Apparatus and methods for replacing a knee joint with an implant prosthesis are described. The method includes distracting the knee, locating the pre-diseased joint line of the knee and referencing cuts to the femur and tibia in the coronal plane relative to the joint line. The method further comprises determining the posterior slope of a cut to the tibia in the sagittal plane such that the gaps between the tibia and femur in extension and flexion are substantially equal. Apparatus for use in the method is described, including a distractor for distracting the joint, a joint level finder for finding the pre-diseased joint line and a flexion gap optimisation device.
