A nonmodular tibial prosthesis having a retainer for a modular bearing on the superior surface of a tibial base, a nonmodular primary bearing directly molded to the base, and a mechanical release member mounted on the tibial base in contact with the nonmodular primary bearing. The release member includes a wedge portion with a superiorly posteriorly inclined face, and is designed to be driven anteriorly upon actuation so as to force the primary bearing off of the tibial base, after which the primary bearing and release member are removed and replaced with a modular bearing.