%0 Journal Article %A A L Williams %A G F Carrera %A J P Grogan %A D P Ullrich %T Vacuum phenomenon as a sign of an ununited, unstable dens fracture. %D 1987 %J American Journal of Neuroradiology %P 1144-1145 %V 8 %N 6 %U https://www.ajnr.org/content/ajnr/8/6/1144.full.pdf