An internal human prosthesis used in surgical procedures to treat flatfoot deformities, including pediatric, adult congenital, and adult acquired deformities. The internal prosthesis is inserted between the human talus and calcaneus bones. In a preferred embodiment, the prosthesis has a truncated conical shape along its long axis and a central bore through the center of its long axis. The base of the prosthesis includes a concave depression with a polygonal recess in its center.