article is part of the Topical Collection on Imaging

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