METHOD AND APPARATUS FOR DETERMINING A LEG LENGTH DIFFERENCE AND A LEG OFFSET
A data processing method, performed by a computer, for determining a leg length difference and a leg offset difference of a patient's leg including a femur connected to a pelvis, comprising the steps of: —determining a first landmark vector between a femoral landmark and a second landmark at a first point in time; —determining a second landmark vector between the femoral landmark and the second landmark at a second point in time which is later than the first point in time; —calculating an orthogonal projection of the first landmark vector into a sagittal plane and using the direction of the orthogonal projection of the first landmark vector into the sagittal plane as a leg length direction; —calculating a direction which is perpendicular to the sagittal plane and using this direction as a leg offset direction; and —calculating the leg length difference in the leg length direction and the leg offset difference in the leg offset direction from the first landmark vector and the second landmark vector.
1 . A data processing method, performed by a computer, for determining a leg length difference and a leg offset difference of a patient's leg including a femur connected to a pelvis, comprising the steps of:
determining a first landmark vector between a femoral landmark and a second landmark at a first point in time;
determining a second landmark vector between the femoral landmark and the second landmark at a second point in time which is later than the first point in time;
calculating an orthogonal projection of the first landmark vector into a sagittal plane and using the direction of the orthogonal projection of the first landmark vector into the sagittal plane as a leg length direction;
calculating a direction which is perpendicular to the sagittal plane and using this direction as a leg offset direction; and
calculating the leg length difference in the leg length direction and the leg offset difference in the leg offset direction from the first landmark vector and the second landmark vector.
2 . The method according to claim 1 , wherein a orientation of the sagittal plane is defined as being horizontal.
3 . The method according to claim 1 , wherein a orientation of the sagittal plane is determined as the orientation of a surface of an operating table on which the patient is located.
4 . The method according to claim 1 , wherein a orientation of the sagittal plane is determined from a plurality of inertial sensor data which are acquired from an inertial sensor attached to a tibia of the leg while the femur is locked in position and the tibia is flexed relative to the femur.
5 . The method according to claim 3 , wherein a first sagittal plane is determined for the first landmark vector, and a second sagittal plane is determined for the second landmark vector, and the orthogonal projections of the landmark vectors into the corresponding sagittal plane are calculated.
6 . The method according to claim 1 , wherein calculating the leg length difference and the leg offset difference involves calculating a first orthogonal projection of the second landmark vector into the sagittal plane, calculating a second orthogonal projection of the first orthogonal projection onto the projection of the first landmark vector, determining the leg length difference as the difference in length between the projection of the first landmark vector and the second projection of the second landmark vector, and calculating the leg offset difference as the difference between the components of the first landmark vector and the second landmark vector in the leg offset direction.
7 . The method according to claim 1 , wherein calculating the leg length difference and the leg offset difference involves calculating an orthogonal projection of the second landmark vector into the sagittal plane, rotating the orthogonal projection of the second landmark vector within the sagittal plane such that its direction matches the direction of the projection of the first landmark vector, determining the leg length difference as the difference in length between the projection of the first landmark vector and the rotated second landmark vector, and calculating the leg offset difference as the difference between the components of the first landmark vector and the second landmark vector in the leg offset direction.
8 . The method according to claim 1 , wherein calculating the leg length difference and the leg offset difference involves calculating a difference vector between the first landmark vector and the second landmark vector and decomposing the difference vector into its components in the leg length direction and the leg offset direction.
9 . The method according to claim 1 , comprising the steps of obtaining neutral position data which define a neutral position of the leg at the first point in time and obtaining second position data which define a position of the leg at the second point in time, wherein the second landmark vector is only determined if the second position data match the neutral position data.
10 . The method according to claim 9 , wherein the position data are acquired from an inertial sensor.
11 . The method according to claim 1 , wherein a landmark vector is calculated from two landmark reference vectors, wherein each landmark reference vector represents a vector between the respective landmark and a common reference point and at least one of the landmark reference vectors is determined using a light beam which is emitted from a light beam source and pointed at an offset point, the light beam source having a known distance from the landmark and a known orientation relative to the direct line from the light source to the landmark, by performing the steps of acquiring the direction of the light beam and the distance between the light beam source and the offset point, and calculating the landmark reference vector from the known distance between the light source and the landmark, the known orientation of the light source relative to the direct line from the light source to the landmark, the direction of the light beam, the distance between the light beam source and the offset point and the reference offset between the offset point and the reference point.
12 . The method according to claim 11 , wherein the second landmark is a virtual landmark, namely the centre of rotation of the acetabulum; one of the landmark reference vectors is a virtual landmark reference vector for the virtual landmark and is determined as the average of two auxiliary reference vectors, wherein each auxiliary reference vector represents a vector between an auxiliary landmark and the common reference point and
comprising the steps of obtaining neutral position data which define a neutral position of the leg at the first point in time and obtaining second position data which define a position of the leg at the second point in time, wherein the second landmark vector is only determined if the second position data match the neutral position data,
wherein the position data are acquired from an inertial sensor, and
wherein the respective auxiliary landmark is used as the landmark.
13 . A non-transitory computer-readable storage medium storing a program which, when running on a computer, causes the computer to perform the steps of
determining a first landmark vector between a femoral landmark and a second landmark at a first point in time;
determining a second landmark vector between the femoral landmark and the second landmark at a second point in time which is later than the first point in time;
calculating an orthogonal projection of the first landmark vector into a sagittal plane and using the direction of the orthogonal projection of the first landmark vector into the sagittal plane as a leg length direction;
calculating a direction which is perpendicular to the sagittal plane and using this direction as a leg offset direction; and
calculating the leg length difference in the leg length direction and the leg offset difference in the leg offset direction from the first landmark vector and the second landmark vector.
14 . A device for determining a leg length difference and a leg offset difference of a patient's leg including a femur connected to a pelvis, comprising a computer that executes a program that causes the computer to
determine a first landmark vector between a femoral landmark and a second landmark at a first point in time;
determine a second landmark vector between the femoral landmark and the second landmark at a second point in time which is later than the first point in time;
calculate an orthogonal projection of the first landmark vector into a sagittal plane and use the direction of the orthogonal projection of the first landmark vector into the sagittal plane as a leg length direction;
calculate a direction which is perpendicular to the sagittal plane and use this direction as a leg offset direction; and
calculate the leg length difference in the leg length direction and the leg offset difference in the leg offset direction from the first landmark vector and the second landmark vector.
15 . The method according to claim 4 , wherein a first sagittal plane is determined for the first landmark vector, and a second sagittal plane is determined for the second landmark vector, and the orthogonal projections of the landmark vectors into the corresponding sagittal plane are calculated