A rotary mill is disclosed comprising a body portion (4) having a milling surface (10) and a central bore (20) extending along an axis of rotation (18) of the body portion (4) and a guide portion (6) having a guide body (22) and a guide peg (24) extending from the guide body (22), the guide peg (24) operable to be received in the central bore (20) of the body portion (4), the guide body (22) having at least one alignment feature (28, 30) which is the same as that of a prosthesis component. A method of implanting a unicondylar femoral component is also disclosed, the method comprising, a) reaming the femoral condylar surface to accept the unicondylar femoral component b) drilling peg holes for affixing the unicondylar femoral component c) affixing a guide portion of a rotary mill onto the prepared condylar surface using the drilled peg holes d) reaming a portion of bone anterior to the affixed guide portion e) removing the guide portion from the bone and f) affixing a unicondylar femoral component to the prepared condylar surface.
