The implantable prosthesis for replacing a human knee joint comprises a femoral prosthetic element (12) and a tibial prosthetic element (14) which are articulated with one another by means of a joint device (30) for the purpose of executing a knee bend. The tibial prosthetic element (14) has a through-opening (40) for the passage of a tool (84), a rod-like shaft anchoring part (32) or an intramedullary lengthening nail (80). In order to reduce the extent of a surgical intervention and thus the risk of infection also for the lengthening of the tibia, the joint device (30) is designed such that it forms a guide channel (42) for the passage of the tool (84), the rod-like shaft anchoring part (32) or the intramedullary lengthening nail (80) from the outside, which guide channel is in alignment with the through-opening (40) and is in connection therewith when the tibial prosthetic element (14) and the femoral prosthetic element (12) are arranged at a predetermined angle relative to one another in a functional position corresponding to a knee bend.
