Mucous cyst https://en.wikipedia.org/wiki/Mucocele
Mucous cyst est cysta benigna digitorum, quae proprie ad articulos interphalangeales superne locatur.

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References Digital Mucous Cyst 32644518 
NIH
Cystae mucosae digitales, quae in manu saepe apparent, sunt forma ganglionis. Ganglion est tumor mollis, qui iuxta iuncturam vel tendinum invenitur. Cysta mucosa digitalis proprie oritur a dorso articulationis interphalangeae superne (COMM iuncturae). Haec saepe occurrunt cum OSTEOARTHRITIS iuncturae subiectae.
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.