What is the ice-cream cone sign on high-resolution CT of the temporal bone?
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Answer:
The ice-cream cone sign is the normal appearance of the malleoincudal complex on axial high-resolution CT at the level of the epitympanum: the rounded head of the malleus forms the scoop of ice cream and the tapering short process and body of the incus form the cone pointing posterolaterally. Recognising it is a rapid way to confirm that the two ossicles are present, correctly articulated and normally orientated, and any distortion of the appearance is an alerting finding. In ossicular trauma the incudomalleolar joint dislocates and the cone separates from the scoop, which is the commonest ossicular injury after temporal bone fracture; in cholesteatoma the ossicles are eroded, most often the long process of the incus and the stapes superstructure, and the soft tissue mass displaces the chain medially; in chronic otitis media the ossicles may be fixed or resorbed. The related sagittal and coronal correlates are the molar tooth appearance of the incus body with its two processes and the parallel course of the incus long process with the stapes. Assessment must be systematic and include the scutum, the tegmen, the lateral semicircular canal, the facial nerve canal and the ossicular chain, since these are the structures the surgeon needs described before middle ear surgery.
Why is it called so?
On the axial image the round malleus head sits on the tapering incus like a scoop of ice cream on a cone.
Pathophysiology
The malleus and incus develop from the first and second pharyngeal arches and articulate at a true synovial joint, the incudomalleolar joint, which lies in the epitympanic recess. Because the joint is orientated obliquely and the two ossicles have characteristic shapes, a single axial section through the attic captures both in a reproducible configuration. Loss of the appearance means the relationship has been disturbed, whether by traumatic disruption of the joint capsule and its ligaments, by erosive resorption from the enzymatic and pressure effects of a cholesteatoma matrix, or by congenital malformation in which the ossicles fail to separate from each other or from the tegmen.
Alternative names: Ice cream on a cone appearance of the ossicles
Other associated named signs: Molar tooth sign of the incus on sagittal reformats and the Prussak space landmarks used in cholesteatoma reporting
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