Apparatus for use in arthroplasty on a knee joint

A method of, and apparatus for, arthroplasty of a knee joint comprising obtaining a desired spacing of the femur from the tibia by inserting a spacer into the knee joint between the femur and the tibia. The spacer is used as a reference for securing a guide jig in position about the knee joint for guiding resection of one or both of the femur and tibia for fitting of tibial and/or femoral prostheses. A method of resecting the femur while moving the tibia through an arc of motion to remove bone to the desired depth, a method of determining the position of the guide jig, the guide jog, an alignment device and the spacer are also described.