Pseudofracture of the dens: Mach bands.

Mach bands are an optical phenomenon in which dark and light lines appear at the borders of structures of different radiodensity on radiographs. In most instances they do not cause a diagnostic dilemma. However, in a patient who has sustained trauma to the cervical spine, the commonly occuring negative Mach band across the base of the dens may be mistaken for a fracture. Failure to recognize this optical illusion may result in unnecessary treatment by skeletal traction. The production of Mach bands, methods of proving their illusory nature, and several illustrative cases are presented.