After entering the middle ear cleft, cholesteatoma can extend into surrounding spaces and destroy adjacent bone.
Sequence and routes of extension
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Cholesteatoma first follows the path of least resistance and subsequently causes enzymatic bone destruction.
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An attic cholesteatoma may extend backwards into the aditus, antrum and mastoid, or downwards into the mesotympanum.
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Medial extension may surround the incus, the head of the malleus, or both.
Bone destruction
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Structures that may be destroyed or eroded include the ear ossicles, bony labyrinth, facial nerve canal, sinus plate and tegmen tympani, resulting in complications.
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Bone destruction is attributed to collagenase, acid phosphatase and proteolytic enzymes liberated by associated osteoclasts and mononuclear inflammatory cells.
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The earlier explanation of bone destruction by pressure necrosis is rejected in the source.