Table of Contents
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Lower body fixed values
The Newington-Gage model is used to define the positions of the hip joint centers in the pelvis segment.
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The position of the top of the lumbar vertebra 5 (the reference point for Dempster data) is then estimated as
(LHJC + RHJC)/2 + (0.0, 0.0, 0.828) * Length(LHJC – RHJC)
where the value 0.828 is a ratio of the distance from the hip joint center level to the top of the lumbar 5 compared to distance between the hip joint centers on the pelvis mesh.
Knee and ankle offsets are then calculated by adding half the measured joint width and marker diameter to give the distance from the center point of the marker to the joint center.
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If the distance between the first and last frame of the LASI marker is less than a threshold of 800mm however, the progression frame is calculated using the direction the pelvis is facing during the middle of the trial. This direction is calculated as a mean over 10% of the frames of the complete trial. Within these frames, only those which have data for all the pelvis markers are used. For each such frame, the rear pelvis position is calculated from either the SACR marker directly, or the center point of the LPSI and RPSI markers. The front of the pelvis is calculated as the center point between the LASI and RASI markers. The pelvis direction is calculated as the direction vector from the rear position to the front. This direction is then used in place of the LASI displacement, as described above, and compared to the laboratory X and Y axes to choose the Progression Frame.
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Pelvis
First the pelvis segment coordinate system is defined from the waist markers. The origin is taken as the midpoint of the two asis markers. The dominant axis, taken as the Y axis, is the direction from the right asis marker to the left asis marker. The secondary direction is taken as the direction from the sacrum marker to the right asis marker. If there is no sacrum marker trajectory, the posterior markers are used. If both are visible, the mean is used. If just one is visible, then that one is used. The Z direction is generally upwards, perpendicular to this plane, and the X axis generally forwards.
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Alternatively, the Knee Alignment Device (KAD) may be used. This must be placed on the patient during the static trial to indicate the plane of the knee joint center. Then the model calculates the relative angle of the Thigh wand marker, and this angle is used in the dynamic trial to determine the joint center without the KAD. This technique relies on the accurate placement of the KAD, rather than the accurate placement of the wand marker.
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In the dynamic model, the KJC is determined using the modified chord function, from the global position of the HJC, the thigh wand marker (THI), and the knee marker (KNE), together with the knee offset (KO), and thigh wand angle offset offset (
) from the subject measurements.
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The ankle joint center is determined in a similar manner to the knee joint center (see Knee joint center).
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In the dynamic trial, and static trials without a KAD, the ankle joint center is calculated from the knee joint center, shank wand marker and ankle marker with the ankle offset and shank rotation offset using the modified chord function. Thus the ankle joint center is at a distance of ankle offset from the ankle marker, and the angle between the KJC-AJC-ANK plane and the KJC-AJC-TIB plane is equal to the tibia rotation offset.
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Tibia
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A second tibia is also generated representing the tibia before tibial torsion is applied, by rotating the X and Y axes of the Tortioned Tibia round the Z axis by the negative of the tibial torsion (i.e. externally for +ve values). This represents the proximal end, and is used to calculate the knee joint angles.
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Foot
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The Static offset angles (Plantar Flexion offset and Rotation offset) are then calculated from the 'YXZ' Cardan Angles between the two segments (rotating from the 'uncorrected' segment to the heel marker based foot segment). This calculation is performed for each frame in the static trial, and the mean angles calculated. The static plantar-flexion offset is taken from the rotation round the Y axis, and the rotation offset is the angle round the X axis. The angle round the Z axis is ignored.
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