Deformable 2D-3D medical image registration is an essential technique in Computer Integrated Surgery (CIS) to fuse 3D pre-operative data with 2D intra-operative data. Several factors may affect the accuracy of 2D-3D registration, including the number of 2D views, the angle between views, the view angle relative to anatomical objects, the co-registration error between views, the image noise, and the image distortion. In this paper, we investigate and assess the relationship between these factors and the accuracy of 2D-3D registration. We proposed a deformable 2D-3D registration method based on a statistical model. We conducted experiments using a hemi-pelvis model and simulated x-ray images. Some discussions are provided on how to improve the accuracy of 2D-3D registration based on our assessment.