Napoleon Hat Sign-inverted hat-like appearance of L5 vertebra on frontal view in severe anterolisthesis of L5 on S1
Naked Facet Sign-lack of coverage of one facet, usually in cervical spine, by its subluxed partner, seen on CT and plain films
Nubbin Sign-On HIDA scan, there is no evidence of the radionuclide in the gallbladder except for a little nubbin representing the cystic duct with tracer. This is a sign of obstruction in the gallbladder neck.
Nutcracker Fracture-compression fracture of cuboid between calcaneous and bases of 4th and 5th metatarsals