NOVELTY - The method involves placing a tibia (1) and a femur (2) in a set of positions relative to each other. A mobility test is performed on a knee joint e.g. patella, in the set of different positions of the tibia relative to the femur. A navigation system (9) determines the positions of two axial points (15) relative to each other during the test by determining the positions and orientations of the two tracking devices, where the two devices are reference stars (4, 5). A stability and mobility of the knee or knee joint is ascertained from the determined positions of the two axial points.    USE - Method for determining stability and mobility of a knee or knee joint e.g. tibia, femur, and patella, of a patient e.g. child and athletic patient, for image-guided surgery by using medical navigation system, and used by a surgeon and physician.    ADVANTAGE - The method provides simple and quick knee or knee joint registration and allows simple and quick determination of knee or knee joint mobility and stability. The method provides a number of calculated positions of the axial points and the positions of the tibia axis and the femur axis. The time needed for determining stability and mobility is greatly reduced. The method allows restoring and maintaining of the previous or original stability and mobility of the knee during and after a knee joint operation, and allows checking of how far the stability or mobility of the knee or knee joint after the operation deviates from the stability or mobility before the operation.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for the following:    (1) a computer-readable medium storing a computer program for performing a method for determining stability and mobility of a knee or knee joint of a patient    (2) a device for determining stability and mobility of a knee or knee joint of a patient.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view illustrating a recording of a reference point on a tibia of a patient using a pointer.    Tibia (1)    Femur (2)    Two reference stars (4,5)    Reference point (6)    Pointer (8)    Tip (8a)    Navigation system (9)    Axial points (15)    Camera (17)