A method and device is provided for identifying a length of a bone screw to be inserted through a bore drilled in a predetermined drilling direction at a predetermined position through a bone. The predetermined drilling direction and the predetermined position is identified in an X-ray projection image of the bone, and the identified direction and position is transferred to a corresponding model of the imaged bone, wherein the bone model is a 3D model. The length of the bone screw is finally determined at the bone model based on the transferred direction and position.
