A knee prosthesis including femoral and tibial implants, a tibial insert and a cam post assembly. The femoral implant is secured to a femur and has a cam feature and condyles. The tibial implant is secured to a tibia. The tibial insert has articulating surfaces that match the contours of the condyles of the femoral implant, and may have a medial boss that aligns with the tibial implant to provide rotation about a medially displaced rotation axis. A cam post is secured to the tibial implant and passes through a channel of the tibial insert. A motion limiting structure provides limits to motion between the tibial insert and the tibial baseplate. The cam post assembly interacts with the femoral implant cam feature and allows more anatomically correct rollback and femoral external rotation during knee flexion, and varus/valgus support to the femur.