DETERMINING A RANGE OF MOTION OF AN ARTIFICIAL KNEE JOINT
A data processing method for determining a range of motion of an artificial knee joint which connects a femur and a tibia via a medial ligament and a lateral ligament, wherein at least the femur comprises an implant which forms a medial condyle and a lateral condyle, the method comprising the steps of: acquiring the maximum lengths of the lateral ligament and the medial ligament for a particular flexion angle of the knee joint; calculating a first virtual position between the femur and the tibia in which the lateral condyle of the femoral implant touches the tibia and the medial ligament is stretched to its maximum length; calculating a maximum valgus angle of the range of motion from the first virtual position; calculating a second virtual position between the femur and the tibia in which the medial condyle of the femoral implant touches the tibia and the lateral ligament is stretched to its maximum length; and calculating a maximum varus angle of the range of motion from the second virtual position.
1 . A data processing method for determining a range of motion of an artificial knee joint which connects a femur and a tibia via a medial ligament and a lateral ligament, wherein at least the femur comprises an implant which forms a medial condyle and a lateral condyle, the method comprising the steps of:
acquiring the maximum lengths of the lateral ligament and the medial ligament for a particular flexion angle of the knee joint;
calculating a first virtual position between the femur and the tibia in which the lateral condyle of the femoral implant touches the tibia and the medial ligament is stretched to its maximum length;
calculating a maximum valgus angle of the range of motion from the first virtual position;
calculating a second virtual position between the femur and the tibia in which the medial condyle of the femoral implant touches the tibia and the lateral ligament is stretched to its maximum length; and
calculating a maximum varus angle of the range of motion from the second virtual position.
2 . The method according to claim 1 , wherein the length of a ligament is defined as the distance between the point on the femur at which the ligament connects and a plane defined with respect to the tibia.
3 . The method according to claim 2 , wherein the plane which is defined with respect to the tibia is a tibial cutting plane.
4 . The method according to claim 1 , wherein calculating a virtual position includes the steps of:
calculating a virtual contact position in which both the lateral condyle and the medial condyle of the femur touch the tibia; and
rotating the femur about the contact point between one of the condyles and the tibia until the opposing ligament is stretched to its maximum length.
5 . The method according to claim 4 , wherein calculating a virtual contact position comprises the steps of:
rotating the femur until both condyles have the same distance from the tibia; and
moving the femur translationally relative to the tibia until both the condyles of the femur touch the tibia.
6 . The method according to claim 1 , wherein calculating a virtual position includes the steps of:
determining a point on the femur at which the ligament connects for a relative position between the femur and the tibia in which the ligament is stretched to its maximum length; and
rotating the femur about said point until the condyle opposite said ligament touches the tibia.
7 . The method according to claim 1 , wherein a surface model of the femur is used for the calculating steps.
8 . The method according to claim 1 , wherein the condyles of the femur are modelled as ellipses.
9 . The method according to claim 4 , wherein when the rotation is performed, the axis of rotation is parallel to a cutting plane of the tibia and lies within the sagittal plane of the tibia.
10 . The method according to claim 1 , wherein the maximum length of a ligament is calculated from a transformation matrix which represents a relative position between the femur and the tibia in which the ligament is stretched to its maximum length.
11 . A computer program embodied on a non-transitory computer readable medium which, when running on a computer or when loaded onto a computer, causes the computer to determine a range of motion of an artificial knee joint which connects a femur and a tibia via a medical ligament and a lateral ligament, wherein at least the femur comprises an implant which forms a medial condyle and a lateral condyle, by performing the steps of:
acquiring the maximum lengths of the lateral ligament and the medial ligament for a particular flexion angle of the knee joint;
calculating a first virtual position between the femur and the tibia in which the lateral condyle of the femoral implant touches the tibia and the medial ligament is stretched to its maximum length;
calculating a maximum valgus angle of the range of motion from the first virtual position;
calculating a second virtual position between the femur and the tibia in which the medial condyle of the femoral implant touches the tibia and the lateral ligament is stretched to its maximum length; and
calculating a maximum varus angle of the range of motion from the second virtual position.
12 . A computer on which the computer program according to claim 11 is running or into the memory of which the computer program is loaded.
13 . A medical navigation system comprising a computer according to claim 12 and at least one of a stereoscopic camera and an electromagnetic receiver, wherein the stereoscopic camera and the electromagnetic receiver are operatively coupled to the computer.