Originally Posted by some internet article

BONE LENGTHENING
This series of treatments involves several surgeries, a long recovery period, and a number of risks. However, it can add up to 6 inches of length to a leg.
Metal pins or screws are placed through the skin and into the bone. Pins are placed above and below the cut in the bone. Stitches are used to close the wound.
A metal device is attached to the pins in the bone. It will be used later to very slowly (over months) pull the cut bone apart. This creates a space between the ends of the cut bone that will fill in with new bone.
When the leg has reached the desired length and has healed, another surgery is done to remove the pins.