A trial system for an implantable joint replacement includes a guide assembly having a baseplate and a guide plate, the guide plate rotatable relative to the base plate about a rotation axis extending through the guide plate and the baseplate while directly connected to the baseplate. An adjustment mechanism is actuable to selectively control the rotational juxtaposition of the guide plate relative to the baseplate. The rotation axis may be medially offset from the center of the guide assembly. A single instrument may be used to rotate the guide plate relative to the base plate and lock the plates in the selected juxtaposition. The guide plate provides in situ positionable cutting guidance for implantation of prosthetic joint components. The system may further include an articulating insert trial mounted on the guide assembly to determine preferred joint alignment and/or optimal positioning of the guide plate.
