Abstract: Objective To investigate the feasibility of using two-and three-dimensional(2D/3D)image registration for establishing a testing system of 3D kinematics of the spine in vivo.Methods CT data of the adult human lumbar spine were collected and the two orthogonal images of the same specimen were captured using an X-ray fluoroscope at two different positions.The 3D computer models of L3 and L4 vertebrae were reconstructed.A virtual fluoroscope was then created with solid modeling software to reproduce the relative positions of the orthogonal images.Two virtual cameras in the software were used to represent the X-ray sources.The 3D computer models of the L3 and L4 vertebrae were then introduced into the virtual fluoroscope respectively and projected onto the orthogonal images by the two virtual cameras.By matching the projections of the 3D model to the orthogonal images of L3 and L4 vertebrae,the 3D positions of L3 and L4 were obtained.After calculation,the relative displacement and angle of L3 were determined.Results After 2D/3D image registration,the relative displacement and angle were calculated.Compared with position I,the positional changes of L3 were represented with an extension of 5.86°,left bending of 1.85° and right rotation of 2.96°.Conclusion 2D/3D image registration allows the simulation of 3D kinematics of the spine in vivo,but the efficiency and accuracy of this technique need further evaluation.