The fossula post fenestram is a sac-like evagination of connective tissue within the otic capsule just posterior to the oval window. The region around the fossula is one of the less common areas of predilection for otosclerosis.
It arises from the vestibule and is thus one of three extensions of the perilymphatic labyrinth through the otic capsule (the others being the perilymphatic duct/cochlear aqueduct and fissula ante fenestram) 1. Unlike the fissula ante fenestram, however, the fossula post fenestram seldom opens to the middle ear cavity 2.
The fossula is not visible on clinical imaging. It can be identified on histology in the majority of temporal bones from fetuses and children but less commonly in adults 2. It contains mesenchymal tissue including fibrous tissue, cartilage, and/or blood vessels 2,3.
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