The present invention relates to an instrument and method for performing a partial knee replacement and, in particular, a single condyle replacement method. The instrument includes a tibial mounted platform which has formed thereon a keyed slot for receiving a spacer which spacer is positioned between the femur and tibia of the knee joint being replaced. The spacer receives thereon a positioning element which positioning element includes guides for guiding a drill to drill mounting holes for mounting further instruments used in the knee operation. In particular, the mounting holes are drilled when the femur and tibia are in a predetermined angular relationship with the spacer providing a predetermined spacial relationship. In this way, the mounting holes are properly determined and positioned with respect to the relationship between the femur and tibia. A saw guide is mounted to the mounting holes and used to guide a saw when cutting the anterior chamfer, the posterior chamfer and the posterior femoral cut. A further drill guide is then mounted to the mounting holes after removal of the saw guide for drilling holes for receiving the lugs of the condyle prosthesis.