The present invention comprises devices, systems, and methods for elongating bone using an extension implant having a first end and a second end. The first end of the extension implant is inserted into an opening in the live bone and the second end of the extension implant is combined with an enlarged implant. A plurality of channels extend through the components to serve as conduits for delivering fluids and physiological signals which induce bone formation. Some embodiments include a subcutaneous cage assembly for helping to support the implant as the bone heals around it.