There is described a method and an apparatus for the optical 3D digitization of bodies and body parts which reveal non-visible regions which therefore cannot be detected by the 3D digitizer. A mechanical aid is fixed at these regions and protrudes into the measurement space visible for the 3D digitizer. On this visible part, it is provided with marks and is digitized together with the remaining, visible body parts. From the spatial position of the marks of these aids, important geometrical information of the non-visible parts, such as the spatial position, circumferential dimensions, etc., can be calculated, and the 3D model of the body or body part incomplete at these points can be completed therewith. Two applications from the field of orthopaedics are described by way of example.