Koerner's septum is an anatomic boundary in the temporal bone formed by the petrosquamous suture between the petrous and squamosal portions of the mastoid air cells, at the anatomic level of the antrum. Along with the middle ear ossicles, it is usually eroded in middle ear cholesteatomas. Superiorly, this continues as the petrosquamous suture, a normal anatomic structure that can be mistaken for fractures on temporal bone CT.
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Koerner's septum Wikipedia(Text) CC BY-SA