A mobile bearing unicondylar tibial knee prosthesis includes a tibial base plate and a tibial articular surface component. The inferior surface of the articular surface component engages the superior surface of the base plate in sliding relationship. The tibial articular surface and a stop on the tibial base plate define a tab and groove engagement between them such that the tibial articular surface component is permitted to slide anteriorly-posteriorly on the tibial base plate while movement of the tibial articular surface component superiorly away from the base plate is restricted.