In order to lengthen bones by several centimetres, 3D-Surgery usually uses the method known as “Distraction Osteogenesis”. The bone is gently, surgically cut at a carefully defined level. The artificial fracture begins to heal after a few days and forms the so called “callus”. Longitudinal growth (“distraction”) can now begin. The two healing ends of the fracture are slowly and continuously (usually 1 mm per day) moved away from each other. Gradually, new, biologically complete bone forms in this interspace. To stabilise the bone during this time, implants are inserted, for example fully implantable, motorised intramedullary nails. Like a ballpoint pen refill, they can be inserted into the bone through tiny incisions and later removed in the same way. Less reliable methods such as external fixators or plates only need to be used in very rare cases