Most difficult site to remove cholesteatoma, i.e. sinus tympani is related with:

(b) Mastoid segment of facial nerve(Ref. Cummings, 6th ed., 2193)The sinus tympani area is the hidden area of the middle ear and is the most common site of residual cholesteatoma. It lies medial to the bulge of the vertical part of fallopian canal, i.e. mastoid segment of facial nerve. It being a hidden area, it is the most difficult site to remove cholesteatoma.It can be visualised through the posterior tympanotomy approach.