A method includes obtaining a data file that includes print instructions for causing a three dimensional printer to print a three dimensional object, wherein the data file describes external surfaces of the object, and providing user instructions for positioning a preformed structure so that the three dimensional printer can print the object around the preformed structure, wherein the preformed structure has a size and a shape to be subsumed within the external surfaces of the object. The method further comprises modifying the print instructions so that the printer will print the object around the preformed structure, and printing material around the preformed structure using the modified print instructions to form the object.