Mucous cyst - 黏液囊肿 https://en.wikipedia.org/wiki/Mucocele
黏液囊肿 (Mucous cyst) 是一种良性神经节囊肿,通常位于远端指间关节。

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References Digital Mucous Cyst 32644518 
NIH
Digital mucous cysts 是手上常见的一种囊肿。Ganglion 是一种软组织肿瘤,常见于关节或肌腱附近。Digital mucous cyst 通常起源于远端指间关节(DIP 关节)的背面。这些囊肿往往伴随 DIP 关节的骨关节炎出现。
Digital mucous cysts are a type of ganglion commonly found on the hand. A ganglion is a soft tissue tumor that is found next to a joint or tendon. A digital mucous cyst is a ganglion that arises from the dorsum of the distal interphalangeal joint (DIP joint). Digital mucous cysts commonly have an association with underlying DIP joint osteoarthritis.