An instrument system for setting the internal-external rotation of a prosthetic femoral component with respect to a proximal tibia has a bone resection instrument having a planar surface for engaging a planar surface of a resected distal femur. The bone resection instrument system has a cutting guide for guiding a tool for resecting a posterior surface of the distal femur and forming a cylindrical surface on the posterior condyle intersecting the planar surface. The cylindrical surface defined by an axis extending in a direction generally perpendicular to the resected planar surface. The system includes a trial component having a planar surface for engaging the planar surface of the resected distal femur and having posterior condyles having a cylindrical anteriorly facing surface engagable with the resected cylindrical surface on the posterior condyle.
