A computer assisted orthopedic surgery system in support of an arthroplasty surgery of a patient's knee joint including a first locatable element attachable to a first bone on one side of the patient's knee joint. The system includes a sensor in the form of a second locatable element. The sensor is movable in proximity of the patient's knee joint. A tracking device locates a plurality of positions of the sensor relative to the first locatable element. The system also includes an initial generic model of a knee joint that is not specific to the patient. A computer is configured to deform the generic model in response to the plurality of positions of the tracking device that are specific to the patient and to determine a position for a knee prosthesis on the deformed model. The determined position upon the deformed model is outputted and displayed on a display connected to the computer.
