Apparatus and method for computer-aided navigation-2D

Apparatus for computer-aided navigation with at least one of these related tracking system (TK), an orientation device (ME) with a first reference unit with tracking markers, a fluoroscopic image acquisition unit (CB), a first arithmetic unit (RCB), a volume image (VB) assigned source coordinate system (K0), a a surgical instrument (OPI) associated with the fourth coordinate system (K4), characterized in that the orientation device (ME) can be angled, a second reference unit variably with or removable side arms with image markers (RM) and a fixed association between the first and second reference unit is formed in that a third co-ordinate system (K3) is assigned to the orientation device (ME), that at least one image marker (RM) of the image marker (RM) is arranged the second reference star within the volume image (VB).