While the child is deep asleep and pain-free (using general anesthesia), the surgeons carefully study the blood vessels and blood supply to the bone. An incision is made in the bone to be lengthened; usually the lower leg bone (tibia) or upper leg bone (femur). Metal pins or screws are inserted into and through the skin and bone above and below the bone incision and the skin incision is stitched closed. A metal device (such as an Ilizarov device) is attached to the screws in the bone and will be used later to gradually "crank" the cut bone apart, creating a space between the ends of the cut bone, which heals to form new bone. The lengthening device is used very gradually, lengthening the bone in extremely small steps, usually over the course of several months.