Registration

This document describes how the physical femur, tibia, and patella coordinate systems are registered to their digital models. Registration is used to define the osseous coordinate systems with respect to the digital model’s coordinate system. The digial models (usually surfaces) were defined using MR images, therefore the models are defined with respect to the MR image’s coordinate system. The MR image’s coordiante system will be referred to as the image coordinate system, and the digial models are initially defined with respect to this coordiante system.

At the end of the registration procedure, there will be femur, tibia, and patella coordinate systems that are defined with respect to to the image coordinate system.

DU Registration

During physical testing, several points around the femur, tibia, and patella were digitized.

This registration procedure registers points that were digitized during physical testing to the segmented osseous surfaces. The points that were digitized during physical testing were taken on the ossous parts of the anatomy, and specific anatomic landmarks were digitized to define the osseous coordinate system during testing. The transform that is used to register the digitized points to the osseous surface will be used to transform the digitized anatomic landmarks, and these transformed landmarks will be used to define the osseous coordinate system in the MR image coordinate system.

Method Overview

Optimization is used to minimize the total distance between the model’s surface and the transformed points. Rotations and translations in the xyz directions (rx, ry, rz, tx, ty, tz) are the control variables that are used to define a transform, and the transform will be applied to the digitized points.

The optimization will likely have several local minima. In an attempt to avoid these issues, an informed initial guess for the transform (rx, ry, rz, tx, ty, tz) will be used. The initial guess will be informed by the osseous landmarks that were digitized during physical testing. Similar landmarks will be defined in the digital model, and these will be used to define the initial transform.

Note

that the digitized points may extend beyond the proximal/distal end of the femur/tibia surfaces. These points should be excluded from the optimization to prevent the registration from erroneously being more proximal/distal. These points should be excluded from the optimization to prevent them from influencing the optimization results.

Specific details on the segmented surfaces and digitized points that are included in the registration optimization are in each section below

Femur

Table 5 Femur registration data - DU registration
Description File Type Comments
Osseous femur points txt The points that were digitized during physical testing. data-MD-DU02/DU02_raw_probed_points/Points_on_Femur/DU02_fem_Bone.txt
Osseous landmarks txt Osseous landmarks that were digitized during physical testing to define the femoral coordinate system. data-MD-DU02/DU02_raw_probed_points/Points_on_Femur/DU02_fem_GSPts.txt Table 6
Femoral cartilage txt The points that were digitized during physical testing around the femur’s articulating surface. data-MD-DU02/DU02_raw_probed_points/Points_on_Femur/DU02_fem_artgeo.txt
Osseous surface stl The segemented femoral surface
Femoral cartilage stl The segemented femoral cartilage surface
Image points fcsv The landmarks that were manually defined from the MR image

The digitized points around the osseous femur do not include any points on the distal end. Given that the articulating surface of the femur is an area of interest in the model, the femoral cartilage segmented surface and the digitized points around the femoral articulating surface (data-MD-DU02/DU02_raw_probed_points/Points_on_Femur/DU02_fem_artgeo.txt) will also be included in the optimization.

There is no description of the digitized points in data-MD-DU02/DU02_raw_probed_points/Points_on_Femur/DU02_fem_GSPts.txt, so the point descriptions were determined by plotting the digitized landmarks and the digitized femoral surface points (DU02_fem_Bone.txt). The descriptions can be found in Table 6.

Table 6 Point order description for DU02_fem_GSPts.txt - DU registration
Row number Point description
1 Medial femoral epicondyle
2 Lateral femoral epicondyle
3 Distal point between condyles
4 Posterior medial femoral condyle
5 Posterior lateral femoral conldyle

Tibia

Table 7 Tibia registration data - DU registration
Description File Type Comments
Osseous tibia points txt The points that were digitized during physical testing. data-MD-DU02/DU02_raw_probed_points/Points_on_Tibia/DU02_tib_bone.txt
Osseous landmarks txt Osseous landmarks that were digitized during physical testing to define the tibial coordinate system. Table 8. data-MD-DU02/DU02_raw_probed_points/Points_on_Tibia/DU02_tib_GSPts.txt
Tibial cartilage txt The points that were digitized during physical testing around the tibia’s articulating surface. data-MD-DU02/DU02_raw_probed_points/Points_on_Tibia/DU02_tib_artgeo.txt
Osseous surface stl The segemented tibial surface
Tibial cartilage stl The segemented tibla cartilage surface
Image points fcsv The landmarks that were manually defined from the MR image

The digitized points around the osseous tibia do not include any points on the proximal end. Given that the articulating surface of the tibia is an area of interest in the model, the tibial cartilage segmented surface and the digitized points around the tibial articulating surface (data-MD-DU02/DU02_raw_probed_points/Points_on_Tibia/DU02_tib_artgeo.txt) will also be included in the optimization.

There is no description of the digitized points in data-MD-DU02/DU02_raw_probed_points/Points_on_Tibia/DU02_tib_GSPts.txt, so the point descriptions were determined by plotting the digitized landmarks and the digitized tibial articulating surface (DU02_tib_artgeo.txt) and the PCL tibial insertion (DU02_PCL_tib.txt). The descriptions can be found in Table 8.

Table 8 Point order description for DU02_tib_GSPts.txt - DU registration
Row number Point description
1 Medial tibial plateau dwell point
2 Lateral tibial plateau dwell point
3 Proximal tip of the medial spine of the tibial eminence
4 Center of the tibial eminence
5 Center of the distal intramedullary canal

Patella

There is no description of the digitized points in data-MD-DU02/DU02_raw_probed_points/Points_on_Patella/DU02_latPFlig_pat.txt, so the point descriptions were determined by plotting the digitized landmarks and the digitized patellar insertion of the quadracept tendon (DU02_QuadTend_pat.txt) and lateral patellofemoral ligament (DU02_latPFlig_pat.txt). These descriptions. The descriptions can be found in Table 9.

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Fig. 46 The digitized points from the articulating surface of the patella.

Table 9 Point order description for DU02_Pat_GSPts.txt - DU registration
Row number Point description
1 Superior patella
2 Inferior patella
3 Medial patella
4 Lateral patella
5 Center of patella