The odontoid process should be scrutinised thoroughly for fractures by working around the outline of the odontoid process and then the body of C2. Peg fractures occur most commonly at the base

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Be aware of the possibility of Mach effect resembling a fracture.

Learning bite:

Be aware of the ‘Mach effect’ wherebynormal structures (typically the base of the skull) superimpose the Peg and falsely resemble an odontoid base fracture.

Click on the x-ray to enlarge.

The Mach effect occurs when the superimposition of normal structures gives the appearance of a fracture. Most typically, this occurs when the base of the skull gives the appearance of an odontoid base fracture.