A modular prosthesis system for replacement of a head portion of a radius. The prosthesis system includes a head component having a first connection portion that connects to a second connection portion and a collar component having the second connection portion and a third connection portion. The system also includes a stem component including a fourth connection portion that connects with the third connection portion, the stem component having a stem anchoring portion that connects to the radius. The collar component provides the modular geometry to the prosthesis without having to have an increased number of head components and stem components with variable lengths and angles.