Most difficult site to remove cholesteatoma in sinus tympani is related with -

The sinus tympani (Posterior facial ridge) is the posterior extension of the mesotympanum and lies deep to both the promontory and facial nerve. The medial wall of sinus tympani becomes continuous with the posterior portion of the medial wall of the tympanic cavity. This is the worst region for access because it is above pyramid, posterior to intact stapes and medial to facial nerve. A retro facial approach via mastoid is not possible because the posterior semi circular canal blocks the access.