Cholesteatoma commonly perforates -
Lat. semicircular canal
Sup. semicircular canal
Promontory
Oval window
Cholesteatoma has the property to destroy the bone by virtue of the various enzymes released by it.
Structures immediately at the risk of erosion are:

– Long process of incus.Q
– Fallopian canal containing facial nerve. Q
– Horizontal/lateral semicircular canal. Q