1. A computer assisted orthopedic surgery system comprising: a first reference marker attachable to a femur: a second reference marker attachable to a patella; a tracking device for tracking a position of the patella attached to the second reference marker with respect to the femur attached to the first reference marker; a model of a femur bone that is not specific to a patient's knee joint; a computer configured to deform the model of the femur bone in response to the tracking device tracking a sensor that Is moved in proximity of the patent's knee joint; and a display connected to the computer for displaying a planned position of a femoral component on the model of the femur and the tracked position of the patella relative to the femur.