A method of producing a modification plan for producing a surgical guide from an impression element includes: obtaining surface data representing a configuration of a surface of an impression element providing an impression of a surgical site obtaining image data of a patients anatomy obtaining surgical plan data providing a surgical plan with respect to features in the image data representing anatomical features of the patients anatomy registering the impression element using the surface data and the image data with anatomical features of the patients anatomy and producing a modification plan from the surgical plan data using the registration of the impression element with anatomical features of the patients anatomy, the modification plan being a plan for modifying the impression element.
