Beware of three-dimensional motion change on Occipito-atlanto-axial joint complex following the fractures of the dens is very important for selecting treatment methods.The purnose of this in vitro study was to quantitatively determine e three-dimensional motions of the Occiputo-C:and C1-C2 oints.Eleven Occipito-atlanto-axial complex of fresh cadaveric specunen were studied,using the three-dimensional spinal montion measure and analysis system to document their physiological three-dimensional six degree of freedom rotation movements.An osteotomy at the base of the dens was performed.Pure movements of a maximum of 1.53N.m were applied,each movement was applied individually and in three load/unload cycles. Parameters of neutral zone,elastic zone and range of motion were computed.We found that their ranges of three-dimensional rotation and their neutral zones increased at different degrees after the dens fractured,and so did their coupling motion;but only were ranges of the main motion for left/right axial rotation and fiexion significantly increased,and coupling rotation motion when accompanied with main motion of flexion/extension and left/right lateral bending.