Once each pilot hole has been drilled the correct length of screw must be selected. The shafts of bones like the radius are hollow cylinders with a cavity inside - this is where the marrow is found. A bone screw must be long enough to go through both sides of the cylinder, however not too long or the soft tissues on the other side would be damaged. A special instrument is used to measure this length, and the correct screw is chosen. This is then carefully screwed into place (pictured right) and tightened to hold the plate onto the bone.