A knee prosthesis including a femoral implant, a tibial implant, a tibial insert and a cam post. The femoral implant is secured to a femur and has a gap for receiving a natural PCL. The tibial implant is secured to a tibia. The tibial insert comprises a medial cavity with a rotational axis. The tibial insert has articulating surfaces that match the contours of the condyles of the femoral implant and a notch for retaining the natural PCL. The tibial insert has a medial boss that aligns with the medial cavity allowing it to rotate about the rotational axis. A post is secured to the tibial implant and enters a hole of the tibial insert providing anterior and posterior stops for the rotation of the tibial insert about the rotational axis.