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Once solved, the FE model was validated quantitatively via comparison of the output of the FE simulation with measurements from the MRI images. Two comparisons were performed: 1) the sitting induced gross displacement of soft tissues; 2) the sitting induced position shift of the muscle underneath the ischial tuberosity (Group1).
1) Sitting Induced Gross Displacement of Soft Tissues Computed by FE Analysis and that Measured from MRI Images: It was assumed that the bony structure remained the same when loaded. A Cartesian coordinate system was defined based on the femur-pelvis bony structure. This coordinate system took the center of the femoral head as its origin. The X-axis was pointing to the distal along the femoral shaft. The Y- and Z- axes were pointing to the medial and the superior, respectively (Fig. 5). Thirty regions of interest (ROIs) were identified over the skin of the sitting area (Fig. 5). For measurement on MRI images, the morphological alteration of buttock soft tissues induced by sitting pressure of 20.34±2.79KPa was identified for these ROIs by comparing MRI images obtained in loaded condition to the matching image from unloaded condition. On each image, vectors pointing from the origin of the coordinate system to the specific locations on the skin were constructed to compute the gross displacement. In order to compare the differences in different portions of the buttock, 4 regions, the distal-medial (DM), distal-lateral (DL), proximal-medial (PM) and proximal-lateral (PL) regions, were defined (Fig. 5).
The same coordinate system mentioned above was established for the FE model. The 30 ROIs were also identified for the FE model for both the “Without Sitting Pressure” and “With Sitting Pressure” configurations. The sitting induced changes of the coordinates of the FE nodes in these ROIs were taken as the FE predicted gross displacement. These displacements were then compared with the measurements from MRI images.
2) Position Shift of the Muscle Group1 beneath the Ischial Tuberosity: In the sitting condition, it is apparent that the tissue underneath the ischial tuberosity is loaded more than tissues at other locations. Therefore, the muscle here may be pushed away from its original position. This shift of position of muscle Group1 can be used as a validation parameter. A small area right beneath the ischial tuberosity was selected to calculate the shift of the muscle Group1 induced by sitting load of 20.34 kPa (Fig. 6). Taking the tip of the ischial tuberosity as the center, this area covered ±15mm in the proximal-distal direction and ±6mm in the medial-lateral direction (Fig. 6). For this volume, in the medial-lateral direction, 12 sagittal slices were taken for the FE/MRI models. On each of these sagittal slices, 11 lines in anterior-posterior direction were defined with a distance of 3mm between neighboring lines. Therefore, altogether 132 anterior-posterior lines were determined in this volume. For MRI model, the proximal-distal shift of this volume under sitting load was measured as the average proximal-distal shift of these 132 lines. For the FE model, the proximal-distal shift of this volume was obtained by identifying the changes of the coordinates of the FE nodes within this volume.
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FE Model Coordinate Systems
The Cartesian coordinate system used in the study. The origin of the coordinate system was at the center of the femoral head. The X-axis was along the femoral shaft pointing to the distal. The Y- and Z- axes were pointing to the medial and the superior, respectively. Thirty regions of interest (ROIs) were identified over the skin of the sitting area. The buttock-thigh area was divided into 4 regions, the distal-medial (DM), distal-lateral (DL), proximal-medial (PM) and proximal-lateral (PL) regions.
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Sagital View of FE Model and Measured Images
A sagittal view of the small area right beneath the ischial tuberosity which was selected to calculate the shift of the muscle Group1 induced by sitting load of 20.34 kPa. This area covered 30mm in the proximal-distal direction and 12mm in the medial-lateral direction and centered at the tip of the ischial tuberosity. This shift was measured from the MRI images (left) as well as from the sections of the FE (right) analysis
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Comparison Between FE Model and Measured Displacement
Gross displacement in anterior-posterior direction from 30 ROIs distributed in 4 main regions of the buttock-thigh: distal-lateral (DL), proximal-lateral (PL), distal-medial (DM) and proximal-medial (PM): A) measured from MRI images, and B) predicted by the FE analysis, and C) The average difference of the displacement between those measured on MRI images and those from FE analysis.
Fig. A gives the actual anterior-posterior gross displacement measured from MRI images (16.8±16.5mm) for the 30 ROIs. ANOVA analysis detected a significant (P<0.001) “region” effect on the anterior-posterior displacement. The largest values were seen in the PM region of the buttock covering the ischial tuberosity (36.6±9.0mm, P<0.002).
2) From FE Model Prediction and Comparison with Those from MRI.
Fig. B gives the gross displacement in the anterior-posterior direction predicted from the FE analysis (10. 7±8.0mm) for the 30 ROIs. ANOVA analysis detected a significant (P<0.001) “region” effect and the largest displacements were seen also in the PM region (18.1±5.8mm, P<0.002) of the buttock covering the ischial tuberosity. The displacement pattern in the posterior-anterior direction was compared with that measured from MRI images (Fig. 7a).
The difference between the anterior-posterior displacement obtained from MRI and the FE model was 6.2±10.4mm over the 30 ROIs (P=0.003).
The difference between the actual and the computed displacement is shown in Fig. C.
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