The principle of the biological growing prosthesis is based on the idea of Dr. R. Baumgart⁽¹⁾ and combines the tumour prosthesis with a extentable intramedullary nail. It is consequently not the prosthesis which is extended, but the bone by means of callus distraction. The finally used intramedullary nails are manufactured by the company Wittenstein intens and have been used for several years. As there will be no difference in leg length at the time of tumour resection and implantation of the prosthesis and no growth of bone during chemotherapy, a „dummy“ nail is attached to the prosthesis for primary implantation. The interim prosthesis is an adequate implant which is designed to bear  the corresponding mechanical loads over several years. As soon as there will be a significant difference in leg length, the „dummy nail“ should be replaced during a second surgery by a computer controlled distraction device. Thereafter the leg lenghtening can begin. The bone can maximally be extended with 80 mm.