Three-Dimensional Preoperative Virtual Planning in Derotational Proximal Femoral Osteotomy

In this protocol, we demonstrate that the axis of rotation of the femoral osteotomy and the axis of rotation of the femur does not coincide. This protocol makes it possible to visualize and correct it. By using the free 3D software in this study, the surgeon can plan the surgery and increase the precision of the osteotomy.

This technique is easily reproducible with simple step. We recommend practicing it many times to gain fluency. To begin, import the CT images in the DICOM format by clicking on the DCM icon on the top left corner of the screen, followed by clicking on Import DICOM Files on the left side of the screen.

Next, click on Dummy Patient Name on the top right of the screen, and click on Load on the lower right margin. To create a 3D biomodel, in the dropdown menu, choose the Legacy and Editor options. Press OK on the message that appears and wait for a new menu to appear on the left side of the screen.

Click on the Threshold Effect icon and move the bar in the lower box until only the bone is painted in the images on the right. This way, select the value of the Hounsfield units to be included in the model. Once the desired paint level is achieved, click on Apply.

The selection is marked with the color green here. Select the option Make Model Effect from the side menu, and select Apply, which generates the 3D model in the upper right window. Click on the golden frame.

Center the 3D view on the scene to center the image in the window. To save the created 3D biomodel, click on Save in the upper left margin. In the box that appears, uncheck other files, and only select the file Tissue.

In the second column in the dropdown menu, select STL. Import the STL image by selecting the Import option in the center of the screen. Look for the Select option in the menu on the left side.

Select the thickness of the brush using the Select tool, and double-click on the femur. Use the Select tool to select the part of interest. Look for the options, Modify and Invert in the side menu, and then press the delete button on the keyboard.

Then navigate to Edit, Generate Face Groups, and move the bar of Angle Threshold until the different structures have a different color. Next, using the Select tool, double-click on the femur, modify, invert, and press the Delete button on the keyboard. to make a solid model, navigate to Edit, Make Solid, Solid Type, and Accurate.

Then maximize the solid accuracy and mesh density values. Select the Export option from the side menu, and select the STL format and the folder to which the biomodel is exported. In the 3D builder software, click on the Insert icon at the top of the screen.

Then click on Add to import the biomodel to the scene. Click on Object, then Settle to fix the object to the work plane. To perform the femoral osteotomy, click on Edit and Split from the top menu.

When a rectangular cut plane appears, select Keep Both. Use the Move mode button on the bar in the lower margin of the screen to move the cutting plane horizontally and vertically. Use the Rotate mode button on the bar in the lower margin of the screen to rotate the plane around the femur.

Put the cutting plane parallel to the x-axis and perpendicular to the Y-axis. Click on Split. To calculate the femoral anteversion, insert the guides by clicking on Insert, Add, and choose the 3MF file.

Select only the proximal part of the femur on the right side of the screen, and press Control+X on the keyboard to cut the selection. The femoral diaphysis appears like this. Select the red circular guide and the purple circular guide together, on the right side of the screen.

Use the commands in the bottom margin panel to move the guides and put them in the center of the femoral diaphysis. Then use the inferior margin panel commands to adjust the size and ensure that all the edges touch the cortex of the bone. Press Control+V on the keyboard to paste the proximal femur again.

Select only the sphere on the right side of the screen and adjust the size, including all the edges touching the bone cortex. Select the proximal femur on the right side and cut it by pressing Control+X on the keyboard. Select only the red plane on the right side of the screen and place it to pass through the center of the sphere and the center of the circular guides.

Press Control+V on the keyboard to paste the proximal femur again. To perform the rotational osteotomy of the proximal femur, select the proximal femur, the red circumference, and the sphere on the right side. Make an internal derotational proximal femoral osteotomy of 20 degrees by adding 20 to the pitch using the commands on the panel of the lower margin.

To measure the new femoral anteversion, select the proximal femur and the red circumference, and press Control+X on the keyboard to cut these two elements. Select only the red plane and place it to pass through the center of the sphere and the center of the purple circular guide. Paste the proximal femur and the red circumference.

To perform the adjustment of the rotational osteotomy, select the femoral diaphysis and the red plane, and press Control+X on the keyboard to cut. Select the proximal femur, the sphere, and the red circumference, and move these three elements en bloc so that the center of the red circumference matches the center of the purple circumference. Recalculate the new femoral anteversion with the adjustment made.

The data here detail the values of the femoral anteversion obtained in two groups for the three magnitudes of the rotational osteotomy, 10, 20, and 30 degrees. When the adjustment was made so that the femur's rotation axis and the osteotomy's rotation axis coincided, the relationship between the planned correction and the final correction was a one-to-one ratio in the three correction magnitudes, but the same did not occur in group 1. The most important thing to remember is accuracy in performing the procedure, especially when creating the bone model, selecting only the bone of interest, and performing the osteotomy measurement before and after the adjustment.

In the field of rotational deformities of the femur, this technique opens the way to study the region of these deformities and the consequences they have on potential femoral pressure and anterior knee pain.